CLINOFORMS AND LOWSTAND DEPOSITIONAL ELEMENTS IN THE LEWIS SHALE, FOX HILLS FM. AND LANCE FM. IN SOUTHERN WYOMING

We are developing a sequence stratigraphic framework for the migrating shelf-margin succession formed by the Lewis Shale, Fox Hills Fm. and Lance Fm. in southern Wyoming. The work is based on well-logs and provides insight on Lewis shelf-margin evolution through time as well as on characterizing a variety of hydrocarbon reservoirs within the succession. A remarkable feature in the study area is the recurrent occurrence of lowstand depositional elements in the sequences, including basin-floor fans, channel-levee complexes and late lowstand prograding wedges. These units lie in clinoforms whose slope angle is conspicuously low; around one degree.

Basin-floor fan deposits exhibit a lobe-like geometry with a typically blocky or less often bell-like gamma-ray log pattern. Basin-floor deposits are generally sandy with only few thin shale layers (assuming gamma-ray values as a proxy for sand/shale content). Formation of basin-floor fans usually requires lowering of relative sea level to a position below the shelf-edge provoking river incision of the shelf-edge and abundant sand delivery to slope and deepwater areas. Channel-levee complex deposits are sand-prone elements on the slope commonly exhibiting a small-scale blocky gamma-ray log pattern. However in contrast to basin-floor fans, the channel-levee complexes tend to have more numerous and thicker shale layers between sandstone beds. The channel-levee complex lies above and landwards of the fan and is likely related to early rise of relative sea level. Late lowstand prograding wedges show a funnel-like gamma-ray log pattern at the shelf-edge and on the upper slope, indicating renewed establishment and progradation of deltas at the shelf-edge. These deltas downlap onto previously formed basin-floor fan and channel-levee complex deposits, suggesting their formation during the terminal stages of lowstand sedimentation.