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Shannon Sandstone depositional model; sand ridge dynamics on the Campanian Western Interior Shelf

Gerard C. Gaynor and Donald J. P. Swift
Shannon Sandstone depositional model; sand ridge dynamics on the Campanian Western Interior Shelf
Journal of Sedimentary Petrology (September 1988) 58 (5): 868-880


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 are 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.

ISSN: 0022-4472
Serial Title: Journal of Sedimentary Petrology
Serial Volume: 58
Serial Issue: 5
Title: Shannon Sandstone depositional model; sand ridge dynamics on the Campanian Western Interior Shelf
Affiliation: Univ. Tex. at Dallas, Programs Geosci., Richardson, TX, United States
Pages: 868-880
Published: 198809
Text Language: English
Publisher: Society of Economic Paleontologists and Mineralogists, Tulsa, OK, United States
References: 53
Accession Number: 1988-078465
Categories: Sedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sects., geol. sketch maps
N43°00'00" - N44°00'00", W107°00'00" - W106°00'00"
Secondary Affiliation: ARCO Oil and Gas Co., USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 1988
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