--> Abstract: Sand Ridge: A Possible Incised Valley-Fill in the Hanna Formation (Paleocene) South-Central Wyoming, U.S.A., by A. F. J. Wroblewski and R. J. Steel; #90937 (1998).

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Abstract: Sand Ridge: A Possible Incised Valley-Fill in the Hanna Formation (Paleocene) South-Central Wyoming, U.S.A.

WROBLEWSKI, ANTON F.-J., and RONALD J. STEEL, Department of Geology and Geophysics, The University of Wyoming, Laramie, WY 82071-3006

Basal strata of the Hanna Formation (?middle Torrejonian-early Wasatchian) in the central Hanna Basin of south-central Wyoming form a series of amalgamated sandstone sheets, known locally northwest of the town of Hanna as Sand Ridge. Sand Ridge is laterally traceable for approximately 10 km before it pinches out to the west and the southeast. At its thickest, the succession is ca. 60 m and forms a nearly vertical cliff face. Throughout the remainder of its extent, Sand Ridge forms a prominent cliff and ridge system with a nearly east-west strike.

Sand Ridge consists of mainly fluvial deposits, but the uppermost strata strongly suggest a tidal influence (Fig. 1). The fluvial portion of the succession is characterized by conglomerate (up to 8 cm maximum diameter) lag deposits that fine upwards into poorly sorted, pebbly trough and occasionally, planar-tabular cross-stratified sandstones. Bedforms approaching 3 m in height register minimum flow depths within the fluvial channels. The majority of bedforms and bars, however, are truncated by erosion surfaces and only the toesets and lowermost part of the foresets are preserved. Soft sediment deformation on a large scale is abundant, strongly suggesting conditions of limited accommodation and bar cannibalization during the development of the early channel belts. In the latest phases of deposition, a hierarchy of sigmoidal bedforms and composite bars or sandwaves shows more accommodating conditions and more complete preservation, not least of the sigmoidal forms. Signs of tidal influence, suggesting that the youngest infill may represent upper estuary sandwaves, include abundant and continuous mud drapes, double mud drapes, and foreset thickness bundling in some of the sigmoidal bedforms.

The Hanna Formation lies unconformably upon the uppermost Ferris Formation along the southern, eastern, and western margins of the Hanna Basin. On the northern margin, the Hanna unconformably overlies strata down to the Upper Cretaceous Steele Shale. These unconformable contacts indicate a pulse of local uplift and erosion during early Paleocene time. At a more local scale, the basal boundary of the Sand Ridge succession can be mapped as an erosional surface with some 10's of meters of relief in places. This geometry, the scale of the basal erosion, the time-trend of the sediment infill, both as regards higher accommodation and brackish/tidal influence, all suggest that Sand Ridge represents a series of Paleocene incised valleys, induced by tectonic uplift and back-filled under the influence of a rising base level which included a component of sea level rise from an encroaching shoreline embayment of the Western Interior sea.

Even though the basal Hanna succession may have been a considerable distance from the time equivalent shoreline, the tidal influence strongly suggests a connection with the marine realm during Torrejonian time.

Channelform sandstone bodies in the Paleocene (earliest Puercan - (?)early-middle Torrejonian) part of the Ferris Formation of the Hanna and Carbon basins have been interpreted traditionally as remnants of braided and/or meandering fluvial channel belts. The geometry, architecture, and sedimentology of some of these channel belts appear similar in some respects to those of Sand Ridge. It is possible that more of the Paleocene sandbodies within the Hanna Basin also represent previously unrecognized incised valley fills.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah