ABSTRACT

Anacapa Island comprises three small islands which lie approximately 12 miles off the southern California coast along longitude 34°0030 N. They rise above a submerged westward-trending anti-clinal ridge which extends seaward from the western terminus of the Santa Monica Mountains. The island is barely 5 miles long and includes an area of about 1 square mile.

Gentle north-dipping Middle Miocene andesitic lavas and pyroclastic rocks of the Conejo Volcanics form the bulk of the island. Two strata of the San Onofre Breccia, each a maximum of 35-40 ft thick, occur as interbeds near the base of the exposed volcanic rocks which are about 1700 ft thick. Fossiliferous marine sands 2-3 ft thick of Early Pleistocene(?) age and alluvial sands 25-30 ft thick of Late Pleistocene age overlie a summit terrace atop the central of the three islands. The Conejo Volcanics crop out over much of the insular shelf east, west, and south of the island. Siliceous and calcareous shale and gray mudstone of the Monterey Formation of Middle Miocene age crop out about one mile west of the island beneath Anacapa Passage.

Two contrasting sedimentary environments, the geology of the island and the surrounding shelf, and the submarine topography control the character and distribution of the unconsolidated shelf sediments. East and west of the island relatively strong currents prevent the accumulation of detrital clasts and the sediment blanket is thin and mainly composed of calcareous animal and plant fragments. In areas of submarine outcrops relict and residual detrital sand and gravel have formed. North and south of the island detrital sedimentation is dominant; the deposits decrease in grain size with distance from shore and are composed mainly of sand size rock and mineral fragments derived from the Conejo Volcanics and to a lesser extent from the San Onofre Breccia. Detrital clasts finer than about .125 mm are able to by-pass the rather steeply sloping (1°30) and narrow southern shelf. A much wider and more gently sloping northern shelf largely prevents by-passing of the finer sand clasts north of the island. Sedimentation has been fastest on the northern shelf during Recent time, and the sediment blanket is here thickest and most uniform.