ALBERT

Beschreibung: The J sandstone comprises less than 46 m (151 ft) of sandstone-dominated strata within the mudrock-dominated lower Upper Cretaceous succession of northwestern Nebraska. The unit is a prolific hydrocarbon producer in this region (Denver-Julesburg Basin), but its lithostratigraphic and sequence-stratigraphic framework, critical for reservoir characterization and mapping, is poorly known. We have achieved an improved understanding of depositional history and sequence stratigraphy by describing and correlating cores and wireline logs from wells within Sioux, Dawes, and Box Butte counties, Nebraska, and Niobrara and Goshen counties, Wyoming. Coals, paleosols, fluvial or inner estuarine sandstones, estuarine mudstones, fluvial conglomerate, shoreface sandstone, and reworked volcanic fallout (bentonite) lithofacies were identified. Trace fossil assemblages representing stressed expressions of the Skolithos and Cruziana ichnofacies are common. These lithofacies are arranged vertically into three erosionally based cycles, each less than 28 m (92 ft) thick, and each grade upward from fluvial or inner estuarine sandstones into estuarine mudstones and in turn into shoreface sandstones. The lateral and vertical stacking patterns of the lithofacies are complex, however, and the upper cycles appear to fill space eroded into the underlying ones. Northeast-southwest–elongate isochore trends appear in all three cycles. Lithofacies transition downdip from inner estuarine sandstones in the northeast to estuarine basin mudstones and shoreface sandstones toward the southwest. Detrital mineralogy indicates an easterly (cratonic) provenance for the entire unit. Our data suggest that the J sandstone in northwestern Nebraska accumulated in wave-dominated estuarine settings, as part of a long-lived transgressive systems tract. The unit as a whole occupies a complexly incised landscape cut during a third-order lowstand ca. 98 Ma. Coeval cycles of similar magnitude throughout the Western Interior suggest that the three cycles represent eustatic fluctuations. The highest quality reservoirs occur at the base of the unit in inner estuarine lithofacies in the central and southwestern parts of the study area. Jonathan Antia received his Ph.D. in geology from the University of Nebraska-Lincoln in 2009. He currently works as staff geologist at Core Laboratories in Houston, Texas. His academic research focused on coastal to shallow-marine siliciclastic depositional systems. Chris Fielding holds the Mr. & Mrs. J. B. Coffman Chair in Sedimentary Geology at the University of Nebraska-Lincoln. He received his Ph.D. from the University of Durham (United Kingdom) in 1982 and previously worked for BP Exploration and the University of Queensland in Brisbane, Australia. He is currently president-elect of SEPM. His research interests lie in the stratigraphy of continental, coastal, and shallow-marine successions. R. Matthew Joeckel is a professor in the School of Natural Resources and Department of Earth and Atmospheric Sciences at the University of Nebraska-Lincoln. He received his Ph.D. from the University of Iowa in 1993. His research interests include Pennsylvanian and Cretaceous stratigraphy, continental depositional environments and sedimentary successions, weathering processes, and paleosols.

Beschreibung: Anomalous carbonate horizons with intercrystalline hydrocarbon residue, cone-in-cone structures, and calcite “beef” veins in adjacent sandstone beds record potential evidence for hydrocarbon generation and seepage in the middle to upper Turonian Frontier Formation from the Uinta Basin, Utah and Colorado. Eight carbonate occurrences, all encountered within distal delta-front facies (thin-bedded sandstones and siltstones), were sampled at outcrop locations from the southern and eastern margins of Dinosaur National Monument. Seven petrographic facies (PF1–PF7) were identified using standard petrographic and cathodoluminescence microscopy: PF1, large and small botryoids and fans; PF2, yellow-brown spherules; PF3, microcrystalline spar cement; PF4, blocky spar; PF5, prismatic spar; PF6, drusy mosaic spar; and PF7, dolomite. Facies PF1–PF3 are synsedimentary phases comprising a large percentage of carbonate horizon volume, whereas PF4–PF7 are late-stage fabrics. The δ13C values of PF1–PF3 (−9.9‰ to −20.0‰) are consistent with contributions from biogenic methane seepage during deposition and early diagenesis. Brecciated PF1 fabrics and blowout depressions within sandstone horizons further indicate significant methane generation during deposition and early burial. Late-stage fabrics contain δ13C (−8.0‰ to −17.3‰) and δ18O (−6.5‰ to −13.5‰) values consistent with progressive burial, during which intercrystalline hydrocarbon residue, cone-in-cone structures, and calcite beef veins were formed by the thermal maturation of organic matter from enclosing distal delta-front facies. Together, these features reveal the potential for the thin-bedded facies of the Frontier Formation distal delta front to serve as a potentially viable petroleum subsystem previously unrecognized in the Uinta–Piceance petroleum province.

Beschreibung: Cretaceous low-accommodation deposits have been extensively studied in the subsurface of the Western Interior of North America because of their prolific hydrocarbon production and remaining potential. Understanding the stratigraphic complexities of these deposits in the subsurface relies strongly on detailed outcrop analogs. In this study, the Dakota Sandstone was examined along 100 km (62 mi) of semicontinuous outcrop between the towns of Hanksville and Ticaboo in the Henry Mountains of southeastern Utah. This region represented a low-accommodation setting located over the forebulge of the Cretaceous Western Interior Basin during accumulation of the unit. The Dakota Sandstone is 0 to 38 m (125 ft) thick, of Cenomanian age, and records multiple cycles of sediment accumulation. The Dakota Sandstone is subdivided into two condensed top-truncated stratigraphic sequences, the upper of which contains two parasequences. The basal parts of both sequences are composed of braided fluvial conglomerates and sandstone overlain by tidally influenced fluvial sandstone, inclined heterolithically stratified estuarine mudstone, carbonaceous shale, and coal. The overlying parasequences consist of coarsening-upward lower to upper shoreface mudstone, sandstone, tidal inlet deposits, and oyster shell concentrations. These facies define tripartite subdivisions of depositional environments typical of wave-dominated estuaries. The fluvial deposits may represent lowstand deposits, but overall sediments accumulated during transgressive systems tracts (TST). The parasequences recorded in the Henry Mountains are similar to the Dakota Sandstone of northwestern New Mexico and to high-frequency sequences identified in the Kaiparowits Plateau, approximately 80 km (∼50 mi) to the southwest, which suggests eustatic driving mechanisms. The best potential for hydrocarbon reservoirs occurs in fluvial sandstones and conglomerates. Jonathan Antia received his Ph.D. in geology from the University of Nebraska-Lincoln in 2009. He currently works as a staff geologist at Core Laboratories in Houston, Texas. His academic research focused on coastal to shallow marine siliciclastic depositional systems. Chris Fielding holds the Mr. & Mrs. J.B. Coffman Chair in sedimentary geology at the University of Nebraska-Lincoln. He received his Ph.D. from the University of Durham (United Kingdom) in 1982 and previously worked for BP Exploration and the University of Queensland in Brisbane, Australia. His research interests lie in the stratigraphy of continental, coastal, and shallow marine successions.

Beschreibung: Sequence-stratigraphic concepts and nomenclature are predicated on the assumption that at any time, accommodation conditions are in phase across a depositional basin. In certain situations, however, such as in evolving retroarc foreland basins, the sense of accommodation may be spatially variable because of the growth of intrabasinal structures. Herein, we present evidence for the accumulation of a fluvially dominated deltaic sandstone under strong forcing from spatially and temporally variable low accommodation within the Cretaceous western Cordilleran foreland basin of North America. The Peay Sandstone Member is a coarsening-upward sandstone body (〈60 m [〈197 ft] thick) that is extensive across much of the eastern half of the present-day Bighorn Basin of Wyoming. It is remarkable for its elongate plan-form geometry, extending many tens of kilometers across the basin, and for distal thickening patterns that are, at face value, difficult to reconcile with conventional facies models for deltaic systems. The sandstone shows some of the characteristics of falling-stage and lowstand deltas (extending far into the basin from the contemporary shoreline to the west, tack of a preserved delta-plain topset) but is not incised into its substrate and does not show a descending regressive trajectory with respect to underlying strata. We submit that the Peay Sandstone Member was formed under a regime of both temporally and spatially variable accommodation forced by the heterogeneous growth of the fore-bulge within the western Cordilleran foreland basin and suggest that the origin of some other apparently isolated sandstone bodies in this and other basins might also be explained in a similar manner.

Beschreibung: Jonathan P. Allen received a B.A. degree in geology-biology from Colby College in 2003 and an M.S. degree in geosciences from the University of Nebraska–Lincoln (UNL). This study stemmed from research he conducted for his M.S. degree. He is currently pursuing a Ph.D. at UNL, studying the evolution of paleoclimate within the Carboniferous of Atlantic Canada. Chris Fielding holds the Mr. & Mrs. J. B. Coffman Chair in Sedimentary Geology at the UNL. He received his Ph.D. from the University of Durham (United Kingdom) in 1982 and previously worked for BP Exploration and the University of Queensland in Brisbane, Australia. His research interests lie in the stratigraphy of continental, coastal, and shallow-marine successions. The sequence-stratigraphic analysis of nonmarine strata has seen several advances in recent years. Such successions, however, are nonetheless difficult to interpret and correlate because the concepts, principles, and methodologies of sequence stratigraphy were developed for coastal and nearshore marine strata where key stratigraphic surfaces that are clearly related to relative changes in base level can be readily recognized (Posamentier and Vail, 1988; van Wagoner et al., 1990; Posamentier and Allen, 1999). Within nonmarine successions, such key surfaces are commonly cryptic, ambiguous, or not easy to correlate. However, several recent publications have attempted to address the sequence stratigraphy of nonmarine successions and their relationship to time-equivalent marine strata (Shanley and McCabe, 1991, 1993, 1994; Shanley et al., 1992; Wright and Marriott, 1993; Gibling and Bird, 1994; Aitken and Flint, 1995; Olsen et al., 1995; Van Wagoner, 1995; Holbrook, 1996; Yoshida et al., 1996; Burns et al., 1997; Rogers, 1998; Legarreta and Uliana, 1998; Posamentier and Allen, 1999; Lang et al., 2001; and see review of Plint et al., 2001). …