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Description
Reconnaissance geologic mapping has generated new information on the stratigraphy, paleontology, and structure of the rocks near Asunción Bay and adjacent regions, Baja California, Mexico. Emphasis is placed on the Cenozoic rock formations and fossils. Rocks of the Asunción Bay region range from Early (?) Jurassic ultramafic rocks to well-preserved Late Pliocene to Pleistocene marine terraces. Middle Jurassic basement rock comprises a large portion of the outcrop within the study area; the headlands of Punta San Pablo, San Roque, and Asunción and the Sierra de Vizcaíno consist of tonalite, mafic-ultramafic rocks, and effusive volcanic rocks of basaltic and andesitic composition. Potassium/argon age determinations indicate the tonalite is Middle to Late Jurassic in age (154.1 ± 1 m.y.). The volcanic rocks of the basement terrane may be genetically related to the plutonic rocks and possibly were generated during subduction of the oceanic crust beneath the continental margin. The Valle Formation of Late Cretaceous age, consists of sandstones, siltstones, and conglomerates resting unconformably on an irregular basement terrane. The basal beds are generally conglomeratic dolomitic biosparrudites containing abundant fragments of shallow-water organisms. The conglomerate is dominated by rhyolite porphyry clasts possibly derived from the Alisitos island-arc of Aptian-Albian age. The Cretaceous rocks are unconformably overlain by the Tortugas Formation of Middle to Late Miocene age. The Tortugas Formation consists of volcaniclastic sandstones, bentonites and siliceous shales. The Miocene strata are richly fossiliferous and contain a diverse assemblage of mega-invertebrates, marine mammalian remains, sharks teeth, and foraminifera. The Tortugas Formation is overlain unconformably by the Almejas Formation of Middle to Late Pliocene age. Fossil bivalves are abundant in some horizons of the Almejas Formation. Based on faunal similarities, the Almejas Formation is correlated with the San Diego Formation of Alta California. A major portion of the study area is covered by thin, slightly west sloping, Late Pliocene to Pleistocene marine terraces. These ancient strand lines occur up to 4 or 5 kilometers inland and form an unbroken sequence stepping down to the present shoreline. Tertiary volcanic rocks of the study area include flood basalts, pyroclastic rock, dikes and sills. Potassium/argon determinations give an age for the flow of Mesa de Auras as 13.12 ± 3.2 m.y. Mesozoic northwest trending structural features gave way to Cenozoic extensional tectonic features during mid-Tertiary time. The Tertiary sedimentary depositional basins are dilational features caused by stretching of the crust, when the East Pacific Rise encountered the continent during Neogene time.
