ABSTRACT: Deformational Structures of Possible Earthquake Origin in Pleistocene Shelf Siltstone, San Francisco, California

H. Edward Clifton

Enigmatic structures, possibly generated by large earthquakes, are common in pleistocene shelf deposits exposed in sea cliffs south of San Francisco, California. The shelf deposits, consisting of very fine sandstone and sandy siltstone, lie within a 1750-m-thick succession, part of the Merced Formation. The structures described here are restricted to the siltstone, where they occupy stratigraphic intervals centimeters to decimeters thick of closely spaced, anastomosing thin laminae of silt and sand. In some beds, the concentrations of sand take the form of flattened rods, 1-2 mm high, 3-4 mm across, and a few centimeters long. Many of the disarticulated bivalve shells and shell fragments and small pieces of wood within these intervals stand upright and, on bedding-plane e posures, are visually aligned parallel to the long axes of the rods of sand in the same interval. Small sand-filled fractures, parallel to the rods and the vertical shells, extend downward from 1 cm to several centimeters from the base of individual silt laminae, particularly where they are bowed into small anticlinal structures. The intervals have sharp lower contacts and less well defined upper contacts. Balls of sand and silt with deformed internal lamination attest to a deformational origin for the anastomosing laminae and the aligned sand rods, vertical shells, and fractures. Some of the intervals are penetrated by tubular burrows, suggesting that they formed no more than a few tens of centimeters below the sea floor. The intervals occur in clusters of three to five within 1-2 m of ection and are separated by 2-4 m of section in which the structure is absent. Although storm stratification occurs in the same part of the shelf sections, its occurrence seems to be unrelated to the deformational zones. The combination of features within the intervals can be explained by alternating extension and compression of the sediment accompanied by rapid dewatering. Large earthquakes provide a feasible mechanism for such a combination of effects. The rate of sediment accumulation suggests a recurrence interval of about 100 to 300 yr within the clusters, with several thousand years elapsing between the clusters.

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