ABSTRACT The Black Hills of South Dakota are the easternmost basement-cored uplift in the Laramide foreland. Uplift occurred during the Latest Cretaceous-Eocene and was coeval with the emplacement of suite of alkaline and peralkaline hypabyssal intrusions. These intrusions were emplaced into Proterozoic metasedimentary rocks and Phanerozoic cratonic strata. Laramide exhumation stripped the Black Hills down to the Precambrian core. This study presents newly dated 54 Ma age peak zircon data of the Mountain Meadow Formation, the lower strata of the White River Group of South Dakota, to the many dates of these and adjacent rocks recently established recently by the authors The Eocene-Oligocene White River Group strata are unconformably overlain by the Arikareean Group of Oligocene-Miocene age. The Oligocene Mountain Meadow Formation is a post-orogenic gravel that consists of locally-derived clasts and rests at high elevations indicating that the Black Hills were at least partially buried following the Laramide Orogeny. Here, we present detrital zircon data (LA-ICPMS at the Arizona Laserchron Center) for the matrix of the Mountain Meadow Formation near Deadwood, South Dakota (n=143). Here, the Mountain Meadow is a crudely stratified, 75 m succession of cobble-boulder gravel that rests on Precambrian basement and early Paleozoic strata. About two-thirds of the zircons are Paleogene (41-63 Ma) in age, with no distinct peaks evident. These were likely derived locally from the Paleogene intrusive rocks. The balance of the zircon age spectrum is dominated by Precambrian grains, again with no prominent age peaks evident. The Mesoproterozoic grains were likely sourced locally from the underlying upper Cambrian Deadwood Formation. The Archean grains could be derived from the Deadwood as well, or perhaps they were derived from Paleoproterozoic basement rocks. Thus, the Mountain Meadow Formation here appears to be all locally derived, with transport distances of less than 10 km likely. This contrasts with coeval strata that cap the Bighorn uplift to the west, and that occur in the adjacent Great Plains and Powder River Basin, each of which have distal, westerly sediment source areas.
ABSTRACT Operations performed on hydrocarbon reservoirs are carried out with the assumption that the reservoirs are in capillary pressure equilibrium when they are initially drilled into. Presently, all evaluation techniques assume reservoirs are in capillary pressure equilibrium. This assumption is made to simplify the mathematics that are needed to quantify fluid flow. Hydrocarbon reservoirs are in equilibrium with respect to energy, fluid saturations, capillary pressure and interfacial tension after the hydrocarbon are emplaced. The forces moving the hydrocarbons into the reservoir are equal to the forces holding the water in place and capillary pressure equilibrium has been reached (McPhee, et al., 2015). Uplift and erosion of overlying strata will lower the pressure and temperature and capillary injection pressure of any reservoirs present and they will no longer be in capillary equilibrium. Restoring capillary pressure equilibrium requires a higher water saturation. (Bennion, et al., 2000). Water has a greater density than hydrocarbons and generally cannot move up into the column to restore capillary pressure equilibrium, so the reservoir will stay in non-equilibrium. If any water-based fluids are introduced into part or all of the reservoir by drilling and production operations, these fluids will imbibe and help to return to capillary pressure equilibrium. This process causes relative permeability damage to the reservoir in and around the wellbore. This wellbore damage can change the fluid composition and potentially the production flow rates.
ABSTRACT Unaweep Canyon, located in Mesa County, Colorado, has long been the subject of controversy due to the fact that no river runs through it. Debates have centered on which river carved it, when was it carved, when and why it was abandoned, and whether it was glaciated. Here we examine the relationship of the canyon to the Uncompahgre Plateau, the structure that it crosses. Prior to the uplift of the Plateau during the Laramide orogeny, the Precambrian basement rock was covered with a little over three kilometers of sediment. Uplift resulted in a structure that is approximately 180 km long, 40 to 60 km wide, and relatively horizontal on top over most of its length. However, at the position of modern Unaweep Canyon, there is an abrupt change and the structure dips northwestward into Utah. This paper explores the hypothesis that the change in dip disrupted the overlying sedimentary column and allowed preferential erosion at that position, creating a valley during the Laramide orogeny that, with continued erosion, eventually became modern Unaweep Canyon.
We report the results of detrital zircon U-Pb analysis from the middle Cambrian Flathead Sandstone from two locations in the eastern Bighorn Mountains of Wyoming and their implications for post-rift tectonics in western Laurentia. The Flathead Sandstone U-Pb zircon age spectra shows prominent age peaks at ~1.8 Ga and minor age peaks at ~2.7 Ga. Detrital zircon samples taken across the extent of the Flathead Sandstone have a much higher proportion of Archean grains nearer to the basal unconformity and further to the west, and age spectra overall get progressively younger in samples taken higher in the section and further east and south. The 2.7 Ga signature observed in the Bighorn Mountains is largely absent from the Wyoming Province rocks and is likely derived from the Superior Province. Proterozoic zircons are sourced from the east and southeast in the Trans-Hudson and Yavapai Province rocks exposed in the Transcontinental Arch. The differences in age spectra between samples correlated to their spatial and temporal differences likely reflects subsidence along the passive margin and progressive southwestward-advancing uplift of the Transcontinental Arch during Flathead deposition. Subsidence, denudation and burial of Wyoming Province rocks and uplift of those exposed in the Transcontinental Arch led to the removal of the Wyoming Province as a source terrane by the time of Flathead Sandstone deposition in the Bighorn Mountains. The Transcontinental Arch was a significant contributor to detrital zircons across Laurentia, and concurrently acted as a barrier to sediment dispersal from more distal and younger Proterozoic terranes to the east and south into northwestern Laurentia.
A detailed regional sequence stratigraphic study of the Cretaceous Greenhorn Transgressive/ Regressive (T6/R6) Cycle within the Denver Basin was conducted using a grid of well-log cross sections, as well as geochemical data from three key cores. This classic T-R cycle in the Denver Basin traditionally includes the Muddy “J” Sandstone, Mowry Shale, Graneros, Greenhorn, and Carlile Formations. In this study, fourteen (14) depositional sequence boundaries and thirteen (13) petrophysically and geochemically distinct depositional sequences were defined within the classic Greenhorn (T6/R6) Cycle within northeastern Colorado, southeastern Wyoming, and southwestern Nebraska. Of these fourteen sequence boundaries, the most significant is a major regional (Mid-Early Cenomanian) angular unconformity (K600sb) located at the base of the Middle Member of the Graneros Formation. Regionally from north to south in the Denver Basin, this angular unconformity sequentially truncates the Lower Graneros, and then the Mowry Shale (D Sand/Huntsman). Thus, in the southern Denver Basin, near Pueblo Colorado, the Middle Graneros is juxtaposed on the Muddy Formation. With the recognition of this angular unconformity within the Graneros Formation, it becomes clear that the Muddy, Mowry (D Sand/Huntsman), and Lower Graneros strata are genetically unrelated to the overlying Middle and Upper Graneros, Greenhorn, and Carlile strata, and should be placed within a separate older (Mowry) T-R Cycle, not within, or part of, any younger (Greenhorn) T-R Cycle, as traditionally interpreted. Within the Mowry Cycle, two third-order composite sequences (M1 and M2) were defined. Two composite sequences were also defined within the updated Greenhorn T-R Cycle (G1 and G2). Within the composite sequence framework for the updated Greenhorn Cycle, the Bridge Creek Limestone represents the basal portions of a trans-gressive sequence set genetically related to the overlying Carlile Formation, and not the underlying portions of the traditional Greenhorn Formation. In the four composite sequences defined within this study, the maximum flooding surfaces (mfs) are placed at the base of siliciclastic-rich mudstones in the Mowry Shale, Hartland Shale, and Blue Hill Shale and within the siliciclastic-rich Muddy J-1 sand. This interpretation differs markedly from previous studies, where maximum flooding surfaces were traditionally placed within carbonate-rich units within T-R cycles (sequences).
Geological studies in the Cretaceous Western Interior Seaway (KWIS) in North America reveal highly variable sedimentological conditions on its western shore caused by rapidly changing sea level and detrital input during the seaway’s closure. Here we examine a 601-foot (183 meters) continuous core through the lower part of the Maastrichtian Lewis Shale in the eastern Washakie Basin, Wyoming, through integrating stratigraphic and geochemical analyses to better understand organic matter deposition and preservation during the final marine transgression within the seaway. The core penetrates eight organic-rich flooding intervals (F1–F8) and a regional condensed section, informally known as the Asquith marker. The lower portions of the core record sediment input from a southern source, likely the Sierra Madre/Park Ranges, while the upper part records sediment input from a northern source, likely the Granite Mountains. This provenance transition is supported by regional stratigraphic analysis and changes in bulk mineral and trace metal composition. The Asquith marker and early flooding surfaces are enriched in oil-prone, marine organic matter deposited under dysoxic to euxinic conditions, whereas younger flooding surfaces show increased terrigenous input and poorer preservation conditions. The Asquith marker is identified as a prime oil-prone source rock. Although younger flooding surfaces also exhibit favorable source-rock properties, their generative potential is reduced due to increased clastic and terrigenous organic matter deposition caused by regression. These results emphasize the importance of sediment source variability, organic matter preservation, and changing redox conditions to provide insight into the sediment provenance and petroleum potential of the Lewis Shale in the context of the final transgression within the KWIS.
This project involved the construction of a detailed geologic map of the Skull Ridge, Wyoming 7.5-Minute Quadrangle (Scale 1:24,000) and supported the broader efforts of the Wyoming State Geological Survey to complete 1:24,000 scale geologic maps of the state. The quadrangle includes parts of the Bighorn National Forest and the Amsden Creek Wildlife Habitat Management Area, which are popular recreation sites for thousands of people each year. This research advances the scientific understanding of the geology of the Bighorn Mountains and the Archean geology of the Wyoming Province. Traditional geologic mapping techniques and FieldMove Clino were used in concert with petrography, geochemistry and isotopic age determinations. Our goal was to further subdivide the various phases of the 2.8–3.0 Ga Archean rocks based on their rock types, age, and structural features. The northern part of the Bighorn Mountains is composed of the Bighorn batholith, a composite complex of intrusive bodies that were emplaced between 2.96–2.87 Ga. Our mapping of the Skull Ridge Quadrangle has revealed the presence of three different Archean quartzofeldspathic units, eight sets of amphibolite and diabase dikes, and a Phanerozoic sedimentary section including the Cambrian Flathead Sandstone through the Pennsylvanian Amsden Formation deformed by two possible blind faults and forming an anticline along the northern section of the mapping area. The Archean rock units’ range in age from ca. 2.91–2.86 Ga, which correlates with other basement rocks reported in the Bighorn batholith. All the Archean rocks have subtle but apparent planar fabric elements, which are variable in orientation and are interpreted to represent magmatic flow during emplacement.
Late Pinedale till deposits mapped in various stream valleys in the Comanche Peak Wilderness Area, north of Rocky Mountain National Park, indicate that Browns Creek Valley was glaciated from two directions simultaneously while the middle of the valley remained unglaciated. Till deposits at the upper end of Browns Creek Valley display a well-developed end moraine in a U-shaped valley. Immediately downstream from the end moraine the valley becomes V-shaped, and shows no evidence of glaciation. Another till is encountered in the lower reaches of Browns Creek Valley. This sediment was deposited by a distributary of the main valley glacier moving down Beaver Creek Valley. This subsidiary lobe of the Beaver Creek Valley glacier moved down gradient, but upstream. The tills are texturally distinct from alluvial deposits, but similar to each other. The tills are muddy and sandy boulder gravels, while the alluvial deposits that lie between them are sandy bouldery gravels. Although produced by two separate glaciers, glaciation of the upper and lower parts of Browns Creek Valley appears to have taken place simultaneously.
The Española Basin is a west-tilted half-graben formed from extensional stresses related to the formation of the Rio Grande Rift. Intrabasinal normal faulting created a series of sub-grabens within the Española Basin. Importantly, this stratigraphic record provides one of North America’s best Miocene vertebrate fossil records. Yet, the composite stratigraphic section necessary to place those fossil occurrences into a biostratigraphic framework is unavailable largely because the tectonic framework creates a complex mosaic of facies, making the correlation of individual stratigraphic sections difficult. However, numerous widespread and distinguishable ash beds occur within the strata that help correlation across complex structural and facies relationships. This paper provides a composite stratigraphic section targeting fossil-rich regions within the Bureau of Land Management’s Sombrillo Area of Critical Environmental Concern, documenting periods of progradation and retrogradation of the alluvial fan facies. Integrating previously published climatic and tectonic data with new paleontological data into a stratigraphic and lithofacies framework bears on the environmental evolution during and after the middle Miocene Climatic Transition (MCT). From approximately 17 Ma to 14 Ma, the Española Basin experienced a period of warmer and wetter conditions during the middle Miocene Climatic Optimum (MCO) reflected by alluvial fan facies. Towards the end of the MCO, alluvial fan facies began to retrograde. We infer these results to indicate that climate provides more influence on sedimentation because alluvial fan retrogradation corresponded with less vegetation and, thus, less sediment stability associated with cooler conditions, as demonstrated by previous stable isotopic studies. As the regional climate cooled and dried, the Española Basin remained wet, although cooler conditions forced the local extinction of palm trees. This is also supported by the increase in antilocaprid abundance and the appearance of gomphotheres attracted to the riparian zones during this time of regional aridity.
Surface mapping, petrographic analysis, isotopic and gravity studies characterize the Professor Valley salt body, located within northeast Paradox Basin between the Salt Valley and Onion Creek salt walls. Gravity analysis identifies the Professor Valley salt body as an isolated diapir that is not connected to Onion Creek, and not likely connected with Salt Valley. Outcrops of the Paradox Formation at Professor Valley are non-caprock carbonates that have undergone meteoric diagenesis that surround unusual sandstone and conglomerate inclusions. We infer the inclusions are derived from Paradox Formation clastic depositional cycles, carried upward during diapirism, and concentrated with salt dissolution. Gravity data and surface bedding orientations on the northern edge of the diapir indicate the adjacent Permian Cutler Group strata represent composite halokinetic sequences extending <1 km from the diapir. The southern edge of the diapir is less constrained but appears consistent with the asymmetry observed in adjacent salt bodies. Smaller scale features are consistent with a burial wedge located near the eastern edge of the diapir. Results demonstrate how the integration of geological and geophysical methods can aid in determining the geometry and evolution of salt bodies.
Distributive fluvial systems are common river environments where the stream is distributary, rather than tributary. These fluvial systems are common in modern basins, where preservation potential is highest, but relatively few have been interpreted from the rock record. This indicates that some previously described fluvial sedimentary units are likely misinterpreted as tributary systems, when in actuality, they were deposited in a distributive system. The Jelm and Popo Agie formations are two poorly understood sedimentary units from west-central Wyoming, which were previously described as part of a tributary fluvial system. In contrast to prior interpretations, we demonstrate that features such as sedimentary structures (i.e., antidunes and chutes-and-pools), facies architecture (i.e., extensive crevasse splays and few channels), and depositional trends, (i.e., fining of sediment and decreased shear stress down-dip) are more consistent with the distributive fluvial system model. Similar to previously described distributive fluvial systems, the Jelm Formation exhibits progradational stacking of the fluvial facies associations. This is contrasted by the overlying Popo Agie Formation, which shows abrupt retrogradational stacking, a pattern that is previously undescribed from any distributive system. The stacking patterns in both the Jelm and Popo Agie formations are interpreted to be the result of changes in the accommodation/sedimentation ratio along depositional dip. Accounting for accommodation and sedimentation in distributive fluvial systems helps better understand how these units were deposited through time and allows for better prediction of stacking patterns found in other distributive fluvial deposits.
Mesozoic eolian oolitic carbonates are rarely documented in the Western Interior of North America, despite the ample presence of exposed carbonate strata spanning the Paleozoic through the Quaternary. This study reinterprets a distinct 15-meter-thick deposit in the Middle Jurassic lower Sundance Formation, located in Wyoming’s Bighorn Basin, traditionally interpreted as high-energy subtidal deposits. These large-scale cross-stratified oolitic strata are interpreted to be eolian deposits, attributed to the deflation of emergent oolitic shoals following structural uplift and sea level fall during the late Callovian. Evidence of an eolian origin is supported by the presence climbing trans-latent stratification produced by migrating wind ripples, composed of alternating laminations of ooids and silt-sized quartz grains. Additional evidence consists of coarsening-upward sequences of fragmented and abraded ooid grains, and evidence of vadose diagenesis. The cross-stratified oolitic bodies’ relationship with surrounding lithofacies also supports the eolian hypothesis, suggesting these oolitic limestones were deposited as isolated bedforms on an emergent deflation surface during a regression of the Jurassic Sundance Sea. The exceptional preservation of these eolian carbonates, was facilitated by low-energy conditions during a subsequent transgression.
Nearly 1,450 resistivity geophysical well logs were correlated over 758 square miles and their stratigraphic information was incorporated with core data to objectively study the geological section from the surface to a depth of nearly 1,950 ft.-deposited during the final retreat of the Western Interior Cretaceous Sea (WICS). This information was utilized to make new interpretations about both the geological and structural events that developed in this area. This included the effect volcanic ash deposition had on the ultimate development of the Boulder- Weld Fault Zone (BWFZ) and the division of this feature into a compressional structural eastern area and a western area comprised of both compressional and extensional features. A stratigraphic correlation of the Fox Hills Sandstone in the western portion of the study area facilitated a detailed structural reinterpretation of its outcrops there.
The Castle Rock Conglomerate is one of Colorado’s most iconic, youngest, and coarsest grained rock units. It is also one of the hardest sedimentary rocks in Colorado and forms prominent buttes in the southwestern Denver Basin. Yet the reasons for its induration and resistance to weathering have not previously been investigated. Sedimentologic observations paired with sedimentary petrology indicate that much of the unit is comprised of a planar-bedded to cross-bedded, mostly poorly sorted, angular to subrounded assemblage of quartz, K-feldspar, quartzite, and unusually large volcanic rock fragments along with some plagioclase and mica flakes. The largest volcanic rock fragments are up to ∼2 m in size and composed of the immediately subjacent Wall Mountain Tuff of late Eocene age. Sedimentary rock fragments and well-rounded quartz grains are rare. Together these features suggest a diverse and relatively proximal provenance for the unit. Pervasive opaline cement coats most grains, and locally exhibits pendant features typical of vadose precipitation. These opal cements formed prior to any grain compaction and indicate early silica precipitation at shallow burial depths. Where the primary pores were not completely cemented by the opal, most were later filled with length-fast chalcedony cement. We hypothesize that cementation of the conglomerate began soon after deposition as weathering of the Wall Mountain Tuff and weathering of clasts of the tuff within the conglomerate, yielded ground water super-saturated with silica. These fluids initially catalyzed precipitation of common opal (hydrous amorphous silica) and later fostered precipitation of length-fast chalcedony. Together, these cements created a silica-cemented “concrete” much more resistant to weathering than any carbonate-cemented sandstone, and much harder than man-made calcite-cemented concrete found in many sidewalks and roadways.
Castlewood Canyon is one of the most distinctive landforms on the Colorado plains—a geomorphology that developed as Cherry Creek and its precursors incised into the Eocene Wall Mountain Tuff and overlying Castle Rock Conglomerate (CRC). Outcrops of the CRC in Castlewood Canyon State Park (CCSP) contain boulders of the Wall Mountain Tuff that are up to two meters in diameter, and the conglomerate itself is composed of large (up to 0.5 m), diverse clasts of Precambrian granite, gneiss, quartzite, and other lithologies eroded from the Colorado Front Range that is 25 km to the west and as much as 100 kilometers to the northwest. These clasts and other evidence suggest transport and deposition by a sequence of flood events. Such flooding events, albeit smaller in scale, continue to occur in modern times, including a catastrophic flood caused by the failure of the Castlewood Dam in 1933, and a canyon-scouring event in 2023. These events and the geologic history of this canyon are described in this paper, illustrating that nature, mild though it may be for millennia, is still shaping the Castlewood Canyon system.
The geometry and deformational mechanisms associated with the growth of the Gobbler Anticline, a north-trending, doubly plunging, tight fold that is interpreted to be a fault-propagation fold above a blind, basement-rooted reverse fault with en echelon tear faults was constrained via a balanced cross-section, virtual outcrop models, and orthomosaic photographs. The Gobbler Anticline consists of tightly folded and faulted mixed lithofacies. Here, thick-bedded (1.0-10.0 m) limestones intercalated with thin-bedded (laminated - 1.0 m) shale, characteristic of the Bug Scuffle Member of the Pennsylvanian Gobbler Formation. Measured sections (totaling 470 m) document the distribution of facies while mechanical rock property analyses document the unconfined compressive strength of the rocks. Thin sections and X-Ray fluorescence indicate lithological controls on deformational mechanisms. Pre-Permian unconformities and thinning of the overlying mid-upper Pennsylvanian strata indicate shallow burial (<500 m) at the time of initial fold deformation (Missourian, ∼307 Ma). We find that argillaceous wackestones are disproportionately weakened when involved in folding soon after deposition and mechanical layering is the dominant control on fracture development in more deformed areas. This study illustrates the value of a tightly constrained mechanical strati-graphic model for the prediction of fracture distribution and fold geometry in deformed carbonate rocks.
The late Eocene Castle Rock Conglomerate occurs mostly in Douglas and Elbert counties, Colorado. It is the uppermost and youngest Cenozoic unit in the southern Denver Basin and its outcrops occur in a swath trending from Sedalia southeast to Calhan. The unit is well exposed and topographically prominent, forming flat mesas, steep cliffs, and narrow canyons. The conglomerate is a fluvial unit deposited by a 3- to 10-km-wide braided stream system. Large-scale crossbedding, massive bedding, angular blocks of welded tuff, a variety of other clast lithologies, cut-and-fill structures, fining-upward sequences, fossil logs, and occasional fossilized bones are readily observable. Because the conglomerate is both geologically and scenically striking, it has interested geologists since the late 1860s. Because of improved access to the unit over the last 60 years (in Castlewood Canyon State Park and in county and municipal open spaces) it has increasingly attracted educators, students, and the public. The purpose of this two-part article is to be a source document for future investigators of the formation. Part 1 of the article (the present publication) is a chronology of the description, nomenclature, and mapping of the formation as presented by various investigators over the decades. Part 2 (for future publication) will cover the formation’s geologic history, depositional environment, age, and diagenesis and will present several suggestions for future research.
Hydrous amorphous silica (aka opal) is a common cement in the Upper Eocene Castle Rock Conglomerate (CRC) of the southwestern Denver Basin. Petrographic study of standard thin sections indicates that this opal forms from 5% to as much as 40% of any given sample. It also commonly occurs as a precursor to fibrous length-fast chalcedony, a crystalline form of quartz cement. Similar opal cement apparently derived from shards of volcanic glass is even more common in the subjacent Wall Mountain Tuff, a welded deposit of volcanic ash that was the most likely source of silica in the opal cement in the CRC. This paper provides a first-of-its-kind attempt to quantify the amount of opal cement in selected samples of the CRC based on X-ray diffraction (XRD) analysis. Because opal is amorphous, its abundance cannot be quantified using standard XRD techniques, but experimenting with heating indicates that it is possible to convert powdered amorphous opal to a crystalline form of silica in less than 48 hours at a temperature of 550°C. Comparison of pre- and post-heat treatment XRD diffractograms thus provides a potential tool for quantifying opal content in the CRC and other opal-cemented samples. This new analytical technique and its results are described in this paper.
The Late Eocene Castle Rock Conglomerate, Tcr on Colorado Geological Survey (CGS) geologic maps, occurs mostly in Douglas and Elbert counties, Colorado. It is the uppermost and youngest Cenozoic unit in the southern Denver Basin and its outcrops occur in a swath trending from Sedalia ∼100 km southeast to Calhan. The unit is well exposed and topographically prominent, forming flat mesas, steep cliffs, and narrow canyons. The conglomerate is a fluvial unit deposited by a 3- to 10-km-wide braided stream system. Large-scale cross-bedding, massive bedding, angular blocks of welded tuff, a variety of other clast lithologies, cut-and-fill structures, fining-upward sequences, fossil logs, and occasional fossilized bones are readily observable. Because the conglomerate is both geologically and scenically striking it has interested geologists since the late 1860s. Because of improved access to the unit over the last 60 years (in Castlewood Canyon State Park and county and municipal open spaces) it has increasingly attracted educators, students, and the public. This article is the second of two parts, and the article as a whole is intended to be a source document for future investigators of the formation. Part 1 (Keller and Morgan, 2024) is a chronology of the description, nomenclature, and mapping of the formation as presented by various investigators over the decades. Part 2 (the present document) covers the formation’s geologic history, age, diagenesis, and possibilities for future research.
Here we present the results of detailed (1:24,000 scale) mapping of the Spanish Point 7.5 Minute Quadrangle, Wyoming, which occurs along the western flank of the Bighorn Mountains. The Quadrangle includes Archean crystalline rocks of the Wyoming Province Beartooth-Bighorn Magmatic Terrane, Paleozoic miogeoclinal clastic and carbonate strata (Flathead, Gros Ventre, Gallatin, Big Horn, Madison, and Amsden formations, and Quaternary (Pinedale Formation) glacial deposits. Isotopic age determinations (LA-ICPMS U-Pb on zircon) of the basement crystalline rocks, which occupy the higher elevations in the eastern part of the Quadrangle, were conducted at the University of Arizona Laserchron Center. Seven samples of grey foliated to massive granodiorite were analyzed, all yielding ages of about 2880–2885 Ma, which is correlative to the Lookout Mountain granodiorite to the north. The dominant basement fabric orientation is ∼N75W90. The Lookout Mountain Granodiorite was uplifted along the N-S striking, west verging Laramide Spanish Point Reverse fault, which dips steeply to the east. Paleozoic strata in the footwall are steeply inclined to overturned. Displacement along the fault is as much as 150 m. To the west, the Paleozoic strata roll into a homocline that dips ∼10 degrees to the west. The Pleistocene Pinedale Formation glacial deposits, which are as much as 50 m in thickness, occur as ground, end, and lateral moraines along the principal drainages.
