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...