Geologic Map of San Miguel Island, California

T. W. Dibblee Jr.¹ and H. E. Ehrenspeck^2
1Department of Geological Sciences, Univ of California, Santa Barbara, Santa Barbara, CA
²Dibblee Geological Foundation, Goleta, CA

A 1:24000-scale full color geologic map of San Miguel Island, recently mapped and compiled by Thomas W. Dibblee, Jr., is now published. San Miguel Island, located 40 km south of Point Conception, is roughly triangular in shape, 14 km wide east to west, and 7 km tall, with its apex pointing north. It is a windswept, erosionbeveled, elevated marine terrace with an average elevation of about 150 m, and two knolls exceeding 250 m elevation. San Miguel is the westernmost of a chain of several islands in the Santa Barbara Channel defining the southern margin of the western Transverse Ranges of Southern California.

San Miguel exposes a series of Cretaceous to Miocene marine sedimentary rocks, including bimodal Miocene volcanics, volcaniclastic rocks and their associated intrusive rocks. The Cretaceous strata, herein newly named the Point Bennett Formation, are 2500 m of mostly tan arkosic sandstone and minor dark shale of mid-fan depositional environment, conformably overlain by lower Tertiary strata, namely 700 m of bathyal, dark gray shale of the Paleocene Canada Formation, and 500 m of tan arkosic mid-fan turbidite sandstones of the Eocene South Point Formation. These units are conformably overlain by nearly 1000 m of Miocene rocks, the Vaqueros and Rincon Formations, and the Monterey Formation that includes 500 m of mafic volcanics on the south to central part of the island. Up to 350 m of Miocene felsic intrusive and volcaniclastic rocks of uncertain stratigraphic association crop out on the northeast part of the island. All these bedrock units are covered extensively by marine terrace deposits and ubiquitous sand dunes.

Structurally, the island appears to be divided by a major hidden fault, herein named the Cuyler fault, which separates the mostly eastward-dipping, Cretaceous through Miocene sedimentary sequence, including the basaltic volcanic rocks, from a westwarddipping sequence of Miocene dacitic volcanics that are found only on the east and north side of the island.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California