Description
The Coronado Islands are four tilted fault blocks located on a shallow submarine shelf 25km southwest of San Diego, California. The islands lie en echelon to one another along a north-northwest trend. North Island lies 7km northwest of South Island and is the westernmost island of the group. North Island is 1.5km long and is composed of 200+m of red sandstone and shale that dips west 20 to 30°. Since clasts of the Eocene Poway Suite (found as reworked clasts in all post-Eocene rocks of the region) were not located on North Island, a pre-Eocene age is likely. North Island rocks are a probable westward equivalent of the Upper Cretaceous nonmarine Lusardi Formation redbeds of the nearby Mainland. Middle Island and Middle Rock, located lkm west of South Island, are the smallest yet most structurally complex of the Coronados. Twelve tilted fault blocks of varying attitudes dominate the structure of the island. The whole of Middle Rock and the western margin of Middle Island are composed of nonmarine red sandstone and conglomerate. Unlike the North Island rocks, these beds contain reworked Poway Suite cobbles and Catalina Schist debris. The remainder of Middle Island contains beds of marine sandstone, shale, and conglomerate. An islet east of Middle Island is composed entirely of volcaniclastic breccia. A probable Miocene age is assigned the strata of Middle Island and Middle Rock on the basis of (1) the presence of Catalina Schist clasts, and (2) the lithologic similarity of some beds to known middle Miocene rocks of South Island. South Island, the largest of the Coronados, is 3.5km long. Three main structural blocks of west-dipping strata comprise the island. The north and south blocks together contain over 250m of conglomerate and sandstone of the San Onofre Breccia facies. These beds contain various clasts of Catalina Schist and yield Foraminifera of middle Miocene age. The middle downdropped block is made up of conglomeratic sandstone, volcaniclastic breccia, siltstone, and sandstone of middle Miocene age. Sedimentary structures in San Onofre-type rocks of the island indicate north-northeast current-slope directions at the time of deposition. Miocene rocks of Middle Rock, Middle Island, and South Island were derived from the west. In middle Miocene time uplift west of the Coronados exposed Cretaceous and Early Tertiary sediments as well as the Catalina Schist complex. The rocks of these southern three islands were laid down as part of a sedimentary fan that prograded eastward in a subaerial, then submarine environment.
