PALEOENVIRONMENTAL RECONSTRUCTION OF MARINE LIMESTONE OF THE UPPER MIOCENE BOLEO FORMATION ALONG THE INCIPIENT GULF OF CALIFORNIA IN CENTRAL BAJA, MÉXICO – BAJA BASINS 2020 IRES

The limestone at the base of the Boleo Formation in the Santa Rosalia basin in Baja California Sur, Mexico developed along the nascent Gulf of California in late Miocene time. Little is known about the environment of deposition of this basal limestone. We measured the basal limestone where it is 2.5 m thick and overlies a sequence of late Miocene basaltic andesite flows and cobble conglomerate in the Caopas gypsum mine north of Santa Rosalia. Samples were collected for petrographic, XRD, SEM/EDS, and microfossil analyses. The base of the limestone is a laminated calcisiltite that contains algal laminations, fecal pellets, and micritic oncoids indicative of a low-energy, lagoonal environment. The presence of coated ooids suggests wave agitation and deposition in shoaling waters. This unit is overlain by a 0.5-m-thick dolostone that possibly replaced an earlier primary phase. Pores in the dolomite were coated with iron oxides, followed by incomplete coatings of Mn oxides, and finally with calcite. Up section, a 1.3-m-thick, fossiliferous calcisilite with some cross-bedding and abundant intraformational angular rip-up clasts, some in channels, record deepening water on a storm-dominated shelf. Genera identified in this unit include Globigerina, Globigerinoides, Bolivina (foraminifera), and Calcidiscus (coccolithophore). These genera are generally associated with upper bathyal water depths up to 200 m (Carreño, 1992). The uppermost 0.6 m is wave-ripple laminated calcarenite with pinch and swell structures suggesting a change to shallower water. In other locations, the basal limestone is capped by ~100 m of gypsum. The Boleo basal limestone at the Caopas outcrop records marine inundation and deepening from shallow to open marine shelf conditions that hosted calcareous foraminifers and nannoplankton. Alteration of the rock seen in the mineralization of the dolostone is related to the Mn and Cu ore deposits that began after the initial marine incursion. The change from open marine deposition to evaporitic conditions recorded in the overlying gypsum likely resulted from fault-controlled subsidence and structural segmentation that isolated the basin from open marine waters, while permitting inflow of saline waters that produced the thick evaporite sequence.

Carreño, 1992, Paleontología Mexicana 59