Shelf-Edge Trajectory as a Predictive Tool for Deepwater Sands:

Application to the Lance-Fox Hills-Lewis System in the Washakie Basin

By

Cristian Carvajal and Ron Steel

University of Wyoming, Laramie, WY

We suggest that the shelf-edge trajectory (for successive shelf-edge positions) serves as a predictor of deepwater sand deposition. The concept suggests that as the “rise angle” involved in the shelf-edge trajectory increases, the amount of sandy sediment positioned beyond the shelf-edge down onto the deepwater slope and basin floor decreases. A low-angle trajectory implies much shelf-slope by-pass of sediment, whereas a high-angle trajectory involves major storage of sediment on the shelf and coastal plain, with little delivery to deepwater areas.

This concept will be tested on the fluvial-shelf-deepwater system of the Lance, Fox Hills and Lewis Shale Formations in the Washakie Basin. Preliminary field observation in the Rock Springs Uplift shows these formations to be overall regressive and aggradationally stacked. The Lewis Shale contains only a few decimeters thick tongues of slope sandstone emplaced during storms. The shelf Fox Hills Formation consists of highly aggradational and amalgamated swaley cross-stratified sandstone formed in the shoreface. The top of the Fox Hills is almost always a transgressive estuarine sandstone overlaid by shale and coal of the Lance Formation.

The predominantly aggradational stacking pattern of these formations suggests that the shelf-edge trajectory was overall of high-angle, implying that sand accumulation on the slope and basin floor was very minor. However, if the trajectory locally flattens, we would expect delivery of sands to the time-equivalent slope and basin floor. Therefore, further testing of the concept will require the establishment of synchronous shelf-slope-basin floor profiles (clinoforms), and associated shelf-edge trajectory and deepwater sand accumulation.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming