Geology of Santa Monica Mountains and Simi Hills, California

Thomas W. Dibblee, Jr.

The Santa Monica Mountains form a low west-trending coastal range 72 km long, transected by Malibu Canyon draining south from the Simi Hills.

Granitic rocks and slate of the Peninsular Range basement terrain are exposed in the eastern part of this range. The basement is overlain by uppermost Cretaceous and lower Tertiary marine clastic deposits exposed in the central part and in the Simi Hills. All these units are unconformably overlain by the Oligocene and Miocene sequence that thickens westward and thins eastward and northward. It is composed of a basal red-bed unit (Sespe Formation), marine clastic sediments (Topanga Formation), volcanic deposits (Conejo Volcanics) that thicken enormously westward, and unconformably overlying marine deposits that include siliceous shale (Monterey Shale). The stratigraphy of this sequence is extremely complex, with rapid local variations in lithology and thickness and several major unconf rmities.

The Santa Monica Mountains were squeezed up anticlinally and thrust southward on the Malibu coast-Santa Monica fault zone along their southern border. The structure of this range becomes increasingly complex toward this fault zone. But major fold axes plunging northwest from this fault zone apparently are the effect of lateral drag on this fault zone with an estimated 22 km of left slip.

The Santa Monica Mountain uplift is separated on the north by a synclinal structure in the Miocene sequence which buttresses northward against the north-dipping Cretaceous-lower Tertiary sequence of the Simi Hills.

Exploratory test holes were drilled on favorable structures but little oil was found.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.