Transient Depositional Shelves: A New View of Ancient Shallow-Marine Systems, Upper Cretaceous Kenilworth Member of the Blackhawk Formation, Central Utah, U.S.A.

Delebo, Nicholas¹, Michael H. Gardner² (1) Amerada Hess Exploration and Production, Houston, TX (2) Montana State University, Bozeman, MT

Sedimentological analysis within a stratigraphic framework based on 21 sections and photomapping defines three ancient wave regimes (WRI, WRII, WRIII) across a 17km segment of the seaward depositional limit of the Kenilworth shallow-marine shoreface sandstone tongue.

Homogeneous, well-sorted sandstones coarsen up and form the most sand-rich intervals in WRI. Amalgamated multi- and unidirectional cross-stratification indicates bedload transport above fair-weather wave base. Non-migrating bedforms responsible for hummocky and swaley cross-stratification in sandstones interbedded with mudstone comprise WR II. Storm-modulated, sand deposition from waning-depletive, strongly oscillatory combined flow alternates with flood-modulated, mud deposition from homopycnal flow. Slumped mudstones and thin sandstone beds with mud-draped asymmetrical ripple marks record deposition from unidirectional homopycnal and hyperpycnal flows and characterize WRIII.

The sand percent in the three regimes changes offshore from 80 to <20 to 60, which indicates significant sand bypass in the wave regime most affected by storms (WRII). Wave intensity was transient, modulated by storms and influenced by river floods. This sand decrease indicates more time represented by bypass and erosion and has several important stratigraphic implications. First, shelf deposition preceded and limited shoreface progradation. Second, upper shoreface clinoforms downlap the shelf diastem and form thin genetic units. Third, transient depositional processes acting at the same profile position through time produced diastems. Fourth, superimposing these wave regimes in a vertical succession apparently conforms Walther’s Law. However, Walther’s Law actually relates changes in water depth, interpreted sedimentologically in profile, to laterally adjacent environments, not the same profile position where deposition is determined by storm lowering of wave base.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana