The geologic history of the west side of the Gulf of California is particularly well recorded near the copper-mining town of Santa Rosalía, Baja California, Mexico. During the Miocene a thick accumulation of volcanic rocks (Comondú volcanics), grading westward into terrestrial sediments, was derived from the present site of the Gulf. These rocks were gently tilted westward over most of the peninsula, perhaps in the form of a tilted fault block, but were locally strongly deformed by faulting near the Gulf. Great subsidence of the former highlands on the site of the Gulf occurred, perhaps as a result of downfaulting. When the first engulfment by the sea occurred near Santa Rosalía, starting probably in early Pliocene time, the topography of the Comondú surface was still in a stage of late youth or early maturity, having a local relief of 450 meters. As a result there existed a rugged shoreline with an irregular topography, characterized by headlands, bays, submarine hills and ridges, and islands. The deposition of Pliocene and Pleistocene sediments over this initial topography resulted in prominent initial structures. Anticlinal structures were produced over buried hills, and synclinal structures were formed over pre-existing valleys. The lowest deposits, particularly a peculiar basal marine limestone, show the highest initial dips, commonly as great as 30°. The higher beds show successively gentler dips as the sediments wedged out against the hills and filled in the basins. The initial structures have been modified by tilting and normal faulting occurring at various times. The conditions of sedimentation and the origin of the initial structures are discussed, and the literature is reviewed concerning initial structures described in other regions.

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