Inner shelf to shoreface hummocky sandstone bodies with evidence for geostrophic flow in the Lower Cretaceous, West Greenland

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.