Lithostratigraphy and depositional setting of the limestone-rich interval of the Lahood Formation (Belt
Supergroup), southwestern Montana
by Adrienne Thornley Bonnet
A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE
in Earth Sciences
Montana State University
© Copyright by Adrienne Thornley Bonnet (1979)
Abstract:
The LaHood Formation crops out along a narrow east-west trending belt which coincides with the
Willow Creek fault zone. This 3500 m thick arkose and arkosic conglomerate deposit contains a
distinctive 400 m thick limestone-rich interval near the top. The limestone-rich interval crops out in the
Horseshoe Hills and Bridger Range and is similar to intervals in various stratigraphic positions in the
formation of other areas.
The limestone-rich interval of the LaHood Formation contains two types of depositional sequences
which alternate and reflect mass flow and suspension deposition in relatively deep water. The arkose
depositional sequences contain 25 m of undeformed arkose beds which range between 1 m and 5 m
thick. The limestone-bearing depositional sequences are 30 m thick and contain both rhythmically and
randomly interbedded limestone, siltstone, shale, and arkose in addition to intraformational slump
structures.
The arkose depositional sequences are interpreted as channel deposits of the lower-upper fan and
middle fan of a submarine fan system. Arkose beds in these sequences are amalgamated, and contain
rip-up and floating clasts of silt-stone, shale, and limestone; both normal and inverse graded bedding;
and basal conglomerates. The limestone-bearing depositional sequences represent interchannel deposits
of a submarine fan system. Small scale deformation features and their distribution in slump structures
reflect opencast slump processes and include: plastically deformed septarian concretions, molar tooth
structures, imbricate thrusts, and truncated decollement folds. The deformation reflects both faulting
and bank undercutting by migrating distributary channels.
The LaHood Formation geometry, lithologies, and association with fault zones which were probably
active during the Precambrian together suggest fault-controlled deposition. The east-west fault zone
may be related to the origin of the Belt basin as an abandoned rift arm or a transform fault zone.
Northwest-southeast trending faults to the south of this zone may be secondary shear faults formed
during transform movement and may also have influenced LaHood Formation deposition. 
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Signature _ ^
LITHOSTRATIGRAPHY AND DEPOSITTONAL SETTING OF 
THE LIMESTONE-RICH INTERVAL OF THE LAHOOD 
FORMATION (BELT SUPERGROUP), SOUTHWESTERN MONTANA
by .
Adrienne Thornley Bonnet
A thesis submitted in partial fulfillment 
of the requirements for the degree
of
MASTER OF SCIENCE 
in
Earth Sciences
Approved:
Chairl
Head, Major DefoaJrtment
GraduatevDean
MONTANA STATE UNIVERSITY 
. Bozeman, Montana
November 1979
iTHESES
61,4 &
iii
ACKNOWLEDGEMENTS
Much gratitude is felt for the moral and other sup­
port provided by my parents, Dr. Donald Smith (advising), 
Sharon Dusenberry (typing), Cindy Farquharson (drafting), 
and Connie Zarndt throughout preparation of this thesis.
TABLE OF CONTENTS
Page
LIST OF FIGURES................. .................- , . v±
INTRODUCTION............. ■.............. ............... I
GEOLOGIC SETTING .......................................  4
Location.....................   4
Modern Setting.............................. .. 4
Precambrian Setting . . .  .................  . . .  6
LITHOSTRATIGRAPHY......................... .............  9
Stratigraphy.............................. .. 9
Introduction........................... 9
Stratigraphy of the LaHood Formation . . . .  12
Stratigraphy of the Limestone-Rich
Interval of the LaHood Formation ......... 15
Lithology......................................  27
Introduction..........       27
Description of Arkose........................ 27
Interpretation of Arkose ...................  32
Description of Carbonates. . ................ 42
Interpretation of Carbonate Beds ........... 46
Description of Siltstone and Shale ......... 57
Interpretation of Siltstone and Shale. . . .  58
SEDIMENTARY-TECTONIC ENVIRONMENT OF DEPOSITION . . . . 59
Introduction............................ i . . . . 59
Previous Interpretations ....................  59
VPage
Author's Interpretation. ; .................  62
Sedimentary Environment of Deposition 67
Channel Deposits.............   68
Interchannel Deposits........................ 7 0
Tectonic Environment of Deposition...............  7 2
CONCLUSIONS......................................... . 7 8
APPENDICES A - K ............. ................. .. In
The
Packet
vi
LIST OF FIGURES
Page
Figure I. Distribution of LaHood Formation
outcrops and lines of cross sections 
shown in Figure 3. ......................  5
Figure 2. Tectonic elements of Belt basin and
pre-Belt source area .................... 7
Figure 3. Cross sections of LaHood Formation and
other Belt r o c k s ........................10
Figures 4a.
4b.
4c.
4d.
Depositional sequences and features of
arkose beds (photos)................... 16
Figure 5. Location and correlation of appended
measured sections...................... . 18
Figure 6. Generalized stratigraphic sections of
depositional sequences .................  19
Figures 7a.
7b.
7c.
7d.
Primary sedimentary and diagenetic
features in arkose beds (photos) . . . .  21
Figures 8a.
8b. 
8c. 
8d.
Stromatolites, concretions, and defor­
mation in carbonate beds (photos). . . .  24
Figures 9a. Deformation and diagenetic features in
9b. carbonate beds (photos)................. 26
9c.
9d.
Figure 10. Submarine fan model 65
ABSTRACT
The LaHood Formation crops out along a narrow east- 
west trending belt which coincides with the Willow Creek 
fault zone. This 3500 m thick arkose and arkosic conglom­
erate deposit contains a distinctive 400 m thick limestone- 
rich interval near the top. The limestone-rich interval 
crops out in the Horseshoe Hills and Bridger Range and is 
similar to intervals in various stratigraphic positions in 
the formation of other areas.
The limestone-rich interval of the LaHood Formation 
contains two types of depositional sequences which alter­
nate and reflect mass flow and suspension deposition in 
relatively deep water. The arkose depositional sequences 
contain 25 m of undeformed arkose beds which range between 
I m and 5 m thick. The limestone-bearing depositional 
sequences are 30 m thick and contain both rhythmically and 
randomly interbedded limestone, siltstone, shale, and arkose 
in addition to intraformational slump structures.
The arkose depositional sequences are interpreted as 
channel deposits of the lower-upper fan and middle fan of 
a submarine fan system. Arkose beds in these sequences are 
amalgamated, and contain rip-up and floating clasts of silt- 
stone, shale, and limestone; both normal and inverse graded 
bedding; and basal conglomerates. The limestone-bearing 
depositional sequences represent interchannel deposits of 
a submarine fan system. Small scale deformation features 
and their distribution in slump structures reflect open­
cast slump processes and include: plastically deformed
septarian concretions, molar tooth structures, imbricate 
thrusts, and truncated decollement folds. The deformation 
reflects both faulting and bank undercutting by migrating 
distributary channels.
The LaHood Formation geometry, lithologies, and asso­
ciation with fault zones which were probably active during 
the Precambrian together suggest fault-controlled deposi­
tion. The east-west fault zone may be related to the origin 
of the Belt basin as an abandoned rift arm or a transform 
fault zone. Northwest-southeast trending faults to the 
south of this zone may be secondary shear faults formed dur­
ing transform movement and may also have influenced LaHood 
Formation deposition;
INTRODUCTION
The LaHood Formation of the Belt Supergroup (Pre- 
cambrian Y ) in southwestern Montana is a 3500 m thick wedge 
of arkose and arkosic conglomerate which contains a 400 m 
thick limestone-rich interval near the top. The limestone- 
rich interval studied in the Horseshoe Hills and Bridger 
Range consists of two types of depositional sequences which 
alternate and may reflect lower-upper fan and middle sub­
marine fan processes and episodic faulting along the south­
ern margin of the Belt embayment.
The LaHood Formation has been studied across the 
130 km outcrop extent in varying detail. The studies in 
the limestone-rich interval of the formation involved 
details of deformation structures and characteristics 
related to the formation as a whole, such as thickness and 
stratigraphic position.
Peale (1893) and Walcott (1899) described general 
characteristics of the coarse Belt rocks, including those 
in the limestone-rich interval, and recognized them as 
nearshore equivalents of the finer-grained Belt rocks to 
the north. In order to map the structure in the Whitehall 
area, Alexander (1955) measured and correlated sections.
2described the type section near LaHood Park, and .described 
the petrography of rocks in that area. Verrall (1955) 
described some lithologic features of the limestone-rich 
interval and mapped the distribution of outcrops as part 
of a larger mapping project in the Horseshoe Hills.
McMannis (1963) was the first to measure sections of the 
formation throughout the extent of outcrops and describe 
the stratigraphy and lithology by area. In addition to 
defining the formation and indicating a principal refer­
ence section, McMannis (1963) referred to a limestone- 
rich interval, first recognized by Verrall (1955), as the 
"'molar tooth1 and algal zone". McMannis (1963) stated 
that this zone is the "only reasonable key horizon" and 
used it to correlate sections in the Bridger Range and 
Horseshoe Hills with each other and with other Belt rocks 
to the north. In an abstract, Hawley (1973) described 
characteristics of the formation and proposed that turbidity 
currents, sand flows, and submarine slides deposited sedi­
ment in a fault-bound basin adjacent to a rugged Precambrian 
crystalline source area. The only other reference to this 
limestone-rich interval was made by Hawley and Schmidt 
(1974) in a field guide. In order to propose a depositional 
system for the formation, Boyce (1975) measured sections
3throughout the extent of the outcrops and defined vertical 
and lateral facies changes, stratigraphic relationships to 
other Belt rocks, and petrographic trends. Although 
McMannis (.1963) and Hawley (1973) interpreted some of the 
paleogeographic conditions during deposition of the forma­
tion, Boyce (1975) was the first to design a detailed 
study with the intention of interpreting the environment 
of deposition. Trunk and Smith (1979) studied details of 
the molar tooth structures in the southern Horseshoe Hills 
and concluded these structures represent soft sediment 
deformation of cryptalgal laminites.
Sections were measured by the author in the Horseshoe 
Hills and Bridger Range limestone-rich interval in order 
to describe details of the stratigraphy and lithology and 
propose a sedimentary-tectonic environment of deposition. 
This paper attempts to show that (I) this interval may 
reflect processes and environmental conditions which were 
present throughout LaHood Formation deposition, (2) the 
two types of alternating depositional sequences represent 
channel and interchannel deposits on the lower-upper fan 
and middle fan segments of a submarine fan system, and 
(3) the deposits may also reflect episodic faulting along, 
the southern margin of the Belt embayment.
4GEOLOGIC SETTING
Location
The limestone-rich interval of the LaHood Formation 
crops out in southwestern Montana north of the East Gal­
latin River in the Horseshoe Hills and in Felix Canyon 
on the west side of the Bridger Range (Fig. I). Similar 
intervals crop out in the Highland Mountains (Thorson, J., 
1979, oral commun.) and near St. Paul Gulch, north of 
Whitehall (Coppinger, W., 1979, oral and written commun.). 
The nearly continuous exposures along 6 km of the East 
Gallatin River in the Horseshoe Hills provide an oppor­
tunity to observe detailed lithologic and stratigraphic 
characteristics of the limestone-rich interval with little 
structural complication. The interval is well exposed in 
Felix Canyon; however, the geometry of the steeply dipping 
beds is difficult to discern in logging road cuts.
Modern Setting
The upper LaHood Formation crops out in a series of 
bevelled Laramide folds which plunge to the north in the 
Horseshoe Hills. Some faults with minor displacements cut 
the formation near the contact with the Cambrian Flathead 
Formation, but do not cut LaHood exposures along the East 
Gallatin River (Verrall,1955).
5THHEJ FORKS
FIGURE la. Distribution of known LaHood Formation outcrops and loca­
tions of stratigraphic sections referred to in text under the heading 
Lithostratigraphy. Author's measured sections lie along the East Gal­
latin River in the southern Horseshoe Hills and in Felix Canyon, the 
Bridger Range. Modified from McMannis (1961).
CWGlOERATlC IAHOOD
IAHOOD HITH FINE-GRAINED INTERBEDS
FIGURE lb. Lines of cross sections shown in Figure 3. W-W', Y-Y', and
Z-Z' modified from McMannis (1963); X-X' modified from Alexander (1955).
6A normal fault on the west side of the Bridger Range 
places pre-Belt crystalline rocks against the LaHood For­
mation (McMannis, 1963). The LaHood Formation is finer- 
grained at the north end of the Bridger Range and the 
limestone-rich interval is unconformably overlain by the 
Cambrian Flathead Formation north of the North Cottonwood 
section (McMannis, 1963).
Precambrian Setting
The wedge of LaHood arkose and arkosic conglomerate 
was deposited along the southern margin of the Belt embay- 
ment, north of the pre-Belt crystalline rock source area 
(Fig. 2). An east-west trending fault zone, parallel to 
the present Willow Creek fault zone, probably formed the 
southern margin of the embayment and may have existed dur­
ing pre-Belt and Belt time (Harrison and others, 1974; 
Casella, 1979). This fault zone and the northwest- 
southeast trending faults to the south, probably influenced 
LaHood Formation deposition and may be related to the origin 
of the Belt basin.
Theories regarding the origin of the Belt basin are:
(I) subsidence in an epicontinental trough (Harrison and 
Reynolds, 1976; Harrison, J., 1977, oral commun.), and
7100
I
200 MI
I
100
I
200 KM
FIGURE 2. Tectonic elements of the Belt basin and the pre-Belt crystal­
line rock source area. LaHood Formation source area lies south of the 
Willow Creek fault zone. Modified from Harrison and others (197^).
.8
(.2) faulting during initial stages of continental rifting 
(Harrison and others, 1974; Boyce, 1975). Subsiding epi­
continental troughs differ from rift arm basins because 
they form in stable tectonic environments and involve 
little or no structural control during development. Struc 
tural and stratigraphic■evidence support the concept of 
faulting and fault-controlled deposition along the south­
ern margin of the Belt embayment. Evidence and theories 
concerning the origin of the Belt basin will be discussed 
under the heading: Sedimentary-Tectonic Environment of
Deposition.
9LXTHOSTRAT!GRAPHY
Stratigraphy
Introduction .
The base of the LaHood Formation is covered.or in 
fault contact with pre-Belt crystalline rocks except for 
one depositional contact described by.McMannis (1963). 
According to Boyce (1975), this depositional contact is 
questionable and may be a weathered and sheared zone. The 
stratigraphic relationship between the LaHood Formation and 
other Lower Belt and Ravalli Group rocks is not fully known 
Most workers correlate the finer-grained upper portions of 
the formation with the Greysoh Shale of the Ravalli Group 
and lower, coarser portions with the Neihart Quartzite, 
Chamberlain Shale, and Newland Limestone of the Lower Belt 
(Fig. 3). The limestone-rich interval in the Horseshoe 
Hills and Bridger Range is found in the upper LaHood For­
mation but is correlated with the Newland Limestone 
(McMannis, 1963; Boyce, 1975).
Y'
FIGURE 3. Cross sections located in Figure lb. W-W shows fining upward and northward 
and the wedge shape of the LaHood Formation; X-X' shows the correlation of the formation 
in the Whitehall area with the Greyson Shale of the Ravalli Group; Y-Y' shows the loca­
tion of fine-grained intervals and the interfingering with both coarse LaHood rocks and 
finer-grained Belt rocks; Z-Z' shows the stratigraphic position of the limestone-rich 
interval in the Horseshoe Hills and the Bridger Range. €  = Cambrian; GS = Greyson Shale; 
LH = LaHood Formation; HT = Neihart Quartzite; P€ XLN = Precambrian crystalline rocks;
NLS EQUIV = Newland Limestone equivalent; LS-RICH LH = Limestone-rich LaHood.
H
O
11
McMannis (1963, p. 417) referred to the limestone- 
rich interval in the Horseshoe Hills and Bridger Range 
as the "'molar tooth' and algal zone' , and stated,
"This appears to be the only reasonable key horizon 
within these sections, and serves to correlate across the 
7-mile-wide covered area between these Belt exposures." 
Stratigraphic intervals similar to the limestone-rich 
interval in the Horseshoe Hills and Bridger Range have 
been recognized in various stratigraphic positions in 
the LaHood Formation .of other areas (Coppinger> W., 1979, 
oral and written commun.; Thorson, J., 1979, oral commun.). 
This suggests the Horseshoe Hills and Bridger Range interval 
reflects sedimentary processes and environmental conditions 
which must have existed throughout deposition of the forma­
tion rather than a brief time of carbonate deposition which . 
is implied by the phrase "key horizon".
Molar tooth refers to structures which resemble the 
molar teeth, of elephants. These structures are discontin­
uous both laterally and vertically and have been interpreted 
as primary sedimentary structures, inorganic diagenetic 
structures, and soft sediment deformation structures in 
cryptalgal laminites. The term has been misapplied since 
Bauerman (1885) first used it to describe structures 
identical to those in the Horseshoe Hills and Bridger 
Range limestone-rich interval (Trunk, J., 1977, unpub. data). 
These structures in the limestone-rich interval are most 
reasonably interpreted as soft sediment deformation struc­
tures in cryptalgal laminites as proposed by. Trunk and 
Smith (1979).
fStratigraphy of the LaHood Formation 
Tlhe LaHood Formation fines both upsection and from 
south to north. The southernmost exposures of the forma­
tion grade northward from conglomeratic units into inter- 
bedded shale and sandstone. The fining upsection may be 
depositional; however, it is possible that coarse deposits• 
at the top of the formation were eroded or faulted out.
Lower portions of the formation crop out in Jeffer­
son Canyon and near LaHood Park (Fig. I). The conglom­
erate in the 1400 m thick Jefferson Canyon section appears 
to be chaotic, but distinct zones of crudely imbricated 
and aligned pebbles and cobbles, some fine-grained inter­
beds, and a channel occur. Contacts may be abrupt or grada-
tional between beds containing amphibolite, marble, gneiss,
\ _
and schist boulders and pebbly arkose and shaly beds.
On the east side of Route 10, approximately I km. 
north of the Jefferson Canyon section and Ik km. south of 
LaHood Park, nearly vertical beds of bouldery and pebbly 
arkose crop out. This section contains the upper, part of 
the lower LaHood Formation and is faulted against pre-Belt 
crystalline rocks (McMannis7\1963). Here, the LaHood For­
mation contains more silty and shaly interbeds than the
12
13
Jefferson Canyon section and resembles some of the Horse­
shoe Hills sections which contain similarly interbedded 
lithologies.
Farther to the northwest of LaHood Park, 2.5 km north 
of the intersection between Route 10 and Route 218, graded 
arkose of the upper LaHood Formation is rhythmically inter­
bedded with shale. Most basal contacts are undulatbry, but 
no sole marks occur on the well-exposed bedding surfaces 
of the pebbly arkose. The LaHood Formation here resembles 
rhythmically, interbedded arkose and shale in the Horseshoe 
Hills and Bridger Range.
In Corbly Canyon, in the Bridger Range, the lower 
LaHood Formation is faulted against pre-Belt crystalline 
rocks and contains 2500 m of pebbly, graded arkose beds.
Few fine-grained or conglomeratic units are associated with 
this part of the section,, but according to McMannis (1963, 
p. 415), "...northeastward and upward in this section
grains becomes less coarse but sporadic boulders and blocks
,
occur within 2,000' of the top...some siltite and fine­
grained arkose are interbedded in the upper 1,000'.".
Fine-grained' intervals of the formation interfinger 
with the arkose and arkosic conglomerate in both the upper 
and lower LaHood Formation. An interval of fine-grained
14
LaHood rocks between St. Paul Gulch and the Boulder River 
grades to the northwest and southeast into lower LaHood 
rocks but may also grade into Greyson Shale to the north 
(Fig. 3). A section near St. Paul Gulch lies above this 
interval and contains a 100 m thick limestone-rich inter­
val (Coppinger, W ., 1979, oral and written commun.). The 
St. Paul Gulch section and the limestone-rich interval .in 
the Horseshoe Hills and Bridger Range interfinger with 
coarse LaHood rocks to the south and Newland Limestone to 
the north. The interval in the Horseshoe Hills and Bridger 
Range occurs in the upper LaHood Formation and the interval 
in the St. Paul Gulch section occurs in the upper-lower 
LaHood Formation but both are correlated with the Newland 
Limestone. Alexander (1955) correlated all LaHood rocks 
in the Whitehall area with Greyson Shale (Fig. 3), but 
Coppinger (W., 1979, oral commun.) stated that in this 
area the LaHood Formation intertongues with the Chamberlain 
Shale and Greyson Shale and the carbonates lie in the cor­
rect stratigraphic position to be correlated with the New- 
Iand Limestone.
Coarse upper LaHood rocks probably interfinger with 
the limestone-rich interval in the Horseshoe Hills and 
Felix Canyon, although at the scale of McMannis' cross
15
section, the interval appears to be a time "horizon"
(Fig. 3). The limestone-rich interval in both areas over- 
lies and underlies relatively coarse upper LaHood rocks 
(McMannis, 1963; Boyce, 1975) and sections measured up 
to the Cambrian Flathead Formation by the author suggest 
this as well (Appendices I and K). The upper 400 m of the 
Felix Canyon section and the upper 300 m of Horseshoe Hills 
section I contain mostly graded arkose beds, no limestone 
beds, and few fine-grained interbeds.
Stratigraphy of the Limestone-Rich Interval 
of. the LaHood Formation
General: Verrall (1955) and McMannis (1963) recog­
nized a limestone-rich interval in the Horseshoe Hills, 
and McMannis (1963) recognized the same interval in the 
Bridget Range. This limestone-rich interval is a 400 m
thick lithostratigraphic unit characterized by two types
i
of depositional sequences : arkose sequences, and
limestone-bearing sequences (Figs. 4a and b). Although 
McMannis (1963, p. 417) referred to this interval
Depositional sequence in the limestone-rich LaHood 
interval refers to a group of beds 10-30 m thick which 
reflects similar depositional conditions. Several rock 
types may be associated with a group and may be common to 
both types of sequences; however, the stratigraphic rela­
tionships and proportions of rock types differ between the 
two types of sequences.
16
FIGURE 4a. Arkose depositions! sequence (channel deposits). Note 
thick, amalgamated arkose beds.
FIGURE 4b. Limestone-bearing depositions! sequence (interchannel 
deposits). Rhythmically interbedded siltstone, shale, limestone, 
and arkose.
FIGURE 4c. Graded bedding and siltstone, limestone, and shale clasts 
in arkose bed. Note scour at base of bed.
FIGURE 4d. Horizontal laminae. Truncated, base cut-out sequence(?) 
in arkose bed.
17
informally as both the "'molar tooth' and algal zone" and 
"key horizon", the phrase "limestone-rich interval" 
describes this interval more precisely. "Limestone-rich 
interval" refers to an enrichment in limestone relative 
to other portions of the formation and implies interbedded 
lithologies.
The two types of depositional sequences alternate four 
times within 200 m of Section and can be correlated across 
most of the study area (Fig. 5). The arkose depositional 
sequence contains mostly massive and graded arkose beds 
and the limestone-bearing depositional sequence contains 
interbedded limestone, shale, siltstone, and arkose (Fig. 6). 
Similar repetitions of the sequences are found in Horse­
shoe Hills section I (Fig. 5) and the Felix Canyon section; 
however, these could not be correlated with the more closely 
spaced Horseshoe Hills sections shown in Figure 5.
Stratigraphic relationships between the uncorrelated 
sections can only be guessed. The Felix Canyon section con­
tains a limestone-rich interval similar and probably equiva­
lent to the Horseshoe Hills interval (McMannis, 1963; Boyce, 
1975), but stratigraphic positions of depositional sequences 
in that interval cannot be determined from the available 
data. The sequences in the Horseshoe Hills and Bridger
18
FIGURE 5. Location and correla­
tion of appended Horseshoe Hills 
measured sections, limestone- 
rich interval of the LaHood 
Formation.
R 3 E
19
50 m
intorbedded nvptiirian con­
cretion::, SiIintone, shale, and 
arkone.
Ofen-CNist (?) Intrufom a t ional 
slump structures containing 
molar tooth structures; deformed 
concretions; soft-sediment bou- 
dina^e; urkose lenses, tongues, 
wedges, blocks; truncated 
deco-1 IemenL folds; re-folded 
folds; imbricate* thrusts.
HhythmIc and random Inter- 
beddinj; of 11 mi-stone, ni I L- 
stone, shale , emit arkoae; 
e-ryptalgal lainlnitea; water- 
escape structures.
Amalgamnteil -irkime beds; lubhly 
•/.uni-::; Inverse and normal 
''railed bi-ddiin.; cross bedding; 
r I !'-Hf and final ini', r lasts of 
limestone, si H stone, and shale; 
water escape structures; clastic 
dikes; basal pebble and cobble 
conglomerates; channel cut- 
and-fill.
Om
FIGURE 6. Generalized stratigraphic sections of depositional sequences 
in the limestone-rich interval of the LaHood Formation.
20
Range reflect the same mechanisms of transport and deposi­
tion even though the thicknesses of the depositional 
sequences vary greatly between uncorrelated and correlated 
sections. The thickness changes may reflect a change in 
the rate or site of deposition.
Most contacts between the two types of depositional 
sequences are tabular and abrupt; however, an undeformed 
arkose sequence may drape over and fill in topography formed 
by a slump in a limestone-bearing depositional sequence.
The amount of draping varies with the intensity of slump 
deformation in the limestone-bearing depositional sequence. 
Some slumps involve 20 m of section and extend laterally 
60 m or more (one of these slumps may be seen just west 
of section I). According to Helwig (1970), a slump struc­
ture in which beds are relatively undeformed may represent 
only a small volume of the total slump and may be a large 
allocthonous block.
Arkose Depositional Sequences: Most arkose deposi­
tional sequences contain 10 m to 25 m of thick arkose beds 
and may be correlated across the study area (Fig 5). Corti- 
relations were made in the Horseshoe. Hills sections by walk­
ing along beds, and using lithologic features and strati­
graphic positions. Most tabular arkose beds are continuous
\
21
FIGURE 7a. Conglomerate bed contains poorly sorted and inversely 
graded cross beds and grades upsection into massive arkose bed.
FIGURE Tb. Cross bedding with stoss side preservation and tangential 
foreset laminae.
FIGURE Tc. Dish-like structures near the upper portion of an arkose 
bed which grades into shale.
FIGURE Td. Arkose pillar structure penetrates shale beds.
22
over 30 m or 40 m, but some may extend laterally 3 0 0 m  or 
more. Channel deposits pinch out within 50 m. Most beds 
in the arkose depositional sequences contain normal grading, 
rip-up and floating (matrix supported) clasts, and pebbly ' 
zones, and some beds contain basal conglomerates and a few . 
sedimentary structures (Figs. 4c and d, and 7).
Individual beds in arkose depositional sequences may 
be traced laterally or correlated between sections as parts 
of sedimentation units. A sedimentation unit consists of 
a massive or graded arkose bed and some fine-grained inter­
beds and may be correlated in spite of thickness changes - 
between sections. For example, laminated siItstone and 
shale separate pebbly arkose beds in sections A, B, C, b>
E 7 and E 1 and contacts are erosional or contain injection, 
structures (Appendices A, B, C, D , E , E'). Basal pebbly 
zones, which are similar to zones scattered throughout the 
beds, are found in the arkose beds in the first two sedimen­
tation units. In section C , a bed which contains large- 
scale planar cross stratification is also associated with 
swirled bedding and correlated with a zone containing clasts 
and pebbles in sections A, B , D , E , and E '. The arkose bed 
in the fourth sedimentation unit contains a basal
Iconglomerate with rip-up and floating clasts of lime­
stone, siltstone, and shale.
Limestone-Bearing Depositional Sequences: . Most 
limestone-bearing depositional sequences of the LaHood 
limestone-rich interval range between 10 m and 30 m thick. 
The depositional sequences may be correlated across most 
of the Horseshoe Hills although many beds within these 
sequences pinch out or cannot be traced in deformed sec­
tions. Most sequences contain: stromatolites (associated
with cryptalgal laminites); concretions; large- and small- 
scale deformation structures; and both randomly and rhyth­
mically interbedded limestone, siltstone, shale, and minor 
arkose (Fig. 8).
Large- and small-scale deformation structures in 
limestone-bearing depositional sequences display lateral 
components of movement, but orientations of the structures 
are very inconsistent between measured sections and some­
times within a section. Slump structures contain small- 
scale deformation structures, plastically deformed con­
cretions, and soft sediment micrite boudins. Deformation 
in the micrite beds and concretions increases with the 
intensity of deformation in the slump structures; deforma­
tion decreases toward the bases of some depositional
23
24
FIGURE 8a. Stromatolites and horizontally interbedded calcite and 
dolomite.
FIGURE 8b. Plastically deformed septarian concretions (scale: h feet).
FIGURE 8c. Molar tooth structures (deformed cryptalgal laminites?). 
Note interbedded arkose tongues.
FIGURE 8d. Truncated decollement folds. Undeformed arkose beds over- 
lie limestone-bearing depositional sequence.
25
sequences. Small-scale deformation structures which occur 
throughout a deformed sequence are: molar tooth deforma­
tion structures (Fig. 8c), contorted bedding, decollement 
folds (Fig. 8d), recumbent folds, imbricate thrusts 
(Fig. 9a), blocks and wedges (Fig. 9b), and normal faults. 
The small-scale deformation is discontinuous and may be 
truncated by undeformed beds.
Undeformed beds in this type of depositional sequence 
are both rhythmically and randomly interbedded. Tabular 
micrite beds 2 cm to 5 cm thick may alternate with 20 cm 
to 30 cm thick beds of laminated shale and minor siltstone. 
Some I m thick arkose beds alternate with equally thick 
siltstone and shale intervals.
Beds which contain concretions are more typical of the 
limestone in the limestone-rich interval of the LaHood 
Formation than beds associated with molar tooth structures. 
Most dense micrite concretions are interbedded, sometimes 
rhythmically, with siltstone and shale in 20 m thick sec­
tions although a few concretions are isolated. Concretions 
range between 15 cm and I m long; however, in some expo­
sures, it is difficult to distinguish between long concre­
tions and continuous beds. Few arkose beds and no molar 
tooth structures are associated with concretions except
iT
FIGURE 9a. Imbricate thrust in plastic micrite bed.
FIGURE 9b. Disaggregated deformation characterized by blocks and 
wedges of arkose interbedded with limestone and shale.
FIGURE 9c. Surface expression of cone-in-cone structures in micrite 
concretion. Note radiating digitate structures.
FIGURE 9d. Water escape structures(?) in dense micrite beds.
27
in section H where limestone and arkose are interbedded 
(Appendix H).
Laminated carbonate beds are interbedded with stroma­
tolites and sandwiched between arkose beds. The carbonate ' 
beds which contain stromatolites may be traced almost 
continuously for 100 m, but molar tooth structures are very 
discontinuous. Many lenses of detrital grains occur within 
the laminae, and some lenses are interbedded. Micaceous 
arkose lenses between I mm and 10 cm thick intertongue with 
laminae in beds which contain molar tooth structures.
Lithology
Introduction
The limestone-rich interval of the LaHood Formation 
contains arkose, limestone, siltstone, and shale. The 
arkose and limestone beds exhibit the most distinctive 
features, so lithologic description focuses on bed thick­
ness and contacts, sedimentary structures, and diagenetic 
features of these beds.
. Description of Arkose
Thickness and Contacts; The thickness of arkose beds, 
in arkose depositional sequences ranges between I m and 5 m, 
and beds in limestone-bearing depositional sequences range
28
between 0.5 m and 1.5 m thick. Discontinuous tongues, 
wedges, and lenses of arkose are found in limestone and 
shale interbeds and range between 2 mm and 30 cm thick.
Basal contacts in arkose beds of both types of deposi- 
tional sequences are abrupt and undulatory or flat. ■ Most 
basal contacts between arkose and varve-like siltstone or 
shale contain cut-and-ifill and injection structures. Most 
cut-and-fill structures are found in the arkose depositional 
sequences. Some arkose beds display gradational upper con­
tacts with siltstone and fine sandstone, but many upper 
contacts are abrupt and flat. Contacts between dis­
continuous arkose tongues, wedges,. and lenses, and lime­
stone, siltstone, or shale beds are irregular and abrupt.
General Sedimentary Structures and Diagenetic Features; 
Various sedimentary structures such as normal and inverse 
distribution graded bedding, pebbly zones, and clasts of 
siltstone, shale, and limestone characterize the arkose 
beds in the limestone-rich interval of the LaHood Formation. 
Conglomerates, cross stratification, load structures, and 
water escape structures are also found in these beds. No 
sole marks or complete Bouma sequences were observed.
Arkose beds in limestone-bearing depositional sequences are 
both massive and normally graded, and are distinguishable
29
from similar beds in‘the arkose depositions^ sequences in 
that they contain fewer clasts and pebbly zones.
Bedding Features: The bases of many beds grade nor­
mally from a pebble conglomerate into a coarse- or medium­
grained sandstone (but some beds are inversely.graded) and 
recurrent graded bedding occurs in some of the thicker 
arkose beds. A few beds which contain swirled, deformed- 
looking patterns can be traced laterally into graded bed­
ding, but otherwise these, beds contain no laminae, traction 
structures, or clasts. The arkose bed in sedimentation 
unit 3 of section A contains swirled patterns and is cor­
related with a thicker arkose bed which contains planar 
cross beds at the base (Appendix A ) .
Wedge-shaped, irregular, and flat pebbly zones are 
distributed throughout the beds and contain 35 per cent 
quartz, 30 per cent plagioclase, 10 per cent microcline, 
and minor biotite, muscovite, hornblende, and rock frag­
ments (McMannis, 1963; Boyce, 1975). Most basal pebbly 
zones contain rip-up clasts of siltstone, shale, and lime­
stone, but these clasts are also scattered throughout the 
beds.
Traction Features: The arkose beds contain floating
and bedded clasts up to 30 cm long which were ripped up from
30
limestone-bearing depositional sequences but do not contain 
arkose or comparable sized pre-Belt crystalline rock clasts. 
Shale and siltstone rip-up clasts range between 10 mm and 
30 cm long and are both angular and slightly rounded. The 
long axes of the floating micrite clasts measure between 
I cm and 30 cm and may be randomly oriented or aligned with 
siltstone and shale clasts both parallel to, and at high 
angles to bedding contacts.
A 2 m thick cobble conglomerate with large-scale 
cross bedding lies at the base of an arkose sequence west 
of section :J and overlies a deformed limestone-bearing 
sequence (Fig. 7a). Small limestone, siltstone, and shale 
clasts are imbricate and large clasts are randomly oriented 
in the poorly sorted cross bed's. Cobbly cross beds 30 cm 
thick alternate with 10 cm thick beds of imbricate rip-up 
clasts.
A few arkose beds in arkose depositional sequences 
contain large- or small-scale cross bedding. Large scale 
cross beds lie at the bases of section D and the previously 
described cobble conglomerate. The cross beds in section D 
appear to be planar and are found at the base of an arkose 
depositional"sequence which thins to ,the east and west
31
(Appendix D). This suggests that the exposure is nearly 
perpendicular to the planar cross beds.
Small-scale cross bedding with tangential foreset 
laminae and stoss side preservation is found near the tops 
of a few normally graded arkose beds (Fig. 7b). The only 
other small-scale cross strata found in the arkose beds are 
a few isolated lenticular beds. Angular feldspar and quartz 
pebbles form the cross beds in the lenticles.
Diagenetic Features; Diagenetic features of the arkose 
beds include dish-like structures (Fig. 7c), pillar struc­
tures (Fig. 7d), and load and injection structures. Pil­
lar structures were observed in two arkose beds in limestone­
bearing depositional sequences. In both cases, the discor­
dant structures were less than 10 cm long and plastically 
deformed the shale beds. . Dish-like structures are found in 
fine sandstone near the tops of graded arkose beds and 
resemble antidune structures. The arkose beds associated 
with these structures grade upward into siltstone or shale 
beds. Load and injection structures are associated with 
the bases of pebbly arkose beds in contact with siltstone 
or fine'sandstone; however, some undulatory contacts which 
do not contain pebbles and are not erosion surfaces may con­
tain load structures.
'5
32
Interpretation of Arkose
General: The distinction between current and suspen­
sion deposition is more clear-cut than the distinction 
between various types of mass flows. In the limestone- 
rich interval of the LaHood Formation, the distribution 
graded bedding and matrix-supported clasts point to suspen­
sion deposition rather than waning current activity. ' The 
paucity of shallow water sedimentary structures such as
ripple marks and cross beds, and the I m to 5 m thick
: '
graded arkose beds indicate deep water sedimentation. Shal 
low water graded bedding rarely exceeds a few centimeters 
thick (Reinecke and Singh, 1975).
Turbidity currents and grain flows have some features 
in common, but represent different amounts of water and 
turbulence. Kuenen and Miglioririi (1950) interpreted beds 
which lack traction features and are similar in thickness 
and lateral extent to beds in the LaHood limestone-rich, 
interval as high density turbidity current deposits, and 
Stauffer (1967) used the same criteria to recognize grain 
flow deposits. Rocheleau and LaJoie (1974) stated that
grain flow deposits should lack traction features, but
/
Lowe (1976) proposed that sedimentary structures may form 
during late states of grain flow .movement or during
Oresedimentation and pore water escape. The recognition of 
transitional flows (flows intermediate between grain flows 
and turbidity currents) by Middleton (1970) explains some 
features common to these two types of mass flow.
The abundance and distribution of amalgamated^ 
arkose beds, graded and nongraded bedding, floating and 
bedded clasts, traction features, swirled bedding, con­
glomerates, and water escape structures point to turbidity 
current deposition for most of the beds and grain flow and 
transitional flow deposition for some, or parts of some 
beds. Pebbles of angular feldspar and quartzite and minor 
amounts of mafic minerals and micas which are found in the 
beds suggest there was sufficient transport to mechanically 
break metamorphic- rocks into their component minerals, but 
that there was little re-working or chemical weathering.
Amalgamation: . The arkose depositional sequences are 
characterized by amalgamated sandstone beds which, accord­
ing to Walker (1966), are associated with channelized tur- 
bite deposits and form when upper divisions of Bouma
i
An amalgamated bed forms when two or more beds are 
deposited in rapid succession. The contact between the 
beds may be difficult to discern except when traced 
laterally into two or more beds with fine-grained inter­
beds.
33
34
sequences are eroded by subsequent turbidity currents.
The amalgamated beds may be traced laterally in the LaHood 
limestone-rich interval into two or more turbidite sequences 
and the junctions may be recognized by load structures, 
grain size changes, and aligned shale and siltstone 
"clasts" which are in situ remnants of the upper Bouma 
divisions.
Graded and Nongraded Bedding: Graded bedding is the
single most common feature in beds of both types of deposi- 
tional sequences. Normally graded beds may form in both 
turbidity currents and transitional flows. In turbidity 
currents, settling from suspension forms division A beds 
of the Bouma sequence. In transitional flows, where the 
base undergoes pseudo-laminar flow and the top acts as a 
dilute turbidity current, resedimentation of the bottom 
may form a division A bed and traction features may form 
in the upper portion of the flow (Middleton, 1970) .
Although no complete Bouma sequences were observed, both 
truncated sequences and truncated, base cut-out sequences 
are common and may reflect erosion by subsequent turbidity 
currents, or interruption of sedimentation by the second 
of two turbidity currents diving under the.tail of the 
first (Kuenen and Menard, 1952; Bouma, 1962). Most Bouma
35
sequences in the limestone-rich interval are truncated after 
the division A bed, and according to Walker (1966) and Mutti 
(1977), this type of truncation is typical of the proximal 
ends of turbidity current deposits and submarine channel 
deposits. The.truncated, base cut-out sequences observed 
contain only the lower division of parallel laminae 
(Fig. 4d).
A few beds which contain division A grading at the 
base and small-scale cross bedding near the top, are mis­
sing the middle divisions of the Bouma sequence and there 
is no evidence of erosion or amalgamation. These beds may 
represent transitional flow deposits in which the base was 
resedimented to form a division A bed, and the top was 
re-worked by the dilute turbidity current portion of the . 
flow.
Inverse graded bedding is found mostly in basal peb­
ble conglomerates associated with arkose depositional 
sequences. Shear stress at the base of a suspension may 
sort larger clasts toward the top and, according to Davies 
and Walker (1974), turbulence and ,dispersive pressure sup­
port the clasts. Hendry (1973) suggested inverse grading . 
forms by progressive liquifaction of unconsolidated sedi­
ment masses on a slope, and Walker '(1975) stated inverse to
36
normal grading in conglomerates represents deposition on 
steep slopes and normal grading represents deposition on 
more gentle slopes.
Most nongraded beds with flat upper and lower 
contacts also lack traction features and floating clasts 
and occur in limestone-bearing depositional sequences.
These features together reflect grain flow deposition and 
may represent deposition in a steep-sided basin (Stauffer, 
1967). The association of nongraded arkose beds with 
deformed limestone-bearing depositional sequences is con­
sistent with deposition on steep slopes.
Traction Features: Graded, amalgamated arkose beds
in the LaHood limestone-rich interval contain traction 
features such as large- and small-scale.cross bedding, but 
no sole marks. Sole marks characterize many turbidite 
deposits; however, a lack of these may indicate very dilute 
or very rapid turbidity currents (Dzulynski and Walton, 
1965). Bedding planes in the limestone-rich interval are 
poorly exposed and load structures may obscure sole marks. 
In addition, the beds lack cross bedding, contain large 
floating clasts, and some have features of grain flow 
deposits. . This evidence suggests the flows were dense
37
and therefore, the apparent lack of sole marks probably 
reflects rapid movement.
Grain flow deposits should have fewer traction features 
than turbidity current deposits although traction features 
occur near the tops of beds in any type of mass flow. 
Large-scale cross bedding occurs at the bases of beds in 
arkose depositional sequences and small-scale cross strati­
fication is usually found at the tops of beds. A few iso­
lated lenticular beds which contain small-scale cross beds 
occur in the middle portions of a few beds, and one lenti-. 
cle occurs near the top of a bed in section G (Appendix.
G) . . '
One bed near the center of a channel contains swirled 
bedding (characteristic of grain flow deposits and high 
density turbidity current deposits) and large-scale planar 
cross bedding. If tractioh at the base formed the cross 
beds, it is likely this part of the bed represents a tur- 
bity current deposit. The cross beds may represent a transi­
tion from the flow-regime which produces graded bedding to 
one in which lower parallel laminae form, and the bed which 
contains.them may represent a truncated, base cut-out 
sequence. Williams (1966) proposed that planar cross beds 
form in migrating braided streams during lateral accretion.
38
Processes of braided distributaries on submarine fans are 
similar to those of terrestrial braided streams, so it is 
likely the planar cross beds indicate migration of a dis­
tributary channel.
According to Stauffer (1967), turbulence of high den­
sity turbidity currents may be damped by excess sand and 
form non-turbulent mass flows and Middleton (1970) sug­
gested grain flows may become transitional flows and 
turbidity currents as a mass accelerates downslope and 
mixes with water. It is possible that the toe of a mass 
flow near the center of the channel mixed with water and 
became turbulent enough to form the cross beds. The bed 
grades laterally into a Bouma division A bed which would 
be consistent with this interpretation. In addition, if 
a turbidity current were damped by excess sand, one would, 
expect the formation of a Bouma division A bed, not large- 
scale cross beds, when the mass resedimented.
.The cobble conglomerate differs from descriptions of 
resedimented conglomerates (Hendry, 1973; Davies and 
Walker, 1974; Rocheleau and LaJoie, 1974; Walker, 1975; 
Walker, 1977; Cossey and Ehrlich, 1979) due to the presence 
of large-scale, high-angle cross bedding. The conglomerate 
beds which contain small clasts and inverse grading
39
alternate with cobbly beds. Although the cross bedding 
indicates traction deposition, the overlying graded and 
nongraded arkose beds must have been deposited by a dense 
flow with less turbulence. It is possible a dilute tur­
bidity current became damped by excess sand or a change in 
slope and caused the large clasts to drop out rapidly.
The lack of traction feature's in the upper part of the bed 
may indicate that the flow became quite dense or rapid.
Small-scale cross beds with stoss side preservation 
and tangential foreset laminae are found in fine sand­
stone which grades upward into siltstone and shale 
(Fig. 7b). Stoss preservation occurs when sedimentation 
rates exceed erosion rates, and tangential foreset laminae 
reflect high current velocities (Blatt and others, 1972); 
both are consistent with deposition by turbidity currents 
and transitional flows. Other small-scale cross beds in 
isolated lenticular bedding contain angular feldspar and . 
quartz pebbles. The grain size distribution in the denti­
cles contrasts with the typical sand-mud association in 
lenticular bedding but also reflects alternating slack and 
turbulent flow. These isolated lenticular beds indicate 
at least some localized turbulence, if not turbulence 
throughout the bed, and would preclude grain flow
40
deposition. A few isolated denticles within beds suggest 
turbulent flow and a limited supply of pebbles during depo­
sition. According to Mutti (1977), turbidity currents which 
by-pass the mouths of•active channels often change from 
confined to unconfined flow and pass through a slope change 
as well. The excess sand deposited may be re-worked by the 
tail of the same current or subsequent currents to form 
lenticular bedding.
Diagenetic Features: Turbidity currents, transitional
flows, and grain flows move rapidly and deposit water-laden 
sediments. Water escape structures often form during con­
solidation of these rapidly deposited "quick" sediments 
(Lowe and LoPiccalo, 1974). In the limestone-rich interval 
of the LaHood Formation, water escape structures include . 
pillar and probably dish structures, and clastic dikes.
The liquifaction and fluidization of sediment masses which 
produce these structures may be generated by gas pressure, 
hydrostatic pressure, and most frequently, by earthquakes 
(Reihecke and Singh, 1975).
Dish-like structures, which resemble antidune, struc­
tures or disrupted climbing ripples, occur at the tops of 
graded arkose beds'. These structures are often associ­
ated with each other in beds of turbidity current origin
41
(Rautman and Dottz 1977) and may form during late stages 
of grain flows or during fluidized sediment escape through 
or around semi-permeable laminae or sediment masses. 
Although Wentworth (1967) proposed a primary origin for 
the dish structures, most workers attribute post- 
depositional pore water escape to the formation of clastic 
dikes, and pillar and dish structures. Dish structures 
cannot be used to distinguish between high and low density 
turbidity current deposits or grain flow and turbidity 
current deposits but characterize rapidly deposited sedi­
ment whether they are primary sedimentary structures, 
water escape structures, or sedimentary structures modi­
fied by pore water escape.
Load structures often form on the soles of beds when 
a sandy layer overlies a water-laden layer. Differential 
loading and density differences in rapidly deposited sedi­
ments result in penetration of the dense layer into the 
less dense one (Dzulynski and Walton, 1965; Reinecke and 
Singh, 1975). Load structures in the limestone-rich inter­
val are associated with amalgamated arkose beds and arkbse 
beds overlying laminated siltstone and shale beds and are 
consistent with rapid deposition by mass flows.
42
Description of Carbonates
Thickness and Contacts: Tabular limestone beds range
in thickness from I cm to 10 cm but most beds are between 
2 cm and 5 cm thick. Some beds are thickened by slump 
deformation and imbrication^and beds which contain load 
casts and concretions vary in thickness from 5, cm to 30 cm. 
Calcite and dolomite laminae range between I mm and I cm 
thick in beds associated with algal stromatolites and molar 
tooth structures. Limestone beds.which contain stromato­
lites range between 2 cm and 10 cm thick.
Contacts between tabular limestone beds and siltstone 
or shale beds, and a few contacts between limestone beds 
and underlying arkose beds are abrupt and flat except 
in slump structures. A small growth fault contact between 
a limestone bed and an overlying shale bed was found in 
section H (Appendix H). Contacts between beds which con­
tain concretions and siltstone and shale beds are undula- 
tory or wavy and some contacts between arkose beds and 
underlying limestone beds are erosional.
43
General Primary Sedimentary Structures and Diagenetic 
Features: Primary sedimentary structures of the limestone
beds include: laminae formed by stromatolites, and con­
centrations of both detrital grains and "organic"^ mate­
rial. Features which are diagenetic include: open folds,
small-scale deformation structures (Figs. 8c and 9a), 
concretions (Figs. 8b and 9c), dolomitization of laminae 
in molar tooth structures and cryptalgal laminites, and 
water escape structures (Fig. 9d).
Laminae:. Most laminae in dense micrite beds are 
visible only in thin section and are formed by concentra­
tions of "organic" carbon which contain angular feldspar 
and quartz grains. Both "organic" carbon and detrital 
material are evenly distributed in laminated and structure­
less beds, but the largest detrital grains occur in the 
laminated beds. Cryptalgal laminites associated with 
stromatolites are identical to laminae in molar tooth 
deformation structures. According to Trunk (J ., 1977, 
unpub. data), dolomite laminae in beds associated with 
molar tooth structures contain two to three times more
4
Chemical analyses by Trunk (J., 1978, oral commun.) 
suggest the carbon is organic; however, the results of the 
analyses have not been verified.
44
"organic" material and six time’s more detritus than the 
calcite laminae. Some of the "organic" material and detri- 
tal grains are imbricate at high angles to the laminae 
and form pod-like chaotic masses.
Stromatolites: Two stromatolite beds occur within
laminated carbonate beds. The stromatolites are both 
mammillary knobs which resemble load structures and branch­
ing columnar structures (Fig. 8a). The undeformed laminae 
and molar tooth deformation structures associated with 
stromatolites contain flaky black micaceous shale and 
varicolored siltstone identical to the shale and silt- 
stone associated with molar tooth structures and some 
horizontally bedded shale and siltstone in other sections 
of the Horseshoe Hills.
Soft-Sediment Deformation: Soft-sediment deformation
in tabular beds includes open folds, normal faults, imbri­
cate thrusts, recumbent folds, and re-folded folds. The 
imbricate thrusts show some draping (Fig. 9a) and folded 
beds are associated with blocks and wedges of arkose, silt­
stone, and shale and overlie undeformed or relatively unde­
formed beds (Fig. 8d). Molar tooth structures and other 
small-scale deformation structures are randomly oriented 
and display configurations which appear to be related to
45
bed or laminae thickness. Thinly bedded or laminated 
carbonates seem to be most contorted and thicker beds are 
broken and" less intensively deformed.
Concretions: Dense, dark gray to black micrite con­
cretions are widespread and abundant and most contain 
septarian structures and rims of radiaxial fibrous calcite 
mosaics. A few small concretions have flat tops and soles 
and contain iron concentrations and calcite veins. Large 
concretions may contain deformed laminae which can be 
traced laterally into the enclosing shale beds. Some soft 
sediment bpudins and load structures also formed in dense 
micrite and superficially resemble septarian concretions. 
"Organic" carbon stringers and detrital grains occur along 
laminae and are evenly distributed in the concretions. 
Isolated concretions display radiating digitate structures 
on the top surfaces and have been referred to as possible 
halite mushrooms (Boyce, 1975). The texture within the 
digitate structures resembles halite hoppers, but is the 
surface expression of cone-in-cone structures in finely 
laminated impure limestone (Fig. 9c). Apical angles in the 
cone-in-cone structures are less than 70 degrees.
Water Escape Structures (?): The tops and bottoms of
many tabular beds contain worm trail-like structures which
46
protrude from the surfaces and are both randomly oriented 
and aligned along preferred fracture systems (Fig. 9d). In 
some beds, these structures are traced to an internal 
silty layer along micro-faults which show reverse drag.
The reverse drag may reflect fluidization and suggests the 
structures are sediment-filled tension cracks.
Interpretation of Carbonate Beds 
General: The carbonate beds reflect both inorganic
and organic origins. The inorganic limestone beds probably 
precipitated during early diagenesis' of suspension deposits 
and the organic carbonate beds represent calcite precipi­
tation and sediment trapping by blue-green algae followed 
by dolomitization of organic-rich layers. Both inorganic 
and organic carbonate beds were plastically deformed and 
may have been resedimented.
Much of the carbonate is recrystallized; however, one 
would expect to find oolites, pisolites, or primary struc­
tures in some of the beds or thin sections if the car-
■
bonates are shallow water deposits. Cathode luminesence 
treatment revealed no relict textures indicative of shallow 
water deposition (Eby, D., 1978, oral commun.). Although 
the carbonate beds contain detrital grains, they reflect
47
relatively low energy conditions during deposition. The 
horizontally interbedded shale, rhythmic alternation of 
shale and siltstone with limestone, and the association 
with deep water sandstone deposits is consistent with low 
energy deposition in deep water.
Stromatolites, molar tooth structures, and relatively 
undeformed cryptalgal laminites reflect biologic origins 
modified by diagenetic processes. The. molar tooth struc­
tures have been interpreted as deformed cryptalgal laminites 
(Trunk and Smith, 1979) and occur in discontinuous blocks 
and sheets associated with slump structures. This suggests 
the beds which contain molar tooth structures were trans­
ported to the depositional site.
Randomly and rhythmically interbedded shale and silt- 
stone are consistent with suspension deposition which sug­
gests that the interbedded, deformed, and load-cast micrite 
also is a product of suspension deposition. Most of the 
concretions are early diagenetic calcite precipitates prob­
ably related to concentrations of "organic" matter which 
settled from suspensions of. pelagic sediments and the tails 
of turbidity currents. It is also possible that periodic, 
in situ calcite precipitation occurred.
48
Laminae; Laminae in dense micrite beds and concre­
tions and cryptalgal laminites are formed by concentrations 
of detrital grains and "organic" matter. The laminae, in 
inorganic limestone beds formed during settling of suspen­
sions. The laminae in molar tooth structures and unde­
formed cryptalgal laminites are related to algal mat accre­
tion.
In deep water, algal mats accrete mostly by direct 
precipitation of calcite by blue-green algae (Hoffman, 1976) 
The absence of oolites in the limestone-rich interval, the 
association of algal laminites with deep water turbidite 
deposits, and the fact that skeletal calcite was lacking 
during the Precambrian also suggest that algal mat accre­
tion in the LaHood Formation was by direct precipitation 
by algae.
Dolomite laminae in cryptalgal laminites represents 
early diagenetic replacement of calcite by dolomite. The 
relationship between dolomitization and organic matter is 
not firmly established; however, Gebelein and Hoffman (1973) 
stated that the decomposition of organic matter frees mag­
nesium ions which replace calcium ions in calcite and form 
dolomite in the relict algal mat layers.
49
In the LaHood Formation concretions, most laminae are 
visible in thin section although laminae in a few concre­
tions may be traced in outcrop as well. Either parallel 
or deformed laminae occur in concretions of the limestone- 
rich interval. According to Raiswell (1971), the presence 
of parallel laminae reflects concretion development during, 
late diagenesis in sediment with less than 40 per cent 
porosity, whereas the presence of deformed laminae indi­
cates early diagenetic concretion development in sediment 
with approximately 70 per cent porosity.
Stromatolites: Stromatolites are intimately associ­
ated with cryptalgal laminites and are known to exist in 
abundance at depths as great as 1000 m (Monty, 1977). In 
deep water where particulate sediment is terrigenous, stro­
matolites accrete by direct precipitation rather than sedi­
ment trapping (Hoffman, 1976). The stromatolites and 
cryptalgal laminites are interbedded with clastic sediment 
in the LaHood Formation which suggests accretion must have 
been by direct precipitation of calcite. The presence of 
clastic grains within laminae does indicate some entrap­
ment of particulate material; however, the calcite and dolo­
mite could not have derived from skeletal carbonates.
50
The stromatolites are associated with laminated carbo­
nate beds (some of which are deformed) and are'interbedded 
with arkose beds typical of deep water turbidite, transi­
tional, and grain flows. The morphology of the stromato­
lites and association with cryptalgal laminites is typical 
of low energy environments (Peryt and Piatkowski, 1977) and 
consistent with relatively deep water deposition. However, 
some workers associate the same morphologies with protected 
intertidal and supratidal environments (Donaldson, 1976; 
Horodyski, 1976). Monty (1977) stated that the use of 
overlying and underlying deposits to infer growth condi­
tions of stromatolites is hazardous. The stromatolites and 
cryptalgal laminites in the LaHood Formation are overlain 
and underlain by arkose beds which indicates either there 
were episodes of clastic influx, or the arkose beds and 
the carbonate beds did not form in the same environment.
The fact that deformed, discontinuous beds of molar tooth ' 
structures are found in sequences which contain turbidite 
deposits and slump structures suggests that some algal . 
beds were transported. The partially coherent and dissagre- 
gated deformation structures which contain cryptalgal lami­
nites are typical of open-cast slump structures on sub­
marine slopes (Corbett, 1973). The fact that algal beds
51
are not associated with any shallow water sedimentary struc 
tures is also consistent with this interpretation.
Concretions: Septarian concretions are widespread and
abundant and represent early diagenetic or syngenetic cal- 
cite precipitation in pore space (Weeks, 1953) and reflect 
the original amount of pore space (Hudson, 1978) . The 
concretions are siliceous, but are composed primarily of 
micrite with rims of radiaxial fibrous calcite mosaics.
The radiaxial fibrous calcite mosaics found in both tabu­
lar micrite beds and concretions probably represent 
'replacement of acicular calcite cement (Kendall and Tucker, 
1973). The replacement of acicular calcite cement occurs 
in deep and shallow water and may be related to concen­
trations of organic matter (Kendall, 1977).
Laminae in beds overlying and underlying a few con­
cretions seem to flow around the concretion and indicate 
early diagenetic development of the concretions (Raiswell, 
1971). The shape of concretions is related to the permea­
bility of the host sediment. Most concretions in the 
limestone-rich interval are ellipsoidal although a few are 
spheroidal or flat. The ellipsoidal concretions reflect 
increased permeability along bedding planes and the great­
est diameters represent "horizons" where the concretions
52
initially formed (Henningsmoen7 1974). These "horizons" 
are fossiliferous in Phanerozoic rocks, but are probably 
related to organic-rich layers in Proterozoic rocks. Bits 
of "organic" material may have settled with pelagic sedi­
ments or at the tails of turbidity currents after being 
ripped-up from areas of accretion. The rhythmic inter­
bedding of concretions with shale and siltstone is con­
sistent with the turbidite interpretation.
. Early diagenetic concretions form within the upper 
10 m of sediment (Bjpfrlykke7 1974) . The early diagenetic 
concretions in the LaHood Formation were plastically 
deformed which indicates that deformation was early dia­
genetic also, and supports the concept of open-cast slump­
ing.
Beds described by Henningsmoen (1974, p. 410) are 
similar to some in the.limestone-rich interval.
There are various transitions, both 
from horizon to horizon as well as within 
one horizon from isolated, roundish nod­
ules to continuous, knobby limestone layers 
and from flat-topped and flat-soled nodules 
to continuous plane-parallel limestone 
layer s... in small exposures it may be hard 
to determine whether part of a large nodule 
or a continuous layer is present.
Henningsmoen (1974) proposed dissolution of once-cpntinuous
'beds as an origin for these nodular beds and Bjjtfrlykke
53
(1973) proposed the same mechanism for some concretions 
he studied. Although most concretions in the LaHood 
Formation are septarian, it is possible that beds which 
lack septarian structures and other attributes of early 
diagenetic concretion development represent early dissolu­
tion of limestone beds.
Isolated concretions which contain cone-in-cone 
structures are believed to be diagenetic and form during 
compaction of calcareous shales following recyrstalIiza- 
tion of calcite (Raiswell, 1971). The compaction of water­
laden sediments is consistent with beds which contain evi^ 
dence of deposition while "quick", such as water escape 
structures and septarian concretions; however, there is 
evidence that these structures may be typical of partially 
compacted sediments as well. Apical angles in cone-in-cone 
structures are a function of the degree of sediment hydro­
plasticity (Franks, 1969). In.this interval of the forma­
tion, apical angles measure less than 70 degrees which is 
typical of partially.compacted shales.
Soft-sediment deformation; The open folds, normal . 
faults, and imbricate thrusts are associated with rela^ 
tively undeformed limestone-bearing depositional sequences 
as well a,s intensively deformed ones. The recumbent folds
54
and re-folded folds are associated with blocks and wedges 
of arkose, siltstone, and shale in more intensively deformed 
sections.
Discontinuous beds which contain molar tooth structures 
occur in large-scale slump structures. The molar tooth 
deformation structures represent gravity and shear deforma­
tion of cryptalgal laminites (Trunk and Smith, 1979).
The density differences between dolomite and calcite laminae 
resulted in primarily vertical deformation, and shearing 
caused lateral displacement of the deformed laminae. It 
is likely algae precipitated calcite, so the density dif­
ferences attributed to initiating deformation may be due 
to replacement of calcite with dolomite during organic 
matter decomposition. It is also possible that the defor­
mation occurred prior to dolomitization and was related to 
density differences between detrital-rich and detrital- 
poor layers.
Evidence for open-cast slumping in the LaHood Forma­
tion is based on the work of Kelling and Williams (1966), 
and Corbett (1973) and includes: truncated deformation
structures which show lateral components of movement, 
plastically deformed septarian concretions, imbrication, 
of plastic beds, and highly disturbed, interbedded arkose,
55
siltstone, and shale. Much of the small-scale deformation 
is associated with decollement surfaces which also supports 
the concept of open-cast slumping. Corbett (1973) identi­
fied beds of turbidity current origin (which are similar 
to beds in the limestone-rich interval of the LaHood For­
mation) as parts of semi-coherent, partially disaggregated, 
and incoherent slump structures.
The orientation of the deformation is rarely consis­
tent and suggests that local slopes and tectonic instabil­
ity controlled deformation. The incorporation of blocks of 
cryptalgal laminites indicates some regional slumping as 
well. Although most deformation is plastic, the imbrica­
tion and faulting indicate some brittle deformation and 
tensional stress. The tensional stress may have been 
related to a pull-apart zone although other tensional fea­
tures are absent.
The association of relatively undeformed beds with . 
coherent slump structures implies gentle regional tilting 
rather than slumping from a channel wall (Corbett, 1973). 
Although the orientation of small-scale deformation is 
inconsistent with larger-scale deformation structures, 
the large-scale structures within a section are usually 
consistent with each other which points to regional
56
tectonic or topographic controls on deformation„ The 
apparent direction of movement varies from section to 
section. The fact that molar tooth structures are not 
interbedded with concretions■may indicate that beds.which 
contain those structures are transported. The deforma­
tion of septarian concretions indicates post-yearly dia- 
genetic deformation and suggests the concretions were 
resedimented within semi-coherent slump sheets. When 
molar tooth structures and concretions are correlated 
laterally in deformed sections, the deformation may reflect 
different episodes from different sources, or may repre­
sent one episode of deformation for the concretions, and 
two episodes for the molar tooth structures.
Water Escape Structures: The worm trail-like struc­
tures represent pore water escape in water-laden micrite 
beds. Many beds which contain these structures have an 
interbedded siltstone layer. Micro-faults which originate 
in the silty layer show "reverse drag"; this may reflect 
escape by fluidized sediment rather than shearing and would 
be consistent with water escape structures. Some of the 
bedding surface patterns follow a preferred fracture system 
which may indicate these structures represent tension 
cracks. Similar structures have been described by
57
Dzulynski and Walton (1965) and may form when liquified 
sandstone layers expand and increase in lateral extent, 
but adjacent shale beds do not increase in lateral extent. 
The effect is a relative decrease in the extent of the 
shale layers which results in the formation of tension 
cracks into which sediment may be forced.
Description of Siltstone and Shale
Thickness, Lateral Extent,_and Contacts: Shale beds
range between 5 cm and 50 cm thick and contain flaky, 
micaceous laminae approximately I mm thick. Some beds are 
poorly indurated and associated with "organic" matter.
Most siltstone beds contain pastel laminae and range 
between 10 cm and 30 cm thick. Some shale and siltstone 
beds are continuous for 50 m and others are discontinuous, 
particularly those associated with deformation structures. 
The most discontinuous beds are remnants found in amalga­
mated sandstone beds.
Contacts between shale beds and limestone and silt­
stone beds are flat or undulatory except when beds are 
intensively deformed. Both shale and siltstone beds may 
be cut by overlying arkose beds..
58
Primary Sedimentary Structures: Small-scale cross
stratification occurs in siltstone beds, especially those 
which underlie graded arkose beds. In section E', cross 
bedded siltstone is interbedded with shale and cut by 
arkose clastic dikes. Both siltstone and shale beds are 
finely laminated and may rhythmically alternate with arkose 
or limestone beds.
Interpretation of Siltstone and Shale 
The rhythmically and randomly interbedded siltstone, 
shale, and limestone beds in limestone-bearing deposi- 
tional sequences are similar to thinly bedded turbidite 
deposits described by Mutti (1977) and represent either, 
deposits at the tails of turbidity currents, or pelagic 
suspensions. Rhythmically interbedded deposits are more 
consistent with periodic influx typical of turbidity cur­
rents. (Keith and Friedman, 1977). Siltstone and shale beds 
associated with amalgamated sandstone beds in arkose 
depositional sequences probably represent suspension 
deposits of turbidity currents also.
59
SEDIMENTARY-TECTONIC ENVIRONMENT OF DEPOSITION 
Introduction
Previous Interpretations
Most workers agree that the LaHood Formation is a 
clastic wedge deposit which was derived from pre-Belt 
crystalline rocks along the southern margin of the Belt 
embayment, but have variously interpreted the formation 
as the littoral equivalent of other Belt rocks (Peale,
1893; Walcott, 1899), a wedge arkose deposit (Alexander, 
1955) ; a moderately deep water debris flow, turbidite, 
and grain flow deposit (McMannis, 1963; Hawley, 1973) , 
and as an alluvial fan or fan-delta deposit (Boyce, 1975). 
Boyce (1975) interpreted the limestone-rich interval of 
the Horseshoe Hills as a braided stream,tidal flat, and 
lacustrine complex.
In contrast to the littoral interpretation (Peale,
1893; Walcott, 1899) ,. Alexander (1955) described the LaHood
Formation as a "wedge arkose" which was deposited in an
5
"intracratonic basin" and stated that the depositional
5Alexander (1955) used the terms "wedge arkose" and 
"intracratonic basin" in the sense of. Dapples and others 
(1948) to describe a linear clastic wedge which develops 
when a positive area adjacent to a basin undergoes strong 
uplift through normal faulting. Once sediment is trapped 
in the basin, little re-working or transport can occur.
60
area must have been subsiding rapidly adjacent to the nor­
mal fault margin. The fact that little re-working occurred 
led Alexander (1955) to conclude that deposition was below 
wave base.
The coarseness, thickness, and mineralogical and 
textural immaturity of the deposit, in addition to the 
presence of disseminated iron sulfide, and the "almost 
complete absence" of shallow water features such as ripple 
marks, cross beds, salt casts, raindrop imprints, and mud 
cracks led McMarinis (1963, p. 433) to believe that tor­
rential streams deposited sediment into a differentially 
subsiding basin adjacent to an east-west trending fault 
zone. He proposed that turbidity currents and slumps 
re-distributed material to stable bottom areas in moder­
ately deep water. Hawley (1973; D., 1977, oral commun.) 
concurred with McMannis' interpretation and also proposed 
that some beds in the LaHood Formation were deposited by 
grain flows.
According to Boyce (1975), the formation represents 
fault-controlled deposition along the southern margin of 
the Belt embayment related to initial stages of continental 
rifting at a triple junction; the embayment formed in the 
failed arm of the triple junction. Boyce (1975, p. 167)
61
also proposed that the LaHood Formation represents an 
alluvial fan or possibly a fan-delta system and stated.
Earlier workers have been impressed by 
graded bedding in the LaHood Formation and 
have missed other important sedimentary struc­
tures . Some areas of the LaHood Formation 
outcrop are possibly related to marine sedi­
mentation in moderate depths of water...the 
base of the St. Paul Gulch section and 1-90 
section are characterized by graded bedding, 
intraformational slump and soft sediment defor­
mation. In these cases, the process of emplace­
ment was turbidity current or fluidized flow in 
a mud-rich environment.
Boyce (1975, p. 190) acknowledged that the "down deposi- 
tional dip" occurrence of "thick mud rock sequences and 
deltaic-like sequences", soft sediment deformation and 
slump structures associated with dish structures, graded 
beds, and algal stromatolites are inconsistent with the 
alluvial fan and fan-delta hypotheses and suggested that 
the alluvial fan or fan-delta deposits represented by the 
LaHood Formation interfinger with other depositional sys­
tems. In Boyce's model, the arkose beds represent braided 
stream deposits, the siltstone and shale beds represent 
levee and overbank deposits, and the limestone beds may 
be interpreted as lacustrine deposits.
62
Author 1s Interpretation
Detailed study by the author shows that the limestone- 
rich interval of the LaHood Formation reflects deep water 
slump, suspension, and mass flow processes typical of sub­
marine fans. Boyce (1975) attributed these processes to 
depositional systems other than alluvial fan and fan-delta 
systems and Coppinger (W., 1979, oral and written commun.) 
believed the St. Paul Gulch section (which Boyce suggested 
may contain turbidite and grain flow deposits) is similar 
to the Horseshoe Hills limestone-rich interval. Thus, the 
LaHood Formation represents two depositional systems if 
the alluvial fan and fan-delta deposits are interpreted 
correctly. Dickenson (W., 1979, oral commun.) visited 
several sections in the Whitehall area and Horseshoe Hills 
which Boyce (1975) interpreted as alluvial fan deposits and 
believed these sections are more reasonably interpreted as 
feeder channels, upper fan, and middle fan deposits of a 
submarine fan system. This suggests the alluvial deposits 
have not been preserved.
Many submarine fans are higher in clay content than 
the LaHood Formation (Boyce, 1975); however, the texture 
of submarine fans is dependent upon the material supplied 
as it by-passes the shelf. Deposits may display little
63
evidence of re-working and be quite coarse if the material 
supplied to the fan is coarse and rapidly burled. The 
fresh feldspars in the LaHood Formation attest to little 
re-working or weathering which could produce clay min­
erals. The possibility that sediment storage on land was 
significantly lower during the Precambrian (Schumm, 1968) 
and sedimentation rates were probably rapid enough to bury 
sediment before much weathering or re-working could occur 
together may.account for unusually coarse fan deposits in 
the LaHood Formation.
The features such as cross bedding, ripple marks, 
stromatolites, possible halite mushrooms, and mud cracks 
which Boyce (1975) believed to reflect shallow Water deposi 
tion are uncommon in the formation and are associated with 
graded, amalgamated arkose beds, and rhythmically and 
randomly interbedded siltstone, shale, and limestone. It 
seems more likely that the cross beds and ripple marks 
reflect traction deposition and the halite mushrooms and . 
mud cracks are more reasonably interpreted as cone-in-cone 
structures and water escape structures or sediment-filled 
tension cracks as discussed in the lithology section. The 
usefulness of stromatolites to infer water depth is ques­
tionable and is also discussed in the lithology section.
64
The stratigraphic relationships and relative abundance of 
these features indicate the beds were deposited in rela- 
-tively deep water.
The alternating depositional sequences in the 
limestone-rich LaHood interval represent proximal turbi- 
diate and grain flow deposits characteristic of channels, 
channel mouths, and interchannel areas of the lower-upper 
fan and middle fan of a submarine fan system similar to. 
deposits described by Nelson and Nilsen (1974), Normark 
(1978), and Walker (1978). Periodic slumping was probably 
related to local and regional faulting in addition to bank 
erosion of submarine channels. Some large-scale slumps on 
the slope may have transported cryptalgal laminite beds 
to the submarine fan environment (Fig. 10).
Submarine fans are associated with active tectonic 
environments such as that proposed for the LaHqod Forma­
tion by Alexander (1955) and Boyce (1975). The evidence 
for faulting during Belt time, fault-controlled deposition 
for the formation as a whole, and the presence of intra- 
formational slump structures in the limestone-rich inter­
val indicate that deposition of the limestone-rich inter­
val was also fault-controlled. East-west transform move­
ment related to initial stages of continental rifting may
65
FEEDER CHANNEL
SLOPE INTO BASIN
UPPER FAN
M ID-FAN
(nt' ,', J1
DEBRIS FLOWS
VKHBLY
SANDSTONESDISORGANIZED BED 
CONGLOMERATES
MASSIVE
SANDSTONESCONGLOMERATES
INVERSE-NORMAL 
GRADED BEDDING
THIN 
BEDDED 
TURBIDITE
THIN BEDDED 
TURUIDITEGRADED-STRATIFIED
TNTERCHANNEL
CHANNEL.
FIGURE 10. Submarine fan model. Cross section X-X1 shows the relation­
ship between channel deposits (arkose depositional sequences) and inter­
channel deposits (limestone-bearing depositional sequences) of the 
limestone-rich interval of the LaHood Formation. Greatly modified 
from Mutti (1977) and Walker (1978).
66
have produced secondary shear faults south of the east-west 
fault zone. Terrestrial streams which supplied clastic 
material may have been associated with the northwest- 
southeast trending shear (?) faults. Both the supply and 
distribution of sediment would have been influenced by the 
intensity and frequency of faulting. Additional work is 
necessary in order to recognize facies changes which might 
indicate the relationship between sedimentary and tectonic 
processes..
The relationships between submarine fan deposits of 
the LaHood Formation and finer Belt rocks which lie to 
the north are not clear. It is likely some of these are 
lower-middle fan and lower fan deposits (Fig. 10). The 
depositional environment of the Newland Limestone may have 
been similar to that of the LaHood limestones but the 
deposits may not intertongue. If the northern margin of ■ 
the Belt embayment was a gentle warp (Harrison and others, 
1974; McMannis, 1965), it is possible the Newland Lime­
stone accumulated contemporaneously with the. LaHood Forma­
tion but the site of deposition did not receive the same 
amount or intensity of clastic influx or tectonism as the 
southern margin of.the Belt embayment.
67
Sedimentary Environment of Deposition
The thick sequences of arkose which contain con­
glomerates, graded, "clast-bearing beds, and evidence of 
grain flow may represent deposits at the distal ends of 
feeder channels on the lower-upper portion of a submarine 
fan (Fig. 10) .
The alternation of depositional sequences may be 
explained in terms of migration of a distributary chan­
nel so that the channel deposits appear to be super­
imposed on the interchannel sequences rather than later­
ally gradational with them (Fig. 5). The graded arkose 
beds which contain pebbly zones and rip-up and floating 
clasts probably represent deposition in distributary chan­
nels on a lobe of the middle fan. The significant thicken­
ing and thinning of channel sequences in the Horseshoe Hills 
sections is probably due to longitudinal outcrop views of 
sinuous distributary channels. Measured sections which 
could not be correlated probably represent deposits of other 
lobes or distributary systems of the middle fan.
The limestone-bearing depositional sequences are inter­
preted as thinly bedded turbidite and pelagic suspension 
deposits in interchannel areas (Fig. 10). The interchannel
68
deposits contain slump structures which reflect local and 
regional topographic and tectonic controls on slumping.
Channel Deposits
The channel deposits are represented by arkose deposi- 
tional sequences. The abrupt lateral thickness changes, 
amalgamation, and sedimentary features of beds in these 
sequences of the LaHood Formation are characteristic of mid­
f a n  proximal^ turbidite and transitional flow deposits 
described by Walker (1966) and Mutti (1977). Few conglom­
eratic units, usually associated with upper fan channel 
deposits, are found in the limestone-rich interval which 
suggests most of the channels occupied the middle fan,. The 
cobble conglomerate at the base of an arkose depositional 
sequence near section H is not typical of resedimented 
conglomerates, but may represent a braided distributary 
channel .deposit of the■lower-upper fan or the upper-middle . 
fan (Fig. 10).
The presence of grain flow and turbidity current 
features in a single bed is consistent with channels in the
^Proximal turbidite deposits do not necessarily 
represent flows on upper (proximal) portions of sub­
marine fans but they exhibit many features of upper fan 
deposits such as amalgamated sandstone beds, a lack of 
sole marks, incomplete Bouma sequences, and a relatively 
high sand:shale ratio.
69
mid-fan region. According to Keith and Friedman ' (1977), 
distributary channels are deepest near their center 
so it is possible for the central portions of flows to 
accelerate more rapidly than the edges. Since liquifac- 
tion and fluid movement are dependent upon thickness, 
acceleration, water content, and turbulence of a mass 
flow under gravity, it is likely single flows in the chan­
nels acted as both grain flows and turbidity currents.
Upper and middle fan channels confine rapid succes­
sions of mass flows. The consecutive mass flows form 
amalgamated sandstone beds which contain incomplete Bouma 
sequences and characterize these portions of submarine 
fans (Fig. 10).
Channel deposits are confined and, therefore are not 
as susceptible to slumping as interchannel deposits. Chan 
nel migration in the braided distributaries causes bank 
undercutting and may generate.slumps in interchannel 
deposits.
Isolated lenticular bedding has been recognized in 
deposits similar to the limestone-rich interval of the 
LaHood Formation and, according.to Mutti (1977); this type 
of bedding may form when turbidity currents by-pass the 
mouths of channels on the inner fan. The sand which is
70
dropped due to a slope change or as the turbidity current 
passes from confined to unconfined flow may be re-worked 
by the tail of the same current or subsequent currents 
to form lenticular bedding.
Interchannel Deposits
Interchannel deposits on submarine fans are charac­
terized by low sand to shale ratios and reflect suspension 
deposition (Fig. 10). Many interchannel deposits contain 
slump structures and small-scale deformation (Mutti, 1977). 
Arkose beds in interchannel deposits of the LaHood Forma­
tion do not differ greatly from channel arkose deposits.
The interchannel deposits of the limestone-rich interval 
are thinner, contain fewer clasts than the channel arkose 
beds, and are mostly nongraded which indicates deposition 
from suspensions at the tails of turbidity currents and . 
transitional flows which spilled over channels, or grain 
flows formed, by damped turbidity currents. The inter- 
bedded siltstone and shale in the limestone-bearing deppsi- 
tional' sequences represent both pelagic and turbidity cur­
rent suspensions and the cross-bedded siltstone beds 
probably represent re-working by bottom currents or 
the tails of turbidity currents. "Organic" matter in shale 
and inorganic limestone beds was probably transported from
71
algal limestone beds on the shelf edge and slope by slump­
ing and turbidity current flows.
Some sections of the limestone-rich interval contain 
relatively undeformed beds and may be portions of large 
allocthonous sheets; however, additional work is neces­
sary in order to determine the mechanisms and nature of 
deformation. Much deformation in submarine slump struc­
tures is inconsistently oriented and points to local 
tectonic or topographic control on deformation. In the 
limestone-rich interval of the LaHood Formation, slump 
structures which fill channels and are overlain by thick 
channel deposits may represent bank undercutting and local 
faulting. ■ However, slump structures which contain blocks 
of cryptalgal laminites and arkose beds, and decollement 
folds are similar to descriptions of open-cast slumps and 
probably reflect regional tectonic instability.
Plastic deformation of early diagenetic concretions 
and the decrease in deformation downsection in. some slump 
structures support the interpretation of open-cast slump­
ing in interchannel regions of the middle fan. The con­
cretions do not occur with molar tooth structures or other 
beds of probable algal origin which suggests that the 
"organic" particles, which probably served as nuclei
72
for the concretions, settled from pelagic suspensions or 
were transported by slumping and did not form in the 
environment of deposition.
Tectonic Environment of Deposition
Although the depositional sequences may be explained 
in terms of normal submarine channel and interchannel pro­
cesses, some features of the limestone-rich interval are 
best explained in terms of regional and local tectonic 
controls. The presence of blocks or sheets of algal lime­
stones in slump structures which do not contain concre­
tions, and the probability that slumps were open-cast 
together suggest that regional and local faulting control­
led slump processes and some small-scale deformation.
The 3500 m thickness, coarseness, wedge shape, and 
association with an east-west trending fault zone suggest 
fault-controlled deposition of the LaHood Formation. 
Reynolds (M., 1977, oral commun.) ,proposed that an ero- 
sional escarpment may have been the sediment source for 
the LaHood Formation. It seems likely that any long-lived 
feature such as erosional escarpment which contributed 
3500 m of sediment, would be in part structurally control­
led. The east-west trending fault zone along the southern 
margin of the Belt embayment was probably related to the
73
basin origin (Harrison and others, 1974; Boyce, 1975). 
Detailed discussion of the Belt basin origin is beyond 
the scope of this paper, and perhaps premature in that 
detailed, mapping, correlations, and environmental inter­
pretations of the Belt Supergroup have yet to be completed; 
however, possible origins will be briefly discussed in 
order to support the theory of fault-controlled deposi- ■ 
tion for the LaHood Formation.
There are several ideas regarding the origin of the 
Belt basin. Harrison (1977; J., 1977, oral commun.) sug­
gested the basin was an epicontinental trough dominated 
by vertical tectonics. Epicontinental troughs slowly 
accumulate sedimentary sequences several kilometers thick 
by sediment loading and continual subsidence; however, 
mechanisms for subsidence in these basins are not well 
understood. if higher heat flow existed during the Pre-. 
Cambrian, thermal doming may have thinned lithospheric 
plates and domed areas may have undergone long-lived 
subsidence during and following removal of the heat sources 
(Fountain, D., 1978, oral commun.). Long-lived subsidence 
permits sediment thicknesses in excess of several kilometers 
to accumulate, possibly with little or no structural
74
control or igneous activity. The Belt Supergroup reflects 
subsidence and sediment accumulation of this magnitude.
Harrison (J., 1977, oral commun.) considered the 
paucity of igneous rock in the Belt Supergroup as evidence 
against rifting in the Belt basin but this does not pre­
clude rifting or possible transform movement in portions 
of the basin. Harrison (1977) also calculated sedimen­
tation rates for various types oi basins and concluded 
that Precambrian Belt sedimentation was too slow for the 
basin to be a rift arm basin. It may be valid to con­
sider the 600 m.y. span of Belt deposition as relatively 
continuous, but hiatuses are abundant and rock packages in 
the supergroup reflect pulses of sedimentation. The sedi­
mentation rates for the pulses of deposition may well have 
been rapid.
Rapid sedimentation rates may not characterize aborted 
rift arm basins through time, although one would expect 
rapid sedimentation during early stages of basin develop­
ment. Gross lithologic and stratigraphic features of the 
LaHood Formation point to rapid sedimentation for this 
portion of the Belt Supergroup and are consistent with 
most concepts of early stages of continental rifting. Fea­
tures of the two types of depositional sequences in the
75
limestone-rich LaHood interval such as beds of turbidity 
current and grain flow origin, and slump structures indi­
cate rapid sedimentation as well.
Although the Belt Supergroup contains few igneous 
rocks, evidence such as Belt age and older faults, and 
abrupt thickness changes in Missoula Group rocks (Winston, 
D., 1979, oral commun.) suggests that basin configuration 
and deposition were structurally controlled. According to 
Stewart (1976, p. 12), "...extension appears to have taken 
place sporadically across the entire craton producing 
rift features and leading to emplacement of mafic intru­
sive and extrusive rocks." Aborted rift arm basins may 
form during initial stages of continental rifting (Hoffman 
and others, 1974). During rifting, two rifts are favored 
and may continue to spread and create oceanic crust, while 
the third is abandoned as a rift but continues to subside 
and receive sediment. Although aborted rift arm basins may 
resemble epicontinental troughs, most are orifented at high 
angles to the craton margin and are associated with ocean 
basins. Aborted rift arm basins may develop without genera 
ting oceanic crust (Hoffman and others, 1974).
The Belt basin lies along the boundary between 
two rifted continental blocks and the embayment is
76
parallel to a major east-west trending fault 
zone (Fig. 2). Precambrian strike-slip faulting and 
reactivation along those faults throughout geologic time' 
indicate a major crustal weakness was associated with the 
basin. The basin may have been associated with a rift- 
rift-transform system analogous to the Benue Trough,,
Africa or basins along the San Andreas fault zone.
Oceanic transform faults undergo relatively little vol- 
canism compared to spreading ridges except during quiescent 
periods along the ridge related to changes in the direction 
of spreading (Ward, 1971). If the Belt basin formed along 
a continental transform fault, one might expect igneous 
activity to be absent or minimal.
Casella (1979) and Schmidt and Garihan (1979) docu­
mented Precambrian movement on northwest-southeast trend­
ing faults. These faults end abruptly along an east-west 
line parallel to the present Willow Creek fault zone and 
the southern extent of the LaHood Formation (Fig. 2). Most 
LaHood Formation outcrops are faulted against pre-Belt 
metamorphic rocks along this zone and Reid and others (1975) 
described lateral offset in metamorphic rocks during the 
Preoairtbrian which indicates the fault zone could have 
influenced LaHood Formation deposition and was probably
77
reactivated during Laramide deformation. It is likely the 
northwest-southeast trending faults influenced deposition 
of the LaHood Formation (Schmidt and Garihan, 1979) and 
may reflect secondary shear stress generated by east-west 
lateral movement.
78
CONCLUSIONS
(1) . Previous interpretations of the limestone-rich 
interval of the upper LaHood Formation in the Horseshoe 
Hills and Bridger Range have treated it as an unusual zone 
or "key horizon". While this interval is distinctive, it 
seems likely that the processes and environmental condi­
tions represented by the deposits were evident throughout 
deposition of the formation. The presence of other simi­
lar intervals in various stratigraphic positions in the 
formation of other areas supports this concept.
(2) . The depositional sequences represent channel and 
interchannel deposits of a submarine fan system. The inter 
bedded limestone, siltstone, and shale of the interchannel 
deposits reflect both suspension deposition at the tails
of turbidity currents and pelagic suspension deposition.
The arkose beds in the interchannel deposits reflect turbid 
ity current deposition probably from channel spill-over.
The arkose beds in the channel sequences are typical of 
proximal turbidite, transitional, and grain flow deposits 
on the lower-upper fan and middle fan segments of submarine 
fan systems. It is probable that migrating, distributary
79
channels undercut banks and initiated some slumping in the 
interchannel deposits.
(3) Open-cast slump processes are typical of sub­
marine fans, particularly on the upper fan or upper-middle
I
fan. Although additional work is necessary in order to 
characterize the deformation in the limestone-rich inter­
val, much evidence points to open-cast slump processes 
related to both local and regional faulting.
(4) The origin of the Belt basin is enigmatic and 
interpretations of the basin origin may be premature
in that detailed mapping > stratigraphic correlations, and 
environmental interpretations of the Belt Supergroup have 
yet to be completed. The LaHood Formation reflects rapid 
deposition and subsidence, and the association with a major 
east-west trending fault zone and northwest-southeast trend­
ing faults together suggest the formation was deposited 
during long-lived tectonic instability. This is consis­
tent with, but not indicative of early stages of continen­
tal rifting at triple rift junctions or rift-rift-transform 
junctions. Additional study of the LaHood Formation, in 
particular, detailed study of the stratigraphy-and lithology 
of unusual or distinctive deposits within it, may provide 
information useful to. interpretation of the Belt basin origin.
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MONTANA STATE UNIVERSITY LIBRARIES
3 1 7 6 2 0 0 1 2 8 8 7  3
DATE DUE
•* V  J S
. : "4; UAV B r 1Q99
M A V  ? n »
I'WM *  ' ■- ■
HrIrTi— ?
./JiV ( 7 #
IS ©  > 5 • "
j y i i i  2 i m »on 7 I N M----- Arm *"
*AAT% |• 111 S-I %
V g A P P  I 9  M f f i
F E B l l R FEB I 2 2001
' T R  . J g R r r  . ' T  r r o n r '.Q l ',
f i r r r k a L O A Hi l l  I LlXy1
SEP I 42M2
I ’ f
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APR 11 am
e r r t Z 3 s 7 E A o 9 7WW
I . V W
A F ^  0 »
HIGH SMITH REORDER #45-230
N378
B642
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