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Accueil du site → Master → Etats Unis → 1975 → Reconnaissance geology of the northern Vizcaino Peninsula, Baja California Sur, Mexico

San Diego State University (1975)

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

Robinson, John William

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

Auteur : Robinson, John William

Université de soutenance : San Diego State University

Grade : Master of Science in Geology 1975

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


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