Wind River Basin Research Articles

Vertebrate microfossil assemblages: their role in paleoecology and paleobiogeography

Contents Preface. Sven Baszio Part 1. Importance of Microvertebrate Sites, Sampling, Statistical Methods, and Taphonomy 1. Information from Microvertebrate Localities: Potentials and Limits Sven Baszio 2. How Much Is Enough? A Repeatable, Efficient, and Controlled Sampling Protocol for Assessing Taxonomic Diversity and Abundance in Vertebrate Microfossil Assemblages Heather A. Jamniczky, Donald B. Brinkman, and Anthony P. Russell 3. Taphonomic Issues Relating to Concentrations of Pedogenic Nodules and Vertebrates in the Paleocene and Miocene Gulf Coastal Plain: Examples from Texas and Louisiana, USA Judith A. Schiebout, Paul D. White, and Grant S. Boardman Part 2. Guild Analysis, Ecological and Faunal Analyses, Biodiversity, and Paleobiogeography 4. The Structure of Late Cretaceous (Late Campanian) Nonmarine Aquatic Communities: A Guild Analysis of Two Vertebrate Microfossil Localities in Dinosaur Provincial Park, Alberta, Canada Donald Brinkman 5. Vertebrate Paleoecology from Microsites, Talley Mountain, Upper Aguja Formation (Late Cretaceous), Big Bend National Park, Texas, USA Julia T. Sankey 6. Terrestrial and Aquatic Vertebrate Paleocommunities of the Mesaverde Formation (Upper Cretaceous, Campanian) of the Wind River and Bighorn Basins, Wyoming, USA David G. DeMar Jr. and Brent H. Breithaupt 7. Lack of Variability in Feeding Patterns of the Sauropod Dinosaurs Diplodocus and Camarasaurus (Late Jurassic, Western USA) with Respect to Climate as Indicated by Tooth Wear Features Anthony R. Fiorillo 8. Diversity of Latest Cretaceous (Late Maastrichtian) Small Theropods and Birds: Teeth from the Lance and Hell Creek Formations, USA Julia T. Sankey 9. Small Theropod Teeth from the Lance Formation of Wyoming, USA Nick Longrich 10. The First Serrated Bird Tooth Philip J. Currie and Clive Coy 11. First Dinosaur Eggshells from Texas, USA: Aguja Formation (Late Campanian), Big Bend National Park Ed Welsh and Julia T. Sankey 12. Review of the Albanerpetontidae (Lissamphibia), with Comments on the Paleoecological Preferences of European Tertiary Albanerpetontids James D. Gardner and Madelaine Bohme 13. New Information on Frogs (Lissamphibia: Anura) from the Lance Formation (Late Maastrichtian) and Bug Creek Anthills (Late Maastrichtian and Early Paleocene), Hell Creek Formation, USA James D. Gardner List of Contributors Index

Precise dating of biotite in distal volcanic ash: Isolating subtle alteration using 40Ar/39Ar laser incremental heating and electron microprobe techniques

Precise dating of K-rich minerals in volcanic rocks via the 40 Ar/ 39 Ar geochronometer has become crucial to resolving many geological problems. In some cases, ash beds containing biotite phenocrysts but lacking sanidine are the only datable horizons at key stratigraphic intervals, necessitating comparison of sanidine and biotite ages within the same chronostratigraphic framework. To assess the integrity of biotite and evaluate the accuracy of ages obtained from this often problematic mineral, incremental heating 40 Ar/ 39 Ar experiments were performed on large, millimeter-sized, euhedral biotite crystals from four key Eocene tuffs in the Green River and Wind River Basins and are compared with sanidine 40 Ar/ 39 Ar ages. Unaltered biotite crystals with homogenous K-compositions from two tuff beds yielded concordant plateau ages that are indistinguishable from cogenetic sanidine ages. In contrast, biotite crystals from two other tuffs yielded discordant spectra, with relatively young initial steps followed by older, down-stepping apparent ages. Plateau ages from the discordant experiments are older than the sanidine ages by 1 to 14%. Integrated (total fusion) ages from these experiments exhibit scatter toward both younger and older ages that correlate with the degree of spectral discordance. Electron microprobe transects reveal that biotite crystals yielding discordant age spectra contain 1-10 μm thick K-depleted (<8% K 2 O) alteration zones along internal cleavage planes that are absent in biotite crystals yielding concordant age spectra. We propose that these altered zones promote open-system behavior. Phenomena such as loss of K or 40 Ar*, 39 Ar K recoil into internal K-depleted phases, and 39 Ar K recoil entirely out of internally corroded biotite crystals are all potential mechanisms that can be related to this alteration. Due to the presence of these multiple potential pathways that promote the gain or loss of isotopes, such altered biotite crystals are unsuitable for high-resolution 40 Ar/ 39 Ar dating, comparisons with sanidine ages, or calibration of the geomagnetic polarity timescale.

Tectonically induced climate and its control on the distribution of depositional systems in a continental foreland basin, Cloverly and Lakota Formations (Lower Cretaceous) of Wyoming, U.S.A.

Continental sediments of the Cloverly and Lakota Formations (Lower Cretaceous) in Wyoming are subdivided into three depositional systems: perennial to intermittent alluvial, intermittent to ephemeral alluvial, and playa. Chert-bearing sandstones, conglomerates, carbonaceous mudrocks, blocky mudrocks, and skeletal limestones were deposited by perennial to intermittent alluvial systems. Carbonaceous mudrocks contain abundant wood fragments, cuticle and cortical debris, and other vascular plant remains representing deposition in oxbow lakes, abandoned channels, and on floodplains under humid to seasonal conditions. Intraformational conglomerates, sandstones, bioturbated and blocky mudrocks with caliche nodules, and bioturbated limestones characterize deposition in intermittent to ephemeral alluvial systems. Bioturbated limestones are encased in bioturbated mudrocks with abundant pseudo-slickensides. The presence of caliche nodules in some of the blocky to bioturbated mudrocks is consistent with supersaturation and precipitation of calcium carbonate from groundwater under semi-arid conditions. Caliche nodules, pseudo-slickensides, and carbonate-rich floodplain sediments are interpreted to have been deposited by intermittent to ephemeral alluvial systems under seasonal to semi-arid climatic conditions. Laminated mudrocks, siltstones, vuggy carbonates, bedded to nodular evaporites, pebbly mudrocks, and diamictites were deposited in evaporative alkaline lakes or playas. Pebbly mudrocks and diamictites are interpreted to represent deposition from channelized and unchannelized hyperconcentrated flows on a playa, resulting from intense rain events within the basin. The areal abundance and distribution of these depositional systems change systematically across the overfilled portion of the Early Cretaceous Cordilleran foreland basin in Wyoming. The lower part (A-interval) of the Cloverly and Lakota Formations is characterized by deposits of perennial to intermittent rivers that existed 300 to 1000 km east of the Sevier fold-and-thrust belt. Proximal to the Sevier fold-and-thrust belt, the A-interval of the Cloverly Formation and upper Ephraim Formation of the Gannett Group are typified by deposits of intermittent to ephemeral rivers and their associated floodplains. In the middle part (B-interval) of the Cloverly Formation, intermittent to ephemeral alluvial systems expand to 600 km into the basin. The upper part (C-interval) of the Cloverly Formation is characterized by playa deposits in the Bighorn and Wind River Basins and intermittent to ephemeral alluvial deposits along the front of the ancestral Sevier Mountains. Deposits of perennial to intermittent alluvial systems in the C-interval of the Cloverly and Lakota Formations are restricted to the Black Hills region, almost 900 km to the east of the Sevier Mountains. The change in the areal distribution of depositional systems through time within this continental foreland basin may be attributed to the development of a rain shadow associated with the uplift of the Sevier Mountains in the Early Cretaceous.

Using pebble lithology and roundness to interpret gravel provenance in piedmont fluvial systems of the Rocky Mountains, USA

Clast populations in piedmont fluvial systems are products of complex histories that complicate provenance interpretation. Although pebble counts of lithology are widely used, the information provided by a pebble count has been filtered by a potentially large number of processes and circumstances. Counts of pebble lithology and roundness together offer more power than lithology alone for the interpretation of provenance. In this study we analyze pebble counts of lithology and roundness in two contrasting fluvial systems of Pleistocene age to see how provenance varies with drainage size. The two systems are 1) a group of small high-gradient incised streams that formed alluvial fans and terraces and 2) a piedmont river that formed terraces in response to climate-driven cycles of aggradation and incision. We first analyze the data from these systems within their geographic and geologic context. After this is done, we employ contingency table analysis to complete the interpretation of pebble provenance. Small tributary streams that drain rugged mountains on both sides of the Santa Cruz River, southeast Arizona, deposited gravel in fan and terrace deposits of Pleistocene age. Volcanic, plutonic and, to a lesser extent, sedimentary rocks are the predominant pebble lithologies. Large contrasts in gravel lithology are evident among adjacent fans. Subangular to subrounded pebbles predominate. Contingency table analysis shows that hard volcanic rocks tend to remain angular and, even though transport distances have been short, soft tuff and sedimentary rocks tend to become rounded. The Wind River, a major piedmont stream in Wyoming, drains rugged mountains surrounding the northwest part of the Wind River basin. Under the influence of climate change and glaciation during the Pleistocene, the river deposited an extensive series of terrace gravels. In contrast to Santa Cruz tributary gravel, most of the Wind River gravel is relatively homogenous in lithology and is rounded to well-rounded. Detailed analysis reveals a multitude of sources in the headwaters and the basin itself, but lithologies from these sources are combined downstream. Well-rounded volcanic and recycled quartzite clasts were derived from the headwaters. Precambrian igneous and metamorphic clasts were brought down tributary valleys to the Wind River by glaciers, and sandstone was added where the river enters the Wind River structural basin.

A new early Eocene Microparamys (Mammalia, Rodentia) from the Wind River Basin, Wyoming

A new early Eocene Microparamys (Mammalia, Rodentia) from the Wind River Basin, Wyoming

Teleseismic P‐wave tomogram of the Yellowstone plume

Inversion of a new data set of teleseismic P‐wave travel‐times from three PASSCAL seismic deployments around the Yellowstone hotspot reveals a 100 km diameter upper mantle plume that extends from the Yellowstone volcanic caldera to 500 km depth and dips 20° to the northwest. A monotonic decrease in the velocity perturbation of the plume from −3.2% at 100 km to −0.9% at 450 km is consistent with a uniform thermal anomaly of 180°C. Where the plume crosses the 410 km discontinuity, previous research shows a depression in the 410 km discontinuity consistent with a warm plume (Fee and Dueker, 2004). Additionally, a region of high velocities extends to 250 km beneath the Wind River basin in NW Wyoming that may represent a convective downwelling of the lithosphere.

Factors Affecting the Occurrence of Saugers in Small, High‐Elevation Rivers near the Western Edge of the Species' Natural Distribution

AbstractFactors affecting the occurrence of saugers Sander canadensis were studied throughout the Wind River basin, a high‐elevation watershed (>1,440 m above mean sea level) on the western periphery of the species' natural distribution in central Wyoming. Adult saugers appeared to have a contiguous distribution over 170 km of streams among four rivers in the watershed. The upstream boundaries of sauger distribution were influenced by summer water temperatures and channel slopes in two rivers and by water diversion dams that created barriers to upstream movement in the other two rivers. Models that included summer water temperature, maximum water depth, habitat type (pool or run), dominant substrate, and alkalinity accounted for the variation in sauger occurrence across the watershed within the areas of sauger distribution. Water temperature was the most important basin‐scale habitat feature associated with sauger occurrence, and maximum depth was the most important site‐specific habitat feature. Saugers were found in a larger proportion of pools than runs in all segments of the watershed and occurred almost exclusively in pools in upstream segments of the watershed. Suitable summer water temperatures and deep, low‐velocity habitat were available to support saugers over a large portion of the Wind River watershed. Future management of saugers in the Wind River watershed, as well as in other small river systems within the species' native range, should involve (1) preserving natural fluvial processes to maintain the summer water temperatures and physical habitat features needed by saugers and (2) assuring that barriers to movement do not reduce upstream boundaries of populations.

REVISIONS TO UPPER CRETACEOUS STRATIGRAPHY NEAR HELL'S HALF ACRE, EASTERN WIND RIVER BASIN, CENTRAL WYOMING

Thick, steeply dipping, and generally conformable Upper Cretaceous and lower Paleogene strata are well exposed on the western Casper arch, central Wyoming. They record deposition prior to, during initiation of, and following onset of subsidence and basement-involved thrusting of the Laramide orogeny. Confusion has permeated the distributions, thicknesses, and ages of these strata, adjacent to the Wind River Basin's east-central margin. New lithostratigraphic mapping near Hell's Half Acre (HHA) revises identification of nonmarine units overlying the marine Lewis Shale (of Campanian and/or earliest Maastrichtian age). The homogeneous, mostly fine-grained Meeteetse Formation (MF) represents paludal conditions following deposition of the Lewis Shale's “lower tongue.” The local MF exceeds 3,200 feet (nearly 1.1 km) in thickness, almost 7.5 times greater than previously reported. The MF near HHA, although much thicker, exhibits characteristics strikingly similar to the Meeteetse elsewhere in the Wind River and Bighorn Basins. Most strata identified here as MF previously were considered as Lance Formation. The top of the MF near HHA matches the previously recognized contact with the Fort Union Formation (mostly Paleocene). Dominance of sandstone above that contact implies a more energetic fluvial regime that may, in part, represent latest Cretaceous time. Reflecting existing uncertainties, rocks above the MF and below the lacustrine Waltman Shale Member (Fort Union Formation) are designated here as “Fort Union (unnamed lower part) and Lance Formations, undifferentiated.” Upper surfaces of the Fort Union Formation were eroded early in Eocene time, and local scouring near HHA cut as deeply as the MF. Thus it is improbable that youngest parts of the Paleocene are recorded paleontologically in strata near HHA. The unexpectedly great thickness of the MF reflects onset of major, differential subsidence and provides evidence that the Laramide structural axis of the Wind River Basin originated several million years earlier than previously considered.

Reservoir characterization of the Mississippian Madison Formation, Wind River basin, Wyoming

Significant heterogeneity in petrophysical properties, including variations in porosity and permeability, are well documented from carbonate systems. These variations in physical properties are typically influenced by original facies heterogeneity, the early diagenetic environment, and later stage diagenetic overprint. The heterogeneities in the Mississippian Madison Formation in the Wind River basin of Wyoming are a complex interplay between these three factors whereby differences from the facies arrangement are first reduced by pervasive dolomitization, but late-stage hydrothermal diagenesis introduces additional heterogeneity. The dolomitized portions of the Madison Formation form highly productive gas reservoirs at Madden Deep field with typical initial production rates in excess of 50 MMCFGD. In the study area, the Madison Formation is composed of four third-order depositional sequences that contain several small-scale, higher frequency cycles. The cycles and sequences display a facies partitioning with mudstone to wackestone units in the transgressive portion and skeletal and oolitic packstone and grainstone in the regressive portions. The grainstone packages are amalgamated tidally influenced skeletal and oolitic shoals that cover the entire study area. The basal three sequences are completely dolomitized, whereas the fourth sequence is limestone. The distribution of petrophysical properties in the system is influenced only in a limited manner by the smaller scale stratigraphic architecture. Porosity and permeability are controlled by the sequence-scale stratigraphic units, where uniform facies belts and pervasive dolomitization result in flow units that are basically tied to third-order depositional sequences with a thickness of 65–100 ft (20–30 m). The best reservoir rocks are found in regressive, coarse-grained dolomites of the lower two sequences. Although the amalgamated shoal facies is heterogeneous, dolomitization decompartmentalized these cycles. Fine-grained sediments in the basal transgressive parts of these sequences, along with caliche and chert layers on top of the underlying sequences, are responsible for a decrease of porosity toward the sequence boundaries and potential flow separation. Good reservoir quality is also found in the third sequence, which is composed of dolomitized carbonate mud. However, reservoir-quality predictions in these dolomudstones are complicated by several phases of brecciation. The most influential of these brecciations is hydrothermal in origin and partly shattered the entire unit. The breccia is healed by calcite that isolates individual dolomite clasts. As a result, the permeability decreases in zones of brecciation. The late-stage calcite cementation related to the hydrothermal activity is the most important factor to create reservoir heterogeneity in the uniform third sequence, but it is also influential in the grainstone units of the first two sequences. In these sequences, the calcifying fluids invade the dolomite and partly occlude the interparticle porosity and decrease permeability to create heterogeneity in a rock in which the pervasive dolomitization previously reduced much of the influence of facies heterogeneity. Hildegard Westphal studied geology in Tubingen, Brisbane, and Kiel, where she received her Ph.D. in 1997. After a postdoctoral position at Rosenstiel School for Marine and Atmospheric Sciences, University of Miami, she became an assistant professor at Hannover University. Currently, she is a member of the Paleontology Department at Erlangen University. Her work focuses on early diagenesis of carbonates; the genesis, diagenesis, and paleoenvironmental record of limestone-marl alternations; and paleoecological interpretation of carbonate platforms.Gregor Eberli received his Ph.D.s from the Swiss Institute of Technology (ETH), Zurich, Switzerland, in 1985 and the University of Miami in 1991. With his colleagues at the Comparative Sedimentology Laboratory, he conducts research in sedimentology, stratigraphy, geochemistry, and petrophysics of modern and ancient carbonates. In several projects, he investigated the influence of sea level changes on sedimentary architecture. He was an AAPG Distinguished Lecturer in 1996–1997 and a Joint Oceanographic Institutions/U.S. Science Advisory Committee Distinguished Lecturer in 1998–1999. Langhorne Smith currently heads the Reservoir Characterization Group at the New York State Museum. He holds a B.S. degree from Temple University, a Ph.D. from Virginia Tech, and did postdoctoral work at the University of Miami. He also worked for Chevron as a development geologist for two years. His current research interests are in carbonate reservoir characterization and hydrothermal alteration of carbonate reservoirs. G. Michael Grammer is an associate professor at Western Michigan University. His research includes high-resolution carbonate sequence stratigraphy and early diagenesis and their application to reservoir characterization. He was an AAPG Distinguished Lecturer for 2002–2003 and is a coleader of AAPG's modern carbonate field course. Previously, he was a senior research associate for Texaco and has consulted for domestic and international oil companies. He received his Ph.D. from the University of Miami in 1991. Jason Kislak received his bachelor's degree from Franklin & Marshall College in Lancaster, Pennsylvania. He is currently working toward his master's degree at the Rosenstiel School of Marine and Atmospheric Science at the University of Miami.

Reservoir characterization of the Mississippian Madison Formation, Wind River basin, Wyoming

Reservoir characterization of the Mississippian Madison Formation, Wind River basin, Wyoming

Dating fluvial terraces by 230Th/U on pedogenic carbonate, Wind River Basin, Wyoming

Reliable and precise ages of Quaternary pedogenic carbonate can be obtained with 230Th/U dating by thermal ionization mass spectrometry applied to carefully selected milligram-size samples. Datable carbonate can form within a few thousand years of surface stabilization allowing ages of Quaternary deposits and surfaces to be closely estimated. Pedogenic carbonate clast-rinds from gravels of glacio-fluvial terraces in the Wind River Basin have median concentrations of 14 ppm U and 0.07 ppm 232Th, with median ( 230Th/ 232Th) = 270, making them well suited for 230Th/U dating. Horizons as thin as 0.5 mm were sampled from polished slabs to reduce averaging of long (≥10 5 yr), and sometimes visibly discontinuous, depositional histories. Dense, translucent samples with finite 230Th/U ages preserve within-rind stratigraphic order in all cases. Ages for terraces WR4 (167,000 ± 6,400 yr) and WR2 (55,000 ± 8600 yr) indicate a mean incision rate of 0.26 ± 0.05 m per thousand years for the Wind River over the past glacial cycle, slower than inferred from cosmogenic-nuclide dating. Terrace WR3, which formed penecontemporaneously with the final maximum glacial advance of the penultimate Rocky Mountain (Bull Lake) glaciation, has an age of 150,000 ± 8300 yr indicating that it is broadly synchronous with the penultimate global ice volume maximum.

GeoHostel: Geology of the Southern Wind River Range and Wind River Basin, Wyoming

GeoHostel: Geology of the Southern Wind River Range and Wind River Basin, Wyoming

Notharctine primates (Adapiformes) from the early to middle Eocene (Wasatchian-Bridgerian) of Wyoming: transitional species and the origins of Notharctus and Smilodectes.

Notharctine adapiform primates are an abundant element of early (Wasatchian) and middle (Bridgerian) Eocene faunal assemblages from the western interior of North America. Early Eocene notharctine samples are dominated by Cantius with Pelycodus and Copelemur being much rarer and more restricted in their geographic distribution. Cantius is replaced in the middle Eocene by Notharctus and Smilodectes, both of which are common but less widespread, being best known from southwestern Wyoming. The origin of these two middle Eocene taxa has not been well understood, due to a lack of transitional Wasatchian-Bridgerian notharctine faunal samples or because known samples had not been adequately studied. Field work at South Pass in the Greater Green River Basin has produced a relatively large sample of earliest Bridgerian notharctines. Combining this sample with a large, but previously under-studied, sample of notharctines from the latest Wasatchian and earliest Bridgerian in the Wind River Basin has clarified the relationships among Notharctus,Smilodectes, and earlier occurring notharctines. Notharctus first appears in the latest Wasatchian (Wa7), represented by N. venticolus. Phylogenetic analysis supports a Notharctus clade that shares sister taxon status with Cantius nunienus and indicates that Notharctus arose through bifurcation of the lineage containing the last common ancestor of C. nunienus and Notharctus. The origins of Smilodectes are less clear. Phylogenetic analysis supports a clade consisting of Smilodectes and Copelemur, but the origins of both taxa are not established as yet. North American notharctines are typified by relatively low taxonomic diversity, but relatively high abundance and high dental morphological variation (disparity). These attributes are opposite to those of North American omomyids, reflecting differences in ecomorphospace between these two primate radiations.

Numerical modeling of fluvial strath-terrace formation in response to oscillating climate

AbstractMany river systems in western North America retain a fluvial strath-terrace rec ord of discontinuous downcutting into bedrock through the Quaternary. Their importance lies in their use to interpret climatic events in the headwaters and to determine long-term incision rates. Terrace formation has been ascribed to changes in sediment supply and/or water discharge produced by late Quaternary climatic fluctuations. We use a one-dimensional channel- evolution model to explore whether temporal variations in sediment and water discharge can generate terrace sequences. The model includes sediment transport, vertical bedrock erosion limited by alluvial cover, and lateral valley-wall erosion. We set limits on our modeling by using data collected from the terraced Wind River basin. Two types of experiments were performed: constant- period sinusoidal input histories and variable-period inputs scaled by the marine δ18O rec ord. Our simulations indicate that strath-terrace formation requires input variability that produces a changing ratio of vertical to lateral erosion rates. Straths are cut when the channel floor is protected from erosion by sediment and are abandoned—and terraces formed—when incision can resume following sediment-cover thinning. High sediment supply promotes wide valley floors that are abandoned as sediment supply decreases. In contrast, wide valleys are promoted by low effective water discharge and are abandoned as discharge increases. Widening of the valley floors that become terraces occurs over many thousands of years. The transition from valley widening to downcutting and terrace creation occurs in response to subtle input changes affecting local divergence of sediment-transport capacity. Formation of terraces lags by several thousand years the input changes that cause their formation.Our results suggest that use of terrace ages to set limits on the timing of a specific event must be done with the knowledge that the system can take thousands of years to respond to a perturbation. The incision rate calculated in the field from the lowest terrace in these systems will likely be higher than the rate calculated by using older terraces, because the most recent fluvial response in the field is commonly downcutting associated with declining sediment input since the Last Glacial Maximum. This apparent increase in incision rates is observed in many river systems and should not necessarily be interpreted as a response to an increase in rock-uplift rate.

Molecular distributions and geochemical implications of pyrrolic nitrogen compounds in the Permian Phosphoria Formation derived oils of Wyoming

Crude oils from the Laramide structures of Wyoming were studied with respect to their geochemical compositions. The sampling areas include the Greater Green River, Wind River and Big Horn basins, and the Casper Arch region. Based on pristane/phytane ratio and various hopane and sterane parameters, the Permian Phosphoria Formation derived oils can be readily differentiated from oils with different origins. Within the Phosphoria Formation derived oils, three subgroups can be identified using Ts/(Ts+Tm) and diasterane/regular sterane ratios, corresponding to sources with subtle variation in organic facies and/or thermal maturity of the Phosphoria Formation. Differences in source organic input, depositional environments, and thermal maturity were observed to greatly influence the saturated hydrocarbon compositions of the Permian Phosphoria Formation derived oils. However, the distributions of pyrrolic nitrogen compounds in these oils do not appear as diagnostic as the conventional hydrocarbon parameters commonly used as indicators of these geological factors. This fact may be related to the more significant role of oil migration in the modification of pyrrolic nitrogen compound distributions in foreland basins as compared to that in rift basins. The recognition of possible source and maturity effects on pyrrolic nitrogen compounds suggests that all of these factors should be taken into proper consideration before the pyrrolic nitrogen compounds are used as indicators for any specific geological process.

Morphology of the Casper Mountain uplift and related, subsidiary structures, central Wyoming: Implications for Laramide kinematics, dynamics, and crustal inheritance

The general east-west trend of the regional-scale, fault-related Cas per Mountain uplift in central Wyoming reflects a preexisting Pre cambrian fabric along which there was Laramide compressional re activation. Initial fault displacement on the south-dipping Casper Mountain fault zone probably predates displacement on the inter secting, northwest-trending, northeast-dipping Casper arch thrust that forms the northeastern border of the Wind River basin. Later phase, incremental, Laramide displacements occurred along both fault zones: left-oblique slip on the Casper Mountain fault zone, dip slip on the crosscutting Casper arch thrust. Basement-involved thrust generation of the subsidiary Laramide, northwest-trending Iron Creek and Emigrant Gap anticlines along the north (footwall) side of the Casper Mountain fault zone also occurred during this later phase Laramide deformation. Perturbation of far-field, northeast-southwest, Laramide max imum horizontal paleostress trajectories (s1) to local, nearly fault-normal orientation at Casper Mountain, together with strain par titioning, is proposed to explain the apparent coeval development of divergent hanging-wall and footwall, basement-involved, fault-related structures. This kinematic and dynamic interpretation may be applied conditionally to other east-west-trending Laramide up lifts in the central Rocky Mountain foreland province.

An integrated 3-D reservoir characterization at Riverton Dome Field, Wyoming

Riverton Dome is a prime example of a mature Rocky Mountain oil and gas field that has benefited from new drilling based on a recent 3-D seismic survey. The field was discovered in 1949 through surface geology and seismic (Hinton, 1957; Borgerding, 1989) and was acquired by Snyder Oil Corporation (SOCO) in 1993. In 1998 SOCO acquired a 38-mi2 3-D seismic survey over Riverton Dome Field with funding from Belco Energy, Tom Brown Incorporated, and the U.S. Department of Energy to determine reservoir distribution, structural configuration, and compartmentalization. The survey was acquired with fractionated bins to image the steep dip on the flanks of the structure as a cost-effective alternative to using a smaller bin size. It was processed independently by two vendors. Results were very similar, providing confidence in the high quality of the data. Geologic maps were integrated with various seismic attributes to define reservoir models and to project reservoir trends beyond the current field. As a result of the Riverton Dome 3-D study, three successful wells were drilled in the Muddy Sandstone and additional opportunities in multiple Upper Cretaceous reservoirs and the Tensleep Sandstone were identified. Riverton Dome, a structurally controlled, mature oil and gas field is on the Wind River Indian Reservation, approximately six miles southeast of the town of Riverton in the southwestern portion of Wyoming's Wind River Basin (Figure 1). A series of thrust-faulted anticlines strikes NNW-SSE, parallel to the large Wind River Mountain thrust to the west. These features formed in the Late Cretaceous-Paleocene during the early stages of the Laramide orogeny (Keefer, 1965, 1970; Gries, 1983). Both Riverton Dome Field and Beaver Creek Field to the south are on the crest of a north-plunging asymmetrical anticline bounded on the west by a steeply dipping thrust fault that originates in basement. The …

Effects of Syndepositional Faulting and Folding on Early Cretaceous Rivers and Alluvial Architecture (Lakota and Cloverly Formations, Wyoming, U.S.A.)

ABSTRACT Quantitatively reconstructed paleochannel hydraulics (e.g., channel discharges, slopes, velocities), geometries (e.g., widths, depths, sinuosities, channel patterns), and alluvial architecture record the local effects of syndepositional faults and folds. These effects are evaluated from Lower Cretaceous rocks in three areas: (1) the northern Black Hills, where the axes of syndepositional folds trend roughly perpendicular to paleoflow, (2) the northwestern Black Hills, where the trend of a syndepositional fault is oblique to paleoflow, and (3) the Wind River basin, where the trends of syndepositional faults are nearly parallel to paleoflow. In each area, channel-belt deposits are laterally and vertically connected where local subsidence rates were high, producing thick, laterally extensive sandstones. Areas of lower subsidence rates are characterized by isolated or laterally connected channel-belt deposits, and thinner sandstones. Surface deformation controlled the positions of some rivers because of the development of antecedent drainages, or by directing rivers toward areas of maximum subsidence. Some reconstructed channel slopes, which range from 0.62 10-4 to 5.43 10-4, record surface deformation. The three-dimensional alluvial architecture was simulated using the model of Mackey and Bridge (1995), together with quantitatively reconstructed paleochannel parameters, in order to provide quantitative insight as to possible conditions and processes that produced the observed alluvial architecture. The effects of deformation on channel slopes, together with depositional topography, were important in controlling avulsion and resultant alluvial architecture. The most important condition that governed the alluvial architecture was comparable rates of local subsidence and sediment accumulation. Paleochannel reconstructions indicate that the rivers were 48-180 m wide and 4.4-13.6 m deep, and had discharges of 64-1073 m3s-1. Channel deposits all appear to represent point bars and associated channel fills of meandering rivers. Sinuosities were comparable, ranging from 1.1 to 1.4, and show no consistent trends with other channel parameters. Hence, with the exception of two paleochannels (out of the thirteen that were reconstructed) differences in hydraulic parameters such as slope and sediment transport rate had no discernible effect on channel pattern or sinuosity.

Cave Gulch 3-D Survey, Wind River Basin, Wyoming: A major gas discovery developed using poststack depth migration

Cave Gulch 3-D Survey, Wind River Basin, Wyoming: A major gas discovery developed using poststack depth migration

The seismic evaluation of a naturally fractured tight gas sand reservoir in the Wind River Basin, Wyoming

Sub-vertical natural fracturing is critical for ensuring economic gas production from tight sand reservoirs in many provinces. Over the last seven years, the US Department of Energy (DoE) has sponsored a series of programmes in the continental United States to develop cost-effective technologies for investigating naturally fractured reservoirs. The results from a study of a Laramide-age faulted anticline reservoir in the Lower Fort Union Formation of the Wind River Basin, Wyoming are presented. At the field site a multi-azimuth, multi-offset 3D compressional wave seismic survey was acquired and processed using azimuth-dependent processing followed by multi-azimuth attribute analysis. Geophysical attributes such as velocity, frequency, reflectivity and amplitude variation with offset, together with geological attributes such as depth below closure and distance from faults, were correlated to estimated ultimate recovery of gas for established wells within the field. The correlations were made using linear statistical methods, non-linear rank methods and neural networks to combine attribute results. The geophysical attributes and other geological information were combined into prospectivity maps for areas of highest fracture density. Based on the geophysical and geological results an 80% success rate was achieved at prospecting well locations. A major aspect of the DoE programmes has been the investigation of costeffective protocols for fracture detection. From this study the following procedure is recommended for cost-effective mapping of fractures and associated gas: (1) perform field and remote-sensing reconnaissance of structural trends; (2) acquire 3D P-wave surveys with offsets equal to or greater than target depth in all azimuths; (3) process in at least two dominant structural azimuths; (4) combine best correlated geophysical and geological attributes to fractures for future fracture zone mapping. The implications of the results from this survey have important consequences for the design and implementation of multi-azimuth, multi-offset data acquisition on land and for marine surveys where multi-component ocean bottom cable surveying is used.