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Description
This thesis involves a detailed sedimentological analysis of the Upper Cretaceous Rosario Formation and the Upper Paleocene/Lower Eocene Sepultura Formation from Playa San Antonio del Mar to Punta Camalu, Baja California, Mexico. Sedimentation began during the late Campanian with the deposition of a transgressive marine conglomerate and sandstone rich in basement clasts and robust oyster shells. As sea level continued to rise during late Campanian/early Maastrichtian, thick deposits of marine sandstone and mudstone retrograded over these deposits. In the northern area, the mudstone and sandstone was deposited on a low energy shelf and slope environment. Inland, the deposits remain fine grained and show evidence of having been deposited in a large protected bay or estuary. Near Camalu, the coastline was apparently steeper, as seen by the development of a submarine fan. During the early Maastrichtian, sea level began to drop and the submarine fan prograded to the west across basin-plain and slope environments. Throughout the Late Cretaceous, sediments were derived primarily from the local Alisitos Formation and Peninsular Ranges batholith (volcanic/plutonic arc terrane) found to the east. A small amount of sediment was also derived from the Jurassic/Cretaceous (?) volcanic and sedimentary rock belt ("flysch" terrane) to the east. Through much of Paleocene time, western North America was above sea level and undergoing erosion. The steep-cliffed coastline of southern California and Baja California had been worn down markedly. Sedimentation between Punta Colonet and San Telmo Canyon began in Thanetian time, with a series of conglomerates and sandstones which were deposited by a well-developed system of braided streams which fanned out laterally toward the coast to form a lobate fan-delta. From Thanetian to Ypresian, sea level was rising, which allowed the deposits to build up. The conglomerates in San Telmo Canyon appear to have "spilled" over their channels to form thin, flat-based deposits toward the top of the section. In late Ypresian, sea level dropped dramatically. At San Antonio del Mar, a thick paralic conglomerate prograded over shelf deposits. The formation was subject to much erosion and humid, tropical weathering. Tectonic influence on the Sepultura Formation is also suggested by the lack of any clear cut transgressive or regressive sequences. The Upper Paleocene/Lower Eocene Sepultura Formation was also derived from the rocks of the Alisitos Formation and the Peninsular Ranges batholith. However, these were deeply dissected by this time and braided streams were bringing a much greater amount of sediment from volcanic and metasedimentary rocks which parallel the Alisitos Formation to the east.
