Reconnaissance geology of the northern Vizcaino Peninsula, Baja California Sur, Mexico

Scale of accompanying map 1:40,800.Center coordinates of accompanying map in decimal degrees [longitude/latitude]: -114.899167?? and 27.759862??.John Robinson used 1:30,000 aerial photographs flown in 1972 to construct his base map. Before the availability of topographic mapping, the use of aerial photographs to construct base maps was typical.The coordinates for Robinson???s geological map were estimated using Google Earth imagery.No title provided for map on Plate I.Title of diagram on Plate II: Generalized cross-sections.Plates in pocketReconnaissance geologic mapping has produced\ud new information on the sratigraphy, structure, and\ud geologic history of the rocks from the northern\ud Vizcaino Peninsula. All of the sedimentary rock units\ud discussed have their type locality within the study\ud area. Exposed Mesozoic and Cenozoic rocks from the\ud northern portion of the peninsula have a maximum\ud thickness of approximately 10,000 m and 600 m, respectfully.\ud The Metamorphic Basement contains mafic to\ud ultramafic rock types, including serpentine, gabbro,\ud and amphibolite. Potassium/Argon age determinations\ud indicate that these rocks originated 187 -+1.4 m.y. B.P.\ud This is the oldest dated specimen from Baja California.\ud The oldest sedimentary unit is the Late Jurassic (Portlandian) \ud Eugenia Formation (2325 m). It\ud consists of basal pillow basalt and pebblestone overlain\ud by interbedded mixed volcanic conglomerate, pebblestone,\ud coarse volcanic arenite and shale. The age of the\ud formation is based on the mollusc Buchia piochii arid\ud deep-water radiolarian. Sediments were derived from\ud pre-Cretaceous volcanic terrane and were deposited in a\ud marginal marine trough. The Eugenia Formation was\ud intruded by hornblende andesite dikes approximately\ud 126 -+3.2 m.y. ago.\ud \ud \ud The Valle Formation lies unconformably upon the\ud Eugenia Formation, and consists of a late Albian to\ud Cenomamian (?) lower member (3500 m), and a Turonian\ud middle member (4200 m). The lower member is comprised\ud entirely of rhythmically alternating beds of feldspathic\ud arenite and poorly laminated shale. The middle member\ud contains similar feldspathic volcanic arenite and shale\ud strata, with additional lenses of massive conglomerate.\ud Uplift and erosion of the Alisitos Arc, provided the\ud sediment represented in the lower member. Continuing\ud uplift and tectonism generated conglomeratic material\ud which migrated westward into the depositional basin.\ud To the east, unroofing of the batholithic core supplied\ud granitic debris to the middle member.\ud No Paleogene rocks are known form the study area.\ud South of the study area, continuous deposition is\ud believed to have taken place from Late Cretaceous\ud through Eocene time. The middle Miocene Tortugas\ud Formation is represented by basal fossiliferous sandstone\ud with overlying siliceous shale. Numerous fluctuations\ud in water depth during this time, locally supported\ud various horizons of diatoms, Foraminifera, Mollusca, and\ud Mammalia. The Almejas Formation, of middle to late\ud Pliocene age, was deposited in similar basins as the\ud Tortugas Formation. The late Pliocene to Pleistocene\ud Marine Terraces locally overlie all of the older\ud formations. The structural pattern of the area consists of\ud large northwest-trending right lateral slip faults with\ud subordinate northeast-trending faults. Paleogeographic\ud reconstructions indicate that Eugenia and Valle Formation\ud depositional basins were located near the present mouth\ud of the Gulf of California. These have since been\ud mobilized along with all of Peninsular California