Provenance of the El Salto Formation (early Oligocene to early Miocene), southern part of La Reforma caldera, Baja California Sur, Mexico

Abstract

This study documented the stratigraphy and provenance of the El Salto Formation in southern Baja California, Mexico, which represents an early Oligocene–early Miocene forearc basin developed during the subduction of the Farallon plate, in the immediate vicinity of La Reforma caldera, central part of Baja California Sur, Mexico. In the study area, El Salto Formation consists of three stratigraphic members. The lower member is characterized by intercalations of sandstones and conglomeratic sandstones that exhibit eolian large-scale cross-stratification. U-Pb detrital zircon geochronology implies maximum depositional ages of ca. 33–31 Ma. The middle member is characterized by successions of conglomeratic sandstones and sandstones with eolian and tidal large-scale cross-stratification. The member also contains ignimbrites, tuff, and andesite deposits, and its maximum deposition age is ca. 30–28 Ma. The ignimbrite collected at the top of this member has a crystallization age of ca. 28 Ma. The upper member is characterized by conglomerates, sandstones, and shales, with maximum depositional ages ranging from 28 to 23 Ma. Petrographically, sandstones of the El Salto Formation are composed of three petrofacies. Petrofacies A is rich in quartz with a greater contribution of felsitic volcanic lithic grains (Q55F21L24; recycled orogenic provenance). Petrofacies B is rich in lathwork and microlitic volcanic lithic fragments with minor contributions of quartz and feldspar (Q39F12L42; recycled orogenic and dissected arc), while petrofacies C is rich in microlitic volcanic fragments and lathwork with subordinate quartz and feldspar (Q21F25L54; transitional arc setting). U-Pb ages of >600 zircon grains from nine samples contained three populations: (1) 35–23 Ma (early and late Oligocene; 22% of all grains), (2) 120–60 Ma (Cretaceous; 32%), and (3) 170–140 Ma (Middle Jurassic–Early Cretaceous; 46%). Detrital zircon grains with ages of ca. 40–20 Ma showed rare earth element patterns and trace-element ratios similar to those formed in a continental arc. Volcanic rocks sampled in this work contained chemical signatures, including Nb, Pb, and Rb anomalies, that indicate their magmas were created in a subduction zone. In addition, high concentrations of heavy rare earth elements (La/Yb = 14–19) suggest that the magmas contain a component of partial melting of the mantle wedge and crust, probably as a result of asthenospheric upwelling. These features support a model in which the El Salto Formation was developed due to the rollback of the Farallon plate in the period 50–25 Ma.