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Lewis Deltaic Shelf Sequences Predict Deepwater Sand Presence/Absence

Cristian R. Carvajal, Department of Geological Sciences, The University of Texas at Austin, 1 University Station C1100, Austin, TX 78712-0254, phone: 512 471-6955, [email protected], Ron Steel, Department of Geological Sciences, The University of Texas at Austin, 1 University Station C1100, Austin, 78712-0254, and Dale Reitz, Devon Energy Corporation.

The likely presence or absence of deepwater sand on the slope or basin-floor of large-scale clinoforms can be predicted from the regime/facies of the shoreline at the shelf-edge. We postulate that it is possible to differentiate between scenarios (1) where there has been little or no sand delivery into deepwater areas because most of the sediment budget during any fall-to-rise base-level cycle has been stored on the shelf and coastal plain, and (2) where there has been much sand delivered and by-passed across the shelf, with a significant portion of the sediment budget partitioned into the deepwater slope and basin-floor.

The model is being tested in the Lance-Fox Hills-Lewis system in southern Wyoming, where we are tracking at a basin scale Late Cretaceous clinoforms. Sediment partitioning and shoreline types exhibit a sharp contrast in the basin. In the east, sandy basin-floor deposits are abundant and the shelf-edge shows feeder deltas that are river dominated (albeit tidally reworked) and toplap truncated by by-pass erosion surfaces. These incision surfaces are filled by sandstones that become progressively slumped towards the shelf-edge rollover, and were apparently connected to deepwater slope channels. In contrast, in the west, shale-prone basin-floors are linked to an un-incised shelf-edge dominated by a storm-wave regime. These strandplains are either the result of low sediment input from small rivers (more time for wave reworking) or are entirely strike fed from the easterly deltas. Thus, the outer-shelf regime at each location predicts the probability of having a sand/shale-prone slope and basin-floor.