Abstract

ARREGUIN-RODRIGUEZ, Gabriela de Jesús  and  SCHWENNICKE, Tobias. Stratigraphy of the western margin of the San José del Cabo basin, Baja California Sur. Bol. Soc. Geol. Mex [online]. 2013, vol.65, n.3, pp.481-496. ISSN 1405-3322.

A stratigraphical analysis performed at the Arroyo San Dionisio provided data on the evolution of the western margin of the San Jose del Cabo basin in Baja California, which evolution started in the late Miocene and is part of the Extensional Province of the Gulf of California. The half graben basin is bounded in its western margin by the listric fault of San José del Cabo. The marginal fault and three stratigraphic formations at the Arroyo San Dionisio are exposed perpendicularly to the basin margin, and were informally grouped into two sequences (lower and upper) separated by an angular unconformity. The lower sequence consists of the Trinidad and Los Barriles formations, while the upper sequence is constituted by the El Chorro Formation. In the western part of the arroyo, next to the San José del Cabo fault and due to the rotation along it, only the El Chorro Formation is exposed. This formation is composed of conglomerate and conglomeratic sandstone deposited in an alluvial fan environment. In outcrops farther to the east, conglomeratic sandstone and conglomerate of the Los Barriles Formation, which reflect an alluvial fan depositional environment as well, underlie the El Chorro Formation. The sediments of both formations are derived from the basement exposed in the sierra La Victoria. The Los Barriles and Trinidad Formations interfinger approximately 7 km east to the marginal fault. The Trinidad Formation is composed of coarse sandstone, muddy sandstone, and mudstone of a shallow marine environment. The interfingering evidences the contemporaneity of both formations, probably in the Pliocene, and reveals the existence of a coastal area with a delta fan during deposition of the sediments. No clinoforms were developed in this delta fan deposits, probably due to the lack of space in an area far from the marginal fault. The angular unconformity between the lower and upper sequences probably reflects a basin reorganization resulting in the partial erosion of the lower sequence in the Pliocene or early Pleistocene. Subsequently, the rotation along the fault confined the alluvial deposits of the El Chorro Formation to the western part of the basin. The youngest deposits of the El Chorro Formation cover the San José del Cabo fault and also show their original sedimentary disposition, suggesting little or no activity along the fault since the late Pleistocene.

Keywords : San José del Cabo Basin; San José del Cabo Fault; Trinidad Formation; Los Barriles Formation; El Chorro Formation.