SYNTECTONIC DEPOSITION OF PLIO-QUATERNARY SEDIMENTS IN THE SANTA ROSALIA BASIN OF BAJA CALIFORNIA SUR, MEXICO

Plio-Quaternary sediments of the Tirabuzón, Infierno, and Santa Rosalía formations record syntectonic deposition in the Santa Rosalía basin—an oblique-rift-margin basin along the Gulf of California in Baja California Sur, Mexico. This study aims to further understanding of the basin’s evolution and reconstruct its depositional history through interpreting the tectonic events and depositional environments present at Mesa Soledad. This was done through the analysis of two stratigraphic sections on either side of a high-angle, NNW-striking fault with 26 meters of vertical displacement. Analyses of macrofossils, microfossils, sediments, and sedimentary petrography help characterize both the marine and fluvial facies within the three sedimentary units present in the study area: the Tirabuzón, Infierno, and Santa Rosalía. These three units lie unconformably atop one another, all with a distinct stratigraphic pattern of regression characterized by fossiliferous marine sandstones and well-sorted beach deposits grading up into poorly-sorted fluvial conglomerates. The displacement seen in this previously unstudied area creates a mismatch of the stratigraphy across the fault, providing evidence of strike-slip and syntectonic subsidence and uplift. The upper-most Santa Rosalía Formation is overlain by the 1.4 Ma La Reforma ignimbrite (Schmidt 2006), indicating that the style of deformation of the basin changed at approximately this time.