Abstract

The stratigraphic succession cropping out along the coast in the Tijuana-Rosarito Beach area is dominated by the Rosarito Beach Formation (new name), a Miocene succession of interbedded basalt flows, pyroclastic rocks, and clastic sedimentary rocks. This thick succession is underlain by unnamed Eocene sandstones and overlain by the Pliocene sandstones and conglomerates of the San Diego Formation and later Pleistocene rocks.

Five members are recognized: the Mira al Mar Member composed of fossiliferous sandstones, shales, limestones, and breccias, which contain abundant clasts of glaucophane, schist, serpentine, greywacke, bedded chert, and minor amounts of acidic volcanic and plutonic rocks; the Costa Azul Member, which is predominately basalts and tuffs; the Amado Nervo Member, which is composed of basalt; the Las Glorias Member, which is predominately sandstones; and the Los Buenos Member, which is predominately basalts.

The structure of the area is one of gently westward-dipping strata cut by numerous high-angle north-south and lesser northeast-southwest, northwest-southeast, and east-west faults. Several monoclines closely related to the faulting are developed in the area.

The presence of detritus of the Franciscan type indicates a source to the west even though at present such an area lies below sea level. Flow structures within the volcanic rocks also suggest a westward source. Crossbedding within the sandstones of the Las Glorias Member suggests that they were derived from the east.

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