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AAPG Bulletin

Abstract

(Begin page 261)

AAPG Bulletin, V. 85, No. 2 (February 2001), P. 261-294.

Copyright ©2001. The American Association of Petroleum Geologists. All rights reserved.

Lowstand deltas in the Frontier Formation, Powder River basin, Wyoming: Implications for sequence stratigraphic models

Janok P. Bhattacharya,1 Brian J. Willis2

1University of Texas at Dallas, Geosciences, P. O. Box 830688, FO 21, Richardson, Texas, 75083; email: [email protected]
2State University of New York at Oswego, Department of Earth Sciences, Oswego, New York, 13126; email: [email protected]

AUTHORS

Janok P. Bhattacharya is associate professor in geology at the University of Texas at Dallas (UTD). His research interests include deltaic sequence stratigraphy , the local control of structure on stratigraphy and reservoir architecture of clastic depositional systems. He received his B.Sc. degree in 1981 from Memorial University of Newfoundland, Canada, and Ph.D. in 1989 from McMaster University, Hamilton, Ontario, Canada. Following a Natural Sciences and Engineering Research Council postdoctoral fellowship at the Alberta Geological Survey in Edmonton, Janok worked for the Bureau of Economic Geology at Austin and ARCO before joining UTD.

Brian Willis received a B.S. degree from the University of Minnesota and an M.S. degree and Ph.D. from Binghamton University. Following a postdoctoral fellowship at the Smithsonian's Natural History Museum, Brian was employed as a research scientist at the University of Texas. He is presently a visiting assistant professor at SUNY-Oswego. An early interest in fluvial systems led to studies of long fluvial successions exposed in Devonian strata of New York and Miocene strata of Pakistan. Later Brian studied several stratigraphic units deposited on shorelines of the Cretaceous Western Interior seaway of North America. His current interests include understanding sequence stratigraphic controls on deposition and quantifying sedimentologic variations to predict reservoir heterogeneity.

ACKNOWLEDGMENTS

This research is a product of the Clastic Reservoirs Group at the Bureau of Economic Geology, University of Texas at Austin, sponsored by Amoco/BP, Chevron, Conoco, Exxon, Intevep, Japan National Oil Company, Maxus, Occidental, Oryx, Saga, Statoil, and Unocal. Sharon Gabel, Chris Swezey, and Charl Broquet assisted in the field; Qing Fang, Matt Uliana, and Yugong Gao helped assemble the data. We thank ranchers in the Kaycee, Wyoming, area for logistical help and access to their private lands. Nicholas Christie-Blick and Kenneth G. Miller reviewed an earlier draft, and we thank Randi Martinsen and A. Guy Plint for their constructive reviews of this version.

ABSTRACT

Deposits of lowstand deltas formed on the floor of the Cretaceous Interior seaway of North America are found in the Cenomanian, lower Belle Fourche Member of the Frontier Formation, central Wyoming. Sandstones located in similar distal basin locations, hundreds of kilometers basinward of highstand shoreline deposits, form important hydrocarbon reservoirs isolated within marine shales, but interpretation of their origin has been highly controversial. The distribution, geometry, and internal facies of these sandstones are documented by an extensive outcrop study and regional subsurface correlations to develop genetic facies models for these deposits. This integrated record of lithofacies, ichnofacies, palynofacies, paleocurrent data, bedding relationships, and isolith maps incorporates observations from nearly 100 measured outcrop sections and about 550 subsurface well logs.

Four episodes of sediment progradation and subsequent transgression each left behind gradually upward-coarsening deltaic sandstones that have eroded tops. These deltaic sandstones have a lobate to elongate geometry, basinward-dipping internal clinoform bedding, radiating paleocurrents, a low to moderate degree of shallow-marine burrowing, and show variable wave influence and tidal influence on deposition. Delta plain, paralic, and nonmarine facies have been eroded from the top of deltaic successions. Erosion surfaces capping progradational deltaic successions are the only stratal Previous HitdiscontinuitiesNext Hit that can be mapped regionally, and they appear to record transgressive ravinement enhanced over areas of structural uplift, compared with lowstand surfaces of erosion, which record the bypass of sediments basinward. Low accommodation during lowstands left little room for sandstones to stack vertically, and successive episodes of delta progradation were offset along strike. More tide-influenced delta deposits formed within shoreline embayments (Begin page 262) defined by the topography of older wave-influenced delta lobes and subtle syndepositional deformation of the basin floor.

Standard sequence stratigraphic terminology is difficult to use in broad lowstand systems like the Frontier Formation because sandstones do not show simple vertical stacking patterns, major stratal Previous HitdiscontinuitiesTop can form by processes other than lowstand fluvial erosion, and minor syndepositional deformation of the basin floor exerts a first-order influence on depositional and sediment preservation patterns. Although many basin-distal sandstones have been interpreted to be deposits of offshore bars, shelf-isolated valley fills, and stranded shorelines, the Frontier Formation examples documented here suggest that many of these deposits may be top-eroded deltas formed where rivers delivered sediment to lowstand coastlines. The external geometry and internal heterogeneities of hydrocarbon reservoirs found in these types of deposits reflect processes active on the low accommodation deltaic shoreline, even in cases where subsequent ravinement has significantly truncated the deposits during transgression.

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