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. The extensive data set available for this economically important unit makes it one of the best known ancient analogs to the sand ridges of modern shelf seas. The Shannon Sandstone was deposited as linear ridges by intermittent, along-shelf, southerly storm flows in a transgressive middle- to outer-shelf environment of the Cretaceous Interior Seaway. Comparison of Shannon sandstone bodies with modern sand ridges shows that they were formed in response to the kind of flow field observed over the modern systems. The long axes of these bodies are aligned northwest-southeast to north-northwest--south-southeast. The sandstone bodies ar asymmetric in transverse section: northeastern flanks are shorter and steeper than the gentle accretion surfaces on the southwestern flanks. Paleocurrent direction is southerly; hence, the sandstone bodies are aligned obliquely with respect to the prevailing transport direction, as required by hydrodynamic theory.

The four basic lithofacies types recognized in these sand ridge deposits are cross-bedded sandstone; thin-bedded sandstone: bioturbated, silty sandstone; and lenticular-bedded to bioturbated siltstone. The cross-bedded lithofacies has tabular, trough, glauconitic trough cross-bedded and bioturbated variants. This lithofacies grades down-section and down-current into a thin-bedded lithofacies comprising flaser- to wavy-bedded sandstone and low-angle, laminated sandstone. This thin-bedded lithofacies overlies a widespread, massively bioturbated sandstone lithofacies which in turn rests disconformably on a siltstone lithofacies.

The distribution of lithofacies within a Shannon sandstone body is most readily explained in terms of the velocity distribution over a sand ridge, and textural modification by the distortion of the regional flow field over the growing sand ridge. The up-current flank underwent erosion, and deposition occurred on the crest and down-current side, so that the sand ridge grew and migrated in a southwestward direction. Each sand ridge served as a fractionation mill in which successive environments on the transport pathway across the ridge extracted successively finer grades of sand from the transported load.