Pierre Shale Research Articles

On the Theory of Solitons of Fluid Pressure and Solute Density in Geologic Porous Media, with Applications to Shale, Clay and Sandstone

We here analyze a new model of transients of pore pressure p and solute density ρ in geologic porous media. This model is rooted in the nonlinear wave theory, its focus is on advection and effect of large pressure jumps on strain. It takes into account nonlinear and also time-dependent versions of the Hooke law about stress, rate and strain. The model solutions strictly relate p and ρ evolving under the effect of a strong external stress. As a result, the presence of quick and sharp transients in low permeability rocks is unveiled, i.e., the nonlinear “Burgers solitons”. We, therefore, show that the actual transport process in porous rocks for large signals is not only the linear diffusion, but also a solitons presence could control the process. A test of a presence of solitons is applied to Pierre shale, Bearpaw shale, Boom clay and Oznam-Mugu silt and clay. An application about the presence of solitons for nuclear waste disposal and salt water intrusions is also discussed. Finally, in a kind of “theoretical experiment” we show that solitons could also be present in higher permeability rocks (Jordan and St. Peter sandstones), thus supporting the idea of a possible occurrence of osmosis also in sandstones.

Nautilid nurseries: hatchlings and juveniles of Eutrephoceras dekayi from the lower Maastrichtian (Upper Cretaceous) Pierre Shale of east‐central Montana

The nautilid Eutrephoceras dekayi (Morton 1834) is relatively abundant in the lower Maastrichtian (Upper Cretaceous) Pierre Shale of east-central Montana. We analysed the morphology, size frequency distribution, and isotope composition of a large collection of 220 well-preserved specimens including hatchlings, juveniles and adults. The newly hatched shell is approximately 14 mm in diameter with a body chamber one-third whorl in angular length terminating in the nepionic constriction. Internally, the embryonic shell contains five septa. Juveniles are abundant and comprise two-thirds of the sample whereas sub-adults, defined by the incipient flattening of the venter, are rare. Adults comprise approximately one-third of the sample and average 100 mm in diameter. The co-occurrence of newly hatched shells, small juveniles and adults suggests that the eggs were laid in the same area in which the hatchlings developed. Based on the excellent preservation of the juveniles, we conclude that they did not float into the area after death, but lived in the region, implying that this area served as a nursery for young animals. The calculated temperatures of the embryonic shells are similar to those of the post-embryonic shells and generally range from 16 to 18 °C. Upon hatching, the nautilids probably followed a demersal mode of life and lived in well-oxygenated water ≤50 m deep. An examination of lethal injuries (puncture holes) suggests that all ontogenetic stages were equally vulnerable to predation. The proximity of the site to the Sheridan Delta suggests that the specimens were smothered by sudden pulses of sediment transported into the area by major storms.

Great Plains polygonal fault system as expressed in Saskatchewan: Late Cretaceous fault initiation and graben formation

An extensive polygonal fault system (PFS) has been recognized in fine-grained Late Cretaceous sediments of the Western Interior Seaway of North America. Polygonal fault systems are pervasive organizations of nontectonic faults with fault traces that coalesce to form distinctive polygonal fault patterns. Interpretation of a three-dimensional seismic dataset from southeast Saskatchewan provides insight into fault initiation, timing, and geometry for the Great Plains PFS (GPPFS). Faulting initiates in the Niobrara Formation, with the largest fault throws occurring over Early Cretaceous Viking Formation sandstone accumulations, suggesting that drape compaction over the channel sand initiated some of the faulting. Above this, faulting increases in vertical offset, and the predominant fault strike angles change in the Lea Park, Belly River, and Bearpaw formations (all homotaxial to the Pierre Shale) throughout Campanian time. By late Bearpaw time, the initially almost random fault strike orientations change to well-defined northwest–southeast- and west–east-striking grabens. These grabens have up to 20 m of throw and can be 125 m wide and 900 m long at ∼400 m current depth. Predominant graben faults are the continuation of some of the deeper PFS faults. Moreover, the grabens are present over a Campanian clinoform bed and may be interpreted to indicate Bearpaw time extension tectonics that is local or regional in scale. The PFS helps to explain near-surface faulting observed in Late Cretaceous sediments in the Western Interior Seaway and could be used as a model to help explain Late Cretaceous geology, subsurface groundwater flow, and shallow natural gas reservoir continuity.

ANATOMICALLY LINKED OCCURRENCE OF CONCRETIONARY PRESERVATION OF MOSASAURS FROM THE PIERRE SHALE OF SOUTH DAKOTA

Fossils of mosasaurs, late Cretaceous marine reptiles, are frequently found partially preserved in concretions (i.e., hard mineral masses), but little is known about this mode of fossilization. Concretionary fossils are difficult to extract, so are often ignored in the field and laboratory. Accounts suggest that skulls, the most taxonomically diagnostic element, are most often preserved in concretions. The presence of concretions was documented in four anatomical regions (head, torso, tail, limbs) in 48 mosasaur fossils from the Pierre Shale of South Dakota. Specimens were compiled from a restricted lithostratigraphic interval to minimize the effects of lithologic variation on concretion formation. In this sample, the skull and torso were preserved more often in concretions than were limbs and tails. The occurrence of concretion formation was found to vary depending on anatomical region. The results suggest that concretion growth begins in the core of the organism with a higher volume of organic matter, and successively precipitates towards the periphery. The skull and torso contain many taxonomically diagnostic features; thus concretionary specimens should not be overlooked. Additionally, increased likelihood of preservation of specific anatomic regions was dependent on concretion formation, suggesting that our view of the fossil record is affected by concretions.

Selenium Speciation in the Fountain Creek Watershed (Colorado, USA) Correlates with Water Hardness, Ca and Mg Levels.

The environmental levels of selenium (Se) are regulated and strictly enforced by the Environmental Protection Agency (EPA) because of the toxicity that Se can exert at high levels. However, speciation plays an important role in the overall toxicity of Se, and only when speciation analysis has been conducted will a detailed understanding of the system be possible. In the following, we carried out the speciation analysis of the creek waters in three of the main tributaries—Upper Fountain Creek, Monument Creek and Lower Fountain Creek—located in the Fountain Creek Watershed (Colorado, USA). There are statistically significant differences between the Se, Ca and Mg, levels in each of the tributaries and seasonal swings in Se, Ca and Mg levels have been observed. There are also statistically significant differences between the Se levels when grouped by Pierre Shale type. These factors are considered when determining the forms of Se present and analyzing their chemistry using the reported thermodynamic relationships considering Ca2+, Mg2+, SeO42−, SeO32− and carbonates. This analysis demonstrated that the correlation between Se and water hardness can be explained in terms of formation of soluble CaSeO4. The speciation analysis demonstrated that for the Fountain Creek waters, the Ca2+ ion may be mainly responsible for the observed correlation with the Se level. Considering that the Mg2+ level is also correlating linearly with the Se levels it is important to recognize that without Mg2+ the Ca2+ would be significantly reduced. The major role of Mg2+ is thus to raise the Ca2+ levels despite the equilibria with carbonate and other anions that would otherwise decrease Ca2+ levels.

Deformational behaviour of a clay-rich shale with variable water saturation under true triaxial stress conditions

This study has investigated the impact of true triaxial deformation and variable water saturation on the static mechanical properties of Pierre Shale. The impact of increasing the stress anisotropy (the difference between the intermediate principal stress, σ2, and the minimum principal stress, σ3) during axial loading was systematically investigated. Low to moderate stress anisotropy resulted in increasing strength and stiffness at any given level of σ3, which then peaked or decreased at the highest stress anisotropy. Systematic decreases in water saturation were also noted to increase strength and stiffness. The increased strength resulting from variations of both these parameters changed the macroscopic fracture patterns observed in the shales. Microscopically, however, dilatancy was suppressed by increasing the intermediate principal stress, often to levels in excess of 90% of peak strength. The experimental results were interpreted in the framework of Wiebols and Cook models as well as Mogi type models for true triaxial behaviour of rock materials and were found to have good fits to these models for all levels of water saturation.

Selenium speciation in the Fountain Creek Watershed and its effects on fish diversity.

Se is an environmental concern as it can be toxic if present in high concentrations even though it is a dietary requirement for all animals. Se levels are a special concern in the Fountain Creek Watershed located in southeastern Colorado whose geological source is the Se-rich Pierre Shale. Segments of Fountain Creek have Se water levels that exceed the current EPA limit of 5µg/l. In the studies described here, the effects of river water containing selenium were examined on fish populations at different sites along the Fountain Creek Watershed. Based on the hypothesis that high levels of Se present in the Creek and resident bryophytes should be an indicator of diversity in the river fish we explored the possibility that the low toxicity of the selenium could be due to speciation. A speciation analysis was conducted to determine the selenium(IV) and selenium(VI). Our results show that sites with higher ratios of the more toxic Se(IV) relative to total selenium exhibit lower fish diversity and numberof fish. Our results indicate that factors, other than total Se, such as Se speciation may be involved in controlling the bioavailability and toxicity of this element to aquatic organisms in Fountain Creek.

PALEOECOLOGY OF ECHINODERMS IN COLD SEEP ENVIRONMENTS REVEALED BY ISOTOPE ANALYSIS IN THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

A cold seep is an extreme environment characterized by a specialized group of organisms generally referred to as “chemosynthetic communities”. Until recently, echinoderms were thought to be rare in cold seep environments and had not been treated as a member of chemosynthetic communities, which otherwise are composed of a variety of taxa. One fossil echinoderm assemblage associated with a cold seep comes from the middle Campanian Pierre Shale Formation of the U.S. Western Interior Seaway. In this study, the taxonomy, morphology, and paleoecology of fossil echinoderms (crinoids and echinoids) from the Pierre Shale are discussed. Elemental chemical analyses of the fossil echinoderm skeletons and stable carbon isotopes were used to clarify the influence of seep hydrocarbons chemistry on the formation and diagenesis of echinoderm skeletons. We show that the crinoids Lakotacrinus brezinai from seep carbonates developed highly specialized morphologies and skeletons with low δ 13 C values, suggesting that it was adapted to cold seep environments and might be an obligate member of the chemosynthetic community. The echinoids from the Western Interior Seaway seep carbonates have morphologies and skeletal δ 13 C values not significantly different from those from non-seep environments, suggesting that the echinoids were not obligate members of the chemosynthetic community, but opportunists living in the periphery of the cold seep habitat.

Description of the lower jaws of Baculites from the Upper Cretaceous U.S. Western Interior

Abstract We report the discovery of lower jaws of Baculites (Ammonoidea) from the Upper Cretaceous U.S. Western Interior. In the lower Campanian Smoky Hill Chalk Member of the Niobrara Chalk of Kansas, most of the jaws occur as isolated elements. Based on their age, they probably belong to Baculites sp. (smooth). They conform to the description of rugaptychus, and are ornamented with coarse rugae on their ventral side. One specimen is preserved inside a small fecal pellet that was probably produced by a fish. Another specimen occurs inside in a crushed body chamber near the aperture and is probably in situ. Three small structures are present immediately behind the jaw and may represent the remains of the gills. In the lower Maastrichtian Pierre Shale of Wyoming, two specimens of Baculites grandis contain lower jaws inside their body chambers, and are probably in situ. In both specimens, the jaws are oriented at an acute angle to the long axis of the shell, with their anterior ends pointing toward the dorsum. One of the jaws is folded into a U-shape, which probably approximates the shape of the jaw during life. Based on the measurements of the jaws and the shape of the shell, the jaws could not have touched the sides of the shell even if they were splayed out, implying that they could not have effectively served as opercula. Instead, in combination with the upper jaws and radula, they constituted the buccal apparatus that collected and conveyed food to the esophagus.

A new homolid crab, Zygastrocarcinus tricki sp. nov., from the Pierre Shale (middle Campanian), Baculite Mesa, Pueblo County, Colorado, USA

Abstract A new species of homolid crab, Zygastrocarcinus tricki sp. nov., is reported from the Late Cretaceous (middle Campanian, Baculites scotti Zone) of the Pierre Shale Formation (Baculite Mesa, Pueblo County, Colorado). This nearly complete homolid, hereto described is the sixth species assigned to the genus and extends our knowledge along with the geographical range and geological age of this taxon.

Encrustation of inarticulate brachiopods on scaphitid ammonites and inoceramid bivalves from the Upper Cretaceous U. S. Western Interior

AbstractThe inarticulate brachiopodDisciniscais a rare faunal element in the Upper Cretaceous of the U.S. Western Interior. We report two occurrences of encrustation ofDisciniscaon a scaphitid ammonite (scaphite) and several inoceramids from the lower MaastrichtianBaculites baculus/Endocostea typicaBiozones of the Pierre Shale at two localities. Six specimens ofDisciniscaare present on a single specimen ofHoploscaphites crassusfrom east-central Montana. They occur along the furrow at the mature apertural margin. Because the brachiopods are restricted to the margin and do not occur on the rest of the shell, it is likely that they encrusted the ammonite during its lifetime. If so, this implies that the soft body of the scaphite did not cover the outside surface of the aperture, leaving this area vulnerable to epizoan attachment. A total of 13 specimens ofDisciniscaare also present on four specimens ofCataceramus?barabinifrom east-central Wyoming. The brachiopods occur in crevices on the outside of the shells and may have encrusted the inoceramids after their death as the shells began to break down and delaminate, resulting from the decomposition of the organic matrix holding them together. Based on the faunal assemblages at both localities, the presence ofDisciniscamay indicate environments with either low oxygen levels and/or few predators or competitors.

A new laboratory apparatus for the measurement of seismic dispersion under deviatoric stress conditions

ABSTRACTA better understanding of seismic dispersion and attenuation of acoustic waves in rocks is important for quantitative interpretation of seismic data, as well as for relating seismic data, sonic‐log data, and ultrasonic laboratory data. In the present work, a new laboratory setup is described, allowing for combined measurements of quasistatic deformations of rocks under triaxial stress, ultrasonic velocities, and dynamic elastic stiffness (Young's modulus and Poisson's ratio) at seismic frequencies. The setup has been used mainly for the study of shales. For such rocks, it is crucial that the saturation of the samples is preserved, which requires fast sample mounting. The design of our setup, together with a technique that was developed for rapid mounting of strain gauges onto the sample and subsequent sealing of the sample, allows for sample preservation, which is of particular importance for shales. The performance of the new experimental setup and sample mounting procedure is demonstrated with test materials (aluminium and polyetheretherketone) and two different shale types (Mancos shale and Pierre shale). Furthermore, experimental results are presented that demonstrate the capability of measuring the impact of saturation, stress, and stress path on seismic dispersion. For the tests with Mancos shale and Pierre shale, large dispersion (up to 50% in Young's modulus normal to bedding) was observed. Increased water saturation of Mancos shale results in strong softening of the rock at seismic frequencies, whereas hardening is observed at ultrasonic frequencies due to an increase in dispersion, counteracting the rock softening. The Poisson's ratio of Mancos shale strongly increases with the level of saturation but appears to be nearly frequency independent. We have found that the different types of shale exhibit different stress sensitivities during hydrostatic loading and that the stress sensitivity is different at seismic and ultrasonic frequencies.

FAUNAL ASSOCIATIONS IN COLD-METHANE SEEP DEPOSITS FROM THE UPPER CRETACEOUS PIERRE SHALE, SOUTH DAKOTA

Study of ancient cold-methane seep deposits provides insight into the changes in seep communities over the lifetime of a seep, which are otherwise difficult to observe in modern settings. We studied 24 cold-methane seep deposits in the Upper Cretaceous (Campanian) Pierre Shale of southwestern South Dakota. These deposits were subdivided into three categories depending on their physical characteristics: (1) those with a single main conduit, few secondary pipes and concretionary bodies, and no carbonate cap, implying strong advective flow to the sediment-water interface; (2) those with a single main conduit, a moderate number of secondary pipes and concretionary bodies, and a small carbonate cap, implying both advective and diffusive flow to the sediment-water interface; and (3) those with a single main conduit, a high number of secondary pipes and concretionary bodies, and a broad carbonate cap, implying extensive flow, but dampening at the sediment-water interface due to the presence of the large carbonate cap. We analyzed the faunal composition at all 24 seeps. The number of species ranges from five to 20. All of the seeps are dominated by baculitid ammonites, inoceramids, and lucinids (“foundation” organisms). These species are the same as those in time-equivalent non-seep sites in the Pierre Shale and are not seep-obligate. However, in seep categories 2 and 3, the number and kind of secondary organisms increases in association with the development of the large carbonate cap. These organisms include oysters, gastropods, echinoids, sponges, crinoids, and scaphitid ammonites. We infer that these organisms appear because (1) the carbonate hardground provides a more diverse habitat allowing attachment and encrustation and (2) the bottom waters are better oxygenated and/or the level of hydrogen sulfide is reduced because the methane rich fluids are diverted away from the carbonate cap, thus providing a more suitable habitat for organisms such as scaphitid ammonites that require a well-oxygenated environment. However, even at these seeps, the number of secondary organisms usually does not exceed that of foundation organisms.

Lakotacrinus brezinain. gen. n. sp., a new stalked crinoid from cold methane seeps in the Upper Cretaceous (Campanian) Pierre Shale, South Dakota, United States

AbstractDespite a rich and varied record, Mesozoic stalked crinoids are relatively rare in the Western Interior Seaway of North America compared to those found in Northern Europe. A unique example of Mesozoic stalked crinoid is described from cold methane seeps (hydrocarbon seep mounds also called “tepee buttes”) from the Upper Cretaceous (upper Campanian) of the Northern Great Plains of the United States; the first crinoids to be described from such an environment. The Late Cretaceous Western Interior Seaway has never before yielded any identifiable stalked crinoid remains. Nevertheless, there have been significant studies on both free living and stalked crinoids from other locations in the Upper Cretaceous of North America that provide a good basis for comparison.Lakotacrinus brezinain. gen. n. sp. is characterized by a tapering homeomorphic column with through-going tubuli, lacking any attachment disc. The arms are unbranched and pinnulate, with muscular and syzygial articulations. The unique morphology of the column justifies the establishment of Lakotacrinidae new family. A new suborder Lakotacrinina n. subord., is also proposed as there exists no corresponding taxon within the Articulata that can accommodate all the characteristics of this new genus. This new crinoid shares many features with other members of the articulates, including bathycrinids, bourgueticrinids and guillecrinids within the Order Comatulida, as currently defined in the revised Treatise of Invertebrate Paleontology. Reconstructing the entire crinoid using hundreds of semi-articulated and disarticulated (well preserved) fossils, reveals a unique paleoecology and functional morphology specifically adapted to living within this hydrocarbon seep environment.

Biostratigraphy of the Campanian-Maastrichtian, Mid-Maastrichtian, and Cretaceous-tertiary boundaries in the Raton Basin: Implications for models of early Laramide tectonism and for the origin of the enigmatic Disturbed Zone

The Campanian-Maastrichtian stage boundary is placed within the upper part of the Pierre Shale about 100 m below the base of the Trinidad Sandstone at Berwind Canyon, Colorado. Correlation of ammonite biostratigraphy with palynostratigraphy places this boundary near the top of the Vermejo Formation in the southwestern part of the Raton Basin, northeastern New Mexico. The informal mid-Maastrichtian boundary should be placed at the level of the Trinidad Sandstone at Berwind Canyon, although it should be placed at the level of the Vermejo-Raton unconformity in northeastern New Mexico based upon palynostratigraphy. High-resolution megafloral biostratigraphy of the K-T boundary is also discussed. Chronostratigraphic and paleoenvironmental indicator species reveal differential sedimentation rates across the basin. The magnitude of the Vermejo-Raton unconformity varies primarily in a north-south direction across the Raton Basin and was greatest in northeastern New Mexico and least in south-central Colorado, which may affect interpretations of the early Laramide history of the Raton Basin. The results of this study preclude correlation of the basal part of the Vermejo Formation with the Disturbed Zone of southwestern South Dakota.

Hesperornithiformes (Aves: Ornithurae) from the Upper Cretaceous Pierre Shale, Southern Manitoba, Canada

A historical collection of hesperornithiform fossils from the Gammon Ferruginous, Pembina, and Millwood members of the Pierre Shale (Campanian, Upper Cretaceous) in southern Manitoba, Canada, was examined to revise their taxonomy. Only two species of Hesperornis have been recognized in previous studies on the Pierre Shale in Manitoba, but our study recognizes six species of two genera, including H. lumgairi sp. nov. H. regalis is the most common species but absent in the uppermost unit within the studied sequence. The result of this study supports the paleobiogeographic subdivision of the Campanian vertebrate fauna within the Western Interior Seaway, but not the faunal boundary that distinguishes the avian fauna of Manitoba from that of South Dakota and Kansas.

Hesperornis escapes plesiosaur attack

Evidence from the Pierre Shale (Late Cretaceous) of South Dakota is presented for an attack on a juvenile Hesperornis by a polycotylid plesiosaur. The wound healed and the Hesperornis grew to maturity. Evidence of survival provides our best information about predator prey interactions in the fossil record but are rare for birds where survival is an unlikely outcome.

Interpretation of wrench faulting and faultrelated pressure compartmentalization, Wattenberg Field, Denver Basin Colorado

A structural interpretation of a 128 km2 3D seismic data set from Wattenberg Field, Colorado indicates a northeast-trending step-over fault system in the Niobrara Formation. These step-over faults are the product of a fault block rotation that is caused by two large-scale dextral wrench faults: the Longmont and the Lafayette wrench fault zones. An investigation into different fault orientations between the Niobrara and the overlying Pierre Shale indicates that a rotation of the maximum horizontal stress has occurred during the Laramide Orogeny. The step-over faults and stress rotation result in reservoir compartmentalization in the Niobrara. Seismic inversion shows the different stress field compartmentalization. Low impedance occurs in a lower effective stress environment caused by high pore pressure. These compartments may constitute ‘sweet spots’ in exploration and production in the Niobrara Formation. The Niobrara Formation is a major hydrocarbon reservoir deposited during the Late Cretaceous (Figure 1a). This formation consists of the Smoky Hill Member and Fort Hays Limestone. The Smoky Hill member consists of A, B, and C chalks, and A, B, and C marls. Bratton (unpublished work, 2015) indicates that the overpressure regime extends into the overlying Sharon Springs Member of the Lower Pierre Formation. Figure 1B shows the geologic events of the Late Cretaceous Petroleum System (Higley and Cox, 2007). The Laramide Orogeny occurred during the Late Cretaceous and Early Tertiary (between 70-50 ma.). This event is thought to be responsible for the structural features in the study area (Higley and Cox, 2007). The products of this movement include several wrench fault zones located in the area mapped by Weimer (1996).

Geochemical evidence (C and Sr isotopes) for methane seeps as ammonite habitats in the Late Cretaceous (Campanian) Western Interior Seaway

Methane seeps in the Upper Cretaceous Pierre Shale of the U.S. Western interior contain a rich fauna including ammonites (Baculites, Hoploscaphites, Didymoceras, Placenticeras, Solenoceras), bivalves (Lucina), gastropods, sponges, and crinoids. Occasionally, the shell material in the seeps is very well preserved, retaining the original mineralogy and microstructure. We explored two such seeps from the upper Campanian Didymoceras cheyennense and overlying Baculites compressus Zones (74.7–73.5 Ma) in southwestern South Dakota. Light values of δ13C in the micritic limestones (–11 to –47 ‰) confirm the impact of anaerobic oxidation of methane on the isotopic composition of the dissolved inorganic carbon reservoir. At the seep from the D. cheyennense Zone, δ13C values in well-preserved specimens of Hoploscaphites and Baculites are significantly lighter than those measured in specimens from approximately age-equivalent non-seep deposits. In addition, the 87Sr/86Sr ratio is elevated in the authigenic carbonates and ammonite shells compared with the coeval marine value. This suggests that seep fluids imprinted with a radiogenic Sr signature, perhaps derived from isotopic exchange with granitic deposits at depth associated with the Black Hills uplift, are transported through the surficial sediments into the overlying water. The persistence of these isotopic tracers of seep fluids in the ammonite shells suggests that these mobile animals were likely demersal and were living in close proximity to the seep. A more restricted data set on a single baculite and nautilid from the seep in the B. compressus Zone shows less divergence of δ13C and 87Sr/86Sr relative to non-seep values, suggesting that fluid transport was not as strong at that seep.

3-D orientation and distribution of ammonites in a concretion from the Upper Cretaceous Pierre Shale of Montana

One of the most common modes of preservation of ammonites in the Upper Cretaceous US Western Interior is in concretions. We examine an accumulation of ammonites from a single concretion in the lower Maastrichtian Pierre Shale of eastern Montana. The concretion is an oblate spheroid 50 cm in length and 26 cm in diameter, with its long axis parallel to the substrate. It contains approximately 90 ammonite specimens representing three species of Hoploscaphites including adults and juveniles. The concretion also contains other fauna, primarily bivalves and gastropods. A total of 33 ammonites, mostly adults, are concentrated in a cluster that spans 71 % of the length of the concretion (called the “sculpture”). 3-D measurements of the ammonites in the sculpture reveal that (1) the shells dip at all angles, with a significant trend toward more horizontal from west to east; (2) the shells dip with a highly significant bias toward the east, suggesting a current from that direction; and (3) a highly significant number of the shells that are non-vertical face with their left side up. Most of the shells show lethal damage as indicated by missing pieces of body chamber. After settling to the bottom, the shells may have piled up against each other, creating a sediment trap. Other organisms such as scavenging gastropods may have been attracted to the site to feed on the stranded ammonite carcasses. The chambers of the ammonite phragmocones and even some of the body chambers are empty, suggesting relatively rapid burial. Oxygen and carbon isotopic analyses of the ammonite shells reveal that they preserve their original isotopic signature. The values of δ13C of the carbonate cement in the concretionary matrix are much lighter than those in the ammonite shells and imply that cementation of the concretion occurred in association with the decomposition of organic matter.