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Abstract

Pleistocene nearshore deposits (part of the Merced Formation) exposed in sea cliffs south of San Francisco, California, contain elongate, gravel-filled casts at the bases of some gravel or sand beds. The size and shape of these linear features resemble those of gutter casts of sand protruding into shale in other deposits. The gravel-filled gutter casts of the Merced Formation show a strongly preferred east to northeast orientation, approximately normal to the shoreline trend inferred on the basis of the modern shoreline trend and the regional structural grain. Truncation and absence of deformation in the lamina of the sand into which the gutter casts extend indicate that the features are erosional rather than constructional (due to loading), and their orientation implies that they were cut by oscillatory currents beneath shoaling waves. As such, they provide a useful indication of the shoreline trends in other ancient gravelly nearshore deposits. in the Merced Formation, high-angle cross-bedding is abundant and dips primarily in a longshore or obliquely offshore direction. The variability of the cross-bedding makes it a less reliable indicator of shoreline trends. Steep, in some cases vertical or overhanging, walls to the gutter casts and the presence of clasts of sandstone in the casts that are indistinguishable from the underlying material indicate that the structures are formed by rapid cutting and filling of nearshore sand that is unexpectedly cohesive.

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