Abstract

Hummocky sandstone bodies were deposited on the margin of an Early Cretaceous rift basin in an overall progradational inner-shelf to lower-shoreface unit. Excellent outcrops of this unit permitted a detailed study of sedimentary structures and paleocurrents in relation to sandstone geometries. The sandstone bodies are lensoid and 0.5-4 m thick with lateral extensions from a few meters to more than 30 m. Laterally they pinch into thin sandstone sheets that continue for at least 130 m along the entire outcrop or swell within a distance of 20-30 m to form another sandstone lens. The lower boundaries are sharp with local erosional relief up to 1 m. Hummocky cross-stratification, wave-ripple cross-lamination, and low-angle cross-stratification are closely interbedded within the sandstone. Laminated mudstone beds (10-60 cm thick) separate the sandstone bodies and are generally continuous over the entire length of the outcrop. Sedimentary structures in the sandstone bodies were produced by high-energy wave-generated oscillatory currents and unidirectional geostrophic currents. Vertical stratification successions from individual storm events reflect deposition during both increasing storm-current velocity succeeded by waning-storm deposits and, more typically, waning-storm facies successions. Deposition of the unusual vertical successions was probably controlled by an episodically high sediment supply. The local presence of the hummocky sandstone bodies on a regional scale was controlled by a paleogeographic setting where the bathymetric contours locally caused geostrophic currents to decelerate.

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