--> ABSTRACT: Delta Morphologies and Progradational Styles: Ferron Sandstone Member of Mancos Shale, East-Central Utah, by F. W. Stapor and R. D. Adams; #91030 (2010)

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Delta Morphologies and Progradational Styles: Ferron Sandstone Member of Mancos Shale, East-Central Utah

F. W. Stapor, R. D. Adams

Stream-modified, Gilbert-type deltas are common in the Ferron Sandstone Member of the Mancos Shale along the western side of the San Rafael swell in east-central Utah. These deltas exhibit steeply inclined delta-front foresets (10°-20°) characteristic of Gilbert-type deltas; however, the deltas are stream-modified, because current-ripple cross-lamination and three-dimensional dune cross-bedding typically occur within the foresets, indicating that traction as well as mass-flow depositional processes were active. These Gilbert-type deltas are 10-50 m thick and can be traced up to 5 km along the outcrop belt.

The Ferron also contains numerous stream mouth-bar deltas with gently inclined delta-front foresets (less than 2° or 3°). These deltas are 5-25 m thick and can be traced for up to 20 km both downdip (outcrop) and parallel with strike (subsurface). They are limited to the northern three-quarters of the outcrop belt and account for greater than 80% of the deltas observed.

In the southern quarter of the outcrop belt, only Gilbert-type deltas are found; they are approximately twice as thick as both types of deltas in the northern three-quarters. Because delta-front and delta-plain facies are present in both areas, this differential thickness implies a significant reduction in accommodation during northward progradation of the deltaic complex.

In the northern three-quarters of the outcrop belt, distal delta-front beds typically contain wave-ripple cross-lamination and hummocky cross-bedding, and when coupled with the predominance of stream mouth-bar deltas in the north indicate an increasing importance of basin processes, especially wave processes, as northward progradation occurred.

A well-log stratigraphic section, subparallel to the outcrop belt and using bentonite beds within the overlying Blue Gate Member as datum horizons, shows a significant down-to-the-southwest monoclinal flexure in the underlying Tununk Member. Bentonite beds and marker beds within and on top of the Tununk have 170 m of structural relief over a distance of 35 km. Correlations of Ferron deltaic units, modeled on outcrop geometry and continuity, suggest that no more than half of this structural relief developed during Ferron deposition. The remainder may represent original topographic relief across which basal units of the Ferron prograded. This differential relief, deeper in the southwest and shallower in the northeast, is reflected in both the northward decrease in thickness of the delta and the northward occurrence of stream-mouth-bar deltas.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.