Abstract: Depositional Environment and Paleogeography of the Topanga Canyon Formation, Eastern Santa Monica Mountains, Los Angeles County, California

MILLER, JONATHAN S., and FRITSCHE, A. EUGENE, California State University, Northridge, CA 91330-8266

Sedimentary rocks of the 17-Ma Topanga Canyon Formation were studied in the eastern Santa Monica Mountains of southern California between Runyan Canyon Park and Coldwater Canyon. East and west of this area, the Topanga Canyon Formation pinches out due to onlap of the basement rocks. Stratigraphic sections were measured at Runyan Canyon Park and Coldwater Canyon and the rocks were divided into 10 lithosomes. A nonmarine, dusky-red siltstone and a very light-gray, coarse- grained sandstone are found only in the east at Runyan Canyon Park near the bottom of the section, whereas a marine, upper shoreface, fine-grained, fossiliferous sandstone, a bioturbated sandstone, and a finegrained, laminated sandstone are encountered at the bottom of the section toward the west. These occurrences indicate simultaneous nonmarine deposition in the east and upper shoreface deposition in the west. Relative sea level rise brought marine transgression, which resulted in deposition of a grayish-olive siltstone and a grayish-orange, medium-grained sandstone as backshore deposits on top of the nonmarine rocks in the east. On top of these eastern rocks, a fine-grained, upper shoreface, fossiliferous sandstone was deposited, followed by a lower shoreface, grayish-orange, mediumgrained sandstone. The shoreline eventually began to retreat seaward, as indicated by an upper shoreface, fossiliferous, sandy conglomerate at the top of the eastern section. In the west, deposition remained in the upper shoreface through the entire section. These wave-dominated, nearshore, marine rocks were deposited in an ocean embayment in the coastline as indicated by the onlap and pinchout of the formation both to the east and the west. In the western Santa Monica Mountains, the formation appears again and is represented by deltaic rocks. The area of pinchout between the wave- dominated and river-dominated regions represents a peninsula or headland between the two areas.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California