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Description
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 the se rocks originated lB7 ~ 1.4 my 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 adiolaria . 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 (7) lower membe r (3500 m), and a Turonian middle member (4200 m). The lower member is comprised entirely of rhythmically alternating beds of feldspathic are nite and poorly laminated shale. The middle member contains similar feldspathic volcanic arenite and shale strata, with additional lenses of massive conglome rate. Uplift and erosion of the Alisitos Arc, provided the sediment represented in the lowe r member . Continuing uplift and tectonism gene r a t ed conglme ratic material which migrated we stward into the depositi6nal basin. To the east, unroofing of the batholithic core supplied g r ~llitic 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 Eoce ne time. The middle Miocene Tortugas Forma tion is represented by basal fossiliferous sandstone with ove rlying siliceous shale . Numerous fluctuation s 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 l arge northwe st-trending right l ateral 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.
