--> Abstract: Shannon Sandstone: Reinterpretation of "Offshore Ridges" as Detached Lowstand Shoreface Sandstones, by K. M. Bergman; #91012 (1992).

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ABSTRACT: Shannon Sandstone: Reinterpretation of "Offshore Ridges" as Detached Lowstand Shoreface Sandstones

BERGMAN, KATHERINE M., McMaster University, Hamilton, Ontario, Canada

The Lower Campanian Shannon and stone now appears to be the "most offshore" of all proposed "offshore ridges", deposited some 70 to 100 mi (112-160 km) east of the contemporaneous shoreline. Despite several recent detailed studies, this "model" poses serious and as yet unresolved problems in terms of (1) sediment transport across the shelf and (2) sediment focusing into coarsening-upward, long, narrow ridges. In the Hartzog Draw-Heldt Draw area of Wyoming the base of the Shannon is a flat surface, marked by the abrupt incoming of bioturbated sandstones on top of black mudstones. Siderite pebbles (<1 cm) occur at this contact. These bioturbated sandstones are separated from the overlying cross-bedded sandstones by a 50 cm thick ubiquitous horizon of eroded mud clasts. The bioturbate and overlying cross-bedded sandstone facies can be traced as sheets from Hartzog Draw to Heldt Draw, both on and off fields, except where they have been truncated by erosion. The top of the Shannon is marked by chert grains, siderite pebbles, and mud clasts, probably representing erosional transgression. The end of Shannon deposition is marked by an abrupt change to laminated mudstones. The sharp basal contact, the mud clast horizon, the truncation of the lower sheet-like facies, and termination apparently by erosive transgression all suggest depositional discontinuities within the Shannon. These can all be explained in terms of relative sea level fluctuations, suggesting a reinterpretation of the "offshore ridge" as a detached lowstand shoreface deposit.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)