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Description
Pre-batholithic metasedimentary rocks exposed beneath Tertiary volcanic rocks east of El Marmol on the 30th parallel of Baja California, Mexico, include Permian and Triassic strata. These rocks are tightly folded, with fold axes plunging steeply to the north, and are metamorphosed to upper greenschist/lower amphibolite facies. Upper Permian(?) and Lower Triassic lithostratigraphic units recognized in the area are informally called the Cerro Volcan and Del Indio units, respectively. The Cerro Volcan unit has an estimated thickness of 1400 m and is divided into three subunits. The lower subunit is 1000 m thick and consists of rhythmically bedded, dark gray and white siliceous argillite, containing intervals of massive calcareous metasandstone, sandy metalimestone, and thinly-bedded metacarbonate with siliceous horizons. These rocks are overlain by up to 50 m of metaconglomerate, pebbly metaquartzite, and calcareous metasandstone of the middle subunit. Angular metacarbonate blocks are randomly distributed throughout this subunit and commonly contain bioclastic debris. The upper subunit is 350 m thick and consists of laminated argillite containing widely-spaced interbeds of argillaceous metacarbonate. The Cerro Volcan unit lies conformably on Lower Permian rocks and is disconformably overlain by Lower Triassic rocks. Based on these age constraints, the Cerro Volcan unit is assigned to the Upper Permian(?). The Del Indio unit has an estimated thickness of 300 m and is divided into two subunits. The lower subunit consists of up to 50 m of chert pebble metaconglomerate, metaquartzite, cross-bedded calcareous metasandstone and sandy metalimestone, and micritic metalimestone. Fossiliferous portions of the micritic metalimestone include ammonites of the Meekoceras fauna, and other bioclastic debris. The upper subunit consists of 250 m of carbonaceous, staurolite-bearing argillite. The Del Indio unit lies disconformably on the Cerro Volcan unit, and is overlain by "middle" Cretaceous rocks along an angular unconformity. The Meekoceras fauna and previously identified conodonts indicate an Early Triassic (Smithian) age for the base of the Del Indio unit. Age-correlative rocks of the southern Cordilleran miogeosyncline of western North America straddle a regional Permian-Triassic unconformity and have similar lithologies and faunal assemblages. The Meekoceras-bearing limestone of the lower subunit of the Del Indio unit is lithologically- and age-correlative to a Meekoceras- bearing stratigraphic marker that characterizes Lower Triassic (Smithian) miogeoclinal rocks from southeastern Idaho to east- central California. Channelized conglomerates similar to those at the base of the Del Indio unit overlie the Permian-Triassic unconformity in part of the Cordilleran miogeosyncline. These similarities suggest that the rocks of this study may have been deposited in a southern extension of the Cordilleran miogeosyncline.
