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Tore M. Loseth1, William Helland-Hansen2, Ron J. Steel3
(1) University of Bergen, N-5007 Bergen, Norway
(2) University of Bergen, N-5007 Bergen
(3) University of Wyoming, Laramie, WY

Abstract: Controls on the basinward extent of shallow-marine sandstone tongues

A combination of literature study and field studies in the Western Interior Basin (USA), the Central Tertiary Basin of Spitsbergen (Norway) and the Ebro Basin (Spain) are providing an improved understanding of the controlling mechanisms for the depositional-dip extent of ancient shallow-marine regressive-to-transgressive sandstone tongues.

The pinchout distance (here defined as the distance, horizontally and normal to the orientation of the palaeo-shoreline, from a sandstone tongue's most basinward non-marine deposits to where it pinches out into offshore shale) of sandstone tongues is greatly variable, and is apparently not dictated only by the nature of the shorezone regime (delta, strandplain etc.). The three factors controlling the pinchout distance are 1) regressive and transgressive shoreline trajectories, 2) type of depositional system at the turnaround from regression to transgression, and 3) amount of erosion of regressively deposited non-marine sediment during following transgression. Shoreline trajectories during regression and transgression are often very difficult to define. However, the turnaround angle (defined by combining the sandstone tongue's regressive and transgressive shoreline trajectories) is much simpler to define and gives practically the same information concerning the pinchout distance.

The study shows that high turnaround angles correlate positively with short pinchout distances, whereas small turnaround angles correlate positively with long pinchout distances. Further, for equal turnaround angles: (1) the pinchout distances are longer if, at the turnaround from regression to transgression, the depositional system is either fluvial or tide dominated rather than wave dominated; and (2) the pinchout distances are longer the deeper the erosion during the transgression.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana