Abstract

Eocene conglomeratic strata in the San Diego area were deposited as a narrow, west-trending progradational system that changed facies from fluvial channel, to alluvial fan, to coastal plain-fan delta, to paralic, to shelf and subsea channels. Continuing this system westward are the large Eocene subsea fan deposits that include inner, middle, and outer subfan and basin plain facies of the southern California borderland. In all environments, 81 to 96 percent of the conglomerate clasts are distinctive, well-rounded, siliceous metavolcanic stones (Poway clasts) that reach 60 cm in size and average 6 cm; 2 to 13 percent are quartzite; and 0 to 12 percent are locally derived (Peninsular Ranges) crystalline basement (granitic and volcanic) rocks. The composition, shape, and average size of the metavolcanic and quartzite clasts are constant in all environments, while the less resistant granitic clasts show the effects of progressive abrasion by a decrease in size and abundance with increased transport distances. Three major sedimentologic trends characterize the Eocene conglomerate. (1) Conglomerate uniformly decreases, and finer grained rocks increase, in abundance and thickness from east to west. (2) The nonmarine and shallow marine conglomerate facies each contain disorganized and organized beds with no grading. Organized conglomerates are framework supported, and have imbricated clasts that dip up-current. The long axes of the clasts are oriented perpendicular or parallel to the paleoflow or are distributed randomly. Some nonmarine sandstone beds contain caliche horizons. (3) The subsea fan conglomerate is both disorganized and organized, and the beds are graded, or inversely graded. The clasts are matrix-supported, imbricated, and dip up-current. Long axes of clasts are oriented both parallel to and perpendicular to paleocurrent trends, and angular mudstone rip-up clasts and reworked marine fossils are present.

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