| Summary: | Scale of map on Plate I: 1:50,000 Polyconic projection Title of map on Plate I, folded: Geologic map, San Marcos, Baja California. Title of diagrams on Plate II, folded: Geologic cross sections, San Marcos, Baja California. Topographic base map is taken from the INEGI [Instituto Nacional de Estadística, Geografía e Informática] Carta Topográfica – Francisco Zarco I11D82. Bibliography: pages 98-108 At San Marcos, roughly halfway between Tecate and Ensenada in northwestern Baja California, an isolated 1 km by 5 km Lower Ordovician allochthon overlies phyllites and metasandstones of Mesozoic (?) age. These distinct stratigraphic units are separated by a mélange consisting of allochthon-derived granule to boulder-size fragments in a pervasively foliated, dominantly phyllite matrix. Granitic rocks, followed by dikes, of the Cretaceous Peninsular Ranges batholith have intruded both autochthonous and allochthonous rocks. The Ordovician allochthon is comprised of: resistant, moderately to well sorted blue-gray ortho-quartzite; brown, gray, and black, commonly argillaceous bedded chart; medium to dark gray, finely to coarsely recrystallized carbonate rock; and minor amounts of brown to gray-green meta-argillite, and clast-supported pebble conglomerate. Because of the general absence of paleontologic and sedimentologic indicators, these rocks may be interpreted as deposited in either a deep or shallow water paleoenvironment. North Atlantic province affinity conodonts recovered from a carbonate outcrop indicate a medial Arenigian age. The autochthonous rocks consist of two distinct lithofacies: alternating phyllite and metasandstone, and “massive” greywacke metasandstone. These are interpreted to represent flysch-facies turbidites and mid-fan channel deposits, respectively, of a submarine fan system. Although age is uncertain due to the absence of fossils, proximity and lithologic similarities suggest that these rocks are part of the Triassic-Jurassic clastic wedge of peninsular California. Structurally, ...
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