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Description
Bahia Soledad Embayment is located about 140 kilometers south of the U.S.-Mexico International Border, along the Pacific Coast of Baja California Norte, Mexico. The embayment strata consist of about 1,225 meters of Upper Cretaceous essentially undeformed, unmetamorphosed marine sedimentary rocks resting unconformably on prebatholithic rocks of Early Cretaceous age, with Pleistocene terrace deposits occurring along the coast in a narrow band. Pre-batholithic Lower Cretaceous rocks assigned to the Alisitos Formation form the basement complex for the Bahia Soledad Embayment section. They consist of a slightly metamorphosed series of interbedded volcanic breccias, porphyritic flows, tuffaceous sediments, and biohermal limestones, Unconformably overlying the Alisitos Formation is the Rosario Formation of Late Cretaceous age. It consists of essentially undeformed, metamorphosed marine sedimentary rocks. The Formation at Bahia Soledad Embayment is divided into a lower Bandstone unit (1,000+ meters), a middle shale unit (25+ meters), and an upper conglomerate unit (200-k meters). The Rosario Formation south of Punta China, although mapped in this study as undifferentiated, can be divided into a lower mudstone-siltstone unit (800-k meters), a middle sandstone unit (135+ meters), and an upper conglomerate unit (50c meters). Textural and mineralogical characteristics of the Rosario Formation sedimentary samples analyzed suggest a nearby area of high relief as the source of the Late Cretaceous sediments. Abundance of Alisitos Formation rock types as conglomerate clasts imply this Formation to have been the principal sediment source; igneous bodies to the east being secondary. Meager primary sedimentary structure data show the source area to have been somewhere from the north to the east. The presence of Ostrea, Tellina, and Glvcymeris in association with abundant petrified logs and cross-bedding in the Rosario Formation sandstone unit suggest a very near shore environment of sediiment deposition. Long, straight, sharp contacts between rocks of the Rosario and Alisitos Formations are considered strong evidence for the shape of the Bahia Soledad Embayment being at least partially fault controlled.
