Syn- and post-Laramide geology of the south-central Gravelly Range, southwestern Montana
by Ernest Jan Luikart
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in
Earth Sciences
Montana State University
© Copyright by Ernest Jan Luikart (1997)
Abstract:
The geologic history of post-Laramide basin evolution in the foreland of southwestern Montana has
been a matter of controversy. A complex assemblage of Upper Cretaceous to Tertiary sedimentary and
volcanic rocks which record some of that history are exposed on and near the crest of the Gravelly
Range. Past interpretations of their relations and tectonic implications conflict. The present
investigation of a portion of the southern Gravelly Range crest helps to resolve the physical
stratigraphy and ages of the post-Laramide deposits and suggests the following sequence of events: (1)
syn- and post-Laramide erosional beveling of the Madison-Gravelly arch; (2) Late Cretaceous
deposition of quartzite gravel from a thrust belt source, locally containing Archean metamorphic clasts
from a foreland source; (3) conformable transition to deposition of limestone conglomerate derived
from the Blacktail-Snowcrest arch, with interbedded siltstone, sandstone and lacustrine limestone,
deposited prior to the end of Laramide deformation; (4) final movement of Laramide faults; (5) erosion
represented by a 28-38 my-long unconformity; (6) deposition of tuffaceous mudstones beginning in the
Duchesnean (40-37 Ma) and proceeding into the Whitneyan (32-29 Ma) interrupted by erosion at about
32 Ma; (7) eruption of basalt flows from local vents between 33 and 30 Ma; (8) minor erosion followed
by early Miocene (23 Ma) eruption of an isolated mafic volcanic center; (9) emplacement of
Huckleberry Ridge Tuff at 2.1 Ma after erosion or nondeposition of Miocene strata; (10) significant
uplift of the range in Quaternary time; (11) Pleistocene deposition of glacial moraines in the deeper
valleys, and ongoing mass-movement and colluvial processes.
Conclusions differ from those of previous workers in that the quartzite gravel is older than the
limestone conglomerate, both units are Late Cretaceous rather than Paleogene, and basal exposures of
Renova mudstones produce a Duchesnean rather than a Chadronian local fauna. The broader
Conclusions support disruption of a broad Paleogene depositional basin by the present geometry of
basins and ranges in southwestern Montana. Extensional faulting did not predate 30 Ma and has offset
the Renova Formation by 3,350-5,300 m (11,000-17,400 ft) relative to adjacent grabens. The dense
welding of the Huckleberry Ridge Tuff on the range crest suggests that the 1,100 m. (3,600 ft.) of relief
relative to outcrops in the adjacent Madison valley is largely the result of tectonism during the
Quaternary. 
SYN- AND POST-LARAMIDE GEOLOGY OFTHE SOUTH-CENTRAL 
GRAVELLY RANGE, SOUTHWESTERN MONTANA
by
Ernest Jan Luikart
A thesis submitted in partial fulfillment 
of the requirements for the degree
of
Master of Science 
in
Earth Sciences
MONTANA STATE UNIVERSITY - BOZEMAN 
Bozeman, Montana
April 1997
11
APPROVAL
of a thesis submitted by
Ernest Jan Luikart
This thesis has been read by each member of the thesis committee and 
has been found to be satisfactory regarding content, English usage, format, 
citations, bibliographic style, and consistency, and is ready for submission to 
the College of Graduate Studies.
V- Z V - f  7  
Date
[ ) / 2 ,
Chairperson, Graduate Committee
Approved for the Major Department
£
2 ^ . ^ 2 ^ 1 ^ _
Head, Major Department
Approved for the College of Graduate Studies
Date Graduate Dean
iii
STATEMENT OF PERMISSION TO USE
In presenting this thesis in partial fulfillment of the requirements for a 
master's degree at Montana State University, I agree that the Library shall make 
it available to borrowers under rules of the Library.
If I have indicated my intention to copyright this thesis by including a 
copyright notice page, copying is allowable only for scholarly purposes, 
consistent with "fair use" as prescribed in the U.S. Copyright Law. Requests for 
permission for extended quotation from or reproduction of this thesis in whole 
or in parts may be granted only by the copyright holder.
Signature
Date f  F
iv
ACKNOWLEDGMENTS
I would like to thank my graduate advisor Dr. Dave Lageson for his 
support, advice and encouragement, and for turning me toward the light of 
structural geology. Thanks to committee members Drs. Jim Schmitt and Tony 
Barnosky for their helpful discussions and advice. Special thanks to committee 
member Dr. Mike O'Neill of the U.S.G.S. for initial inspiration, training in 
geologic mapping, and feedback through all the phases of this project. I 
profited from discussions with Dr. Bill Locke and my undergraduate advisor at 
Wesleyan University, Dr. Jim Gutmann.
Dr. Malcolm McKenna of the American Museum of Natural History 
introduced me many years ago to the joys of field vertebrate paleontology in the 
northern Rockies. More recently, he curated the fossils from this study, had key 
specimens prepared, and allowed me to wander unguarded in the Frick 
collection.
I cannot find words to thank Wendy Herrick for her help, faith and 
support, and for waiting patiently for our honeymoon during the course of this 
thesis.
I
TABLE OF CONTENTS
Page
INTRODUCTION....................................................................................................  1
Purpose of Study...........................
Geologic Setting..........................
Methods.........................................
Tertiary Basin-Fill Nomenclature
Tectonic Models..........................................................................................  n
PALEOZOIC AND MESOZOIC GEOLOGIC FRAMEWORK.......................... 13
Pre-Laramide Stratigraphy......................      13
Laramide Structures...................................................................................  14
SYN- AND POST-LARAMIDE GEOLOGY.........................................................  20
Erosion Surface..........................................................................................  20
Sedimentary and Volcanic Rocks.........................    21
Gravel of Black Butte....................................................................... 23
Description and Distribution..............................................  23
Age Limits and Regional Correlation................................ 25
Conglomerate of Red Hill...............................................................  27
Description and Distribution..............................................  27
Age Limits and Regional Correlation................................ 30
Renova Formation.......................................................   33
Description and Distribution...........................................  33
Age Limits and Regional Correlation...............................  37
Basalt and Phonolite of Black Butte.............................................  39
Description and Distribution..............................................  39
Age Limits and Regional Correlation...............   40
Huckleberry Ridge Tuff................................................................... 41
Description and Distribution ................   41
Age Limits and Regional Correlation...............................  42
Quaternary Deposits................................................................................... 43
Glacial T ill.................................................................................. ’....  43
Boulders of Uncertain Origin.........................................................  44
Landslide Deposits.......................................................................... 44
Individual Stratigraphic Sections........ .....................................................  45
Exposures at Black Butte................................................................  46
Exposures at Lion Mountain..........................................................  46
Exposures at Red Hill/Lazyman Hill.............................................  46
Exposures at Tepee Mountain.................................    50
Generalized Stratigraphic Column..........................................................  50
CM CO CO CO
Vl
TABLE OF CONTENTS— Continued
VERTEBRATE PALEONTOLOGY OF THE RENOVA FORMATION ............  52
Mammalian Biochronology...................................   52
Paleontology of the Field Area..................................................................  55
Tepee Mountain Locality..................    59
Black Butte Locality......................................................................... 65
Rapamys S ite ............................................................    66
Proposed Duchesnean Local Fauna.............................  68
TECTONIC HISTORY AND DISCUSSION.........................................................  70
Structural Disruption of the Renova Formation...............................    76
REFERENCES CITED.....................................................................    80
APPENDIX...............................................................................................................  88
XRF Analysis of Basalt from Tepee Mountain........................................  89
Radiometric Analysis of Basalt from
Tepee Mountain..........................................................................................  90
----------- ' 11 I . X . I M  , I Ii I I M ^ ' k
v ii
LIST OF TABLES
Table Page
1. Mammalian faunal lists for Renova Formation localities..........................  55
2. North American Land Mammal Age ranges for taxa from the
Tepee Mountain locality............................................................................. 61
3. North American Land Mammal Age ranges for taxa from the
Black Butte locality.........................    66
4. North American Land Mammal Age ranges for taxa from the
RapamyssAe...............................................................................................  68
5. Previous age assignments for the syn- and post-Laramide
deposits of this report..............................................   73
V lll
LIST OF FIGURES
Figure Page
1. Simplified tectonic map of southwestern Montana showing
the Rocky Mountain foreland bounded by the margins of
the Sevier thrust belt and the Snake River Plain...................................  4
2. Index map of contemporary ranges and basins of southwestern
Montana and east-central Idaho...............................................................  5
3. Index map of Gravelly Range, showing map area and selected
features........................................................................................................ . 7
4. Comparison of Tertiary stratigraphic schemes for
southwestern Montana basins.................................................................. 10
5. Schematic paleotectonic map of southwestern Montana,
showing interpreted structural pattern of Laramide-styIe
structures in the Late Cretaceous foreland.............................................  15
6. Silicified fault surface exposed in the thrust of Bighorn Mountain,
showing kinematic indicators........................................................ ...........  17
7. Anticline inferred to be cored by a blind continuation of the
Bighorn Mountain thrust.....................................................    18
8. Bighorn Mountain: an example of considerable local relief on
the exhumed Eocene erosion surface of the range............................... 22
9. Gravel of Black Butte, exposed in a landslide scarp immediately
west of Black Butte, showing clast lithology dominated by
very well-rounded Proterozoic quartzite cobbles.................................. 24
10. Crushed quartzite cobbles from the gravel of Black Butte,
shown on striated Muddy Formation sandstone which
underlies the gravel at this locality...........................................................  26
11. Outcrop of the conglomerate of Red Hill showing conglomerate composed
dominantly of Paleozoic limestone clasts and
minor Archean clasts, overlain by lacustrine limestone........................ 29
Figure Page
12. View of the west side of Lion Mountain, showing Renova
sedimentary rocks overlying the Madison Group and
capped by basalt flows............................ ....................... ..........................  35
13. View of the Black Butte volcanic neck, looking northwest......................... 36
14. Schematic stratigraphic section for exposures near the western
side of Black Butte....................................................................................... 47
15. Schematic stratigraphic section for exposures at the eastern
side of Lion Mountain.................................................................................. 48
16. Schematic stratigraphic section for exposures in the Red
Hill/Lazyman Hill area of Monument Ridge......................................... . 49
17. Schematic stratigraphic section for Tepee Mountain................................ 51
18. Recalibration of part of the Paleogene time scale.....................................  54
19. Paleontologic index map showing the Tepee Creek locality (T),
Black Butte locality (B), Rapamys site (R), and upper Lion
Mountain locality ( L).................................................................................... 58
20. Renova Formation mudstones, sandy siltstones, and thin
granule to pebble conglomerate layers which produce 
a Whitneyan fauna, upper slope of Lion Mountain .................. ............  60
21. Juvenile Hyaenodon skull from the newly described Tepee
Mountain locality, showing milk dentition (in preparation-
jaws not yet separated)...................................................................   62
22. Occlusal views of upper cheek teeth (top), and lower cheek
teeth (bottom) of the Eomyid rodent Protadjidaumo.................... ..........  63
23. Sketch of right jaw fragment showing P3-M1 of a large
unidentified Iagomorph resembling Megalagus, from the
Tepee Mountain locality ............................................................................... 64
24. Skull and lower jaw (image reversed) of Rapamys, a
Duchesnian last occurrence, from the Rapamys site............................. 67
ix
LIST OF FIGURES— Continued
XFigure Page
25. Simplified diagrammatic cross-sections showing the proposed
Late Cretaceous sequence of tectonic and depositional
events in the region of the field area........................................................  71
26. Simplified depiction of two possible models for Paleogene
deposition of Renova Formation sediments in the Gravelly
Range area and surrounding regions.....................................................  77
LIST OF FIGURES— Continued
LIST OF PLATES
Plate Page
1. Geologic map of the south-central Gravelly Range with
cross-section and description of map units.....................................in pocket
2. Generalized stratigraphic column of this report shown on
right, with correlation chart on left showing mammalian
local faunas and stratigraphy of Cenozoic basins of
southwestern Montana....................................................... ................... in pocket
xii
ABSTRACT
The geologic history of post-Laramide basin evolution in the foreland of 
southwestern Montana has been a matter of controversy. A complex 
assemblage of Upper Cretaceous to Tertiary sedimentary and volcanic rocks 
which record some of that history are exposed on and near the crest of the 
Gravelly Range. Past interpretations of their relations and tectonic implications 
conflict. The present investigation of a portion of the southern Gravelly Range 
crest helps to resolve the physical stratigraphy and ages of the post-Laramide 
deposits and suggests the following sequence of events: ( I)  syn- and post- 
Laramide erosional beveling of the Madison-Gravelly arch; (2) Late 
Cretaceous deposition of quartzite gravel from a thrust belt source, locally 
containing Archean metamorphic clasts from a foreland source; (3) 
conformable transition to deposition of limestone conglomerate derived from the 
Blacktail-Snowcrest arch, with interbedded siltstone, sandstone and lacustrine 
limestone, deposited prior to the end of Laramide deformation; (4) final 
movement of Laramide faults; (5) erosion represented by a 28-38 my-long 
unconformity; (6) deposition of tuffaceous mudstones beginning in the 
Duchesnean (40-37 Ma) and proceeding into the Whitneyan (32-29 Ma) 
interrupted by erosion at about 32 Ma; (7) eruption of basalt flows from local 
vents between 33 and 30 Ma; (8) minor erosion followed by early Miocene (23 
Ma) eruption of an isolated mafic volcanic center; (9) emplacement of 
Huckleberry Ridge Tuff at 2.1 Ma after erosion or nondeposition of Miocene 
strata; (10) significant uplift of the range in Quaternary time; (11) Pleistocene 
deposition of glacial moraines in the deeper valleys, and ongoing mass- 
movement and colluvial processes.
Conclusions differ from those of previous workers in that the quartzite 
gravel is older than the limestone conglomerate, both units are Late Cretaceous 
rather than Paleogene, and basal exposures of Renova mudstones produce a 
Duchesnean rather than a Chadronian local fauna. The broader Conclusions 
support disruption of a broad Paleogene depositional basin by the present 
geometry of basins and ranges in southwestern Montana. Extensional faulting 
did not predate 3b Ma and has offset the Renova Formation by 3,350-5,300 m 
(11,000-17,400 ft) relative to adjacent grabens. The dense welding of the 
Huckleberry Ridge Tuff on the range crest suggests that the 1,100 m. (3,600 ft.) 
of relief relative to outcrops in the adjacent Madison valley is largely the result of 
tectonism during the Quaternary.
IINTRODUCTION
Efforts to unravel the post-Laramide sedimentary and tectonic history of 
southwestern Montana have resulted in a number of possible scenarios for 
regional Cenozoic basin evolution (Reynolds, 1979; Fields et aL, 1985; Fritz 
and Sears, 1993; Ruppel, 1993). Southwestern Montana has been shaped by 
a complex tectonic history from Late Cretaceous through Cenozoic time. Major 
episodes of deformation include spatially and temporally overlapping thin- 
skinned (Sevier) and thick-skinned (Laramide) contractional deformation in the 
Late Cretaceous to early Tertiary, followed by at least one phase of extension in 
the mid-Tertiary, and late Tertiary through Quaternary tectonic influence of the 
passage of the Yellowstone hot spot (Anders and Sleep, 1992; Pierce and 
Morgan, 1992; Fritz and Sears, 1993). Although most models of regional 
geologic events share these basic elements, there is ongoing debate about the 
number, timing, and style of basin-forming episodes during the Cenozoic.
Numerous studies have focused on the stratigraphic record of Tertiary 
basin-fill in intermontane basins of southwestern Montana as the key to 
unraveling post-Laramide geologic history. Similarities among basins include 
hornotaxiaI Iithostratigraphic sequences and vertebrate fgunal successions, 
with closely matching radiometric age limits where volcanic rocks are available. 
This suggests that the individual basins share a common history to a large 
degree (Fields et al., 1985). It was recognized that this observed "basin unity" 
(Monroe, 1976) required mechanisms, such as climate, which operate on a
regional scale (Kuenzi and Fields, 1971; Monroe, 1976; Thompson et al., 1982). 
Other proposed tectonic models and geologic histories propose that the semi- 
isoiated extensional basins which preserve Tertiary basin-fill are not the basins 
in which those strata were deposited. The "basin unity," at least for the lower 
part of the Tertiary system, may reflect deposition as a broad, relatively 
continuous sheet which was subsequently partitioned during one or more 
extensional episodes (Thompson et al., 1981; Fritz and Sears, 1993; Thomas, 
1995). Remnants of the Tertiary system which exist outside of the present 
extensional fault-bounded basins preserve important clues about the 
complexity of basin evolution in southwestern Montana.
Purpose of Study
A heterogeneous assemblage of syn- and post-Laramide coarse- and 
fine-grained sedimentary rocks and volcanic flows overlie an erosional surfade 
between about 2,740 and 3,050 m (9,000-10,000 ft) on the crest of the Gravelly 
Range in southwestern Montana. Previous conclusions about the tectonic and 
depositions! significance of these rocks vary significantly, because the field 
relations and ages of the scattered outcrops were interpreted in conflicting ways 
(Scott, 1938; Atwood and Atwood, 2945; Mann, 1954; 1960; Hadley 1969b;
1980; Gutmann et al., 1989; Ruppel, 1993).
The primary goal of this study is to resolve the chronology of Late 
Cretaceous through Tertiary geologic events in the southern Gravelly Range 
based on an in-depth, mapping-based investigation of high-elevation syn- and 
post-Laramide strata, and to discuss the tectonic implications of that chronology* 
in the light of proposed tectonic models. On a local scale, the chronology is
2
3relevant to the uplift history of the range relative to adjacent grabens. On a 
regional scale, it can help test hypotheses of Cenozoic geologic evolution in 
southwestern Montana. Additionally, newly reported fossil mammal faunas 
which contribute age control at several localities are recognized as a significant 
Duchesnean local fauna.
Geologic Setting
The Gravelly Range lies in the Rocky Mountain foreland of southwestern 
Montana (Fig. 1). This region is bounded on the north by the "southwest 
Montana transverse fault zone", a 120-km-long east-trending fault zone which 
forms the southern margin of the Helena salient of the Sevier thrust belt 
(Schmidt and O'Neill, 1983). To the west, the southwest Montana reentrant of 
the Sevier thrust belt is characterized by structural overlap and interference of 
Sevier- and Laramide-style deformation (e.g. 'Dillon cutoff' of O'Neill et al., 
1990). The foreland is bounded on the southwest and south by frontal thrust 
sheets and the Snake River Plain. To the east, the foreland grades into the 
northern Great Plains.
The present physiography of the region surrounding the Gravelly Range 
is characterized by broad intermontane basins separated by high mountain 
blocks (Fig. 2). The underlying extensional structure represents a northernmost 
extension of the Basin and Range structural province, flanking the northern 
margin of the Snake River Plain (Reynolds, 1979, but see Ruppel, 1993). The 
Gravelly Range also lies within a region of tectonic disruption caused by 
thermal effects of the passage of the Yellowstone hot spot (Anders and Sleep, 
1992; Pierce and Morgan, 1992; Fritz and Sears, 1993).
4Butte
TransverseMontana Southwest Montana
Idaho
Ennis
Mountain
Foreland
Lima
Figure 1. Simplified tectonic map of southwestern Montana showing the Rocky 
Mountain foreland bounded by the margins of the Sevier thrust belt and the 
Snake River Plain. Location of Gravelly Range is outlined; other ranges not
Figure 2. 
Index m
ap of contem
porary ranges and basins of southw
estern 
M
ontana and east-central Idaho (from
 R
uppel1 1993).
6Methods
Field work by previous authors (Mann, 1954, 1960; Hadley, 1960,1969b,
1980; Gutmann et al., 1989) and reconnaissance by J.M. O'Neill and the author 
was used to choose a field area on the crest of the Gravelly Range for detailed 
geologic mapping. This area consists of most of the U.S.G.S. 7,5" Bighorn 
Mountain topographic quadrangle, spanning T.10S and T.11S, R.2W, in the 
Beaverhead National Forest, Madison County, Montana (Fig. 3). The field area 
was selected for its outcrops of post-Laramide sedimentary and volcanic rocks, 
in particular for localities that might show the field relations of different 
lithosomes, or that might yield vertebrate fossils.
Mapping of Paleozoic through Tertiary rock units and geologic structures 
was done directly onto the 7.5" Bighorn Mountain topographic map, with strike 
and dip measurements, lithologic descriptions, and fossil locality data kept in a i
separate field notebook. Completion of mapping for a few areas on the eastern 
side of the map was done using USDA color stereo aerial photographs housed 
at the U.S. Forest Service Beaverhead District headquarters in Ennis, Montana.
Nomenclature for Mississippian to Lower Cretaceous strata shown on the map 
and cross-section A-A' follows that of Mann (1954,1960), Hadley (1960 ,1969a,
1969b, 1980) and Tysdal et al. (1989).
One sample of unweathered basalt for 40Ar/39Ar dating and XRF analysis 
was collecfed from a basalt flow overlying Tertiary sedimentary rocks, thus 
providing age Control and testing the correlation of flows in the field area to 
those previously dated elsewhere in the Gravelly Range, The radiometric
i i
analysis was performed by the New Mexico Geochronological Research
Laboratory (Appendix); the XRF analysis was performed by the Washington ; j
7G r e e n h o r n
M a d i s o n
R i v e r
R a n g e
G r e e n h o r n
T h r u s t
S n o w c r e s t
T h r u s t
G R A V E  LLY 
R A N G E
BedKill •
Bighorn Mtn.
Standard Creek
Black Butte
Lion Mtn.
S n o w c r e s t
R a n g e
Figure 3. Index map of Gravelly Range, showing map area and selected 
features (modified from Gutmann et al., 1989).
8State University GeoAnaIyticaI Laboratory (Appendix).
Mammalian fossils were collected from four localities, three of which had 
been identified prior to this project. Specimens were prepared at the American 
Museum of Natural History in New York, using standard micropreparatory 
techniques. All specimens from this study are permanently curated at the 
AMNH, in accordance with the U.S. Forest Service paleontological permit 
stipulations.
The paleontologic data from this study and from previous vertebrate 
collections made in the field area by the Museum of Comparative Zoology at 
Harvard in the 1d50's, and by the American Museum of Natural History in the 
1980's, were combined into taxonomic lists for the four localities. The 
biochronologically significant taxa from these lists were used to establish the 
probable North American Land Mammal Age of each locality. Assignment of 
numerical time intervals and epochs to the relevant North American Land 
Mammal Ages follows the calibration of Prothero and Swisher (1992).
The lithologies, mutual stratigraphic relations and time-significant 
information compiled from several sets of exposures within the map area were 
combined to generate a generalized stratigraphic column with associated time 
scale. This stratigraphic column was compared with the generalized results of 
basin studies in southwestern Montana (Fields et al., 1985), in order to clarify 
issues of Cenozoic basin evolution.
Tertian/ Basin-Fill Nomenclature
The earliest attempts to describe and interpret the significance of Tertiary 
basin-fill sequences preserved in fault-bounded basins of southwestern
Montana.led to the misleading term "Bozeman lake beds" (Peale, 1896), based 
on an overestimation of the role of lacustrine systems during deposition. The 
nomenclature was revised by Robinson (1963), who defined the Bozeman 
Group as: "...the Tertiary fluvial, aeolian and lacustrine rocks which 
accumulated in the basins of western Montana after the Laramide orogeny..." 
The present widely accepted scheme of Bozeman Group Iithostratigraphy 
recognizes two unconformable, lithologically distinct depositional sequences 
(Fig. 4). The Renova Formation (Kuenzi and Fields, 1971), of late Eocene to 
early Miocene age, is predominantly fine-grained and rests on an unconformity 
cut into pre-Tertiary rocks, or locally on volcanics and interbedded volcaniclastic 
rocks associated with the middle Eocene Challis and Lowlapd Creek volcanic 
fields. The Renova is overlain with angular unconformity by the predominantly 
coarse-grained Sixmile Creek Formation (Robinson, 1967) of late Miocene to 
Pliocene age.
Hanneman and Wideman (1991) argued against the use of Bozeman 
Group terminology, citing the presence of fine- and coarse-grained rocks in 
both formations "to the extent that it is in many cases impossible to recognize 
either one." They used a sequence stratigraphic (allostratigraphic) approach 
based on calcic paleosol zones tied to seismic reflection data to recognize five 
depositional sequences in southwestern Montana valleys. Both the 
conventional and allostratigraphic schemes are constrained by radiometric 
dates and mammalian biostratigraphy, and can be correlated with the numerical 
time scale, the North American Land Mammal Ages, and the epochs (Fig. 4). It 
should be noted that some features of regional importance are recognized by 
both systems, particularly the late Eocene and early to middle Miocene angular 
unconformities.
9
10
= -  Holocene
Epoch | ^  ; North American Bozeman Group I A  I Mammal Ages Stratigraphy
PteisioceDe
Pliocene
Miocene
Eocene
1.8 /
Rtrcn La wean Alluvium
Hemphillian
Clarendonian
Barstovian
20.5
Sixmile Creek 
Formation
' 29.5
Oligocene I320 Whitneyan
Orellanxo_________
! 37 0 Chadronian
<2.5 Duchesnian
so.b Bridgerian
Renova
Formation
Paleocene
Southwest Montana Cenozoic 
Sequence Stratigraphy
Sequence 5
SyAZVXZv
Ij H I I I I I I I i
XkX A Z v a  v X /^ x Z - V s Z v 'y v '
YYYYTfYT
Sequence 4
X /X /y X Z Z X /X Z x A Z v y A Z X /
^!Alz<ziAiAlAiAi^|AAiA^
Seouence3
Sequence 2 
Sequence 1
Figure 4. Comparison of Tertiary stratigraphic schemes for southwestern 
Montana basins: conventional Bozeman Group Iithostratigraphy shown on the 
left and the allostratigraphy of Hanneman and Wideman (1991) shown on the 
right. Sequence I is primarily volcanic rocks associated with the Challis and 
Lowland Creek volcanic fields; the other sequences are predominantly 
sedimentary (modified from Hanneman and Wideman, 1991).
U  H I
This report retains the naitie 'Renova Formation' in reference to the 
dominantly fine-grained, felsic tuffaceous sedimentary rocks which overlie a 
regional unconformity in the field area and contain Eocene to Oligocene 
mammal faunas. These rocks constitute a mappable unit within the field area, 
and match the criteria for extension of the term lRenova Formation' from its type 
area in the Jefferson basin (Kuenzi and Fields, 1985). No equivalents of the 
Sixmile Creek Formation (Robinson, 1967) were found in the map area.
tectonic Models J
i
Conflicting theories concerning the post-Laramide tectonic history of 
southwestern Montana can be categorized according to the number and style of 
distinct basin-forming episodes proposed for the Cenozoic. Some workers 
have argued that the existing extensional fault-bounded basins have been
!
active since the Eocene, with the late Eocene through Oligocene Renova !
Formation and equivalent strata deposited in the structural basins which now ;
preserve them (Chadwick, 1981; Fields et al., 1985; Ruppel, 1993). Fields et al.
(1985) stated that, "Delineation of the intermontane basins took plape during I
early and middle Eocene time as a response to regional intra-arc extension." j
Pardee (1950) also proposed that the Tertiary "Lake beds" accumulated in I
1,!
areas corresponding approximately to the present basins, although his model ]
i
included erosion of the ranges plus basin aggradation (= "peneplanation") to |
i
create a surface of low relief during the Tertiary, followed by re-elevation of 
ranges during a late Cenozoic (Pliocene) stage of block uplift. -1
Based on a general geologic investigation of the Gravelly Range, Martn j
(1954, 1960) proposed Oligocpne burial of post-Laramide low-relief topography j
11
by volcaniclastlc material, hinting at a regionally broad Paleogene depositional 
setting. The idea of disruption of older basin geometries has been incorporated 
in more recent tectonic models. Thompson et al. (1981) argued that the 
regionally developed Hemingfordian angular unconformity between the 
Renova and Sixmile Creek formations marks the disruption of a widespread 
Renova blanket and the beginning of differentiation into the modern basins and 
ranges. Such a broad "Renova basin" may have developed partly as a flexural 
basin extending from an uplifted rift shoulder in extreme western Montana 
(Janecke, 1994; 1995; Vandenburg et al,, 1996). Alternatively, the massive 
early to middle Eocene Challis and Absaroka volcanic fields might have 
provided the western and southeastern basin margins, and possibly generated 
slight subsidence of the intervening area through isostatic adjustment, sufficient 
for aggradation of Renova sediments.
Other studies have added a younger phase of uplift and extension, 
linked to the arrival of the Yellowstone hot spot during the last 6 Ma (Anders and 
Sleep, 1992; Fritz and Sears, 1993; Thomas, 1995; Thomas et al., 1995; Sears, 
1995). In these recent models, much of the existing extensional topography of 
southwestern Montana is latest Miocene or younger ahd may not be strongly 
reflective of Basin and Range extension (Thomas, 1995).
Information from this Study can help rule out some of the geologic 
histories and tectonic models mentioned above.
12
13
PALEOZOIC-MESOZOIC GEOLOGY AND LARAMIDE STRUCTURES
Pre-Laramide stratigraphy
Only Mississipian and younger strata crop out in the field area (geologic 
map, Plate 1). The Paleozoic section in the Gravelly Range is more than 440 m 
thinner than in the Snowerest and Greenhorn ranges immediately to the west. 
The thinning occurs abruptly across the Snowcrest-Greenhdrn thrust fault 
system which lies between them (Mann, 1954, 1960; Hadley, 1960). The units 
absent in the Gravelly Range are the Cambrian Park Shale, Pilgrim Dolomite, 
Snowy Range Formation, and Mississipian Big Snowy Group. Drillihg data 
from the Centennial Valley south of the Gravelly Range demonstrate a similar 
abrupt thinning of the Paleozoic section southeastward from the Snowcrest 
Range. This may indicate recurrent activity along a zone of crustal weakness 
between the Snowcrest-Greenhorn and Gravelly ranges (Hadley, 1960). This 
zone of weakness was structurally inverted during Cretaceous contractional 
deformation to become the Snowcrest-Greenhorn thrust fault system (Perry et 
al., 1981).
In contrast, the Mesozoic sectioh is largely similar to that in adjacent 
regions. An exception is a 90 m-thick section of Triassic red beds in the field 
area (Woodside Formation) found only in the southern Gravelly Range; the unit 
pinches out within a few km to the north of the field area.
The youngest pre-Laramide unit in the field area is a sandstone.unit
14
overlying shales of the Thermopolis Formation. It was included in the 
Thermopolis Formation by Hadley (1960; 1980), but is herein referred to as the 
Muddy Formation following Tysdal et al. (1989).
Laramide Structures
The entire pre-Laramide section dips generally 8-£0 degrees westward 
throughout the field area, with local deviations near folds and faults. This is 
characteristic of the Gravelly range as a whole, and reflects Late Cretaceous 
uplift of the area as the western limb of a NNW-trending Laramide-style uplift, 
the Madison-Gravelly arch (Scholten, 1967) (Fig. 5). The Gravelly Range area 
was also the eastern limb of a huge asymmetric syncline (Ruby syncline) which 
developed between the Madison-Gravelly arch and the Blacktail-Snowcrest 
arch to the west. These structures resulted from predominantly E-W shortening 
across the foreland of southwestern Montana.
Timing of Laramide shortening in the southwest Montana foreland is 
based on palynoldgy of synorogenic strata shed from the basement-cored 
uplifts, and on limited radiometric dates. The uplift of the Madison-Gravelly arch 
is dated in part by a Maastrichtian palynomorph assemblage in the middle part 
of the Sphinx Conglomerate, a synorogenic alluvial fan deposit which was 
derived from the Scarface thrust plate in the Madison Range (Decelles et al.,
1987). The Scarface thrust is one segment of a major system of thrust faults, the 
Hilgard fault system, which controlled eastward translation of the Madison- 
Gravelly uplift (Tysdal et al., 1986), Movement on the Hilgard fault system 
predates intrusions which "stitch" the thrusxt system, radiometrically dated at 68- 
69 Ma, providing a Maastrichtian upper limit on uplift (Tysdal et al., 1986).
15
Butte I ZONE %
Mz Pz 9 '
Bozemamc
MzPz
Ennis
MzPf
MzPz
LimaT
M O N TA N A
IDAH O
M Miles
jo 50 Kilcmeiers
EXPLANATION
Thrust-Selt leading edge; 
arrow snows direction 
of transverse movement
Lower Tertiary and Uooer Cretaceous 
synorogenic conglomerates
Kesczoic and Paleozoic 
sedimentary rocks Kajor foreland thrust
Major foreland right-reverse 
s l io fau lt
Inferred hinge of principal 
foreland arch
Arcnem metamorohic ccmolev
Inferred l i tho log ic  boundary
All  features dashed where covered
Figure 5. Schematic paleotectonic map of southwestern Montana, showing 
interpreted structural pattern of Laramide-style structures in the Late Cretaceous 
foreland (from Schmidt et al., 1988).
16
The Lima Conglomerate of the Antone Peak Formation (Beaverhead 
Group) is a syhorogenic conglomerate shed from the Blacktail-Snowcrest arch 
(Fig, 5) (Ryder and Scholtenj 1973). Based on palynomorphs from the top of 
the conglomerate, uplift of the Blacktail-Snowcrest arch appears to have been 
essentially completed by middle Campanian time (78-81 Ma) (Nichols et al., 
1985). It is therefore somewhat older than final Madison-Gravelly uplift.
The relatively early contractional history of this region produced a 
buttressed foreland on which the Sevier thrust belt impinged. Eastward
■ I
propagation of the thrust belt was impeded, accounting for the southwest 
Montana recess of the Sevier thrust belt (Fig. I ) (Eldredge and Van der Voo, 
1988; Schmidt et al., 1988).
In the field prea, thrust faults dip westward at 20-40 degrees. Translation 
on these faults was probably not large; the greatest observed throw is about 
220 m, placing the Amsden Formation on top of Dinwoody Formation siltstones 
and dolomites along the Bighorn Mountain thrust (Mann, 1960) (see map, Plate 
I). Slickenlines preserved on a silicified fault surface at the top of Bighorn 
Mountain record dip-slip motion of the hanging wall to the ESE (Fig. 6). 
Northward, the fault splays through Madison Group limestone in a fault zone 
characterized by breccia-filled karst. It is not known whether the faulting 
exploited a karst zone or karsting developed subsequently along the fault zone. 
South of Bighorn Mountain the fault dies out rapidly into an anticline (Fig. 7), 
and east-west contraction is transferred to another thrust which continues 
southward. The anticline is therefore interpreted as a fault-propagation fold, 
cored by a blind thrust (see cross-section A-A1 of Plate 1). These and other 
relatively small east-vergent thrust faults characteristic of the Gravelly Range 
may be out-of-thersyncline thrusts generated by the space problem in the core
17
Figure 6. Silicified fault surface exposed in the thrust of Bighorn Mountain, 
showing kinematic indicators. Pencil is parallel to slickenlines indicating ESE 
dip-slip movement of the hanging wall. Note the set of right-lateral shear 
fractures.
18
Figure 7. Anticline inferred to be cored by a blind continuation of the Bighorn 
Mountain thrust, view looking SSW from Bighorn Mountain. The anticline is 
cored by Madison Group limestone. The upper cliff-forming unit is the Quadrant 
Formation overlain by Shedhorn Sandstone.
of the Ruby syncline as it became tightly folded and locally overturned to the 
east. Alternatively, they may be small synthetic basement-cored thrusts 
developed on the gentle west limb of the Madison Gravelly arch. The post- 
Laramide erosion surface underlying the gravel and conglomerate discussed in 
the next section probably began to develop Synchronous with Laramide 
shortening.
19
x.
20
SYN- AND POST-LARAMlDE GEOLOGY 
Erosion Surface
One of the striking features in the field area is a widespread, generally 
low-relief angular unconformity which truncates the west-dipping Paleozoic and 
Mesozoic strata and all Laramide folds and faults in the map area. Much of the 
present low-relief topography which characterizes the range crest is an 
expression of this surface due to ongoing removal of the overlying, poorly 
Iithified strata. The erosion surface has been interpreted as a remnant of 
subdued Late Cretaceous to middle Eocene topography which developed 
during and/or after Laramide contraction, but before initiation of Tertiary basin 
formation (Mann, 1954; 1960; Fields et a!., 1985; Ruppel, 1993). At most 
localities within the map area, the erosion surface is best dated by 37-40 Ma 
mammal faunas in overlying tuffaceous mudstones (see Vertebrate 
Paleontology of the Renova Formation). However, the mudstones are not the 
oldest deposits above the unconformity. Where remnants of older gravels and 
conglomeratic units are present, the angular unconformity beneath them is 
probably Late Cretaceous and coeval with late-stage Laramide deformation 
(see discussion of the conglomerate of Red Hill).
In the northern part of the Gravelly Range, Hadley (1960, 1980) noted a 
several degree northeastward dip of the erosion surface toward the Madison 
valley. Within the field area, its average elevation is 2,800-2,840 m (9,200-
9,300 ft). The topography of the erosion surface is consistent with a generally 
horizontal orientation, with the exception that the overlying conglomerate of Red 
Hill is found at successively lower elevations eastward across the north end of 
the map area, indicating significant Late Cretaceous topography developed 
where the Morrison through Quadrant formations were eroded (Map, Plate 1).
The steep-sided and deeply incised drainages, such as the headwaters 
of Cottonwood, Standard, Horse and Ruby creeks, cut through the projected 
level of the erosional surface, and are more recently superimposed features;
No Tertiary rocks are preserved within them or at elevations lower then 2,780 m 
(9,120) ft.. However, other smaller drainages appear to be exhumed 
paleovalleys; a small steep circjue-like drainage on the south side of Bighorn 
Mountain, 0.5 km east of the Bighorn thrust, contains traces of white Renova 
ash at 3,000 m (9,800 ft) and unusually polished blocks of Shedhorn Sandstone 
are present in the upper part of the drainage (Fig. 8).
Sedimentary and Volcanic Rocks
The angular unconformity beveling the range top is overlain by several 
lithologically distinct sedimentary deposits and volcanic rocks. Two factors 
contribute to the uncertainty and disagreement in the literature concerning the 
mutual relations and age limits of some of these strata: only patchy remnants of 
the rocks are left on the Gravelly Range due to glaciation and ongoing erosion, 
and each of the facies is limited in its distribution such that there is no single 
outcrop belt in which all lithologies are present.
21
22
Figure 8 . Bighorn Mountain: an example of considerable local relief on the 
exhumed Eocene erosion surface of the range crest (view looking north). The 
small snow patch on the upper eastern slope (right side of photo) marks a 
paleovalley containing traces of Renova Formation deposits at 2,990 m (9,800 
ft), and a basaltic vent just behind the left-hand peak (on the skyline) preserves 
Renova mudstones within it at about 3,050 m (10,000 ft). The western slope of 
Bighorn Mountain descends to 2,780 m (9,120 ft) (left side of photo) where 
deposits of Renova mudstone are also present. Bighorn Mountain was 
therefore apparently buried in Eocene-Oligocene time.
23
Gravel of Black Butte
Description and Distribution . The stratigraphically lowest, and therefore 
oldest, post-Laramide deposit in the field area is an unlithified pebble to boulder 
gravel, herein informally referred to as the gravel of Black Butte after its best- 
known exposure at the head of a landslide immediately west of Black Butte. At 
that locality, the clast composition consists almost entirely of very well-rounded 
quartzite pebbles and cobbles in a deposit up to 10 m thick (Fig. 9). Minor 
amounts of Archean metamorphic and Paleozoic limestone clasts exhibit a 
lesser degree of rounding, and several large (3 m) angular boulders of cherty 
Shedhorn Sandstone are present. A few rounded clasts of mafic igneous rock 
were noted.
The gravel of Black Butte is everywhere characterized by a coarse 
angular quartz sand matrix and varies from 0-10 meters thick, it varies laterally 
in its clast composition from nearly all quartzite cobbles to dominantly Archean 
metamorphic cobbles and boulders. Remnants of a mixed clast assemblage is 
scattered along Monument Ridge at 2,835 m (9,300 ft) elevation. At the north 
end of Monument Ridge the composition varies from dominantly quartzite 
pebbles and cobbles cropping out around the base of Red Hill, to dominantly 
Archean gneiss and schist boulders near the southern side of Lazyman Hill. J. 
Michael O'Neill (pers. comm., 1997) reports that in outcrops south of the field 
area, Archean metamorphic gravel can be distinguished from, and underlies, 
quartzite cobble gravel. This distinction was not observed within the field area, 
although it cannot be ruled out since clasts of unconsolidated material are 
subject to downslope mixing as they are exposed.
The gravel of Black Butte pinches out rapidly eastward. Tepee Mountain 
(noted on the map as benchmarks 9603T and 9560AT on the western margin of
24
Figure 9. Gravel of Black Butte, exposed in a landslide scarp immediately west 
of Black Butte, showing clast lithology dominated by very well rounded 
Proterozoic quartzite cobbles. Geologic hammer for scale.
25
Tepee basin) marks its easternmost remnant, where a small outcrop of 
dominantly Archean boulders is exposed below late Eocene mudstones on the 
west face of the mountain. The gravel is not present in exposures on the east 
side of the mountain. South of Red Hill and Tepee Mountain, the gravel is 
restricted entirely to the western edge of the map area, from Monument Ridge to 
the west side of Black Butte.
Age Limits and Regional Correlation . The age of the gravel of Black 
Butte is poorly bracketed. It lies on sandstone of the Early Cretaceous Muddy 
Formation we6t of Black Butte, and both it and a conformably overlying 
conglomeratic unit are overlain by late Eocene rocks. Furthermore, the gravel 
exposure west of Black Butte has a long history of conflicting age and 
depositional interpretation in the literature. The crushed cobbles within the 
deposit (see discussion in Gutmann et al., 1989) and striations trending nearly 
down-dip (westward) in the sandstone underlying this deposit (Fig. 10), 
contributed to early interpretations aS an Eocene glacial till (Scott, 1938;
Atwood and Atwood, 1945), and to Mann's (1954,1960) hypothesis of mudflow 
emplacement during middle Eocene to early Oligocene time.
Gutmann et al. (1989) correlate the gravel with the informally named 
"Divide quartzite conglomerate" of the Upper Cretaceous Beaverhead Group. 
The Beaverhead Group is an assemblage of foreland- and thrust belt-derived 
syntectonic deposits in southwestern Montana which contain several poorly 
correlated quartzite conglomerate units (Schmitt et al., 1995). The closest 
outcrop belt of Beaverhead Group quartzite conglomerate lies in the Monida 
Pass area about 60 km to the southwest of the field area. Paleocurrent trends in 
the Monida Pass area are northeast to north (Ryder and Scholten, 1973), so it is
26
Figure 10. Crushed quartzite cobbles from the gravel of Black Butte, shown on 
the striated dipslope of Muddy Formation sandstone which underlies the gravel 
at this locality (immediately west of Black Butte). Pencil for scale.
conceivable that distal parts of this deposit could have onlapped the western 
limb of the Madison-Gravelly arch. Proterozoic quartzites which crop out in the 
Pioneer Mountains 90 kilometers west of the Gravelly Range have also been 
suggested as a possible source area for the gravel of Black Butte (Ruppel, 
1993). The subrounded cobbles and boulders of Archean gneiss were 
probably derived from a foreland uplifts, probably the Blacktail-Snowcrest arch, 
given its early unroofing history.
Alternatively, the gravel of Black Butte could conceivably have been 
recycled sometime after the Late Cretaceous from quartzite-rich units of the 
Beaverhead Group. Both Mann (1960) and Ruppel (1993) interpreted it as 
being only slightly older than the overlying late Eocene succession (thought at 
that time to' be early Oligocene mudstones). However, arguments for the age of 
the locally overlying conglomerate of Red Hill presented in the next section 
indicate that the gravel of Black Butte is most likely Campanian or Maastrichtian.
Conglomerate of Red Hill
Description, and Distribution . In the northern half of the map area, the 
gravel of Black Butte is overlain, apparently conformably, by a complex 
succession of interbedded red siltstone, sandstone and lacustrine limestone, 
with lenticular bodies of limestone ponglomerate. Because the limestone 
conglomerate is the most widely distributed of these facies and often occurs 
without the other facies as isolated lenticular bodies, both in the map area and 
elsewhere in the range, this succession is informally referred to as the 
conglomerate of Red Hill.
In the Red Hill/Lazyman Hill area, basal siItstones and limestones of this 
sequence contain lenses of quartzite gravel in an angular quartz sand matrix*
27
28
essentially identical to the underlying gravel of Black Butte except that the 
average clast size is smaller, ranging from pebbles to small cobbles. The 
lacustrine limestone facies consists of a sparry matrix, locally containing fossil 
snails, stromatolites and oncolites. Discontinuous stringers of coarse 
sandstone and intraformational siItstone and limestone rip-up clasts are also 
common within the limestone facies. The most numerous type of freshwater 
snail from the limestone facies is a Lymnaeid, probably Stagnicola ; similar 
Lymnaeids are known from Jurassic to Pleistocene lacustrine rocks. A less 
common snail was referred to Biomphalaria, which has been noted in pre- 
Chadronian rocks in the Qenozoic basins of Wyoming (Emmett Evanoff, pers. 
comm., 1996). The presence of this snails is consistent with a pre-late Eocene 
age limit, but add no further age refinement.
The large lenticular conglomerate bodies contain rounded to subangular 
Cambrian to Pennsylvanian limestone pebbles, cobbles and locally boulders, in 
a red Sandy siltstone matrix (Fig, 11), Minor amounts of Archean schist and 
gneiss clasts and sandstone cobbles are present as well. The conglomerate 
appears to be mostly clast-supported but is locally matrix supported. It tends to 
be unstratified to crudely stratified.
To the south along Monument Ridge and eastward at Tepee basin, 
outcrops of the limestone conglomerate facies are found by themselves in what 
appear to be remnants of paleodhannels cut into Mesozoic and Paleozoic 
strata. The conglomerate is exposed at lower elevations from west to east 
across the northern part of the map area.
Limestone conglpmerate of the same description is found along the 
range crest in the Varney quadrangle to the north (Hadley, 1969b, 1980), and 
other exposures are found well south of the map area. However, no part of the
29
Figure 11. Outcrop of the conglomerate of Red Hill, showing conglomerate 
composed dominantly of Paleozoic limestone clasts and minor Archean clasts, 
overlain by lacustrine limestone at the north side of Red Hill. Geologic hammer 
for scale.
. conglomerate of Red Hill is found in the sections at Black Butte, Lion Mountain 
and other localities in the southern part of the map area, indicating either 
ndndeposition or erosion there prior to deposition of late Eocene mudstones.
At the south end of Lazyman Hill, the uppermost Iithosome in a 
dominantly siltstone-lacustrine limestone sequence is a thin bed of coarse 
Iitharenite composed of very angular quartz grains and biotite-bearing 
metamorphic fragments. Of significance for age interpretation, the beds at this 
locality appear to be gently folded where they onlap a thrust wedge of 
Shedhorn Sandstone which pinches out rapidly southward beneath them (Plate 
1, northwest corner of map). The aspect of the bedding elsewhere is 
approximately flat-lying.
On the west face of Tepee mountain, an enigmatic outcrop of angular 
blocks and breccia composed of cherty Shedhorn Sandstone in a red silty 
matrix was mapped as part of the conglomerate of Red Hill. It seems likely that 
the deposit is of sedimentary origin, and the angular clasts were eroded from 
immediately adjacent topographic highs in the Shedhorn Sandstone to the 
northeast and southeast. Alternatively, the breccia may be of fault-cataclastic 
origin, in which case the fault responsible for the brecciation is not obvious. The 
breccia is not traceable laterally, and typical limestone conglomerates of the 
conglomerate of Red Hill are exposed on the east side of Tepee Mountain with 
no sign of the breccia.
Aae Limits and Regional Correlation . The broad age limits for the 
conglomerate of Red Hill are similar to that for the gravel of Black Butte: it could 
be as old as Late Cretaceous syntectonic conglomerates (Beaverhead Group) 
in the region, and it cannot be younger than the overlying late Eocene
30
(Duchesnian) mudstones. However, a number of observations considered 
below strongly suggest that it is Late Cretaceous.
Hadley (1969b, 1980) assigned the limestone cobble conglomerate in 
the northern Gravelly Range to the Sphinx Conglomerate in the Matjison 
Range. Decelles et al. (1987) also referred to the sequence at Red Hill as a 
distal equivalent of the upper Sphinx, deposited on fhe western, lower-gradient 
flank of the Madison-Gravelly arch. The Sphinx Conglomerate was deposited 
between about 75 and 58 Ma (Dedelles et al., 1987). If the correlation is correct, 
the conglomerate of Red Hill would likely be of late Maastrichtian or Paleocene 
age (Decelles, et al., 1987).
There are several difficulties with previous correlations of the limestone 
conglomerate of the Gravelly Range with the Sphinx Conglomerate, however. 
Neither the presence of an oxidized matrix, nor an abundance of Paleozoic 
carbpnate clasts are particularly exclusive features on which to base a 
correlation. Oxidized matrix could have been derived locally from redbeds of 
the Woodside Formation during fluvial transport, or could have developed 
during intense weathering in a warm, humid Late Cretaceous or Paleocene 
climate (Berggren and Prothero, 1992), or could even be of diagenetic origin. 
Potential source areas for Cambrian to Pennsylvanian limestones would have 
existed in all of the Laramide foreland uplifts, as well as in the thrust belt to the 
west and south.
A description of clast composition in the Sphinx Conglomerate sensu 
Sfricto did not note any Archean clasts, and Cambrian Flathead Sandstone 
clasts appear only in the uppermost part of the Sphinx section (Graham et al., 
1986). In their provenance model for the Sphinx, Graham et al. (1986) noted 
only "Precambrian possibly exposed" at the top of their sequence. Slight
31
32
deformation of the conglomerate of Red Hill on the margin of a thrust fault 
suggests that deposition of the conglomerate of Red Hill predated the end of 
Laramide deformation. If the Madison-Gravelly arch was ever sufficiently 
unroofed to provide Archean lithologies, it was almost certainly not early 
enough to predate Laramide deformation in the field area.
In contrast, the Lima Conglomerate was derived from the Blacktail- 
Snowcrest arch during Santonian through middle Campanian time (Ryder and 
Scholten, 1973; Nichols et al., 1985), significantly before Sphinx deposition. 
The Lima Conglomerate contains Archean clasts in its upper part (Schmitt, 
1095).
Additionally, the outcrops of limestone conglomerate step-down more 
than 60 meters in elevation from west-to-east across the northern part of the 
map area. In the absence of intervening CenozoiC normal faults, this suggests 
transport from an uplifted source area to the west. The alternative, that of 
complete infilling of the erosional toppgraphy between Monument Ridge and 
Tepee basin, followed by erosional removal of all but a few remnants, seems 
less likely.
Based on the above evidence, the most likely correlative for this unit is 
the Lima Conglomerate, in which case the conglomerate of Red Hill was 
deposited at about 78 Ma (Nichols et al., 1985), toward the end of Laramide 
tectonism west of the Gravelly Range.
The lack of intertonguing relations of the conglomerate of Red Hill with 
overlying late Eocene (Duchesnean) tuffaceous mudstones suggests that the
x
top of the succession is marked by an unconformity. Both the structural 
evidence for the late-stage synorogenic deposition of the conglomerate of Red
Hill, and its correlation with either the Sphinx or Lima conglomerate, indicate 
that this unconformity represents a gap of 28-38 million years.
Renova Formation
Description and Distribution . Scattered outcrops of poorly indurated, 
light-colored, tuffdceous mudstones assigned to the Renova Formation 
comprise the most widely distributed post-Laramide, pre-Ouaternary 
sedimentary deposit in the field area and across the Gravelly Range as a 
whole. The widely scattered outcrops of this tuffaceous mudstone are usually 
capped by Oligocene basalt flows, and sometimes intruded by small vents of 
scoriaceous agglomerate. The sedimentary outcrops are often fossiliferous, 
yielding mammal taxa on which its late Eocene to middle Oligocene ape limits 
are partly based. Carbonate concretions are locally abundant in the 
mudstones, and sometimes produce well-preserved fossils, although they do 
not appear to nucleate specifically around fossil bone.
Most of the deposits in the western and northern portions of the field area 
overlie one or both of the more coarse-grained sequences previously 
described. Elsewhere, the mudstones and basalt flows directly overlie the west­
dipping Paleozoic-Mesozoic section with angular unconformity.
By far the thickest section of Paleogene sedirhentary rocks in the 
Gravelly Range is exposed on the west side of Lion Mountain. The 
sedimentary section is preserved here because Lion Mountain is a major 
Oligocene volcanic center; the infrastructure of dikes and small sills extending 
outward from the central vent, as well as the basalt flows at the summit, have 
protected the poorly consolidated sedimentary strata from erosion. About 240 m 
of sedimentary section rests on the Madison Group and is capped by basalt
33
34
flows (Fig, 12). The lower part of the section here is typical of the other outcrops 
of Paleogene fine-grained, felsic, poorly-bedded mudstone, but the upper 80 
meters, which is significantly younger than any Renova deposits elsewhere in 
the field area, contains some thin layers and small trough cross-stratified 
lenticular bodies of granule to psbble conglomerate within sandy siltstone and 
mudstone. This is one of the few places where the structural attitude of bods in 
the Renova can be discerned. They are very close to flat-lying, with eastward 
dips of a few degrees in some areas (Gutmann et al„ 1989). The clasts in the 
conglomeratic layers are mostly granules and pebbles of Paleozoic limestone 
with basaltic scorid and lapilli. The basaltic clasts may have accumulated 
during early local eruptions at about 31 Ma (Gutmann et al., 1989).
Around the margin of the Black Butte volcanic neck, basal Renova 
mudstone and small IensOs of tuffaceous limestone have also been protected 
from total erosion by younger volcanic rocks, in this case the resistant Black 
Butte plug and the basaltic talus fields surrounding it (Fig. 13). At Tepee 
Mountain, a 30 m-thick fossiliferous mudstone section is intruded by a basaltic 
vent and capped by a basalt flow. Scattered localities show only a few meters 
to a few centimeters of mudstone beneath basalt flow remnants, and are 
sometimes unmappably small at the 1:24,000 scale. Thin outcrops of mudstone 
are capped by basalt flows at the southern end of Monument ridge, and a very 
small deposit which is not protected by basalt is found on top of the 
conglomerate of Red Hill at the southern end of Lazyman Hill. Evidence for the 
thickness of the original Renova mudstone blanket, in addition to the Lion 
Mountain section, includes the traces of mudstone on and near Bighorn 
Mountain (Fig. 8), where an original mudstone thickness of 250 m is indicated,
35
Figure 12. View of the west side of Lion Mountain, showing the thick section of 
soft Renova sedimentary rocks overlying the Madison Group and capped by 
basalt flows. Note the large north-trending diabase dike cutting the section at 
upper left.
36
Figure 13. View of the Black Butte volcanic neck, looking northwest. The base 
of the butte is surrounded by basaltic rubble capping massive light-colored 
mudstones of the Renova Formation.
closely comparable to the total thickness of about 240 m preserved at Lion 
Mountain,
Age Limits and Regional Correlation . The tuffaceous rocks fit the 
description of the Renova Formation as proposed by Kuenzi and Fields (1971) 
in the Jefferson basin, both in terms of their stratigraphic position above a 
widespread unconformity and primarily fine-grained felsic lithology. Mammal 
faunas and radiometric dates support this correlation.
Kuenzi and Fields (1971) noted that the composition of the lower part of 
the Renova Formation (Climbing Arrow Member) in the Jefferson basin was 
approximately 81 perpent volcanic glass- and pumice-bearing montmorillonite 
mudstone. The Climbing Arrow Member, as it was extended to the upper Ruby 
basin 25 kilometers west of the Gravelly Range, was described as mostly sandy 
montmorillonite mudstone (Monroe, 1976). In the Gravelly range, the 
mudstone at Black Buttp is dominated by shards of volcanic glass, with 
interstitial carbonate and 5-10 percent silt-size fragments of plagioclase and 
biotite (Gutmann et al., 1989). In the northern part of the Gravelly Range, 
Hadley (1980) reported that correlative tuffaceous Paleogene sediments 
consisted of 70-98 percent glassy shards and 2-25 percent mineral fragments.
A sample from Black Butte was composed of 68 percent silica; this high silica 
content is probably representative of much of the Renova Formation, and 
indicates a rhyolitic to dacitic source (Gutmann et al., 1989).
Deposition of the Renova began at 40-37 Ma, based on mammalian 
biochronology and recent time scale calibration (Prothero and Swisher, 1992), 
and continued at least until 30.8 ±  0.7 Ma based on a radiometric date from the
37
top of Lion Mountain (Gutmann et al.f 1989). If deposition continued beyond 
this time, the younger rocks have been eroded.
The Renova Formation in the field area appears to have a disconformity 
within it, representing 3-4 Ma of the early Chadronian to late Orellan or early 
Whitneyan, based on evidence at Lion Mountain and Tepee Mountain. At Lion 
. Mountain, a Chadronian titanothere jaw was recovered low in the section, 
below a 31.4 + .7 Ma (Whitneyan) airfall ash and overlying Whitneyan taxa. 
Despite extensive prospecting, no paleontological evidence for an Orellah 
sequence has been found there, although there is a significant section which 
has not produced many fossils. At Tepee mountain, Duchesnean (40-37 Ma) 
taxa are found in mudstones directly overlain by a basalt flow dated at 32.38 ±  
0.64 Ma (see Appendix), which calibrates to the late Orellan/early Whitneyan 
(Prothero and Swisher, 1992). This indicates a gap of 4 to 5 Ma, accounting for 
the entire Chadronian and most or all of the Orellan interval. The most likely 
explanation is that both the Tepee Mountain and Lion Mountain sections 
contain a disconformity which is overlain by early Whitneyan or late Orellan 
rocks, and which varies laterally in its removal of underlying strata. The section 
exposed at Black Butte is thick enough that the disconformity could be present 
there, but further collecting with associated stratigraphic information is needed 
to determine this.
The Renova Formation in the Gravelly Range fits into a regional scenario 
of widespread aggradation of fine-grained, ash-rich sediments deposited in a 
setting dominated by low-energy floodplain, pond and stream channel 
environments throughout southwestern Montana (Kuenzi and Fields, 1971; 
Fields et al., 1985). Atypical features of the depositional sequence in the 
Gravelly Range include the probable erosional disconformity removing much of
38
39
the Chadronian-Orellan interval and the 33-30 Ma alkali basalt flows and 
scattered eruptive centers. These volcanics are distributed throughout the 
Gravelly Range, and correlate with a much thicker sequence of flows in the 
Virginia City area approximately 40 kilometers to the north. The flows there 
range in age from about 34 to 30 Ma and total almost 550 meters in thickness. 
All of these basaltic rocks have been assigned to the middle Dillon Volcanics 
Member of the Renova Formation, interpreted as mantle wedge melts extruded 
on the eastern margin of the Paleogene Renova basin prior to Basin and 
Range extension (McDowell and Fritz, 1995; McDowell et al., 1997, in press). 
However, the basalt flows and intrusions of the Gravelly Range are mapped as 
a unit distinct from the sedimentary Renova Formation rocks in this report (Plate 
D-
The Renova Formation of southwestern Montana may have been 
physically continuous with the White River Formation (or Group) of Wyoming 
and the northern Great Plains during much or all of the Oligocene, although this 
may never be possible to test conclusively. The lithologic and faunal similarities 
between the exposures near Black Butte and outcrops of White River Formation 
rocks just south of Yellowstone Park, northwestern Wyoming, were noted by 
Love et al. (1976). Traces of White River Formation in many areas of Wyoming 
suggest that it had a wide distribution there prior to erosional removal (Emry et 
al., 1987).
Basalt and Phonolite of Black Butte
Description and Distribution , Black Butte, the highest peak in the 
Gravelly Range, is located in the southwest corner of the map area (Figure 13). 
It is interpreted as an early Miocene volcanic neck composed primarily of alkali
40
basalt and diabase (Spaid-Reitz, 1980,), with pods and dikes of the potassio 
nepheline-bearing rock tephritic phbnolite intruding the diabase near the 
summit (Gutmann etal., 1989).
Black Butte intruded a section of beveled, west-dipping Lower 
Cretaceous through Permian strata Overlain by the gravel of Black Butte and 
basal Renova mudstones (see cross-section A-A', Plate I). The only eruptive 
product attributed to this volcanic feature are remnants of mineralogically 
distinctive thin tan tuff lying on Renova mudstones near the butte. These may 
have slumped downhill since emplacement due to massive downslope 
movements of the mudstones (Gutmann et al. 1989).
A number of hydrothermal deposits of massive gypsum with chert 
nodules are found in the Morrison Formation at the south end of Monument 
Ridge, 3 to 4 km north of Black Butte. These formed along a northeast-trending 
fault zone, possibly as a result of hydrothermal circulation driven by heating of 
the country rock as Black Butte was emplaced (Munger, 1995). The deposits 
form a number of funnel-shaped karst holes near the Gravelly Range road, but 
occur as a linear trend of large protruding blocks exposed at the head of a 
landslide on the southern terminus of Monument Ridge. The karst morphology 
is due to the dissolution of gypsum by near-surface water. Expression of some 
of the deposits as blocks, rather than sinkholes, probably reflects the recency of 
the landslide which exposed them. No other rock types related to the eruption 
of the Black Butte volcanic center are found in the area.
Aae Limits and Regional Correlation . The Black Butte volcanic neck is 
dated by a whole-rock K-Ar analysis at about 22.9 + 1.2 Ma (early Miocene) by 
Marvin et al. (1974). Assuming this date is correct, Black Butte is anomalous in
that no other volcanic centers of similar age are documented in the region. The 
northern Rocky Mountain region as a whole was experiencing relative volcanic 
quiescence during this time (Chadwick, 1981). It is possible that this date may 
not be accurate. Given the advances in isotope dating over the past 20 years, 
redating of Black Butte should be done, but it was beyond the limits of this thesis 
project.
Huckleberry Ridae Tuff
Description and Distribution . Welded ash-flow tuff, 1 to 2 m thick, was 
found in one small outcrop capping a 2,926 m (9,600 ft) peak which forms the 
southern boundary of Tepee basin in the northeastern part of the field area.
The base of the unit is a densely welded, dark grey vitrophyre containing 
sanidine phenocrysts up to 3 mm long. It becomes lighter in color and more 
porous, but still firmly welded, toward the top with the upper third a light pinkish- 
grey color, phenocryst-poor, and containing numerous flattened pumice 
fragments. The characteristics of this deposit match the criteria for 
emplacement as a high-temperature pyroclastic flow (Fisher and Schmincke, 
1994)
The glassy welded base of the ash-flow tuff lies partly on Oligocene 
basalt, and partly bn an unidentified 3 m thick, dark grey to brown, welded ash- 
flow tuff which contains abundant sanidine phenocrysts, This enigmatic unit 
underlying the zoned welded tuff described above is massive, Igoks any 
obvious cooling zonation or bedding and is densely welded. It cannot be traced 
laterally for any distance. It has been suggested that this unit is a remnant of the 
tuff of Kilgore (Lisa Morgan, pens, comm,, 1996), which was erupted from the 
Heise volcanic field of the eastern Snake River Plain 70 km to the south at 4.3
41
Ma (Pierce and Morgan, 1992). Alternatively, it could be a lower flow unit within 
the same composite sheet as the zoned upper unit. A 2 m-thick unconsolidated 
white ash underlies these welded tuffs; although it does not show stratification, it 
may represent a ground surge deposit (Fisher and Schmincke, 1994) 
associated with the emplacement of the ignimbrite sheet.
Aae Limits and Regional Correlation . The 1 to 2 m thick welded zoned 
rhyolitic ash-flow tuff described above is most likely the Huckleberry Ridge Tuff, 
erupted from the Yellowstone Plateau volcanic field at 2.1 Ma (Obradovich and 
Izett, 1991). In the absence of a radiometric date, this identification is based on: 
(1) proximity to the eruptive center; (2) physical characteristics including color, 
grain size and phenocryst content of the cooling zones (L. Morgan, pers. comm., 
1996); (3) widespread distribution of the Huckleberry Ridge Tuff, including 
outcrops in other ranges surrounding the Yellowstone Plateau (Izett and Wilcox, 
1982; Pierce and Morgan, 1992). Other outcrops of similar welded ash-flow tuff 
assigned to the Huckleberry Ridge Tuff are found at high elevations south of the 
field area (Mann, 1960; O'Neill et al, 1995) and in the Varney quadrangle in the 
northern Gravelly Range, although the highest outcrop there lies below 2,135 
meters (7,000 feet) (Hadley, 1969a; 1969b; 1980).
Samples from the basal, middle and upper cooling zones of the tuff y/ere 
examined by L  Morgan of the U S. Geological Survey. Their high degree of 
welding favors Quaternary faulting, rather than surge of the ignimbrite up a 
major topographic slope, as the dominant mechanism to account for the tuff's 
present high elevation (1,100 m) above correlatives in the adjacent Madison 
Valley (L. Morgan, pers. comm., 1996). This is because a high particle-density 
pyroclastic flow capable of dense welding would tend to follow ground contours
42
43
and remain at lower elevations, whereas a pyroclastic surge sufficiently 
expanded to surmount high barriers would not be hot enough to weld densely 
upon emplacement.
Quaternary Deposits
Deposits younger than the Huckleberry Ridge Tuff include a few 
scattered boulders of uncertain origin, small amounts of glacial till, and 
widespread landslide deposits, in addition to talus fields and colluvium locally 
reworked into alluvium by small mountain streams. The Quaternary deposits 
discussed here are omitted from the stratigraphic sections and the generalized 
stratigraphic column shown in later chapters, because they do not contribute 
significant Cenozoic tectonic information.
Glacial Till
Small hillocks of poorly sorted, poorly exposed, unconsolidated material 
in the lower parts of Standard, Cottonwood, Ruby and Horse creeks are 
probably remnants of Pleistocene lateral and, ground moraines. Suspected 
small glacial deposits were mapped together with the younger Quaternary 
colluvial and alluvial deposits in which they are found, because differentiating 
them was not deemed relevant to this report. The glacial component of 
Quaternary deposits is more evident at elevations lower than those in the map 
area; the Standard Creek drainage, for example, contains several large lateral 
moraines a few km east Of the map area and about 550 m lower than the crest 
Of the range.
44
Boulders of Uncertain Origin
A few scattered large metamorphic boulders, usually of granitic gneiss, 
were observed in the field area and adjacent parts of the range top. They were 
initially interpreted to be large relicts from eroded deposits of the gravel of Black 
Butte, which is locally dominated by Archean metamorphic boulders. However, 
one such boulder was found on top of the coarse Iitharenite capping the 
conglomerate of Red Hill on the southern side of Lazymdn Hill, at 2,871 m 
(9,420 ft) elevation. Other isolated boulders, 2 to 3 m in diameter, rest on 
Paleozoic and Mesozoic rocks at somewhat lower elevations north of the 
Bighorn Mountain quadrangle. If these few allochthonous boulders are not part 
of the gravel of Black Butte, how and when they were brought to the high 
surface of the range is unclear, and hypotheses regarding them may be difficult 
to test. Pleistocene glacial origin can be ruled out, because Archean lithologies 
are only exposed in the Gravelly Range at elevations much lower than that of 
the field area.
Landslide deposits
Evidence of mass movement at various scales in the Gravelly range is 
almost ubiquitous (O'Neill et al., 1995). Deposits mapped as landslides usually 
lie downslope from arcuate scarp surfaces, and are characterized by hummocky 
topography and numerous ponded depressions. Most of the landslides moving 
toward the Ruby River on the western side of the field area are on dip slopes of 
relatively resistant Muddy Formation sandstone and the basal sandstone of the 
Kootenai Formation.
Landslides extending eastward from the Red Hill area of Monument 
Ridge are composed of a jumbled mixture of all the lithologies found in the
45
gravel of Black Butte and the conglomerate of Red Hill. Most other landslides 
involve the more fine-grained, poorly indurated facies abundant in the Mesozoic 
and Tertiary systems, and some of these deposits may be more accurately 
classified as earthfldws, probably subject to ongoing intermittent movement at 
various rates dictated by groundwater saturation. The clay-rich rocks most often 
involved in mass movements include the Morrison Formation^ the Kootenai 
Formation above the basal sandstone member, the Thermopolis Shale, the 
Renova Formation, and to a lessor degree the Dinwoody and Woodside 
formations.
Landslide movement in the Gravelly Range could be triggered by 
earthquake activity, especially when the poorly consolidated units are water 
saturated. Tree-ring analyses indicated reactivation of landslides in the 
southern Gravelly Range/Madison Valley/Madison Range area, in response to 
the 1959 Hebgen Lake, 1973 Yellowstone Park, and 1983 Borah Peak 
earthquakes (O'Neiliet al., 1995).
Individual Stratigraphic Sections
Four sets of exposures were identified for which schematic stratigraphic 
sections could be drawn, showing the mutual relations, age limits and 
lithologies of the post-Laramide deposits (other than the Huckleberry Ridge 
Tuff). Other exposures of po$t-Laramide strata support the physical stratigraphy 
and age assignments shown for these localities.
46
Exposures at Black Butte
The landslide scarp just west of Black Butte exposes the gravel of Black 
Butte (as discussed previously), resting on a dip slope of Muddy Formation and 
overlain by tuffaceous mudstones of the Renova Formation. The thickness for 
the RenOva Formation shown in figure 14 is based partly on better-exposed 
outcrops around the periphery of Black Butte. The Renova Formation is 
locally overlain by remnants of a mafip tan tuff sourced from the Black Butte 
volcanic center (Gutmann et al., 1989).
Exposures at Lion Mountain
The thickest section of Renova Formation in the Gravelly Range is 
exposed particularly well on the west side of Lion Mountain (Fig. 15). The 
Renova Formation rests on uppermost limestones of the Madison Group, which 
show a few thin traces of red basal Amsden Formation siltstones remainihg on 
them. The Renova Formation sedimentary sequence is capped by basalt flows 
from the Lion Mountain volcanic center.
Exposures at Red Hill/Lazvman Hill
The gravel of Black Butte lies on folded Mesozoic rocks at Red Hill and 
the adjacent southern margin of Lazyman Hill. The gravel apparently grades 
conformably upward into the siltstone, limestone and conglomerate of Red Hill, 
based on the observation of small lenticular deposits of the gravel in the lower 
parts of the intertonguing siltstones and limestones exposed at the south end of 
Lazyman Hill. A very small patch of Renova tuffaceous mudstones is also 
present on the south edge of Lazyman Hill (Fig. 16).
47
Key:
O
C-Jr - - S
In it Ii i
III H n #
(Exposures at Black Butte)
= Carbonate 
nodule
=Tuffaceoua
limestone
= MammaI 
fossil
= Basaltictuff
= Mudstone
= Sand
= Quartzite 
cobble
= Unconformity
Schematic graphic 
columnar section
77:0
Figure 14. Schematic stratigraphic section (not actually measured) compiled 
from exposures near the western side of Black Butte. Tbb = tuff facies of basalt 
and phonolite of Black Butte.
48
(Exposures at west side of Lion Mtn.)
Key:
C =Carbonate 
nodule
= Mammal 
fossil
= Basalt flows
=Mudstone
IW I  =Pebbleconglomerate
= ArfaIIash 
= Unconformity
<S)
QJ
u
QJV)
I
O
0)
S
a>
>
s
SI
CU
Z
250-
Schematic graphic 
columnar section
30.8 + 0.7 Ma
200 -
I
§ 150-j
§
CE
ioo-
' - I r - M - - M - 3 1 . 4 + 0 . 7Ma
Figure 15. Schematic stratigraphic section (not actually measured) for 
exposures at the eastern side of Lion Mountain. Radiometric dates from 
Gutmann et al. (1989).
49
(Exposures at Red Hill/Lazyman Hill)
Kev: 
S  =
^  =
S=
= snail 
stromatolite 
oncolite 
sand, sandstone 
siltstone 
mudstone 
limestone 
quartzite cobble 
= Archean boulder 
limestone clast
unconform ity
unconform ity
Figure 16. Schematic stratigraphic section (not actually measured) for 
exposures in the Red Hill/Lazyman Hill area of Monument Ridge.
50
Exposures at Teoee Mountain
Tepee Mountain forms the eastern margin of Tepee basin, between 
slightly higher adjacent peaks exposing Shedhorn Sandstone. Here, thin 
deposits of the gravel of Black Butte and the conglomerate of Red Hill are 
overlain by fossiliferous Renova Formation mudstones and basalt flows from 
local vents (Fig. 17).
Generalized Stratigraphic Column
Observations from the four localities shown in figures 14-17, and from the 
outcrop of Huckleberry Ridge Tuff, along with mammalian biostratigraphic age 
control, provide sufficient information to compile a generalized Late Cretaceous 
through Tertiary stratigraphic column for the southern Gravelly Range. This can 
be compared to the results for southwestern Montana basins (Plate 2).
Because of the recent changes in time scale calibration (Cande and Kent, 1992; 
Prothero and Swisher, 1992) it was necessary to choose which geochronologic 
datum plane to line up between the results from this study and those of Fields et 
al. (1985). Because one of the major points of plate 2 is to indicate the age 
equivalence between the base of the Renova Formation in the field area and 
the base of the Renova Formation in the modern intermontane basins (see 
Structural Disruption of the Renova Formation, p. 76), the Uintan/Duchesnean 
boundaries of the two schemes are lined up horizontally; other ternporal 
boundaries may not coincide.
51
(Exposures at Tepee Mtn.)
Key:
£§3 = basalt flow 
= mammal fossil 
(22) = carbonate concretion 
= mudstone 
« breccia
= Archean boulder 
( y  = limestone clast
I r
£
r
Z
I
I
I3
Jll
I i
I
-T
50 -
30 -
20 -
10-
Schematic graphic 
columnar section
l i l UIL^iyjTurlrh 32.38 ±0.64 Ma
Z 1
unconform ity
_-J unconform ity
gravel of Black Butte
Figure 17. Schematic stratigraphic section (not actually measured) for Tepee 
Mountain, including radiometric date for basalt flow at top (see Appendix).
52
VERTEBRATE PALEONTOLOGY OF THE RENOVA FORMATION 
Mammalian Biochronology
Vertebrate paleontologists were among the first workers to investigate 
the Tertiary clastib rocks of western Montana basins. They noted the strong 
similarity, both lithologically and in terms of shared faunal elements, of localities 
such as Pipestone Springs (Jefferson basin) and the Canyon Ferry area 
(Townsend basin) to the White River Group of the northern Great Plains 
(Douglas, 1901; Matthew, 1903; Osborn, 1909; White, 1954). In reference to 
these and other late Eocene to Oligbcene localities in western Montana basins, 
early publications often used terms such as " Titanotherium beds" o r" Oreodon 
beds," first developed as basic biostratigraphic subdivisions of the White River 
Group strata.
The early recognition of the utility of mammalian biostratigraphic range- 
zones as a means of correlating Cenozoic continental sections ultimately led to 
a biochronology based in North America: the North American Land Mammal 
Apes (NALMA). These were first formalized as the "North American Provincial 
Ages" by the Wood Committee (Wood e t al., 1941), and the ongoing effort 
since that time has been to: ( I)  free them from the Iithostratigraphic criteria 
which were originally mixed in with biostratigraphic criteria; (2) more clearly 
define their temporal boundaries such that they neither overlap nor leave a gap; 
(3) add to the database of taxa and their associated range-zones, from which
>,
the biochronology is generated; (4) more accurately correlate the NALMA with 
the other time scales in widespread use for the Cenozoic.
Despite disagreements between authors regarding some NALMA 
definitions, they are extremely useful in discerning intervals of Cenozoic time.
In general, even with the advent of modern radiometric dating techniques and 
the use of magnetic polarity stratigraphy, mammalian biostratigraphy has the 
potential to resolve finer time increments within the Paleogene than any other 
geochronologic method (Flynn, et. al., 1984; Woodbume, 1987).
Ongoing debate centers on the validity and defining criteria of some 
NALMA, particularly the Duchesnean. The recent redefinition and faunal 
characterisation of the Duche$nean by LuCas (1992) is primarily followed here.
Calibrations of the NALMA with respect to other time scales, such as the 
epochs, the magnetic polarity time scale, and the numerical time scale, are 
subject to shifts as new or corrected data become available. Unfortunately, 
NALMA have often been used in the geological literature as if they were 
subdivisions of the epochs, despite the fact that the epochs are defined in 
Europe on entirely different criteria. Since the epochs are in such widespread 
use, it is important to clearly state which time scale calibration is being used, 
although as Emry et al. (1987) put it, "As long as one is considering only the 
mutual temporal relationships of North American units, where the Eocene- 
Oligocene boundary is drawn is irrelevant".
Figure 18 shows the recent recalibr&tion of part of the Paleogene time 
scale (Duchesnean through Whitneyan interval) relevant to this research 
project (Prothero and Swisher, 1992). The major features of this revision 
include the much shorter duration of the Chadronian, and its shift from primarily 
early Oligocene entirely into the late Eocene, with the Chadronian-Orellan and
53
54
Previous 40K-40Ar Calibration Revised 40Ar/ 35Ar Calibration
Figure 18. Calibration of the Duchesnean through Arikareean chronology in 
North America. Calibration based on Evernden et al. (1964) and Berggren et al. 
(1985) shown on left. New correlation shown on right (modified from Prothero 
and Swisher, 1992).
Eocene-Oligocene boundaries nearly coinciding at about 34 Ma. This shift 
produces the need to 'translate' the statements of some earlier authors 
regarding the epochs correlated with a particular mammal fauna. For instance,, 
in the Gravelly Range, Mann (1954,1960) asserted that fossils from the 
mudstone exposures near Black Butte indicated an early Oligocene ago; those 
fossils now indicate a late Eocene age.
Paleontology of the Field Area
The stratigraphically highest and lowest exposures of Renova Formation 
sedimentary rocks within the field area are given age limits based on 
mammalian fossils collected by: (1) Mann (1954) and identified by G. L  
Jepsen; (2) A. Lewis and others from Harvard University's Museum of 
Comparative Zoology (in 1952 and 1958); (3) M. C. McKenna and associates 
from the American Museum of Natural History; and (4) the author and J. M. 
O'Neill during this study and curated at the American Museum of Natural 
History.
These sources were combined to form taxonomic lists (Table 1) for four 
localities within the field area (Fig. 19).
Table 1. Taxonomic Lists for Localities in the Field Area
Upper Lion Mountain Locality
Marsupialia
Didelphidae
Copedelphys Stevenson i 
Nanodelphvs sp.
55
56
Table 1. (continued)
Indectivora
Talpidae
. gen. & sp. indet.
Lagomorpha
Leporidae
Palaeolagus cf. burkei
Rodentia
Heteromyidae
Indet. Heteromyine 
Cricetidae
Leidvmvs sp.
Carnivora
Canidae
Hesoerocvon sp. 
Oxetocyon sp.
Perissodactyla
Rhinocerotidae
Diceratherium Tridactylum
Artiodactyla
Agriochoeridae
gen. & sp. indet. 
Merycoidodonfidae 
gen. & sp. indet. 
Hypertragulidae
HvDertraauIus sp. 
Leptomerycidae
Leotomervx sp.
Teoee Mountain Locality
Lagomorpha
Leporidae
TMeaaIaous sp.
Rodentia
Eomyidae
Protadiidaumo so.
57
Table 1. (continued)
Creodonta
Hyaenodontidae
Hvaenodon sp.
Perissodactyla
Titanotheriidae
gen. & sp. indet.
Black Butte Locality
MCZ = collected by A. Lewis, curated at Harvard 
Mann = reported by Mann (1954)
Carnivora
Felidae
gen. & sp. indet. (MCZ)
Perissodactyla
Helaletidae
gen. & sp. indet. (MCZ) 
Titanotheriidae
gen. & sp. indet. (Mann) 
Hyracodontidae
Hvracodon sp. (MCZ)
Artiodactyla
Agriochoeridae
Protoreodon sp. 
Leptomerycidae
. Leotomervx sp. (MCZ; Mann)
Raoamvs Site
MCZ = collected by A. Lewis, curated at Harvard
Rodentia
Paramyidae
Raoamys sp. 
Cylindrodontidae
Pareumvs so. (MCZ)
Carnivora
gen. & sp. indet.
Artiodactyla
Agriochoeridae
Protoreodon oearcei (MCZ)
58
SCALE 1:62500
4 MILES
Figure 19. Paleontologic index map showing the Tepee Creek locality (T), 
Black Butte locality (B)1 Rapamys site (R)1 and upper Lion Mountain locality (L).
59
Fossils from three of the localities, Tepee Mountain locality', ’Black Butte 
locality' and 'Rapamys site', were collected from the basal tuffaceous mudstone 
of the Renova Formation preserved on the range crest. The stratigraphically 
highest fossiliferops sedimentary strata in the field area are the 'upper Lion 
Mountain' exposures, within 25 meters of the basalt flows capping the summit 
(Fig. 20). The taxa from this locality indicate a Whitneyan age (Gutmann et al., 
1989). The Whitneyan NALMA is presently calibrated at about 29 to 32 Ma 
(Prothero and Swisher, 1992); this age range agrees well with radiometric 
dates bracketing this locality: 31.4 ±  0.7 Ma for an underlying airfall ash, and 
30.8 ±  0.7 Ma for the overlying basalt flows (Gutmann et al., 1989). Since no 
significant additions to the upper Lion Mountain fauna have been made since 
Gutmann et al. (1989), its fauna and age brackets will not be discussed further.
The biochronologically significant taxa from each of the three sites 
representing the base of the Renova section can be used to constrain the age 
range of each site in terms of the. North American Land Mammal Ages. The 
ranges of the taxa discussed below are compiled primarily from the syntheses 
contained in Emry (1981,1992), Krishtalka et al. (1987), Emry et al. (1987), and 
Lucas (1992).
Teoee Mountain Locality
This set of exposures has seen little, if any, collecting prior to this study. 
The thin (10-30 m-thick) section of cream-colored tuffaceous mudstone is 
pierced by a small vent of scOriabeouS agglomerate and overlain by a basalt 
flow dated at 32.38 ±  0.64 Ma (Appendix).
Carbonate concretions are numerous here, as elsewhere in the basal 
Renova Formation. One of the concretions yielded a juvenile creodont skull,
60
Figure 20. Renova Formation mudstones, sandy siltstones, and thin granule- 
pebble conglomerate layers which produce a Whitneyan fauna, upper slope of 
Lion Mountain. This locality is bracketed by an underlying air-fall ash dated at 
31.4 ±  0.7 Ma and an overlying basalt flow dated at 30.8± 0.7 Ma (Gutmann et 
al., 1989); this serves as a confirming instance of the numerical calibration of 
the Whitneyan NALMA shown in figure 21.
identified as Hyaenodon. This specimen, still in preparation at the AMNH (Fig. 
21), appears to preserve the most complete example of the milk dentition of this 
genus (M. McKenna, pers. comm., 1996).
A partial skull of the Eomyid rodent Protadjidaumo was also recovered 
from a concretion (Fig. 22). The temporal ranges of these taxa are shown 
below:
61
Table 2. Biochronology of Taxa from the Tep^e Mountain Locality
UINTAN DUCHESNEAN CHADRONIAN
I-? Protadjidaumo--------- - --1
\—* Hyaenodon
There is some disagreement in the literature as to whether 
Protadjidaumo first occurs in the latest Uintan (Emry, 1981) or Duchesnean 
(Lucas, 1992); however, it makes a last appearance by the end of early 
Chadronian time. Hyaenodon is a Eurasian immigrant with a range that passes 
through four NALMA boundaries, yet its Duchesnean first appearance helps 
limit the age of the enclosing sedimentary rocks to Duchesneah or early 
Chadronian.
The Tepee Mountain locality also yielded fragments "of turtle postcrania, 
large weathered bone cores and a single large tooth crown of a titanothere (= 
brontothere), and two partial skulls and a right dentary fragment of a large 
primitive lagomprph. The dentary fragment, with moderately worn lower third 
and fourth premolars and first molar (P3-M1), shows a simple P3 with a single 
external reentrant (Figure 23), characteristic of the extinct basal subfamily 
Paleolaginae, as discussed by Dawson (1958). In terms of general size and
62
Figure 21. Juvenile Hyaenodon skull from the newly described Tepee 
Mountain locality, showing milk dentition (in preparation- jaws not yet 
separated). (Courtesy of the American Museum of Natural History).
63
I f
LtnjcoaI SicLz.
Buccal Side.
Figure 22. Occlusal views of upper cheek teeth (top), and lower cheek teeth 
(bottom) of the Eomyid rodent Protadjidaumo from the Tepee Mountain locality. 
(Drawings by Nancy Hong, American Museum of Natural History).
O I
CM
Figure 23. Sketch of right jaw fragment showing P3-M1 of a large unidentified 
Iagomorph resembling Megalagus, from the Tepee Mtn. locality. Note the 
relatively simple P3 morphology (tooth on left), showing a single external 
reentrant. Outline of teeth in occlusal view (top) and lateral view (bottom). 
(Drawings by Frankie Jackson, Museum of the Rockies).
primitive dental pattern of the P3-M1 and P3-M2, the specimens appears to most 
closely resemble the Chadroniah taxon Megalagus among the Eocene and 
Oligocene Leporids in the Frick collection at the AMNH. Megalagus is 
apparently not reported from earlier than the Chadronian, but is closely related 
to the earlier genus Mytonolagus, known from the Uintan. Preparation of the 
two partial skulls may help resolve whether the specimens can be referred to 
either of these genera. .
Black Butte Locality
Locality data for fauna collected by Mann (1954) noted only that they 
were from exposures in the vicinity of Black Butte. Collections made by A.
Lewis of Harvard (1952 and 1958, unpublished data) included a general map 
indicating the sample site to be located along the eastern margin of Black Butte. 
Since the exposures in this area constitute as much as 140 m of poorly 
consolidated mudstones, there is the possibility of 'contamination' of the basal 
age estimate by significantly younger fossils from higher in the section. Local 
slumping of the mudstones may further complicate this problem. Thb only taxon 
reliably associated with the basal Penova strata at this locality is the 
agriochoerid Protoreodon, which ranges from the early Uintan into the 
Chadronian (Krishtalka et al., 1987), although Lucas (1992) notes it as a 
Duchesnean last occurrence.
65
66
Table 3. Biochronology of Taxa from the Black Butte Locality
UINTAN DUCHESNEAN CHADRONIAN
I— Hyracodon------------
I---------------------------------- mmmmProtoroocIon —— — —I
—Leptomeryx----------- >
There seems to be substantial disagreement in the literature about the 
range of Leptomeryx-, Emry et al. (1987) note its abrupt Chadronian 
appearance, whereas its presence did not conflict with the Duchesnean age 
assignment of a fauna mentioned in Krishtalka et al. (1987, p.105). The Black 
Butte fauna, with the possibility of contamination noted above, has age limits 
similar to that of the Tepee Mountain locality, i.e. Duchesnean to early 
Chadronian.
Raoamvs Site
This locality is a small exposure of mudstones only 3-5 meters thick, 
preserved in a Paleogene paleovalley between Black Butte and Lion Mountain. 
The sediments here are also cream-colored tuffaceous mudstones containing 
carbonate concretions, but with an unusual concentration of small (3 mm) 
cylindrical pellets present in the matrix, interpreted to be fossilized earthworm 
casts (E. Evanoff, pers. comm. 1996). Relatively complete skulls of Rapamys 
were recovered from this site (Fig. 24). Pareumys and Protoreodon are also 
present. In addition to these time-significant taxa, this site yielded a potentially 
identifiable carnivore jaw and a partial snake skull.
67
68
Table 4. Biochronology of Taxa from the Rapamys Site
UINTAN DUCHESNEAN CHADRONIAN
I? ..................................... ------Rapamys-------------1
I-?------------------------------- — Pareumys------------ 1
I- _____________ _____ __ I
Based on these ranges, this locality could be as old as Uintan, but is 
clearly no younger than Duchesnean.
Proposed Duchesnean Local Fauna
It is possible that the base of the mudstone is time-transgressive over a 
small distance, i.e. that the Rapamys site is significantly older than the other 
localities. However, the simplest hypothesis consistent with the paleontological 
data is that the taxa from the three localities constitute a Duchesnean local 
fauna present at the base of the Renova section, and that deposition of these 
mudstones therefore began between 37 and 40 Ma (Prothero and Swisher, 
1992).
Because of the largely unfossiliferous nature of the Duchesne River 
Formation on which the Duchesnean NALMA was based, the principle 
correlatives of this "type area" have contributed the majority of mammalian 
genera known from this NALMA. The heavy reliance on widely geographically 
separated local faunas is the source of some uncertainty, especially regarding 
which local faunas should be included in the Duchesnean and which should be 
subsumed in the early Chadronian (see discussions in Emry1 1981; Emry et al., 
1987). Given these difficulties, the local fauna of this report may, with continued
69
collecting, contribute to the faunal characterization of the contentious 
Duchesnean interval.
70
TECTONIC HISTORY AND DISCUSSION
The most Iikply geologic history of the Gravelly Range consistent with the 
available data in the literature and from this study, begins with Late Cretaceous 
uplift of the area as the western limb of the Madison-Gravelly arch, with syn- 
and post-tectonic erosion of the arch. Far-traveled quartzite cobbles, most likely 
from a southwestern thrust belt source (see Ryder and Scholten, 1973) or 
possibly a northwestern thrust belt source (Pioneer Mountains according to 
Ruppel, i 993), ohlapped the western edge of the erosionally beveled arch in 
Late Cretaceous time (Fig. 25A). Features of the gravel deposit west of Black 
Butte suggest that some of the deposition involved mass-movement 
mechanisms (Gutmann, et al., 1989). More proximally sourced Archean 
metamorphic cobbles and boulders from an unroofed foreland structural 
culmination, probably the Blacktail-Snowcrest arch, mixed locally with the 
quartzite clasts. Major uplift of the Blacktail-Snowcrest arch took place several 
Ma earlier than the Madison-Gravelly arbh (Perry et al., 1992), and could have 
provided a source for basement lithologies prior to the end of Laramide fault 
movement in the field area. Conformable transition to deposition of dominantly 
Paleozoic limestone clasts with some Archean basement cla$ts took place as 
Lima Conglomerate equivalents (conglomerate of Red Hill) reached the field 
area from the Blacktail-Sndwcrest arch, with the conglomerate facies locally 
intertonguing with siItstone and freshwater limestone representing lower-energy 
floodplain and lacustrine depositional environments. Their deposition is
71
Blacktail-Snowcrest
Arch
Ruby Syncline
Field Area
3 o o°<r
Figure 25. Diagrammatic cross-sections (not to scale) from the eastern side of 
the Blacktail-Snowcrest arch through the northern part of the field area, showing 
the proposed Late Cretaceous sequence of tectonic and depositional events.
(A) Deposition of gravel of Black Butte derived from a thrust belt source to the 
southwest (see Ryder and Scholten, 1973), onlapping the western flank of the 
Madison-Gravelly arch. (B) Deposition of Lima Conglomerate equivalents 
(conglomerate of Red Hill) derived from the Blacktail-Snowcrest arch just prior 
to final movement on thrusts in the field area. (pG = Precambrian, undivided;
PZ = Paleozoic; MZ = pre-Laramide Mesozoic rocks; Kl = Lima Conglomerate; 
Kg = gravel of Black Butte; Krh = conglomerate of Red Hill).
probably syntectonic, as shown by their slight folding adjacent to a thrust at 
Lazyman Hill (Fig. 25B). Erosion had cut deeply into the Phanerozoic section of 
the field area by this time; the limestone conglomerate drops in elevation from 
west to east, resting on Kootenai sandstone on the west and on upper Madison 
Group limestone in Tepee basin.
A major erosional episode prior to about 40 Ma is indicated by the 
unconformity below the Renova Formation. Paleozoic to Lower Cretaceous 
rocks were exposed in most areas, with some patches of the gravel of Black 
Butte and conglomerate of Red Hill remaining by the beginning of mudstone 
deposition at 40-37 Ma. In those places where the Renova overlies the gravel of 
Black Butte and conglomerate of Red Hill, there is no evidence of intertonguing. 
If the conglomerate of Red Hill is correlative with the Campanian Lima 
Conglomerate (Nichols et al., 1985), as proposed, the unconformity above the 
conglomerate of Red Hill and below the Renova Formation represents a gap of 
about 38 million years. A minimum estimate of this time gap is 28 million years, 
based on the slight deformation of the unit by a thrust fault and the 68 Ma upper 
age limit for Laramide contraction of the Madison-Gravelly arch (Tysdal et al., 
1986).
The possibility of conformable relations between the conglomerate of 
Red Hill and the Renova Formation should be considered. A number of 
enigmatic 'red conglomerates' have been identified in several basins of western 
Montana, with gradational relations between them and overlying fine-grained 
Renova Formation strata reported in the Three Forks basin (Robinson, 1963; in 
Fields et al., 1985), North Boulder basin, lower Ruby River basin and 
Beaverhead basin (Fields et al., 1985). This raises the possibility that the 
limited exposures of the contact may prevent recognition of interbedded,
72
conformable relations, and that the slight folding of the older unit at Lazyman 
Hill is due to Cenozoic deformation of some sort. In this scenario, the 
conglomerate of Red Hill would be upper Eocene. Previous workers in the 
Gravelly Range assigned Paleocene to Oligocene ages to the quartzite-rich 
gravel, limestone conglomerate and mudstone of the Gravelly Range as shown 
below:
73
Table 5. Previous age assignments for the syn- and post-Laramide rocks of this 
report
Mann
(1954; 1960)
Hadley
(1980)
Gutmann et 
al. (1989)
Ruppel
(1993)
tuffaceous
mudstone
early
Oligocene
early
Oligocene
Chadronian-
Whitneyan
(Oligocene)
early to 
middle 
Oligocene
limestone
conglomerate Paleocene
"Sphinx" (age 
uncertain) N/A N/A
quartzite
gravel
middle to late 
Eocene
Eocene to 
early Miocene
Late
Cretaceous
early
Oligocene
The following factors should be noted with regard to this summary of age 
assignments: 1) Mann (1960) essentially correlated his limestone . 
conglomerate (= conglomerate of Red Hill) with the Lima Conglomerate by 
stating that, "...fanglomerate deposits were developed along the front of the 
rising mass of the Snowcrest Range overthrust to the west" although he 
believed the conglomerate to be Paleocene and also older than the gravel of 
Black Butte; 2) some of the age assignments based on mammal faunas would 
now be "late Eocene" rather than "early Oligocene" due to recalibration 
(Prothero and Swisher, 1992); 3) Hadley (1980) investigated deposits in the 
northern part of the Gravelly Range. His descriptions of deposits overlying a
widespread erosion Surface closely match those of this report and the gravel 
and sedimentary rocks are presumed to be correlative.
The conclusions of Mann (1954; 1960), Hadley (1969b; 1980) and 
Ruppel (1993) differ in their age assignment for the conglomerate of Red Hill by 
about 38 Ma relative to the conclusions presented here. The Campanian age 
assignment given in this report is potentially testable through collection of 
palynomorphs from the lacustrine limestone facies of the conglomerate of Red 
Hill; it would not be hard to distinguish a Late Cretaceous from a late Eocene to 
Oligocene palynomorph assemblage. Additionally, a femur, tibia and fibula 
found in the lacustrine limestone appear to be those of a pterosaur (0.
Varicchio, pers. comm., 1997), which would rule out a post-Cretaceous age for 
the conglomerate of Red Hill. Acid preparation may allow more certain 
identification of this specimen.
No information was obtained in this study to distinguish a tectonic versus 
climatic mechanism for the shift from erosion to deposition in the early late 
Eocene. Thompson et al. (1981) argued for a mechanism of rapid climate 
change from a middle Eocene wet interval to a late Eocene arid or semi-arid 
interval in southwestern Montana, in which sediment production overwhelmed 
the carrying capacity of the streams, resulting in aggradation. Numerous lines 
of paieoclimatic evidence indicate global cooling and increased aridity in a 
number of 'steps' from the middle Eocene to early Oligocene (Berggren and 
Prothero, 1992). Within this interval, the most dramatic turnover in North 
American terrestrial fauna marks the Uintan/Duchesnean transition. At this time, 
animals and plants adapted to warm climate and tropical forests were 
decimated by a major shift to a cooler dryer climate (Berggren and Prothero, 
1992; Wolfe, 1992). Deposition of the Rendva Formation in the field area
74
began at 40-37 Ma, apparently shortly after these dramatic climatic and faunal 
changes.
The area experienced some erosion of older Renova strata just prior to 
32 Ma1 but remained a site of deposition, receiving basaltic volcanics and 
sediment until at least 30 Ma. Some erosion of the Renova cover had taken 
place by the time eruptions began at Black Butte at 22.9 + 1.2 Ma (Marvin, 1974; 
Gutmann et al., 1989). Timing of subsequent Tertiary deformational and 
depositional events in the field area is poorly defined. If any strata younger than 
middle Oligocene were deposited, they were eroded prior to 2 Ma when the 
Huckleberry Ridge Tuff was emplaced. The dense welding of the tuff at 2,926 m 
(9,600 ft) in the field area probably rules out surge to up a major topographic 
slope during its emplacement, as discussed previously. This suggests that 
much of the 1,100 m of relief on the tuff between the field area and the southern 
Madison Valley, only 20 km to the east, is due to Quaternary movement on 
extensional faults bounding the eastern side of the Gravelly Range. Essentially 
the same conclusion was arrived at based on outcrops of Huckleberry Ridge 
Tuff further south in the range (O'Neill et al., 1995). High-elevation (2,700-2,900 
m) outcrops of Huckleberry Ridge Tuff are found in other ranges within the 
Yellowstone crescent of high terrain, including the Madison and Gallatin ranges 
(data of R. L. Christiansen, cited in Pierce and Morgan, 1992), indicating that 
the structural and physiographic differentiation of contemporary ranges and 
basins across southwestern Montana may be quite young (< 2.0 Ma).
75
Structural Disruption of the Renova Formation
76
Two possible models to account for deposition of fine-grained Renova 
Formation strata on the crest of the Gravelly Range and in surrounding basins 
are shown diagrammatically in figure 26. The model shown in figure 26A 
shows sediments progressively filling basins already delineated, onlapping and 
finally burying the Gravelly Range. This scenario requires that the basal 
Renova Formation of the Gravelly Range be younger than that in the adjacent 
basins. Figure 26B shows deposition of a Renova "blanket" (Thompson et al., 
1981; Fritz and Sears, 1993) across a generally low-relief broad basin in which 
the location of the Gravelly Range is not significantly higher than surrounding 
areas, followed by offset via extensional faulting after deposition. This scenario 
requires that the basal Renova Formation of the Gravelly Range be the same 
age as that in the adjacent basins.
The oldest date for any Renova Formation equivalents overlying the 
regional Eocene unconformity in southwestern Montana is late Uintan or 
Duchesnian, assigned to exposures in the upper Ruby basin just west of the 
Gravelly Range (Plate 2). This Uintan or Duchesnean age is based on the 
presence of a small species of Protoreodon low in the Climbing Arrow Member 
of the Renova Formation (Monroe, 1976; unpublished data of Tabrum cited in 
Fields et al., 1985). The fauna reported there is not necessarily older than the 
Duchesnian fauna reported here.for the field area (which also includes 
Protoreodon). Duchesnean local faunas are reported from the Three Forks 
basin, lower Ruby basin, and Beaverhead basin (summarized in Fields et al., 
1985) (Plate 2). All other age determinations, radiometric or paleontologic, for 
the Renova Formation are younger than these. Based on this, model A of
77
Figure 26. Simplified depiction of two possible models for Eocene-Oligocene 
deposition of fine-grained Renova sediments on top of the Gravelly Range and 
in surrounding regions. (A) Progressive filling of already-delineated basins, 
ending with burial of the Gravelly Range crest (1), followed by erosion (2).
(B) Deposition within a broad low-relief basin (1), with subsequent extensional 
partitioning of the Renova "blanket" (2).
,1
figure 26 can be ruled out as a scenario for deposition of the Renova 
Formation. Rather, when aggradation began after the middle/late Eocene 
transition, the Gravelly Range was at an elevation similar to any known areas of 
Renova Formation deposition in the region. The basin floor now exposed on 
the crest of the Gravelly Range was not offset relative to adjacent areas until 
after deposition of the upper sediments of Lion Mountain (about 30 Ma), and 
possibly not until much later.
Similar extensional disruption of Renova Formation equivalents was 
noted in the Big Belt Mountains and adjacent basins east of Helena (Reynolds, 
1979), although the total displacement there is less than a fourth of the 
magnitude calculated below for the Gravelly Range region. Estimates of the 
maximum structural offset between the Renova Formation on the Gravelly 
Range and its equivalents in adjacent extensional grabens are based on 
thickness of Cenozoic fill inferred from gravitational data (Schofield, 1981; 
Ruppel, 1993), added to the difference in surface elevations. Offset with the 
deepest part of the Centennial Valley is inferred to be 2,860-3,780 m (9,400- 
12,400 ft); offset with the deepest part of the upper Ruby basin is at least 3,400 
m (11,200 ft); and offset with the deepest part of the southern Madison basin, 
only 20 km east of the field area, is about 5,300 m (17,400 ft).
If the regional 16 Ma Hemingfordian unconformity above the Renova 
Formation marks initial extensional disruption of the Rpnova basin and 
foundering of these grabens (Thompson et al., 1981), an average offset rate of 
0.33 mm/yr is obtained between the southern Gravelly Range and southern 
Madison Valley. If the influence of the Yellowstone hot spot is the primary 
cause of this extension (see Sears et al., 1995), the offset rate is probably closer 
to 1 mm/yr or more. Evidence for neotectonic activity in the Centennial
78
Valley/southern Madison Valley area suggests rapid ongoing uplift of the range 
(O'Neill et al., 1995); the presence of abundant poorly indurated and clay-rich 
rocks which are rapidly being eroded from the crest of the range by fluvial, 
colluvial, and mass-wasting processes supports this interpretation.
Based on the data from this study, scenarios for the tectonic evolution of 
southwestern Montana which include the modern basin-bounding faults as 
controlling deposition of the Renova Formation (e.g. Fields et al., 1985) can be 
ruled out unless the tectonic history of the Gravelly Range is discarded as 
anomalous with respect to the rest of the region.
79
80
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Geological Society Field Conference Guidebook, p. 261-273.
Pierce, K. L., and Morgan, L  A., 1992, The track of the Yellowstone hot spot: 
volcanism, faulting, and uplift: in Link, P. K., Kuntz, M. A., and Platt, L  B., 
eds., Regional geology of eastern Idaho and western Wyoming: 
Geological Society of America Memoir 179, p. 1-53.
Prothero, D. R., and Swisher, C. C., 1992, Magnetostratigraphy and
Geochronology of the Terrestrial Eocene-Oligocene transition in North 
America in Prothero, D. R., and Berggren, W. A., eds., Eocene-Oligocene 
climatic and biotic evolution, p. 46-73.
Reynolds, M. W., 1979, Character and extent of Basin-Range faulting, 
western Montana and east-central Idaho: Basin and Range 
Symposium, Rocky Mountain Association of Geologists, and Utah 
Geological Association, p. 185-193.
85
Robinson, G. D., 1963, Geology of the Three Forks quadrangle,
southwestern Montana: U.S. Geological Survey Professional Paper 370, 
143 p.
Robinson, G. D., 1967, Geologic map of the Toston quadrangle,
southwestern Montana: U. S. Geological Survey Miscellaneous 
Investigations Map I-486.
Ruppel, E. T., 1993, Cenozoic tectonic evolution of southwest Montana and
east-central Idaho: Montana Bureau of Mines and Geology Memoir 65, p. 
1-62.
Ryder, R. T., and Scholten, R., 1973, Syntectonic conglomerates in
southwestern Montana: their nature, origin, and tectonic significance: 
Geological Society of America Bulletin, v. 84, p. 773-796.
Schmidt, C. J., and O'Neill J. M., 1983, Structural evolution of the southwest 
Montana transverse zone, in Powers, R. B., ed., Geologic studies of the 
Cordilleran thrust belt, 1982: Denver, Colorado, Rocky Mountain 
Association of Geologists, v. 1, p. 193-218.
Schmidt, C. J., O'Neill J. M., and Brandon, W. C., 1988, Influence of
foreland uplifts on the development of the frontal fold and thrust belt, 
southwestern Montana in Schmidt, C. J., and Perry, W. J., eds., I 
interaction of the Rocky Mountain foreland and the Cordilleran thrust belt: 
Geological Society of America Memoir 171, p. 171-201.
Schmitt, J. G., Haley, J. C., Lageson, D. R., Horton, B. K., and Azevedo, P.
A., 1995, Sedimentology and tectonics of the Bannack-McKnight 
Canyon-Red Butte area, southwest Montana: new perspectives on the 
Beaverhead Group and Sevier orogenic belt in Mogk, D. W., ed:, Field 
Guide to Excursions in Southwest Montana: Northwest Geology, v. 24, p. 
245-313.
Schofield, J. D., 1981, Structure of the Centennial and Madison Valleys based 
on gravitational interpretation, Montana Geological Society Field 
Conference Guidebook, p. 275-284.
Scholten, R., 1967, Structural framework and oil potential of extreme
southwestern Montana: Montana Geological Society 18th Annual Field 
Conference Guidebook, p. 7-19.
Scott, H. W, 1938, Eocene glaciations in southwestern Montana: Journal of 
Geology, v. 46, p. 628-636.
86
Sears, J. W., Hurlow, H., Fritz, W. J., and Thomas, R. C., 1995, Late 
Cenozoic disruption of Miocene grabens on the shoulder of the 
Yellowstone hotspot track in southwestern Montana: field guide from 
Lima to Alder, Montana in Mogk, D. W., ed., Field Guide to Excursions in 
Southwest Montana: Northwest Geology, v. 24, p. 201-220.
Spaid-Reitz, M. K., 1980, Petrogenesis of a bimodal assemblage of alkali-basalt 
and rhyolitic ignimbrite, Gravelly Range, southwestern Montana: M.S. 
thesis, Kansas State University, Manhattan, 92p.
Thomas, R. C., 1995, Tectonic sigificance of Paleogene sandstone
deposits in southwestern Montana: Northwest Geology, v. 24, p. 237- 
244.
Thomas, R. C., Sears, J. W., Ripley, A. A., and Berg, R. B., 1995, Tertiary 
extensional history of southwestern Montana: field trip guide for the 
Sweetwater and upper Ruby valleys, Montana: Northwest Geology, v.
25, p. 5-25.
Thompson, G. R., Fields, R. W., and Alt, D., 1981, Tertiary paleoclimates, 
sedimentation patterns and uranium distribution in southwestern 
Montana: Montana Geological Society Field Conference Guidebook, p. 
105-109.
Thompson, G. R., Fields, R. W., and Alt, D., 1982, Land-based evidence for 
Tertiary climatic variations: Northern Rockies: Geology, 
v. 19, p. 413-417.
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southwestern Montana foreland: American Association of Petroleum 
Geologists Bulletin, v. 70, p. 360-376.
Tysdal, R. G., Dyman, T. S., and Nichols, D. J., 1989, Correlation chart of 
lower Cretaceous rocks, Madison Range to Lima Peaks area, 
southwestern Montana: U. S. Geological Survey Miscellaneous Field 
Studies Map MF-2067.
Tysdal, R. G., Marvin, R. F., and DeWitt, E., 1986, Late Cretaceous
stratigraphy, deformation, and intrusion in the Madison Range of 
southwestern Montana: Geological Society of America Bulletin, v. 97, p. 
859-868.
Vandenburg, C. J., Janecke, S. U., and Nichols, R., 1996, Multiple
episodes of Tertiary extension in the west-central Horse Prairie basin, 
southwestern Montana: Geological Society of America Abstracts with 
Programs, v. 28, no. 5, p. 120.
87
White, T. F., 1954, Preliminary analysis of the fossil vertebrates of the Canyon 
Ferry Reservoir area: Proceedings of the U. S. National Museum, v. 103, 
p. 395-438.
Wolfe J., 1992, Climatic, floristic, and vegetational changes near the
Eocene/Oligocene boundary in North America in Prothero, D. R., and 
Berggren, W. A., eds., Eocene-Oligocene climatic and biotic evolution, p. 
421-436.
Wood, H. E. II, Chaney, R. W., Clark, J„ Colbert, E. H., Jepsen, G. L, Reeside, J. 
B. Jr., and Stock, C., 1941, Nomenclature and correlation of the North 
American continental Tertiary: Geological Society of America Bulletin, v. 
52, p. 1-48.
Woodburne, M. O., 1987, Mammal ages, stages, and zones: in Woodburne, M. 
O., ed., Cenozoic Mammals of North America, p. 77-117.
88
APPENDIX
XRF AND RADIOMETRIC ANALYSES OF BASALT FROM TEPEE MOUNTAIN
89
Date
S i02  
AI203 
1102 
FeO* 
MnO 
CaO 
MgO 
K20 
Na20 
P205 
I o ta !
Si02
AI203
Ti02
FeOt
MnO
CaO
MgO
K20
Na20
P205
Ni
Cr
Sc
V 
Ba 
Rb 
Sr 
Zr
Y
Nb
Ga
Cu
Zn
Pb
Th
1195, Ernest Jan Luitcart, Montana State University
LUI
BI
17-Dec-95
Unnormalized Results (Weight %):
48.91 
16.99 
1.459 
8.50 
O . 159 
9.22 
8.99 
1.49 
3.04 
0.383 
99.14
Normalized Results (Weight %):
49.33 
17.14 
I . 472 
8.57 
0.160 
9.30
9.07 
1.50
3.07 
0.386
Trace Elements (ppm):
125
212
24
182
990
43
716
142
26
T35
17
47
64
2
3
Major elements are normalized on a volatile-free basis.
"R" denotes a duplicate bead made from the same rock powder. 
‘ Total Fe is expressed as FeO.
" f "  denotes values >120% of our highest standard.
WSLI GcoAnnlytical Laboratory Analyses by XRF
90
Report On ^OAr/^Ar Analysis for:
Ernest Luikart 
Montana State University
Prepared by:
Lisa Peters
New Mexico Geochronological Research Laboratory
Co-directors 
I)r. Matthew Heizler 
Dr. William C. McIntosh
(NMGRL)
91
Introduction
For the 40ArZ39A r variant o f the K-Ar technique, a sample is irradiated with fast 
neutrons thereby converting 39K  to 39Ar through a (n,p) reaction. Following irradiation,
,the sample is either fused or incrementally heated and the gas analyzed in the same manner 
as in the conventional K -A r procedure, with one exception, no argon spike need be added.
Some o f the advantages o f the 40ArZ39Ar method over the conventional K -Ar 
technique are:
1. A  single analysis is conducted on one aliquot o f sample thereby reducing the sample 
size and eliminating sample inhomogeneity.
2. Analytical error incurred in determining absolute abundances is reduced by 
measuring only isotopic ratios. This also eliminates the need to know the exact 
weight o f the sample:
3. The addition o f an argon spike is not necessary.
4. The sample does not need to be completely fused, but rather can be incrementally 
heated. The 40ArZ39VAr ratio (age) can be measured for each fraction o f argon 
released and this allows for the generation o f an age spectrum.
The age o f a sample as determined with the 40ArZ39A f method requires comparison 
o f the measured 40ArZ39A r ratio with that of a standard o f known age. Also, several 
isotopes o f other elements (Ca, K, Cl, Ar) produce argon during the irradiation procedure 
and must be corrected for. Far more in-depth details o f the determination o f an apparent 
age via the 40ArZ39Ar method are given in Dalrymple et al. (1981) and McDougall and 
Harrison (1988).
Analytical techniques
Whole rock sample Luikart-I was crushed, sieved, treated with ~15% hydrochloric 
acid and hand picked to provide as clean a groundmass concentrate as possible. It was then 
placed in a machined A l disc and sealed in an evacuated Pyrex tube along with 
interlaboratory standard Fish Canyon Tuff (Age = 27.84 Ma). The standard was used to 
monitor the neutron dose received during the 7 hour irradiation in the D-3 position o f the 
reactor at the Nuclear Science Center, College Station, TX. Following irradiation,
92
monitors were placed in a copper planchet and analyzed in an ultra-high vacuum argon 
extraction system with a IOW Syrad CO2 continuous laser. Evolved gases were purified
for two minutes using a GP-50 SAES getter operated at -450° C. The whole rock sample
was step-heated in a double vacuum Mo resistance furnace. The samples were step-heated 
in a double vacuum Mo resistance furnace. The gas was gettered during heating for seven 
minutes with a SAES AP-IO getter, and additionally cleaned following heating with the 
GP-50 for an additional eight minutes. Argon isotopic compositions were determined with 
a MAP 215-50 mass spectrometer operated in multiplier mode with an overall sensitivity of 
3-Ox 10"17 moles/pA. Extraction system and mass spectrometer blanks and backgrounds 
were measured numerous times throughout the course o f the analyses and were very 
reproducible. Typical blanks (including mass spectrometer backgrounds) were; 2000, 1,1, 
2, 8.9 x 10"18 moles at masses 40, 39, 38, 37, 36 respectively. J-factors were determined 
by using 4 separate, 4 crystal aliquots from 4 radial positions around the irradiation vessel. 
Correction for interfering nuclear reactions were determined using K-glass and CaF2.
These values are; (40ArZ39Ar) K = 0.00020+0.0002, (36ArZ37A r)ca = 0.00026+0.00002 and 
(39ArZ37Ar) Ca = 0.00070+0.00005. A ll errors are reported at the two sigma confidence 
level and the decay constants and isotopic abundances are those suggested by Steiger and 
Jager (1977).
Results and Discussion
Luikart-I 32.38 + 0.64 Ma
The preferred age for this sample is the isochron age (Figure I) as it ’ s 40ArZ36Ari of 
300.2± 2.8 is higher than the atmospheric value o f 295.5. The increasing radiogenic yield 
combined with decreasing ages, as seen on the age spectrum (Figure 2), is also indicative 
o f excess argon.
Ot7ZjV
 9£
Figure I Luikart I Whole Rock Isochron Steps D-O
0.0034
0.0032
Isochron Age = 32.38 ±  0.64 Ma 
40ArZ36A ri= 300.2 ±  2.8 
M SW D= 1.15
0.0030
0.0028
0.0026
0.0024
0.0022
0.0020
0.0018
0.0016
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
O.O0O
Ap
pa
re
nt
 A
ge
 M
a 
%
 R
ad
io
ge
ni
c
Figure 2 Luikart I Whole Rock Age Spectrum
-32.1 ±  1.6 Ma-
Integrated Age = 40 + 60 Ma
Cumulative %^Ar Released
K
/C
a
Table 1 Luikait-I Data
Run ID# Temp 40ArZ39A r1 37ArZ39A r2
Luikart-1,19.82mg whole rock J=0.0007545
6171-01C 750 3034 1.982
6171-01D 825 1595 1.597
6171-01E 900 558.5 1.194
6171 -01F 975 210.9 0.9171
6171-01G 1050 79.06 1 . 0 2 2
6171 -01H 1125 58.12 0 .9058
6171-011 1 2 0 0 45.99 0 .8395
6171-01J 1275 47.52 0.9046
6171-O IK 1350 60.68 0 .9194
6 1 7 1 -0 1 L 1425 52.49 0 .8435
6171-01M 1500 52.48 0 .9096
io 6171-01 N 1575 52.27 0 .9324
05 6171 -010 1650 50.47 0 .8762
total gas age
(39ArZ37A r)C = 0.0007010.00005
(36ArZ37Ar)ca = 0.0002610.00002
(40ZlrZ39ZXr)K = 0.000210.0003
1 Corrected for blank
2 Corrected for 37Ar decay
36ArZ39A r 1
10.03 
5 .205 
1.777 
0 .6260  
0.1841 
0 .1139  
0 .0712  
0 .0763 
0 .1225  
0 .0972 
0 .0918 
0 .0978 
0 .0898 
n = 13
% 39Ar39ArK mol 
(X 1 0 '16)
KZCa oZ=40Ar1 0Zo
rad released
Age
(Ma)
± I s.d. 
(Ma)
2.83 0 .257 2.3
7.06 0 .320 3.6
16.0 0 .427 6 . 0
17.7 0 .556 12.3
28.3 0 .499 31.3
19.4 0 .563 42.2
1.84 0 .608 54.4
2.07 0 .564 52.7
5.48 0 .555 40.5
3.12 0 .605 45.4
2.72 0.561 48.4
2.17 0 .547 44.9
1.80 0 .582 47.5
2 . 6 93 44
8.9 76 1 6
23.4 45.2 5.2
39.5 35.0 1 .8
65.0 33.37 0.57
82.6 33.10 0.41
84.3 33.7 1.1
8 6 . 2 33.78 0.76
91.1 33.15 0.58
93.9 32.16 0.75
96.4 34.29 0.75
98.4 31.67 0.80
1 0 0 . 0 32.39
39.5
0.92
3.6
96
References cited
Dalrymple, G.B., Alexander, E.C., Jr., Lanphere, M .A. and Kraker, G.P., 1981.
Irradiation o f samples for 40ArZ39A r dating using the Geological Survey TRJGA reactor. 
U.S.G.S., Prof. Paper, 1176.
McDougall, I. and Harrison, T.M., 1988. Geochronology and thermochronology by the 
40Ar-39Ar method. Oxford University Press.
Steiger, R.H. and Jager, E., 1977. Subcommission on geochronology: Convention on 
the use o f decay constants in geo- and cosmochronology. Earth and Planet. Sci. Lett., 
36: 359-362.
r'
SERI E S /  
EPOCH
P L I O C E N E
M I O C E N E
O L I G O C E N E
EOCENE
L a l e
P A L E O C E N E
NORTH 
AM ER IC AN  
LAND M A M M A L  
AGES
H e m p h l l l l e n
B e r e l o v l  en
I  5-
H e m l n g l o r d l s n
A r l k e r e e e n
W h l t n e y e n
O r e U e n
C h e d r o n l e n
UPPER
RUBY
RIVER
BASIN
Tlmbef Hie beeel l  4 I J
S l im l l e  421 
C r e e k  
Fm.
4 3 *  
4 4 *
B r l d g e r l e n
W meetchien
C l e r k f o r k l e n
T l f l e n l e n
P a s s e m s r l
M b r
Dunbsr  Cr eek Mbr.
C l i m b i n g  4 9 0  
A r r o w
imni
R h y o l i t e  a t  4 a i  
R u b y  Oa m
LOWER
RUBY
RIVER
BASIN
S l i m i l e  
C r e e k  
F m.
_Dwber _Cr eek_Mb f ; _
P a s s a m a r i
C l i m b i n g
A r r o w
M b r .
R h y o l i t e  a t  4 
R u b y  Dam
[[[[[[[[[[
A n d e s l t e - D a c i t e
p o r p h y r y  in 
V i r g i n i a  5 4  
C i t y  a r e a
'T-rr
BEAVERHEAD
BASIN
S i *  mi l e  
C r e e k  
Fm.
R e n o v a  Fm.  5 7*  
SB* 
5 8 *
I
Basal ts at Block Mtn. j 
and m g Q
Beaverhead Canyon '
JEFFERSON
RIVER
BASIN
]____L 6 I* 
6 2 *
SI  * m i l e  
C r e e k  
Fm.
Dunbar  Creek M b r ® ? ?  
-------
Cl imbing I B o n ,  
Ar row 8 e?  B „ , , n
MPr 8 9*1  M b f—
THREE
FORKS
BASIN
S l i m l l e  C r e e k
Fm.  2O-
M e d l s o n  
V e l l e y  
Fm.  2
7 ?e
7 3  =
Dunoer  Creek Fm T e o
C l i m b i n g
A r r o w
Fm.
-Miira^7CT Tm- "
S^ph<n*7 [
R h y o l i t e  
a t  R e d  Mt n .
Plate 2. Generalized stratigraphic column of this report shown on right; correlation chart 
on left shows basin-fill strata and mammalian biochronologic age control from Cenozoic 
basins of southwestern Montana (Modified from Fields et al., 1985; numbered commentaries 
and references not shown). Open circle = fair to good age control based on fossil mammals, 
solid circle = good to excellent age control based on fossil mammals. The position and age 
of the Milligan Creek and 'Sphinx' formations in the Three Forks basin (Robinson, 1963) are 
poorly understood. Note the shift of time scale of this report (Cande and Kent, 1991; 
Prothero and Swisher, 1992) relative to time scale shown on left. The numerical time scales 
have intervals of equal size and the Uintan/Duchesnean boundaries are lined up 
horizontally; other boundaries are not necessarily coincident.
NORTH 
AMERICAN 
LAND MAMMAL 
AGES
SOUTHERN 
GRAVELLY RANGE 
CREST
EPOCH or 
AGE
PleistoceneHuckleberry Ridge Tufl Blancan
Pliocene
Hemphillian
Clarendonian
Barstovian
Miocene
Hemingfordian
basalt and phonolite 
OfBIacjcButte
-2 5 4  Arikareean
Oligocene
Whitneyan-Renova Fm.
Orellan
Chadronian
Renova Fm.
Duchesnian
Uintan
Eocene
Bridgerian
Wasatchian
Campanianconglomerate of Red Hill
gravel of Black Butte
Santonian
ueuiazog - W m ^ j g y 9 l e
57' 30"
44° 52 ' 30" I 
< m ° 5 2 ' l30"
u iT .ic .n o  i u u i
CONTOUR IIvTERVAL 40 FEET
QUADRANGLE LOCATION
A
NORTHWEST
Plate 1. G eolog ic Map, South-C entra l G rave lly Range
A 1
SOUTHEAST
FEET
Black Butte Lion Mountain
METERS 
3.500
h- 3.000
—  2.500
-  2.000
— 1.500
Ernest Luikart, 1997
Qls
km
Je
Ps
Pq
PMd
MDt
p€u
DESCRIPTION OF MAP UNITS
Talus (Holocene)--- Unconsolidated deposits of angular blocks that accumulate at the base 
of cliffs or very steep slopes; includes basaltic rubble surrounding the Black Butte volcanic 
neck
Landslide deposits (Holocene)— Angular fragments of bedrock mixed with soil or
heterogeneous mixtures of boulders and finer-grained material derived from adjacent hill 
slopes; characterized by hummocky topography and small ponded depressions. Also 
includes earthflows composed of clay rich, easily erodable Mesozoic and Tertiary rocks 
Colluvial deposits (Holocene and Pleistocene)— Unconsolidated deposits of silt, sand, and 
angular pebbles, cobbles and boulders formed by downsbpe creep or surface wash; 
locally reworked as alluvium in stream drainages. Includes small remnants of 
unconsolidated Pleistocene glacial moraines in the lower parts of Standard Creek 
Huckleberry Ridge Tuff (Pliocene)— Welded rhyolitic ash-flow tuff, consisting of a basal 
densely welded, dark-grey to black, glassy vitrophyre containing sankJine phenocrysts up 
to 3 mm, grading upward into a middle medium-grey unit which contains sanidine 
phenocrysts and is slightly less densely welded; the unit is capped by porous, light-greyish- 
pink welded tuff with a few small (1 mm) phenocrysts and numerous flattened pumice 
fragments. Total thickness for the tuff is about 1.5 m 
Basalt and Phonolite of Black Butte (Miocene)— Alkali base It and diabase of the Black 
Butte volcanic neck, including pods and small dikes of tephritic phonolite near the top. 
Small outcrops of a thin, tan, mafic tuff overlying an eroded Renova mudstone blanket 
around the margins of Black Butte probably are an eruptive product; the tuff is not in 
place due to slumping and earthflow and is therefore not mapped (Gutmann et ah. 1989). 
The diabase was given a whole-rock K-Ar date of 22.9 ±  1.2 Ma (Marvin et ah, 1974). 
Outcrops of hydrothermal gypsum and chert, exposed in the Jurassic rocks northeast of 
the Butte, may have formed during cooling of the volcanic neck; they are denoted by 
asterisks
Basalt flows and associated vent facies (OIIgocene)— Alkeli basalt and scoriaceous 
agglomerate extruded from several vents within the field area. These basalts are included 
in the Renova Formation by McDowell et ah (1997, in press), but are mapped separately 
here. Where present, thickness varies from I to 30 m. -A -  indicates scoriaceous vent 
agglomerate. Two radiometric analyses have been done on flows in the map area; a 
whole-rock 40Ar/39Ar date of 32.38 ± 0.64 Ma on a flow capping Tepee Mountain (see 
appendix), and a whole-rock K-Ar date of 30.38 ± 0 . 7  Ma (Gutmann et ah, 1989) on a 
flow at the top of Lion Mountain
Mafic dikes (OIIgocene)— Near-surface intrusive alkali basalt and diabase, usually 
associated with local basaltic vents and basalt flows. May be included as part of the 
Renova Formation (as noted above), but mapped separately here 
Renova Formation (Ollgocene and Eocene)— Pale cream-colored to light-tan tuffaceous 
montmorillonitic mudstone containing mammal fossils and locally abundant carbonate 
concretions. Numerous small (3 mm) fossilized earthworm casts are found in some 
outcrops. The mudstones are poorly indurated and massive, showing little or no sign of 
bedding except near the top of Lion Mountain, where thin granule-pebble conglomerate 
interbeds contain clasts of Paleozoic limestone and Tertiary basalt. Thickness ranges 
from 0 to as much as 240 m
Conglomerate of Red Hill (Late Cretaceous)— Dominantly large lenticular bodies of clast- 
supported, limestone pebble to boulder conglomerate in a red to orange argillaceous, 
partly calcareous silt and sand matrix. In the Red HiIIZLazyman Hill area, the conglomerate 
is interbedded with white to reddish lacustrine limestone, red silt stone and coarse 
yellowish-brown Iitharenite sandstone containing abundant schist and phyllite fragments. 
The lacustrine limestone locally contains fossil freshwater snafe, oncoiites, stromatolites 
and vertebrate bones. A deposit of angular breccia of Shedhom Sandstone clasts in a 
calcareous red matrix is included as a locally derived IKhosome at Tepee Mountain. 
Thickness ranges from 0-40 m
Gravel of Black Butte (Lower Cretaceous)— Unconsolidated gravel of extremely well- 
rounded quartzite pebbles and cobbles wKh moderately-rounded Archean metamorphic 
cobbles and boulders locally dominating the clast composKion. Minor Paleozoic limestone 
clasts and rare angular boulders of Shedhom Sandstone are present. The matrix of the 
gravel consists of a coarse, angular, whKe- to yellowish-orange-stained quartz sand whh 
some siH. Thickness ranges from 0-10 m; the unK is present only along the western 
margin of the map area
Muddy Formation (Lower Cretaceous)— Grey to dark-brown, medium-grained, calcareous 
argillaceous sandstone, thin- to medium-bedded, containing carbonized plant remains; 
about 40 m thick
Thermopolls Shale (Lower Cretaceous)— Basal sandstone of the Thermopolis is a 
quartzKic sandstone wKh thin shale interbeds; upper part is medium-grey to black fissile 
shale, containing brown-weathering concretions and thin sandstone interbeds. Total 
thickness is about 30-40 m
Kootenai Formation (Lower Cretaceous)— Lower part consists of grey to light-brown, 
commonly cross-stratKied sandstone, wKh numerous pebbly layers and a chert-pebble 
conglomerate at the base; middle part is silty, light-tan to light-grey, poorly IKhKied 
mudstone and sandy siKstone, poorly exposed; upper part contains a dark-grey limestone 
consisting of small gastropod shells in a micritic matrix. Total thickness is about 120 m 
Morrison Formation (Upper Jurassic)— Generally poorly-exposed maroon, pinkish-grey , 
green, and light-grey medium-bedded mudstone and siKstone, with some discontiuous 
thin sandstone beds. A limb-bone of a dinosaur (sauropod) was recovered in mudstone 
at the south end of Monument Ridge. Thickness is about 45-50 m 
Ellis Group, undivided (Upper and Middle Jurassk)-- Medium- to coarse-grained 
sandstone wKh rounded chert-pebble conglomerate at the base; some grey limestone 
and light brown to tan siKstone in the upper part. Thickness varies from 10-15 m 
Woodside Formation (Lower Trlasslc)— Thin- to medium-bedded red siKstone, sandstone, 
and shale; thickness is 70-90 m, but the unK pinches out raoidly northward of the map 
area
Dlnwoody Formation (Lower Trlasslc)— Thln-bedded yellow to light-brown siKstone 
interbedded wKh yellowish- to pinkish-grey dolomKe. Lingula fossils locally abundant on 
bedding surfaces. About 120-125 m thick
Shedhom Sandstone (Lower Permian)— Light-grey, fine- to r ted'um-grained, brown­
weathering quartzKic sandstone which contains whKe and b ack chert grains; medium- to 
thin-bedded, often cemented and partly replaced wKh chert. Large (5 cm-wide) burrow 
structures filled wKh yellowish- to reddish-brown chert locally abundant in the upper part. 
Thickness is 60-65 m
Quadrant Formation (Pennsylvanian)— Fine- to medium-grained, well-sorted, whKe to pale- 
grey quartz sandstone; bedding usually indistinct but locally Dross-stratified; mostly cakKe- 
cemented, but quartz-cemented in some outcrops. Beds of I to 3 m-thick dolomKe occur 
in the middle and lower part. Thickness ranges from 120 to 150 m 
Amsden Formation (Lower Pennsylvanian and Upper Mlsslasipplan)— I to 4 m-thick 
blue-grey fossiliferous limestone beds wKh red argillaceous sandy siKstone and shale in 
the middle part. Basal unit consists of fissile yellow to tan shale and fine sandstone. Total 
thickness varies from 15 to 40 m
Madison Group, undivided (Upper and Lower Mlsalselpplan)- Olive-grey. light-grey- 
weathering, medium- to thick-bedded, micritic to medium-grained limestone; locally 
fossiliferous, especially in middle and lower parts. Large karsts filled wKh solution breccia 
and siKstone found locally near the top, associated wKh fault zones. Thickness ranges 
from about 470 to 600 m (Hadley, 1960; Mann, 1960)
Map unKs below not exposed in field area. Stratigraphy based on Hadley (1960) and Mann 
(1960)
Three Forks Formation (Misslsslplan and Upper Devonian)— DolomKe and limestone 
breccia in a red argillaceous matrix at base; thick-bedded grey fine-grained limestone and 
grey to black shale in middle part; yellowish-orange calcareous sandstone and sandy 
siKstone in upper part. Thickness ranges from 60-75 m 
Jefferson Dolomite (Upper and Middle Devonian)— DolomKe, moderate- to dark-yellowish- 
brown to dark-grey, thick- to medium-bedded, vuggy and petroliferous. Thickness is about 
90 m
Meagher Limestone (Upper and Middle Cambrian)— Mottled light- to medium-yellowish- 
brown dolomKe and limestone, thin- to thick-bedded, locally weakly petroliferous.
Reported thickness ranges from about 300 m (Mann, 1960) to 110-120 m (Hadley, 1960) 
Wolsey Shale and Flathead Sandstone, undivided (Middle Cambrian)™ Light-brown to 
reddish-brown or whKe quartzKic sandstone wKh thin interbeds of greenish-gray shale, 
overlain by green fissile shale and sandy shale wKh thin interbeds of glauconitic 
sandstone. Thickness is about 60 to 100 m 
Precambrian rocks, undivided (Proterozolc(?) and Archean)- Quartzofeldspathic 
gneiss, amphibolKe, schist. quartzKe, mafic intrusives, and iron-formation
(Contacts, fauKs, anticlines, and synclines are shown by solid lines where accurately located, 
dashed lines where approximately located, dotted lines where concealed)
Contact
Hlgh-angle fault- ball and bar on downthrown side
.A-A A a  A -AJV Thrust fault- sawteeth on upper plate
----------£ --------  Anticline
-------- ^ --------- Syncline
— I—  Strike and dip of beds
L - 1.000
7*