Description
Turbidite deposits located in the Vizcaino Peninsula of Baja California Sur, Mexico, contain several conglomerate units of Mid Cretaceous to Early Paleocene age. They contain the detrital history of material eroded from, presumably, the Peninsular Ranges batholith and its supra-crustal cover to the east. Petrographic analysis of quartz-rich metasedimentary clasts collected from these conglomerates reveals that these cobbles are true quartz arenites (contain > 95% quartz grains) and meta-quartzites. The quartz grains are mostly silica cemented, but hematite and carbonate cements are abundant in some clasts. Individual contacts between framework grains vary between different cobbles ranging from tangential and concavoconvex to highly sutured near-triple-point contacts. Undeformed quartz arenite cobbles have preserved original well-rounded grain outlines typical of multi-cylce reworked sandstone with clearly visible quartz overgrowths. More deformed clasts exhibit various strain fabrics including highly tectonized shear-induced foliation. The main accessory minerals in the quartz arenites are rounded zircon and tourmaline grains. The texturally and compositionally supermature quartz arenite sandstone and meta-sandstones clasts must have been derived from a continental source. Potential sources for quartz-rich sediments occur in prebatholithic sedimentary rocks and farther east on the Mexican mainland. Some of the sedimentary clasts examined are arkosic sandstone with feldspar and lithic clasts in addition to quartz and appear to be more locally derived first cycle sediments. These clasts contain include biotite, chlorite, carbonate, epidote, clay, hematite, as well as zircon and tourmaline grains. Some clasts also contain reworked schist and chert lithics.
