--> Fades Architecture of Shingled Sandstone Bodies: Evidence for Geostrophic Flow, by H. H. Midtgaard; #90986 (1994).

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Abstract: Fades Architecture of Shingled Sandstone Bodies: Evidence for Geostrophic Flow

Helle H. Midtgaard

Shingled sandstone bodies constitute part of a lower shoreface unit, deposited on the margin of a Lower Cretaceous rift basin in West Greenland. Excellent outcrops have enabled a detailed study of the sand body geometry and sedimentary structures.

The sandstone bodies are 0.5 to 4 m thick with lateral extensions from a few meters to more than 30 m paralleling depositional strike. Laterally they grade into thin sand sheets which continue for at least 130 meters along the entire outcrop or swell within a distance of 20-30 meters to form another sandstone body. The lower boundaries are sharp with local erosional reliefs up to 1 m. Quasi-planar lamination, hummocky cross-stratification and low angle cross-stratification compose the sandstone bodies. Internal bounding surfaces are characterized by symmetrical wave ripples. Interbedded are 10-60 cm thick laminated mudstone beds.

The sedimentary structures are produced by unidirectional and oscillatory currents and the observed interbedding demonstrate a close genetic relationship. Results from recent flow experiments by other workers suggest that only minor changes in the proportions between oscillatory and unidirectional currents in a combined flow determines whether 2 or 3 dimensional bedforms or a plane bed develops. In the present study sediment transport was predominantly

obliquely offshore as indicated by the orientation of wave ripple crests, parting lineations and foresets in cross-stratified beds. The evidence for co-existence of differently oriented oscillatory and unidirectional currents strongly suggests deposition by geostrophic flows on the lower shoreface during major storms.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994