Late Miocene volcanism and marine incursions in the San Lorenzo Archipelago, Gulf of California, Mexico

Abstract

During Late Miocene time extensional deformation and restricted volcanic activity related to the vanishing Miocene volcanic arc occurred in the margins of the northern part of the present Gulf of California. As a result, local basins with periodical influence of marine water were formed in some areas like Bahía de los Ángeles, Bahía Las Ánimas, Ángel de la Guarda Island and the San Lorenzo Archipelago.

The stratigraphic sequence of the San Lorenzo Archipelago is formed by a series of evaporitic and clastic deposits that overlie a crystalline basement and indicate alternate transgressive and regressive periods. Minor effusive andesitic volcanism, normal faulting and sedimentation occurred simultaneously; the end of local sedimentation coincides with an increase of basaltic to andesitic volcanic activity 5 ± 1 Ma ago. The stratigraphic sequences of San Lorenzo Island and eastern Sierra Las Ánimas in the peninsula are correlative. In the Sierra Las Ánimas, the volcanic and coarse-grained sedimentary rocks predominate, and represent the western limit of a basin. In the Sierra Las Ánimas, the age of andesitic lava underlying the sedimentary rocks is 7.8 ± 0.2 Ma, bracketing the age of the marine sedimentary sequence in the Late Miocene (approximately 7.8 to 5 Ma). The present position of the northern part of the archipelago with respect to the sierra indicates that the islands moved towards the southeast as a rigid block, along the San Lorenzo and Partida fault zones.