Late Cenozoic History of the Southern California Islands

The floor of the ocean off southern California consists of a series of low depressions and basins separated by high ridges, some of which protrude above sea level to form the Southern California Islands. The basins and many of their intervening sills are at depths that are much greater than usual for the continental shelf. This peculiar region of complicated topography has been called the "continental borderland" by Shepard and Emery (1941). It lies offshore from the continental shelf of the mainland but inshore of the continental slope which extends down to regions of oceanic depths and structures. The basins and ridges are somewhat elongated, and most are aligned in a northwest-southeast direction, parallel to the major structural features of the mainland; a few of these ridges bear islands which are also elongated along a northwest-southeast trend (fig. 1). However, in the north there is a transverse ridge, locally rising above sea I eve 1 to form an east-west chain of islands. This ridge is separated by the transverse Santa Barbara Basin from the east-west mainland coast formed along the flanks of the Santa Ynez Mountains. For convenience, the islands will be grouped here as Northern Channel Islands (Anacapa, Santa Cruz, Santa Rosa, and San Miguel, from east to west) and Southern Channel Islands (Santa Catalina, San Clemente, Santa Barbara, and San Nicolas). The bedrock geology of the Northern Channel Islands indicates that they represent a westward extension of the general geologic and structural style of the Santa Monica Mountains, with a granitic core (exposed on Santa Cruz Island) intruded into what are now slates, phyllites, and schists, overlain by Cretaceous and Cenozoic sedimentary and volcanic rocks. In fact the Northern Channel Islands and the Santa Monica Mountains have been united as the structural province of "Anacapia" by Reed and Hollister (1936). The Southern Channel Islands, together with the Palos Verdes Hills area, which is stratigraphically similar to these southern islands and was an island itself until joined to the mainland during the Quaternary (Pleistocene or Recent), are assumed to have a basement of metamorphic rocks of the "Franciscan" type, which crop out on Catalina Island and at Palos Verdes Hills . These islands and Palos Verdes Hills were included in a "south Franciscan area" or a "southern Geosynclinal Basin" by Reed and Hollister (1936). The Franciscan rocks are overlain by Cenozoic sedimentary and volcanic rocks. Pliocene rocks are not known to crop out on any of the islands but have been dredged from submarine slopes and ridges chiefly south of the Northern Channel Islands (Emery, 1960, p. 68); marine Pliocene rocks are exposed at Palos Verdes Hills and in the Santa Monica Mountains. The chief cause of the elevation of the ranges and subsidence of the basins is faulting, although some displacements may involve steep folds; the inferred fault pattern of the area is well displayed in a figure by Emery (1960, p. 79). The underlying causes are not known but must involve processes at depth in the crust and upper mantle of the earth. At any rate, our knowledge of the origin and significance of this topography does not preclude the assumption of vertical movements on the order of the elevation of the islands, whenever such movements seem required as a parsimonious explanation of the geologic record. Basinrange style of topography probably began developing in the Miocene (Corey, 1954; Emery, 1960), and by at least the later Pliocene the major topographic features present today seem to have become established. The major ridges and islands are usually pictured as standing above sea level during the Pliocene since they are not known to bear marine Pliocene sediments (Corey, 1954), but it is possible that they were submerged at that time.