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Book Chapter

The Shannon Sandstone: A Review of the Sand-Ridge and Other Models

By
Roderick W. Tillman
Roderick W. Tillman
Consultant, 2121 E. 51st St., Ste. 112, Tulsa, OK 74105, U.S.A.
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Published:
January 01, 1999

Abstract

The Shannon sandstones in the Powder River Basin of Wyoming have been interpreted by Tillman and Martinsen and others as having been deposited as sand-ridge complexes during transgression on a wide shelf in the Western Interior Seaway, which stretched from Alaska to Mexico in early Campanian time. In recent papers by other workers, the Shannon is interpreted to be a series of incised lowstand shoreface deposits or estuarine valley-fill deposits. The sand-ridge complexes are considered by different workers to have been deposited in a variety of sequence stratigraphic settings ranging from regression to sea level still-stand to transgression. The shelf sand-ridge model infers offshore deposition, possibly as far as 110-160 km from shore at middle shelf depths by south to south-southwest-flowing, shore-oblique currents intensified frequently by storms. A probable source of sediments for the ridges probably does not include shoreline sandstones to the west, but instead the Eagle Sandstone deltaic deposits of southern Montana which he 320 km to the north-northwest.

A model inferring that Hartzog Draw Field, a major Shannon sandstone oil producer, is a remnant of several sequence boundary incisions was recently developed by the current operators of the field. They recognize a sequence boundary at the base of the Shannon and two sequence boundaries within the Shannon. They infer that deposition of the Shannon is primarily by tidal processes. The incisions at the sequence boundaries are projected to cover tens of km laterally and if these are truly incisions they are believed by this author to have formed large open bays. To accommodate the presence of marine fossils that occur below, above, and between the thick fine- to coarse-grained Shannon Sandstone accumulations, where they occur on the Salt Creek anticline, several abrupt changes in sea-level would be required prior to, during, and following Shannon deposition.

A third model has been developed by Canadian authors who have used shoreface models developed for the Cretaceous in Alberta, Canada to explain the Shannon. Their model infers that the Shannon sandstones are a series of lowstand incised shorefaces.

This paper gives a historical perspective of interpretations of the Shannon during the last 25 years. The pros and cons of the various models are considered, and some of the problems with each model are discussed.

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SEPM Special Publication

Isolated Shallow Marine Sand Bodies: Sequence Stratigraphic Analysis and Sedimentologic Interpretation

Katherine M. Bergman
Katherine M. Bergman
Department of Geology University of Regina Regina SK S4S OA2 Canada
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John W. Snedden
John W. Snedden
Mobil Exploration & Producing Technical Center PO Box 650232 Dallas TX 75265USA
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SEPM Society for Sedimentary Geology
Volume
64
ISBN electronic:
9781565761865
Publication date:
January 01, 1999

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