ARCHITECTURE OF A SINUOUS SUBMARINE-CHANNEL COMPLEX, LEWIS SHALE, SOUTHERN WYOMING

Sinuous, fine-grained, slope channels are of interest from a variety of perspectives including their role in submarine fan evolution, the insight they provide on sediment gravity-flow processes and forcing mechanisms for sinuosity, and their importance as hydrocarbon reservoirs. The Upper Cretaceous Lewis Shale in southern Wyoming provides suitable outcrops for the study of sinuous, fine-grained, slope channels at small to medium (subseismic) scales.

The slope channel outcrops comprise stacked, multistory, lenticular, channel-fill sandstones, separated by thin-bedded sandstones and laminated to structureless mudstones. Little offset stacking occurs between individual channel sandstones. Exposures of channel-fills reveal sharp, erosional, basal contacts and asymmetric internal stratigraphy, consisting of sandy turbidites, muddy debrites, and slump deposits on one side of the channel-fills, and low-angle, cross-bedded, turbidite sandstones on the stratigraphically equivalent opposite side. Ground penetrating radar images of these slope channels also reveal a sharp contact between the channel-fill sandstones and adjacent fine-grained strata, asymmetric channel geometry, possible lateral accretion surfaces, and a possible meander bend.

The nature of the channel margin suggests a significant time interval between channel formation and channel filling. The asymmetric stratigraphy is what would be expected from a sinuous channel deposit, where one channel wall periodically slumped and the other side of the channel received sediment by cross-channel traction transport and deposition. Additionally, the fact that debrites and slump deposits only occur on one side of the channel thalweg might result in progressive diversion of post-slump, sediment-gravity flows, leading to erosion of the opposite channel wall and the initiation/enhancement of channel sinuosity.

Our observations may explain some of the complex production behavior documented for submarine channel reservoirs including the lack of pressure and fluid communication between channel sandstones and adjacent thin beds, complicated flow paths within the internally complex stratigraphy of a single channel, and the vertical compartmentalization caused by separation of individual channel-fills.