Petrology of a triassic marine section, Vizcaino Peninsula, Baja California Sur, Mexico

Abstract

A 2,400-m thick section of Triassic marine sedimentary rocks of the San Hipolito Formation crops out on the 20-km 2 area of Punta San Hipolito. The Triassic rocks are a fault block that has been placed against the Cretaceous Valle Formation, which covers much of the Vizcaino Peninsula. The San Hipolito Formation was deposited on pillow basalts with low potassium content of oceanic origin. Some interpillow areas are filled with red, silicified, dolomitized limestone that contains the hemipelagic pelecypod Halobia of Middle to Late Triassic Age. The formation consists of chert, limestone, breccia, and sandstone members (in ascending order) that were deposited oceanward of a volcanic-island chain. The volcanic islands, and fringing carbonate sediments, were not only the source of sediment for the San Hipolito Formation, but also formed a barrier that prevented any influx of cratonic silt, sand and gravel. The basal member is 0–245 m of green, celadonitic, Halobia- and radiolarian-bearing, aphanocrystalline to very finely crystalline chert interbedded and lensed with subordinate red radiolarian chert. Above the chert is the 95–210 m thick limestone member which is comprised of very finely recrystallized limestone, sandy limestone, and plagioclase volcanic litharenite. Some limestone contains radiolaria and Halobia. Near the top of the member the hemipelagic pelecypod Monotis cf. M. subcircularis of Late Triassic (Norian) age occurs in abundance in a few thin beds. Chaotic folds and autobreccias formed by soft-sediment sliding are common and suggest that deposition occurred on a slope. A submarine erosion surface with up to 100-m relief was cut into the limestone member and filled by 0–105 m of limestone megabreccia. The breccia member is comprised of richly fossiliferous, limestone megabreccia blocks of shallow-water facies that were transported downslope in a sandy volcaniclastic sediment by debris flows. The uppermost member is 1840 m of poorly sorted, calcite-, chlorite-, and zeolite-cemented andesitic litharenite and volcanic plagioclase feldsarenite interbedded with tuff and calcitized, fossiliferous sandstone. Andesitic granule to boulder conglomerate beds increase in abundance upward through the member. The San Hipolito Formation probably accumulated in a trench external to a Middle to Late Triassic volcanic-island arc.