Shannon Sandstone, Powder River Basin: Hydrodynamic Control of Sand Body Geometry and Facies Sequences in Western Interior Cretaceous Seaway: ABSTRACT

The Campanian Shannon Sandstone Member of the Cody Shale forms the reservoir for several significant oil fields in the western Powder River basin of Wyoming. Linear Shannon sand bodies were deposited on the muddy shelf of the Cretaceous Interior Seaway up to 200 km (125 mi) east of the paleoshoreline. The sandstone bodies are asymmetric in transverse section: northeastern flanks are shorter and steeper than the gentle accretion surfaces on the southwest flanks. The long axes of these bodies are aligned northwest-southeast to north-northwest-south-southeast. Paleocurrent direction is generally southerly, hence the sandstone bodies are aligned at a small angle to the prevailing transport direction, as required by hydrodynamic theory. Three major facies types are recognized in cores, in subsurface wireline logs, and in outcrop. In a typical coarsening-upward vertical sequence, a large-scale cross-bedded facies overlies a thin-bedded facies which in turn overlies a bioturbated facies. These facies are genetically related lateral equivalents. The large-scale cross-bedded facies lies on the upcurrent flank and crest of the sandstone body, and grades down the down-current flank into a thin-bedded facies, which grades in turn into bioturbated facies. The cross-bedded facies is typified by current-rippled surfaces, and by trough to tabular cross-bed sets. The sandstone is glauconitic, very fine to medium-grained, and commonly contains sideritic and shale intraclasts. The thin-bedded facies consists of sand and shale in flaser-, wavy-, and lenticular-bedded associations which may exhibit significant burrowing. Thin-bedded sandstones are generally very fine to fine-grained with a significant silt and clay component. The bioturbated facies is a shaly fine sandstone to siltstone interpreted as a distal thin-bedded facies extensive y reworked by burrowing. The Shannon appears to have been deposited by intermittent storm flows in an outer shelf environment where the water was deep enough for sharp-crested rather than hummocky megaripples to develop. Numerical modeling of geostrophic circulation due to wind stress forcing of Campanian shelf waters demonstrates a good correlation between measured paleotransport indicators and model results. The time-averaged bottom circulation induced by a typical mid-latitude storm is southerly. However, a significant onshore or offshore component of bottom flow may be present due to interaction of storm setup and resultant geostrophic currents with a varying coastal and bathymetric configuration. In this model, storm currents decelerating across the crests of subtle topographic highs on the shelf surface would deposit preferentially the coarser fraction of their transported load, so that the aggrading sea floor would become enriched in sand. When a portion of the Campanian shelf End_Page 1338------------------------------ attained a critical sand content, large-scale linear sand bodies were triggered, which were dynamically analogous to the sand ridges of modern storm-dominated shelves. These features tended to migrate southward across the aggrading shelf surface by means of the accumulation of successive facies packages on their down-current slopes. End_of_Article - Last_Page 1339------------