Permian and Lower Triassic stratigraphy along the 30th parallel, eastern Baja California Norte, Mexico

Prebatholithic metasedimentary rocks east of El Marmol on the 30th parallel of Baja California include tightly folded Permian and Lower Triassic rocks metamorphosed to upper greenschist/amphibolite facies. These rocks are divided into one informal and three formal formations. The rocks of El Marmol (informal formation) have a minimum thickness of 2,000 m and consists of thin-bedded argillite, sandstone, and chert with sparsely fossiliferous carbonate rock interstratified with more thickly bedded lenses of calcareous quartzarenite, impure carbonate rock, and chert-quartzite-carbonate clast conglomerate. The rocks appear to have been variously deposited by sediment gravity flows and intervening quiet-water deposition in a hemipelagic setting. The rocks of El Marmol contains no datable fossil material but are Lower Permian or older. The Arroyo Zamora Formation has an estimated thickness of 500 m, conformably overlies the rocks of El Marmol, and is composed primarily of massive or crudely laminated silty metaargillite that locally exhibits fine-grained turbidite successions and pebbly mudstone. Slumps, boudined sheets, and channelized sediment, gravity-flow deposits composed of coarser carbonate and quartzose clasts, including carbonate blocks up to several meters, are scattered throughout. Fossils in both the matrix and clasts of these coarse-grained deposits include Lower Permian (Leonardian) fusulinids, bryozoa, brachiopods, and crinoid columnals. The Cerro El Volcan Formation has an estimated thickness of 1,400 m and is divided into three members. The lowermost member (A) consists of 1,000 m of rhythmically bedded, dark siliceous argillite that contains intervals of metamorphosed massive calcareous sandstone, sandy limestone, and thinly bedded carbonate rock with siliceous partings. The overlying member (B) consists of 50 m of metamorphosed conglomerate, pebbly quartzite, and calcareous sandstone. Angular blocks throughout this member commonly contain bioclastic debris. The uppermost member (C) consists of 350 m of laminated argillite containing widely spaced interbeds of metamorphosed argillaceous carbonate rock. It is considered Late Permian in age because it lies conformably on rocks of Early Permian age, and is overlain with possible disconformity by the Lower Triassic De Indio Formation. The De Indio Formation has an estimated thickness of 300 m and is divided into two members. The lower member (A) contains up to 50 m of metamorphosed chert-pebble conglomerate, quartzite, cross-bedded calcareous sandstone, and limestone. The upper member (B) consists of 250 m of carbonaceous, staurolite-bearing argillite. Ammonoids and conodonts from member A are of Early Triassic (Smithian) age. These four formations have lithologies and faunal assemblages similar to coeval rocks deposited along the outer edge of the Cordilleran miogeocline of western North America, and were probably deposited in a southern extension of this miogeocline.