The 680 square mile area lies between lat. 28°30' and 29°00' N., and long. 114°00 and 114°40' W. on the Pacific slope of the Baja California Peninsula. Cretaceous eugeosynclinal rocks of the Aptian-Albian Alisitos Formation and quartz diorite and granodiorite of the Peninsular Range Batholith occur as an eastward-rising crystalline basement complex. North-south faults and folds were apparently formed contemporaneously with emplacement of the batholithic rocks. The Rosarito Fault, a northwest-trending left lateral (?) fault, disrupted the north-south structures before the deposition of Paleocene sedimentary rocks. The position of this fault suggests it may be part of a major transverse fracture system extending across the peninsula. The basement complex is overlain by a thin veneer of generally undeformed west-dipping Cenozoic sedimentary and volcanic rocks. Paleocene marine deposits contain a brackish water "Martinez" fauna, including Turritella pachecoensis. Interfingering with these beds are continental sediments from which W. J. Morris (1966) described ungulates of the orders Tillodontia, Perissodactyla, and Pantodonta, forms characteristic of the Clarkforkian lower Tertiary of the Rocky Mountains. Unconformably overlying the Paleocene sedimentary rocks are Miocene eolian(?) sandstone, rhyolite tuffs, and basalt flows which thicken eastward toward the Gulf of California. Most of the region was emergent during the late Tertiary and in the Quaternary. Intermittent Quaternary volcanism appears to have been associated with reactivation of a segment of the northwest-trending Upper Cretaceous Rosarito Fault.
