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ABSTRACT: Geostrophic Circulation or Shelf Turbidity Currents? The Dilemma of Paleoflow Patterns in Storm-Influenced Prograding Shoreline Systems

William L. Duke

In ancient storm-influenced prograding shoreline sequences, tool marks from hummocky cross-stratified sandstones commonly are oriented normal to paleoshoreline. Asymmetrical tool marks indicate flows directed offshore. Many workers have attributed them to storm-generated turbidity currents. This interpretation conflicts with observations from modern shelves, where storm circulation is geostrophically balanced and time-averaged bottom currents parallel bathymetric contours and the shoreline. The resolution of this conflict may lie in the realization that tool marks (and many other small paleoflow indicators) form almost instantly as the result of instantaneous flow conditions near the bed. Beneath storm-generated flows in the shallow ocean, instantaneous and time-averaged haracteristics of the bottom boundary layer generally exhibit little similarity. Storm-generated tool marks form within the thin (less than 1 m) inner boundary layer resulting from the superimposition of waves and currents. The orientation of peak instantaneous shear stress under such flows mainly reflects the orientation of the wave-induced shear stress, which typically is normal to shore. The magnitude of stress is greatly increased in the offshore direction (and decreased in the onshore direction) by superimposition of a steady current with an offshore flow component; however, the direction of stress is only slightly affected.

Thus, ancient shore-normal tool marks generally were not formed by turbidity currents; rather, their orientation is best attributed to shoaling waves approaching the coast at a very high angle. Assymetrical tool marks are directed offshore due to enhanced shear stress on the offshore stroke of waves superimposed on a geostrophic current with an offshore flow component. These tool marks do not reflect time-averaged bottom-flow direction.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990