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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Petrology
Vol. 34 (1964)No. 2. (June), Pages 229-253

Grain Orientation and Imbrication in Miocene Turbidity Current Sandstones, California

J. H. Spotts


Orientation studies of thirty-five sandstone samples of turbidity-current origin show statistically significant preferred grain orientation. There is a consistent angular divergence between sand-grain orientation and the linear sole features such as groove casts and flute casts associated with individual sandstone beds. This relationship persists throughout the 80-foot stratigraphic section studied. Sole features show divergences of 40°-60° (47° average) from preferred grain orientation in the overlying sandstone. Differences in lineation of the two types of features are due to differences in direction of erosional and depositional currents, even though the currents may not be separated greatly in time.

Imbrication studies throughout the section show consistent southeast inclination of elongate grains with respect to the bedding. Flute casts, small-scale cross-bedding, and regional paleogeography indicate a southeast-ward sediment source; thus the imbrication data are in agreement with the theory of up-current imbrication. Three-dimensional orientation analysis indicates that grains are oriented parallel with the depositing current. This suggests that sand grains were oriented parallel with the current through original deposition from suspension, or if some grains rolled along the bottom, they were re-oriented before reaching a final position of rest.

Grain orientation can provide general information on source direction for turbidity-current deposits; imbrication indicates the sense of current movement along that direction. Limited paleogeographic inferences can be drawn form local studies of the relationship between grain orientation and erosional sole features.

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