Pierre Shale Research Articles

A pteranodontid wing with traces of extensive invertebrate scavenging from the Pierre Shale (Sharon Springs Member, Campanian–Maastrichtian), east central Wyoming (Niobrara County)

Here we report the associated and semi-articulated partial wing of a mid-sized pterosaur from an exposure of the Pierre Shale, in east central Wyoming. All elements suffer from compaction but are identified as those of a pteranodontid based on the warped deltopectoral crest. Most probably, the wing is that of Pteranodon longiceps, although the length of the ulna and of the metacarpal IV relative to that of the humerus slightly exceeds that reported from any previously documented Pteranodon specimen. The surfaces of all wing elements are littered with circular indentations, ranging from 1.1 to 6.8 mm in diameter. These indentations formed postmortem and appear to be borings left by invertebrate scavengers.

A long-snouted and long-necked polycotylid plesiosaur from the Late Cretaceous of North America

SummaryPlesiosaurs are a group of Mesozoic marine diapsids. Most derived plesiosaurs fall into one of two typical body forms: those with proportionately small heads, short snouts, and elongated necks, and those with large heads, elongated snouts, and short necks. Serpentisuchops pfisterae is a polycotylid plesiosaur from the Upper Cretaceous Pierre Shale that presents the trait combination of both an elongate snout and elongate neck (consisting of 32 vertebrae). Phylogenetic analysis places Serpentisuchops within the Polycotylinae, indicating that its long snout is an ancestral trait, while its long neck is secondarily derived and convergent with that of ancestral plesiosaurs, contemporaneous elasmosaurids, and some more basal members of the Polycotylidae. The conical, recurved, and narrow teeth are consistent with a piscivorous diet. The tall and anteroposteriorly broad cervical neural spines indicate large epaxial muscles, suggesting that Serpentisuchops used both its neck and snout in fast lateral strikes aimed at proportionately small prey.

Effects of chemical osmosis on swelling clays and the elastic properties of the Pierre shale with its implications for oil recovery

The main objective in this study is to experimentally investigate how two types of shales different in swelling characteristic and organic carbon content behave in terms of the mechanical and infiltration properties when being subjected to changing osmosis with varying brine salinity under in-situ stress conditions. This research also aims to determine the swelling tendency of thePierre shale and replicate the effect of selection of brine salinity on oil recovery in liquid-rich shales. A series of coupled measurements from the triaxial experiments at reservoir conditions was conducted on Pierre shale core samples to determine rock properties and behavior under fluid pore pressure intrusion. The main use of the measured properties is to demonstrate the impact of fluid transport induced by low salinity water injection accompanied by osmosis, clay swelling, and rock elasticity change on oil recovery from rock matrix. The Young's modulus, bulk modulus, and Poisson's ratio of the Pierre shale were determined from measurements on the two types of Pierre shale samples obtained from an outcrop having high 65 wt% of smectite content and well core having lean clay content and 3.5 wt% TOC (total organic carbon). The first two experiments were performed on the outcrop samples comprising of similar characteristics of 65 wt% smectite with negligible TOC while the last experiment was performed on the well core sample containing negligible smectite content with 3.5 wt% TOC. The membrane efficiencies of the shale samples were experimentally determined to range from 5% to 28% depending on clay content and stress. The outcrop shale showed 1% of swelling that is significantly higher than 0.07% of swelling developed in the well core shale lean in clay content. One of the most important findings of this research is that the membrane coefficient representing the stress sensitivity of membrane efficiency of organic-rich well core shale was found to be 11.5%/kpsi which is significantly higher than that of the swelling shale ranging from 2.2 to 3.5%/kpsi. This is interesting because the approximately 5.5 times stiffer matrix of organic-rich shale shows the significantly higher stress sensitivity in terms of the membrane efficiency. The Young's modulus of swelling shale declined with water saturation as well as the water transfer driven by osmotic pressure with low salinity brine. In contrast, the well core shale showed significant increase in the dynamic Young's modulus. The experimental data were used to evaluate the mass exchange between the fractures and rock matrix using a coupled fluid flow and geomechanics model accounting for shale swelling and osmosis transport to evaluate how fluid and rock interaction affect the oil recovery from the rock matrix of the Pierre shale during low salinity water injection (LSWI). A 25% increase in oil recovery factor was determined after considering the effects of changing membrane efficiency due to clay swelling resulting from LSWI. The modeling results therefore suggest that the variation of membrane efficiency due to swelling can be an important factor affecting the oil recovery from matrix block during LSWI enhanced oil recovery.

Significance of U-Pb detrital zircon geochronology for mudstone provenance

AbstractDetrital zircon U-Pb studies of mudstone provenance are rare but may preferentially fingerprint distal zircon sources. To examine this issue, Pierre Shale and Trinidad Sandstone deposited in a Late Cretaceous deltaic environment in the Raton Basin, Colorado (USA), were measured for detrital zircon U-Pb age by laser ablation–inductively coupled plasma–mass spectrometry. Two major detrital zircon age peaks at ca. 70 and 1690 Ma are found in both Pierre Shale and Trinidad Sandstone but in inversely varying proportions: 68% and 16%, respectively, for the finest zircon fraction (~15–35 μm) in the shale, and 25% and 32%, respectively, for the coarsest zircon fraction (~60–80 μm) in the sandstone. Proximal sources in the Sangre de Cristo Mountains, directly west of the Raton Basin, contain coarse-grained, ca. 1690 Ma zircon, whereas distal sources in Laramide uplifts and basins in Colorado, New Mexico, and Arizona contain fine-grained, ca. 70 Ma zircon. The results indicate that U-Pb zircon provenance of mudstone reflects availability of volcanic and other fine-grained source rocks rather than simply distal sources. U-Pb zircon provenance studies should routinely include mudstone units because these units may identify fine-grained zircon sources more reliably than sandstones alone.

An Unsplit CFS-PML Scheme for the Second-Order Wave Equation With Its Application in Fractional Viscoacoustic Simulation

The unsplit complex frequency-shifted perfectly matched layer (CFS-PML) has been widely used in the first-order wave equation in velocity and stress while rarely formulated for the wave equation recast as a second-order system in displacement. Among different variants of PML, the unsplit CFS-PML for the second-order wave equation enjoys better absorbing performance and numerical stability, compared to the traditional PML, due to the presence of the general form of CFS stretching factors, as well as higher computational efficiency over the split PMLs since it avoids wave equation order reduction and splitting the state variables into multiple directional components. This study aims to develop an unsplit CFS-PML scheme for the second-order wave equation and devote specific attention to fractional viscoacoustic simulation where fractional time derivatives are involved and hard to be reformulated into a first-order form. In the complex space, PML is typically regarded as an analytical continuation of the real coordinates; thus, we define an explicit coordinate stretching operator acting on the Laplacian operator. This stretching operator consists of several convolution terms; each of them can be efficiently resolved by a recursive convolution updating strategy. Viscoacoustic simulations on homogeneous Pierre Shale, Marmousi model, and 3-D SEG/EAGE overthrust model verify the feasibility and absorbing the performance of our proposed scheme.

Methane seeps as refugia during ash falls in the Late Cretaceous Western Interior Seaway of North America

Abstract Methane seeps host rich biotic communities, forming patchy yet highly productive ecosystems across the global ocean. Persistent hydrocarbon emissions fuel chemosynthetic food webs at seeps. Methane seeps were abundant in the Western Interior Seaway of North America during the Late Cretaceous. This area also experienced intermittent ash falls, which negatively impacted the marine fauna. We propose that methane seeps acted as refugia during these environmental perturbations. We report a laterally continuous bentonite within the upper Campanian Baculites compressus Zone of the Pierre Shale in southwestern South Dakota (USA) that fortuitously cuts across a methane seep deposit. We compare the macroinvertebrate record below and above the bentonite at seep and non-seep sites. Our results reveal that the paleocommunity (measured by abundance and diversity) was largely unaffected by the ash fall at the seep site, whereas it was significantly altered at the non-seep site. Thus, methane seeps in the Western Interior Seaway may have provided refuges or served as oases in the aftermath of severe environmental perturbations.

Effect of Water–Rock Interaction on the Axial Capacity of Drilled Caissons Socketed in Claystone Bedrock

AbstractA large number of bridges and structures in Colorado are supported by drilled caissons and shafts socketed in weak clay-bearing rocks (e.g., Denver blue claystone and Pierre shale). During ...

On the low-frequency elastic response of Pierre Shale during temperature cycles

SUMMARYTemperature affects elastic rock properties but is less-studied and thus less-understood than pressure and stress. Thermal effects on dispersion are experimentally observed herein from seismic to ultrasonic frequencies: Young’s moduli and Poisson’s ratios plus P- and S-wave velocities are determined by forced oscillation (FO) from 1 to 144 Hz and by pulse transmission at 500 kHz. Despite being the dominant sedimentary rock type, shales receive less experimental attention than sandstones and carbonates. To our knowledge, no other FO studies on shale at above ambient temperatures exist. Temperature fluctuations are enforced by two temperature cycles from 20 via 40 to 60 °C and vice versa. Measured rock properties are initially irreversible but become reversible with increasing number of heating and cooling segments. Rock property sensitivity to temperature is likewise reduced. It is revealed that dispersion shifts towards higher frequencies with increasing temperature (reversible if decreased), Young’s moduli and P-wave velocity maxima occur at 40 °C for frequencies below 56 Hz, and S-wave velocities remain unchanged with temperature (if the first heating segment is neglected) at seismic frequencies. In comparison, ultrasonic P- and S-wave velocities are found to decrease with increasing temperatures. Behavioural differences between seismic and ultrasonic properties are attributed to decreasing fluid viscosity with temperature. We hypothesize that our ultrasonic recordings coincide with the transition-phase separating the low- and high-frequency regimes while our seismic recordings are within the low-frequency regime.

On the stability of shale: the role of zeta potential (ζ) and Debye Hückel length (κ−1) on shale swelling

This work utilized electrochemical potential, linear swelling and water and ions uptake tests to correlate zeta potential (ζ) measurements with swelling results when different shales interacted with NaCl, KCl and CaCl2 solutions of different concentrations (water activities). Experimental results show that shale swelling and shrinkage seems to correlate well with changes in zeta potential and Debye Hückel length (κ −1) of shale’s charged surface. I believe that reduction of zeta potential and κ −1 leads to shale shrinkage while increase in zeta potential and κ −1 translates into shale swelling. Results also show that zeta potential and κ −1 of Pierre and Arco shale depends strongly on water activity (concentration) difference between shale and aqueous solutions. It was found that the formation and break up of an ionic shield cloud determines swelling and shrinkage of shale depending on the direction of water and ions transport. Results also suggest that, for constant temperature and medium viscosity (ƞ), the term (z/r) determines the effectiveness of ions to screen particle surface charge and it represents two opposing forces (electrostatic force and viscous force). The (z/r) for NaCl equals 0.28 m−1 which is lower than that of CaCl2 (z/r = 0.46 m−1 ) and KCl (z/r = 0.303 m−1 ).

Codell carrier-bed play, Denver Basin

Carrier-bed plays are an emerging type of unconventional oil play in which reservoirs are generally of low quality because they are characterized by: 1) thinly bedded heterolithic strata; 2) significant compaction and/or diagenesis; and 3) burrowing that has mixed sandstones and mudstone lithologies (i.e., heterogeneous lithologies). In this type of play, the carrier beds are pervasively hydrocarbon saturated and can be areally extensive (>50 mi2 or 130 km2). These low-quality reservoirs generally do not meet traditional petrophysical cutoffs and because of their high clay contents can have low resistivity, low contrast pays. The reservoirs may be composed of siliciclastics or carbonates or both. Due to reservoir quality and degree of oil migration, carrier-bed plays like the Codell are being developed with horizontal drilling and multistage hydraulic fracturing. Traditional vertical drilling yields marginal to uneconomic wells that can provide a clue to the existence of a carrier-bed play. The Codell Sandstone is a low-resistivity, low-contrast pay in parts of the northern Denver Basin. The area of oil and gas production is in the deeper part of the basin between and including Silo and Wattenberg fields of Wyoming and Colorado, respectively. The thickness of the Codell in this part of the Denver Basin ranges from 15 to 25 ft (4.5 to 7.6 m). Keys to Codell production are source rock maturity, and oil entrapment in the carrier bed. Oil in the Codell carrier-bed traps was generated in various intervals including the Niobrara (mainly the “B” marl), Sharon Springs Member of the Pierre Shale, Greenhorn/Carlile, and, rarely, the Mowry Shale.

Orientational mapping of minerals in Pierre shale using X-ray diffraction tensor tomography.

Shales have a complex mineralogy with structural features spanning several length scales, making them notoriously difficult to fully understand. Conventional attenuation-based X-ray computed tomography (CT) measures density differences, which, owing to the heterogeneity and sub-resolution features in shales, makes reliable interpretation of shale images a challenging task. CT based on X-ray diffraction (XRD-CT), rather than intensity attenuation, is becoming a well established technique for non-destructive 3D imaging, and is especially suited for heterogeneous and hierarchical materials. XRD patterns contain information about the mineral crystal structure, and crucially also crystallite orientation. Here, we report on the use of orientational imaging using XRD-CT to study crystallite-orientation distributions in a sample of Pierre shale. Diffraction-contrast CT data for a shale sample measured with its bedding-plane normal aligned parallel to a single tomographic axis perpendicular to the incoming X-ray beam are discussed, and the spatial density and orientation distribution of clay minerals in the sample are described. Finally, the scattering properties of highly attenuating inclusions in the shale bulk are studied, which are identified to contain pyrite and clinochlore. A path forward is then outlined for systematically improving the structural description of shales.

Temperatures of Late Cretaceous (Campanian) methane-derived authigenic carbonates from the Western Interior Seaway, South Dakota, USA, using clumped isotopes

Abstract Methane seep deposits, comprising large, carbonate-rich mounds formed from hydrocarbon seepage, were widely distributed in the Late Cretaceous Western Interior Seaway (WIS) of North America. Well-preserved, methane-derived authigenic carbonates (MDACs) from these deposits have been shown to retain petrological, paleontological, and geochemical imprints of their ancient depositional setting, all of which are important for understanding the dynamics and evolution of the shallow, epeiric WIS. To better characterize the environmental conditions of WIS seeps, we applied clumped isotope paleothermometry to magnesium calcite MDAC samples from five seep localities in the upper Campanian Pierre Shale, South Dakota, USA. We measured 21 subsamples, including 18 micritic carbonates and demonstrated apparent clumped isotope equilibrium between MDACs and WIS bottom waters. Extreme 13C depletion in most samples (δ13C ranging to −45.44‰) indicates they were precipitated with oxidized methane as a major source of dissolved inorganic carbon, which itself implies a close association with ancient methanotrophic metabolism. The average clumped isotope paleotemperature from the micritic carbonates is 23 ± 7 °C (1σ standard deviation), which agrees with bottom water paleotemperatures inferred from δ18O measurements of MDACs and well-preserved mollusk shells at similar localities in the WIS. The calculated average δ18Ow value for these samples is −0.5 ± 1.7‰ (1σ SD), which is indistinguishable from previously reported calculation on Campanian seawater δ18Ow from fossil mollusk shells, but elevated above younger fossils collected from other locations in the WIS. Our conclusions are inconsistent with previously hypothesized disequilibrium for WIS MDAC clumped isotope and therefore we propose that fossil MDAC deposits may be used as paleotemperature archives.

Permeability and porosity modelling for resedimented mudrocks – Applications for compaction dominated mudrock systems

Mudrocks have complex petrophysical properties due, in part, to their very fine grain size and the associated high specific surface area (m2/g) and cation exchange capacity (CEC, meq/g). The role of the cation exchange capacity and its associated bound water must be included to account for mudrock electrical, transport, and mechanical properties. A widely varying set of samples from the deep water Gulf of Mexico Miocene section, the arctic seafloor, the Pierre Shale, and Brazos River sand (used for varying sand content) were resedimented to study the effects of these variations on compaction and permeability. The mudrocks were characterized in terms of CEC, clay mineral content, bulk mineralogy, and particle size distribution. Resedimentation techniques were employed to measure the mudrock properties under controlled conditions i.e. slurries were made with known clay mineral type, bulk mineralogy, particle size distribution, CEC, and pore fluid salinity, and subsequently compacted in an oedometer cell.The measured permeabilities were found to be strongly dependent on the clay mineral CEC and on initial pore fluid salinity. Two models were developed to explain the variation in measured permeability: a modified Carman-Kozeny model and a staged differential effective medium model (SDEM). The Carman-Kozeny model fits the permeability data to within a factor of three but underestimates the stress dependence. The SDEM model includes the effects of both bound water saturation and porosity on fluid flow. The model was also able to estimate permeability for all the different mudrocks to within a factor of three with an improved match to the stress dependence. These two models were also successfully tested using previously published permeability data for both resedimented and intact Boston Blue clay samples.A compaction model for porosity was also developed and applied to published data for artificial mixtures of smectite and kaolinite. The model predicts the stress dependent porosity of the resedimented clays to within four percent error. It is then extended to incorporate the influence of the inclusion of sand sized material and applied to our resedimentation data and previously published Boston Blue Clay data. The required inputs for the model include the weight fraction of non-clay sized material, and the cation exchange capacity (CEC, meq/gram). No consistent salinity dependence was observed for compaction.

Baseline evaluation of legacy hydrochemical data for the Republican River watershed, southcentral Nebraska, USA

The objective of this study was to comprehensively evaluate and interpret hydrogeochemically the extensive legacy surface-water data for the expansive Republican River watershed in southcentral Nebraska. Historical (1968–1991) hydrochemistry data [major cations and anions, total salinity [specific conductance (SC)] and pH supplied by the Nebraska Department of Environmental Quality and the US Geological Survey were hydrogeochemically and statistically analyzed. Interpretation of this large legacy dataset (315 sample analyses at 12 Republican River sampling stations) revealed that the Republican River’s range in SC is relatively low (200–1020 µS/cm), with a river-wide median of 587 µS/cm (~ 380 mg/L). The range in SC for 10 tributaries (204 sample analyses) is 190–2910 µS/cm with a median value of 550 µS/cm (~ 360 mg/L). The Ca–HCO3 hydrochemical facies dominates throughout the watershed followed by the Na–HCO3 facies. Hydrochemical equilibrium modeling reveals that most samples occurred in an aqueous environment characterized by gypsum undersaturation but slight calcite and dolomite supersaturation; low supersaturation values comprise a line of evidence for the thermodynamic equilibrium or near equilibrium of the studied surface waters. Scatter plots of meq/L concentrations for selected anions and cations reveal the impact of silicate mineral (e.g., feldspars) weathering on the aqueous hydrochemistry throughout the watershed. These ubiquitous feldspar minerals most likely originated along the eastern slope of the Front Range during Late Cretaceous time (Laramide Orogeny) and were subsequently transported into the study area. The relatively high concentrations of sulfate and sodium in numerous samples collected from the Arikaree River probably originate in the underlying Pierre Shale, rich in S-bearing minerals.

Geochemistry and age of groundwater in the Williston Basin, USA: Assessing potential effects of shale-oil production on groundwater quality

Thirty water wells were sampled in 2018 to understand the geochemistry and age of groundwater in the Williston Basin and assess potential effects of shale-oil production from the Three Forks-Bakken petroleum system (TBPS) on groundwater quality. Two geochemical groups are identified using hierarchical cluster analysis. Group 1 represents the younger (median 4He = 21.49 × 10−8 cm3 STP/g), less chemically evolved water. Group 2 represents the older (median 4He = 1389 × 10−8 cm3 STP/g), more chemically evolved water. At least two samples from each group contain elevated Cl concentrations (>70 mg/L). Br/Cl, B/Cl, and Li/Cl ratios indicate multiple sources account for the elevated Cl concentrations: septic-system leachate/road deicing salt, lignite beds in the aquifers, Pierre Shale beneath the aquifers, and water associated with the TBPS (one sample). 3H and 14C data indicate that 10.8, 21.6, and 67.6% of the samples are modern (post-1952), mixed age, and premodern (pre-1953), respectively. Lumped-parameter modeling of 3H, SF6, 3He, and 14C concentrations indicates mean ages of the modern and premodern fractions range from ~1 to 30 years and 1300 to >30,000 years, respectively. Group 2 contains the highest CH4 concentrations (0.0018–32 mg/L). δ13C–CH4 and C1/C2+C3 data in groundwater (−91.7 to −70.0‰ and 1280 to 13,600) indicate groundwater CH4 is biogenic in origin and not from thermogenic shale gas. Four volatile organic compounds (VOCs) were detected in two samples. One mixed-age sample contains chloroform (0.25 μg/L) and dichloromethane (0.05 μg/L), which are probably associated with septic leachate. One premodern sample contains butane (0.082 μg/L) and n-pentane (0.032 μg/L), which are probably associated with thermogenic gas from a nearby oil well. The data indicate hydrocarbon production activities do not currently (2018) widely affect Cl, CH4, and VOC concentrations in groundwater. The predominance of premodern recharge in the aquifers indicates the groundwater moves relatively slowly, which could inhibit widespread chemical movement in groundwater overlying the TBPS. Comparison of groundwater-age data from five major unconventional hydrocarbon-production areas indicates aquifer zones used for water supply in the TBPS area have a lower risk of widespread chemical movement in groundwater than similar aquifer zones in the Fayetteville (Arkansas) and Marcellus (Pennsylvania) Shale production areas, but have a higher risk than similar aquifer zones in the Eagle Ford (Texas) and Haynesville (Texas, Louisiana) Shale production areas.

Can Heating Induce Borehole Closure?

The current study shows that heating a cased borehole in low-permeability shale rock can induce plastic deformation, leading to the closure of the casing annulus and decreasing annulus connectivity. The thermally induced borehole closure is interesting for the field operation of plug and abandonment (P&A), as it potentially saves operation cost and time by avoiding cutting casing and cementing. Lab experiments and numerical simulations are implemented to investigate the thermally induced borehole closure. Pierre shale and a field shale are tested. The lab experiments are performed by heating the borehole wall in a 10-cm-OD hollow cylinder specimen. Here, a novel experimental setup is applied, allowing for measuring temperature and pore pressure at different radii inside the specimen. Both the experimental data and the post-test CT images of the rock samples indicate the rock failure by borehole heating, and under certain conditions, heating results in an annulus closure. The decrease of hydraulic conductivity through the casing annulus is observed, but this decrease is not enough to form the hydraulic-sealed annulus barrier, based on the results obtained so far. Lab-scale finite-element simulations aim to match the lab results to obtain poro-elastoplastic parameters. Then the field-scale simulations assess the formation of shale barriers by heating in field scenarios. Overall, (i) the lab experiments show that heating a borehole can increase the pore pressure in shale and hence induce rock failure; (ii) the numerical simulations match the experimental results reasonably well and indicate that the heating-induced borehole closure can sufficiently seal the casing annulus in the field-scale simulation.

Large Scaphitid Ammonites (Hoploscaphites) from the Upper Cretaceous (Upper Campanian–Lower Maastrichtian) of North America: Endless Variation on a Single Theme

We describe three species of large scaphitid ammonites (Ammonoidea: Ancyloceratina) from the Upper Cretaceous (upper Campanian–lower Maastrichtian) of the Western Interior of North America. Each species occurs as two dimorphs, referred to as macroconch and microconch. All three species share a similar pattern of ornamentation consisting of long, thin, nonbifurcating ribs on the adoral part of the phragmocone, suggesting that they constitute a single monophyletic clade. Macroconchs of Hoploscaphites crassus (Coryell and Salmon, 1934) are characterized by a globose whorl section, with closely spaced ventrolateral tubercles on the body chamber, usually persisting to the aperture. Macroconchs of Hoploscaphites plenus (Meek and Hayden, 1860) differ from those of H. crassus in having a more subquadrate whorl section with flatter flanks, and fewer, larger, and more widely spaced ventrolateral tubercles. Macroconchs of Hoploscaphites peterseni, n. sp., closely resemble those of H. crassus, but differ in being nearly circular in side view with a more compressed whorl section. All three species lived at approximately the same time in the same general area and depositional environment. They are abundant in the Baculites baculus Zone but also occasionally occur in the B. eliasi Zone and possibly lower part of the B. grandis Zone. They are present in the Pierre Shale of east-central Montana and east-central Wyoming, the Lewis Shale of south-central Wyoming, and the Bearpaw Shale of northeast Montana. It is possible that these three species represent subspecies within a single species or a “flock” of very closely related species, similar to the “species flocks” observed in modern cichlid fishes.

Palaeoecological analysis of a methane seep deposit from the Upper Cretaceous (Maastrichtian) of the U.S. Western Interior

Methane seeps were a common feature in the Late Cretaceous Western Interior Seaway of the United States. We document the occurrence of methane seep deposits in the Pierre Shale on the Cedar Creek Anticline in east-central Montana for the first time. The seep deposits occur in the lowermost part of the Baculites baculus Zone (the Endocostea typica Zone), corresponding to the lowermost Maastrichtian. They are therefore the youngest seeps yet described from the Western Interior Seaway. We conducted a detailed faunal analysis of a single seep deposit, together with geochemical investigation of both seep carbonates and molluscan shell material to determine palaeoenvironmental conditions. Oxygen isotope analysis of well-preserved molluscan shell material reveals water temperatures of between 19 and 27°C, while depleted carbon isotope values of seep carbonates are indicative of the anaerobic oxidation of methane. The morphology of the seep deposit suggests a strong advective flux of methane to the sediment–water interface. Comparison to a nearby contemporaneous non-seep site reveals that similar groups of organisms occur in both settings, albeit with varying relative abundances – the seep is numerically dominated by the lucinid bivalve Nymphalucina occidentalis. Substrate appears to be the major control on the diversity and palaeoecological composition of both seep and non-seep sites.

Eastern Boulder-Weld fault zone, Colorado: A gravity slide with pop-up structures

The Boulder-Weld fault zone, located southeast of Boulder, Colorado, is about 10 km (6 mi) wide, 34 km (21 mi) long, and involves at least 335 m (1100 ft) of upper Cretaceous sedimentary rocks. It affects the Cretaceous upper Pierre Shale, Fox Hills Sandstone, and the coal–bearing lower Laramie Formation. This study is a detailed examination of the eastern portion of the fault zone which consists of undisturbed areas separated by three long, narrow, fault-bounded uplifts that have received a variety of interpretations over the years. The fault zone geometry is determined from 21 closely spaced cross sections that use more subsurface data than previous studies, incorporate the elevations of the major economic coal seam derived from a published composite structure-contour map, and are area balanced using area-depth-strain (ADS) analysis. The most common structural style is a pop-up structure in which the uplifts are bounded on both sides by reverse faults. At larger-displacement the pop-ups are at the tip of the ramp and a second fault has formed close to the base of the ramp. A few sections show simple ramp anticlines developed above listric thrusts. The lower detachment for all structures is the distinctive Kp2 marker in the upper Pierre Shale. ADS analysis of the best-controlled uplifts shows that the uplifts are area balanced and confirms the lower detachment to be near Kp2. The structures are interpreted to have formed as a gravity slide because they formed in a break-back sequence, a characteristic of gravity gliding, and because the transport direction is approximately down the current southeast dip of the Kp2 detachment.

Structure and genesis of the Boulder-Weld allochthon, Denver Basin, Colorado - Gravity slide or Laramide thrust sheet?

This study was undertaken to determine the structure and genesis of the Boulder-Weld allochthon (BWA), the 216 mi2 (559 km2) remnant of a once larger feature, that moved east from the flank of the Front Range into the western part of the Denver Basin. This review of surface and subsurface data revealed new aspects of the BWA, especially in its western part. There, the decollement of the BWA ramps 900 feet up-section to the east from a near bedding-parallel detachment low in the upper transition member of the Pierre Shale to a bedding-parallel detachment near the base of the Fox Hills Formation. Repeated sections found in wells east of the decollement ramp demonstrate up to two miles of translation in the system. Secondary faults in the hanging wall of the allochthon include antithetic thrusts bounding pop-up structures and occasional normal faults that almost exclusively overprint the decollement ramp. The hanging wall is also cut by a postulated tear fault separating areas exhibiting different amounts of translation. The western, trailing edge of the decollement shows attenuation in its hanging wall that increases to the west. This part of the decollement either represents a very low-angle breakaway normal fault or a thrust fault cutting slightly down-section in the direction of transport. Past studies perceived a southeast transport direction for the BWA in contrast to the northeast slip directions on nearby Laramide thrusts, a difference used to interpret the allochthon as a gravity slide. However, similar east-west oriented slickenlines on thrusts across the western part of the allochthon and into the neighboring Front Range leave open the possibility the BWA originated as a Laramide thrust sheet. Furthermore, both the BWA and Laramide thrusts in the neighboring Front Range utilized detachments near the top of the Pierre Shale, suggesting a possible common genesis. Given the available data, both the gravity slide and Laramide thrust models provide viable explanations for the BWA.