Conglomerate Clast Populations of the Upper Paleogene Sespe Formation, Southern California

Upper Eocene-Oligocene conglomerate clast populations in the fluvial-deltaic Sespe Formation were characterized with pebble counts, size data and paleocurrent measurements. West-directed paleocurrent vectors, eastward coarsening, and increasing quantities of less durable clasts eastward imply that paleotransport was mainly from east to west across the Sespe basin. Locally along the northern margin of the basin, south-directed paleotransport occurred during the latter part of Sespe deposition. Well defined stratigraphic changes in conglomerat e clast populations suggest that two major cycles of sedimentation characterize the Sespe Formation over a wide geographic area. Upsection changes include the abrupt appearance of coarse detritus; an increase in abundance of granitoid and gneiss clasts; the first appearance of andesite and green quartzite clasts; and an increase in green metaporphyry and gray metatuff clasts at the expense of red and brown metaporphyry clasts.