Abstract

A new depositional model for hummocky cross-stratified sandstones is based on the following observations. (1) Storms transport coastal sand to the inner shelf under oscillatory-dominant combined flows exerting peak instantaneous bed shear stresses roughly normal to shore. (2) Isotropic hummocky cross-stratification is formed experimentally by large, three-dimensional, symmetrical bed forms generated by long-period, purely oscillatory flow and very strongly oscillatory-dominant combined flow. Anisotropic hummocky cross-stratification is formed by strongly oscillatory-dominant flow. (3) Grain fabric in hummocky sandstones indicates rapid reversals of bed shear stress consistent with deposition under a shore-normal oscillatory flow, initially superimposed on a relatively weak bottom current with a seaward-directed component of motion. Thus, shore-normal transport of coarse bedload on the inner shelf during storms (inferred from studies of ancient units) is caused by the interaction of high-speed oscillatory bottom motions under long-period shoaling waves and a relatively slow shore-oblique bottom current driven by geostrophically balanced coastal downwelling. Turbidity currents are not required to form shore-normal paleoflow indicators from hummocky beds. Large three-dimensional wave ripples generated by waning-storm or swell waves are responsible for much of the hummocky cross-stratification in the stratigraphic record.

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