GEOLOGIC MAP OF SAN MIGUEL ISLAND, CALIFORNIA

A 1:24,000-scale full-color geologic map of San Miguel Island, recently mapped and compiled by Thomas W. Dibblee, Jr., is now published. Located 40 km south of Pt. Conception, San Miguel Island is 14 km long east to west, and 2 to 7 km wide north to south. It is a windswept, erosion-beveled, largely sand-covered terrane surrounded by seacliffs with an average elevation of about 150 m, and two knolls exceeding 250 m in elevation.

San Miguel exposes a series of Cretaceous to Miocene marine, mostly clastic sedimentary rocks, including bimodal Miocene volcanics, volcaniclastic rocks and associated intrusive rocks. The Cretaceous strata, newly named the Point Bennett Formation in the article on the back of the map, are 2500 m of mostly tan arkosic sandstone and minor dark shale of a mid-fan depositional environment, conformably overlain by lower Tertiary strata, namely 700 m of bathyal, dark gray shale of the Paleocene Canãda Formation and 500 m of tan arkosic mid-fan turbidite sandstones of the Eocene South Point Formation. That formation is unconformably overlain by nearly 1000 m of Miocene Vaqueros and Rincon Formations, and the Monterey Formation that includes 500 m of basaltic volcanics. Up to 350 m of Miocene felsic intrusive and volcaniclastic rocks of uncertain stratigraphic association crop out on the NE margin of the island. All these bedrock units are covered by marine terrace deposits along the island's margins and extensively by wind-deposited drift and dune sand.

Structurally, the western part of the island exposes Upper Cretaceous and lower Tertiary marine sedimentary formations that dip about 45°NE. The eastern part exposes Eocene and Miocene marine formations, including the Miocene basalt, that dip NE into a syncline. These structural segments may be separated by a hidden fault striking NE across the central part of the island. The NE margins of the island expose Miocene intrusive dacitic volcanic and extrusive volcaniclastic rocks that dip low southwestward on both sides of Cuyler Harbor.