Theses and Dissertations at Montana State University (MSU)
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Item The effect of permafrost thaw and geologic substrate on dissolved organic carbon mobilization and transformation in northern streams(Montana State University - Bozeman, College of Agriculture, 2019) Wologo, Ethan Andrew; Chairperson, Graduate Committee: Stephanie A. Ewing; Sarah Shakil, Scott Zolkos, Sadie Textor, Stephanie Ewing, Jane Klassen, Robert G.M. Spencer, David C. Podgorski, Suzanne E. Tank, Michelle A. Baker, Jonathan A. O'Donnell, Kimberly P. Wickland, Sydney S.W. Foks, Jay P. Zarnetske, Joseph Lee-Cullin, Futing Liu, Yuanhe Yang, Pirkko Kortelainen, Jaana Kolehmainen, Joshua F. Dean, Jorien E. Vonk, Robert M. Holmes, Gilles Pinay, Michaela M. Powell, Jansen Howe, Rebecca Frei and Benjamin W. Abbott were co-authors of the article, 'No evience of dissolved organic matter priming in permafrost stream networks: a circumpolar assessment' submitted to the journal 'Global biogeochemical cycles' which is contained within this thesis.; Stephanie Ewing, Jonathan A. O'Donnell, Jim Paces, Rob Striegl, Duane Froese and Joshua Koch were co-authors of the article, 'Groundwater connection and doc transport in the Yukon River Basin: uranium and strontium isotopes in permafrost catchments' submitted to the journal 'Global biogeochemical cycles' which is contained within this thesis.Permafrost landscapes exhibit unique hydrology that is linked both chemically and physically to nutrient cycling and geochemical processes. Permafrost thaw is expected to result in a positive feedback to Earth's climate system through carbon release to the atmosphere; this potential demands better understanding of hydrologic pathways in permafrost landscapes in the face of global change. The work that follows is divided into two main bodies of research that explore both carbon dynamics and isotope geochemistry of river waters draining permafrost catchments in the Yukon River Basin (YRB). The first study uses in-vitro incubations of stream water from seven permafrost regions to investigate how biolabile carbon additions (acetate) and inorganic nutrients (nitrogen and phosphorus) 'prime' water-column dissolved organic carbon (DOC) decomposition. No priming effect from biolabile carbon addition was evident through changes in DOC concentrations or compositional transformations, but consumption of added acetate was correlated with ambient nutrient concentrations. Sites with fine-textured, ice-rich substrate and proximal thermokarst features had higher ambient DOC and nutrient concentrations, and consequently the fastest rates of acetate consumption. We conclude that the fate of biolabile DOC released from degrading permafrost will depend largely on inorganic nutrient availability in receiving waterbodies. The second part of this thesis focuses on hydrology of intermediate-sized catchments in the YRB. We evaluate uranium isotope activity ratios ([234U/238U]) as tracers of groundwater-surface water connection in thawing permafrost landscapes. Streams draining loess-mantled areas had [234U/238U] values moderately increased relative to meteoric values. Streams draining low-order catchments with rocky substrate and surface disturbance exhibit dramatically increased [234U/238U] values, consistent with groundwater connection. In addition, we observed higher DOC concentrations both in areas influenced by recent thaw and where flow is restricted by ice-rich silt. The transformation of northern stream chemistry will likely continue as northern permafrost environents warm, with greatest resilience of ground ice in loess-blanketed areas not subject to thermal erosion by groundwater. As subsurface storage expands and groundwater exchange intensifies, ecosystems within and connected to northern streams will also be transformed, with implications for resource managers concerned with fish and wildlife management in these systems.Item Construction and modification of debris-flow alluvial fans as captured in the geomorphic and sedimentary record: examples from the western Sangre de Cristo Mountains, south-central Colorado(Montana State University - Bozeman, College of Letters & Science, 2020) Nicovich, Sylvia Rose; Chairperson, Graduate Committee: Mary S. Hubbard; James G. Schmitt (co-chair); James Schmitt was a co-author of the article, 'Deposition and modification of debris-flow alluvial fans, western rangefront of the Sangre de Cristo Mountains, south-central Colorado' submitted to the journal 'Journal of sedimentary research' which is contained within this dissertation.; Jim Schmitt, Ralph Klinger and Shannon Mahan were co-authors of the article, 'Late Pleistocene record of surface-modifying processes on the Pioneer debris-flow alluvial fan, San Luis Valley, Colorado' submitted to the journal 'Geological Society of America bulletin' which is contained within this dissertation.; James Schmitt was a co-author of the article, 'Impact of wind-blown sediment on the Pioneer debris-flow alluvial fan, south-central Colorado: concepts of fan activity' submitted to the journal 'Geosphere' which is contained within this dissertation.Alluvial fans and their deposits in the stratigraphic record are key in unraveling intricacies of landscape, tectonic, and climatic dynamics, though integrative geomorphologic and sedimentologic studies that comprehensively evaluate processes which build and modify fans are lacking. Therefore, a gap within the current body of literature exists concerning the sedimentological signature of depositional and surface-modifying processes on alluvial fans. This dissertation presents the sedimentological characteristics, both surficial and in the sedimentary record, of processes that build and modify alluvial fans while revisiting the contemporary concept of what defines an active surface. Detailed analysis of a suite of Quaternary active debris-flow alluvial fans on the western range front of the Sangre de Cristo Mountains in south-central Colorado was made using integrative sedimentological and geomorphic analysis, facies and soils mapping, along with infrared stimulated luminescence (IRSL) geochronology to document timing of fan construction and modification processes recorded in the alluvial fan deposits. Analysis of surface geomorphology, facies assemblages, and particle-size distributions of matrix from various facies of exposed alluvial fan deposits were also applied. These data show a clear distinction between sedimentary facies that represent processes of the primary depositional lobe surfaces versus those operating during periods of non-primary deposition, dominant on abandoned lobes. Primary processes on depositional lobes are debris flow and hyperconcentrated flow with minimal secondary modifying processes. Overland flow, input of eolian material, pedogenesis, and rock and mineral weathering are the main secondary modifying processes that govern abandoned lobes. Addition of wind-blown material, a secondary modifying process, plays a significant part in the sedimentary processes that operate on alluvial fan surfaces, ultimately influencing fan smoothing by mobilizing material derived from primary processes.Item Sequence stratigraphic framework of the late Devonian (Frasnian) Duperow Formation in western and central Montana(Montana State University - Bozeman, College of Letters & Science, 2019) Steuer, Christopher Johann; Chairperson, Graduate Committee: David W. BowenThe Late Devonian Duperow Formation in western and central Montana and it's equivalent lower Jefferson Formation, is comprised of shallow marine carbonate strata deposited on the western margin of North America. It has produced significant volumes of oil and natural gas in the Alberta and Williston basins where the sequence stratigraphic framework of the formation is well-documented. However, in western and central Montana, the Duperow remains largely understudied. Additionally, at Kevin Dome, in northwest Montana, the Duperow hosts a large naturally occurring carbon-dioxide (CO^2) accumulation which is a potential economic resource and an analog for CO^2 sequestration over geologic time scales. The goal of this study is to determine the facies relationships and sequence stratigraphic architecture of the Late Devonian Duperow Formation in western and central Montana. This interpretation could help in exploration for oil and natural gas and provide useful information to aid in future carbon sequestration efforts. Multiple data sets are used in this study to best constrain depositional environments on the platform during Duperow deposition. Seven measured sections, three drill cores with associated well-logs, and forty-one thin sections are used to characterize facies, facies associations, parasequences, parasequence sets and sequences of the Duperow Formation and to construct the sequence stratigraphic framework within which these strata occur. Ten lithofacies comprising six lithofacies associations allow the interpretation of six depositional environments responsible for deposition of the Duperow Formation. The Duperow thins from the west and north onto the Central Montana Uplift, a paleohigh at the time, and thickens into the Central Montana Trough, a sub-basin on the platform. Two 2nd order and seven 3rd order sequences are interpreted from measured sections. Sequences are comprised of a transgressive systems tract and a highstand systems tract with no evidence for lowstand strata on the shelf. Transgression across the Central Montana Uplift did not occur until after the basal sequence boundary of the upper 2nd order sequence. Prior to this transgression, sequences lapped out before reaching the Central Montana Uplift. Overall, the Duperow in central and western Montana exhibits retrogradational stacking and thus is part of the transgressive systems tract of a lower-order megasequence.Item Stratigraphic framework and provenance of the lower belt (Newland Formation), Belt Supergroup, Helena Embayment, central Montana(Montana State University - Bozeman, College of Letters & Science, 2017) Anderson, Colter James; Chairperson, Graduate Committee: David R. LagesonThe Belt Supergroup is exposed in western Montana where most of the unit is allochthonous due to Sevier thrusting. However, east of the Helena salient several autochthonous sections of Newland Formation pin it to central Montana. This study focuses on the stratigraphy, provenance, and structural framework of the Newland across this boundary. Measured sections in the Big Belt, Little Belt, and Big Snowy Mountains, along with previously published sections (Zieg 1981) were used to correlate the Newland and interpret its origin. Detrital zircon geochronology (U-Pb by LA-ICP-MS) was used to establish provenance. Clay mineralogy was used to determine burial depth across the embayment. Deposits of the lower Newland locally contain intraformational conglomerates suggesting basinal reorganization at the onset of Newland deposition. Core from a well south of the Volcano Valley fault demonstrate thickening of the lower Newland in the footwall and indicate down-to-the-south faulting (buttress fault). The Volcano Valley fault was reactivated as a thrust fault during Sevier shortening. Above these local units, the lower Newland is a regionally extensive parallel laminated tan calcareous shale containing thin beds of microspar, interpreted as a deep-water depositional system across the entire Helena embayment, indicating the Belt Basin extended further east than most current depictions. Detrital zircon geochronology of the Newland Formation from the Big Belt Mountains suggests diverse sources. The primary source of ~2.45-2.61 Ga, may correspond to the Medicine Hat Block and/or crystalline basement in the western part of the basin. A younger Paleoproterozoic source (~1.79-1.98 Ga) matches the Great Falls Tectonic Zone, and two less prominent Archean sources of ~2.61-2.8 GA and ~3.0-3.2 Ga may reflect contributions from intra-basinal crystalline basement. Elemental analyses complement the detrital zircon geochronology, but do not distinguish distal Archean contributions (e.g., Canadian Shield) from a more proximal Wyoming Province. Xrd analysis of clays reveals the Helena embayment experienced a minimum burial depth of 3,300 m. It has been proposed that the Belt Supergroup is an allochthonous block that translated 1300 km north (Hildebrand, 2013). However, this study suggests the Lower Belt in the Helena embayment has not undergone significant latitudinal translation relative to cratonic North America.Item Sedimentary tectonics of the Mesoproterozoic Lahood Formation, southwest Montana(Montana State University - Bozeman, College of Letters & Science, 2017) Fox, Nicholas Reid; Chairperson, Graduate Committee: David W. MogkThe LaHood Formation plays a critical role in constraining the sources and tectonic setting during the initial stages of formation of the Mesoproterozoic Belt Basin, particularly in the Helena Embayment. In the Bridger Range the LaHood Formation is composed of coarse conglomerates, lithic arkoses, calcareous shales and siliceous shales. Twelve measured stratigraphic sections show a general fining to the northeast with calcareous shales exposed only in the northern half of the Range. Lithology provides the basis for the LaHood Formation to be divided into three Members: A) conglomerate; B) lithic arkose and sandy micaceous shale; C) interbedded Member B with calcareous and siliceous shales that are interpreted as correlative rocks of the upper Newland Formation. U-Pb ages of detrital zircons (LA-ICP-MS) from four sandstones show: 1) The basal conglomerate and coarse arkosic facies in the lower and southern two sections have a primary age distribution at ~3.2 Ga with a minor ~3.5 Ga component that corresponds to the major TTG crust-forming event in the northern Wyoming Province; 2) zircons from an arkosic sandstone inter-fingered with calcareous shales have a dominant ~3.2 Ga signal, but includes a significant secondary concentration at 2.8 Ga, which corresponds to the age of the Beartooth-Bighorn Magmatic Zone; 3) the northernmost section exhibits a concentration near 3.2 Ga, a more significant ~2.8 Ga signal, and the first occurrence of ~1.8 Ga grains, indicative of the Great Falls Tectonic Zone. These detrital zircon ages confirm the results of Guerrero et al. (2015) in the Bridger Range and from LaHood exposures in the Horseshoe Hills to the west, but differ significantly from the dominant 3.4-3.5 Ga signal from LaHood exposures in Jefferson Canyon and north of Cardwell, MT (Ross and Villeneuve, 2003; Mueller et al., 2016). Collectively, the detrital zircon ages demonstrate a) occurrences of the LaHood Formation across the Belt Basin received sediment from temporally distinct sources that do not necessarily include proximal Archean basement, and b) the stratigraphic succession reflects contributions from progressively younger source areas. The significant variety of ages and abrupt facies changes in the LaHood Formation support a series of compartmentalized sub basins.Item Histovariability and growth in the basal ceratopsian dinosaur Psittacosaurus mongoliensis from the lower Cretaceous Khulsangol Formation, central Mongolia(Montana State University - Bozeman, College of Letters & Science, 2016) Zorigt, Badamkhatan; Chairperson, Graduate Committee: David Varricchio; John R. Horner (co-chair); John R. Horner and Khatanbaatar Purevdorj were co-authors of the article, 'Taphonomy and depositional environment of a monodominant Psittacosaurus bonebed from the lower Cretaceous Khulsangol Formation of central Mongolia' submitted to the journal 'Journal of vertebrate paleontology ' which is contained within this dissertation.; John R. Horner was a co-author of the article, 'Intraskeletal histovariability in the hind limb of Psittacosaurus mongoliensis' submitted to the journal 'Journal of vertebrate paleontology' which is contained within this dissertation.; John R. Horner was a co-author of the article, 'Ontogenetic histovariability in a population of Psittacosaurus mongoliensis from the lower Cretaceous Khulsangol Formation, Uvurkhangai Province, Mongolia' submitted to the journal 'Journal of vertebrate paleontology' which is contained within this dissertation.Large datasets are required in order to study the details of growth in non-avian dinosaurs. The Cretaceous dinosaur sites in Mongolia offer tremendous opportunities to collect a sufficient data for such study. This dissertation introduces a brief history of dinosaur research in Mongolia. As a result of many expeditions since the 1920s, numerous new dinosaurs were named and many new dinosaur localities have been discovered. Whereas the majority of studies of Mongolian dinosaurs have focused on the fossils of Late Cretaceous taxa, dinosaur fossils are also abundant in the Early Cretaceous dinosaur sites. One of the richest Early Cretaceous dinosaur sites is Builst Khudag (BLK) in central Mongolia. Sedimentologic and taphonomic studies of the BLK site suggest that the depositional environment of the locality indicates the occurrence of a debris flow and sheet-flood in an alluvial setting. The fossils of the parrot-beaked, herbivorous dinosaur Psittacosaurus is the most common taxon from the BLK site. A Psittacosaurus aggregation consisting of juveniles reflects a catastrophic mortality event. Previously, the humerus, femur, tibia, and fibula were suggested to represent the ideal bones for growth determination studies. Since various elements grow in different developmental regimes, it is necessary to further determine an ideal element for Psittacosaurus growth assessment. Associated femora, tibiae, and fibulae from four individuals were examined. The femur and tibia show comparable bone tissues in all individuals; the tibia and fibula present comparable numbers of growth lines. Based on this finding, the tibia appears to provide the most comprehensive view of growth in Psittacosaurus. The Psittacosaurus growth strategy was further assessed using 58 tibiae collected from the BLK site. The Psittacosaurus growth rate reaches its peak (3.19 microns/day) after passing the threshold size at the age of two. During the ages of 10-13, the body size reaches 170.3 cm in length and the growth rate is considerably slowed down. This study confirms that Psittacosaurus growth fits in a sigmoidal shaped curve.Item Dinosaurs and time: chronostratigraphic frameworks and their utility in analysis of dinosaur paleobiology(Montana State University - Bozeman, College of Letters & Science, 2016) Fowler, Denver Warwick; Chairperson, Graduate Committee: John R. Horner; John B. Scannella and John R. Horner were co-authors of the article, 'Transitional evolutionary forms and stratigraphic trends in Chasmosaurine ceratopsid dinosaurs' which is contained within this dissertation.; Holly N. Woodward, Elizabeth A. Freedman, Peter L. Larson and John R. Horner were co-authors of the article, 'Reanalysis of 'Raptorex kriegsteini': a juvenile tyrannosaurid dinosaur from Mongolia' in the journal 'PLOS ONE' which is contained within this dissertation.; This dissertation contains two articles of which Denver W. Fowler is not the main author.Stratigraphy is the study of the position of rock strata, in order to determine their age. Dinosaur fossils have been recovered from North America for over 150 years, yet the stratigraphy of the localities from which they were collected has rarely been analysed at high resolution, either due to lack of original locality data, or that precise correlation between depositional basins was not technologically possible. This dissertation analyses what effect the introduction of high-resolution chronostratigraphic data has on our understanding of dinosaur paleobiology. Terrestrial sequence stratigraphy was used in combination with previously published data in order to subdivide the uppermost Cretaceous Hell Creek Formation of Montana into lower, middle, and upper units, shown to be consistent across the formation type area. Similar stratigraphic data was gathered for each of the Late Cretaceous dinosaur-bearing formations of the North American Western Interior, which was combined with nearly 200 radiometric dates (newly recalibrated here) to plot a comprehensive high-resolution correlation chart. The stratigraphic occurrence of dinosaur taxa was consequently plotted upon this chart. Similarly, the new Hell Creek Formation stratigraphic framework was used to plot the stratigraphic occurrence of Triceratops fossils. From this it can be seen that many dinosaur groups form stacks of stratigraphically separated species, a pattern indicative of linear, non-branching evolution (anagenesis). A similar pattern is observed for two new taxa of chasmosaurine ceratopsid (horned) dinosaurs from New Mexico, which form morphologic and stratigraphic intermediates between the slightly older taxon, Pentaceratops, and the younger Anchiceratops. Phylogenetic and geometric morphometric analysis supports the hypothesis that the posterior embayment of the parietal deepens and closes in on itself over ~ 2 million years from Pentaceratops through the new taxa, to Anchiceratops, and suggests a deep split within Chasmosaurinae that occurs before the Middle Campanian. These findings imply that dinosaur evolution in the Late Cretaceous Western Interior was characterized mostly by anagenesis, punctuated by occasional speciation events, perhaps triggered by high sealevel creating a north / south geographic barrier. The evolutionary process of reinforcement is discussed as a possible mechanism for the development of cranial display organs, linked to speciation.Item Can quantitative and qualitative deep marine depositional models be reconciled?: a case study of the sites member turbidite sequence, Cortina Formation (Upper Cretaceous), Cache Creek, California(Montana State University - Bozeman, College of Letters & Science, 2015) Queirolo, Rosalba; Chairperson, Graduate Committee: James G. SchmittTwo disparate sedimentological and stratigraphic models have been presented for the interpretation of the Upper Cretaceous, Sites Member of the Cortina Formation. This study addresses these discrepancies and proposes a unifying model. The first model is a qualitative-based interpretation of depositional environment by Ingersoll (1978), which focuses on analysis of process facies- and sedimentary body-scale attributes. The second model, presented by Murray et al. (1996), is statistically-based and exclusively focuses on sedimentation event-scale attributes. Ingersoll (1978) interpreted the Sites Member as a series of asymmetric, thickening- and coarsening-upward turbidite packages. However, Murray et al. (1996) interpreted the Sites Member to represent a turbidite sequence in which sedimentary event beds thin-upward as commonly as they thicken-upward. In this study the methods and results of Ingersoll (1978) and Murray et al. (1996) were reproduced and an integrated hierarchical stratigraphic framework is presented. One continuous sedimentological section, totaling 223.2 m, of the middle Sites Member was measured along the northern bank of Cache Creek, California, and was used to classify sedimentary process facies, sedimentation events, and sedimentary bodies. A three-fold hierarchical stratigraphic framework was constructed by correlating upsection changes in these sedimentary attributes. Low-order cyclicity, defined by a regional interpretation of depositional environment, shows that the middle Sites Member represents a component of an overall prograding submarine fan system. Intermediate-order cyclicity, defined by changes in subenvironment, shows that the middle Sites Member represents longitudinal oscillations within the unconfined outer fan environment. High-order cyclicity, defined by statistical analysis, is representative of autogenic compensational lobe stacking. If any of these scales of cyclicity are interrogated in isolation, a unique and misrepresentative interpretation is likely. These three orders of cyclicity combine to form an integrated hierarchical stratigraphic framework that resolves the interpretive discrepancies between Ingersoll (1978) and Murray et al. (1996). Additionally, it was determined that at the event bed-scale, multi-scale and coincident, allogenic and autogenic mechanisms are responsible for hierarchical nested cyclicity. Therefore, event bed thickness is an unreliable criterion for interpretation of profile position and for cycle definition. Instead, grain size is a more consistent proxy for profile position and changes in system energy.Item Deformation and metasomatism of the Qomolangma Formation: a geochemical and microstructural analysis of the summit limestone, Mount Everest, Nepal(Montana State University - Bozeman, College of Letters & Science, 2015) Corthouts, Travis Leo; Chairperson, Graduate Committee: David R. LagesonThis study is based on a suite of new samples that represent a structural transect across the Qomolangma Formation, an Ordovician limestone that comprises the uppermost ~125 meters of Mount Everest. Past studies posit Everest's summit limestone to be mildly sheared and unmetamorphosed; however, this study shows that the Qomolangma Formation has endured more strain and metamorphism than previously thought. Ti-in-biotite and Ti-in-quartz geothermometry, electron backscattered diffraction analysis, and geochemical analysis of tourmaline has been used to infer the metamorphic and deformational history of this formation. Results show that the Qomolangma Formation has experienced significant ductile shear throughout, though samples preserve a stronger shear fabric toward the top of the formation. Furthermore, our data show a gradation in the degree of metamorphism across the Qomolangma Formation, increasing toward the base of the unit. Samples collected from the structural top of the formation (Summit samples) have a penetrative foliation with significant grain size reduction of calcite (~8 microns) and yield temperature estimates of > or = 250°C. In contrast, samples from the base (South Summit samples) are distinguished by an increase in recrystallized grain size of calcite (~66 microns) and yield temperatures estimates of 500-600°C. Another important difference between Summit and South Summit samples is aggregates of the coexisting minerals muscovite, chlorite, tourmaline, rutile and biotite found throughout South Summit samples. Geochemical analysis of tourmaline suggests these minerals crystallized from metasomatic fluids (hydrothermal fluids) released from leucogranite bodies emplaced into rocks structurally subjacent to the summit limestone. As a result, metasomatic fluids caused localized metamorphism at the base of the Qomolangma Formation, producing the significant increase in metamorphic grade observed in South Summit samples. It is interpreted that the fabric preserved in Summit samples was ingrained during initial thrust faulting in the Eocene to Oligocene, whereas the fabric and metamorphic grade observed in South Summit samples is the result of detachment faulting and leucogranite emplacement in the Early Miocene. Lastly, microstructural observations suggest that metasomatic fluids may have promoted faulting on the Qomolangma detachment, a splay of the South Tibetan Detachment believed to be at the base of the Qomolangma Formation.Item Microfossil paleontology and biostratigraphy of the early Mesoproterozoic Belt Supergroup, Montana(Montana State University - Bozeman, College of Letters & Science, 2014) Adam, Zachary Robert; Chairperson, Graduate Committee: David W. Mogk; Mark L. Skidmore (co-chair)The Belt Supergroup is one of few early Mesoproterozoic sites worldwide that record paleobiological evidence of enigmatic ancestral eukaryotes. Using low manipulation acid maceration techniques, two new assemblages of microfossils have been recovered from the Greyson and Chamberlain Formations of the early Mesoproterozoic Belt Supergroup of Montana. These microfossils provide a window into the paleobiology of early Mesoproterozoic Laurentia for the first time. By comparison with fossils from other deposits of comparable age, these assemblages may be used to infer key biochronological, paleoenvironmental and ontogenetic aspects about the original organisms. The Greyson Formation assemblage from outcrops along Newlan Creek includes populations of Tappania, Valeria, Satka, Dictyosphaera, Coneosphaera, Caudosphaera and longitudinally striated tubes. The assemblage also includes microbial mat networks of Siphonophycus and isolated occurrences of Oscillatoriopsis, Rugosoopsis and Obruchevella. The Newlan Creek microfossils are conspicuously similar to those from the broadly coeval Roper Group of Australia, pointing to the utility of recognizing an early Mesoproterozoic (~1550-1450 Ma) assemblage zone represented by Tappania plana, Valeria lophostriata, Satka favosa, and Dictyosphaera delicata. The Chamberlain Formation assemblage from drillcore near Black Butte includes Valeria, Leiosphaeridia, Synsphaeridium, Coniunctiophycus, Satka, Symplassosphaeridium and longitudinally-striated tubes. The Black Butte microfossils expand the assemblage diversity previously reported from Chamberlain Formation outcrops near Neihart, Montana. The Black Butte assemblage partially overlaps with, but is notably distinct from, the assemblage from Newlan Creek. The differences between the respective assemblages are most likely attributable to paleoecological zonation of the original organisms. The Black Butte microfossils are broadly consistent with assemblages reported from supratidal to intertidal shallow water deposits of the late Mesoproterozoic. By contrast, the Newlan Creek microfossils are comparable to the distal shelf environment assemblage of the Roper Group of Australia. Correlations between morphometric attributes of the fossils indicate the presence of modestly diverse, environmentally-partitioned ecosystems that included protistan-grade organisms capable of a variety of cell replication processes by the early Mesoproterozoic. Microfossils and macrofossils of the Belt Supergroup provide an unparalleled opportunity for resolving ecological and macroevolutionary relationships among some of the Earth's oldest known eukaryotic organisms.