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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
AAPG Bulletin, V.
2012. The American Association of Petroleum Geologists. All rights reserved.
DOI:10.1306/03261211119
Deltaic process and architectural evolution during cross-shelf transits, Maastrichtian Fox Hills Formation, Washakie Basin, Wyoming
Mariana I. Olariu,1 Cristian R. Carvajal,2 Cornel Olariu,3 Ronald J. Steel4
1Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, Texas; [email protected]
2Energy Technology Company, Chevron Corporation, 1500 Louisiana, Houston, Texas; [email protected]
3Geo EcoMar, Bucharest, Romania; present address: Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, Texas; [email protected]
4Jackson School of Geosciences, University of Texas at Austin, 1 University Station C1100, Austin, Texas; [email protected]
ABSTRACT
The topset compartments of two Maastrichtian basin-scale clinothems are characterized, with focus on the function they played in constructing the Lance–Fox Hills–Lewis shelf-margin sedimentary prism in the Laramide Washakie Basin, south Wyoming. Approximately 1000 well logs were used
to map the delta lobes and complexes on the Fox Hills shelf and to detail their depositional character, dimensions, and orientation as they autogenically shifted during transit from an inner-shelf to shelf-edge position. The regressive transits of the deltas initiated up to 40 km (25 mi) landward from the preexisting shelf-edge and preserved river and
wave
-dominated deltaic deposits that thicken and concentrate sand on the outer shelf. Tidally influenced deltas (now outcropping) also occur in localized areas along the paleoshelf edge, probably where
wave
influence was reduced along invaginated coastal segments. Net sandstone maps of the individual clinothem topsets show that (1) coeval delta lobes exist within each clinothem, suggesting multiple rivers; (2) delta lobes have a likely autogenic compensational stacking pattern; and (3) deltas thicken and storm-
wave
influence become dominant closer to the shelf edge.
Our results support the ideas of (1) predictable increased wave
influence and (2) change to strike-elongate architecture as deltas transit the shelf. In addition, along-strike changes in process dominance cause deltaic reservoirs to be highly variable in their orientation, external shape, and internal character. Some process changes are interpreted to be autogenic responses during overall shoreline progradation. The study also provides new data on delta-lobe and delta-complex thicknesses as well as on deltaic coastline versus shelf-edge progradation rates.
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