JURASSIC SEDIMENTARY ROCKS IN EAST-CENTRAL BAJA CALIFORNIA SUR: THE LOS PARGOS FORMATION

The newly proposed Los Pargos formation (LPF), which is exposed near the settlement of Timbabichi (east-central Baja California Sur), represents the first report of Jurassic sedimentary rocks along the east side of the Baja California Peninsula. In the main outcrop, which we consider as the type locality, a 153 m thick succession of thin rhythmic layers of calcareous and siliceous mudstone, limestone, calcareous and silicified sandstone and greywacke is exposed. Bed thickness generally varies from 5 to 20 cm. The mudstones are dark colored which is caused by the high content of carbonaceous material. Poorly preserved calcareous microfossils are common. The light colored limestones are abundant in the lower and upper part of the type section. Most limestones contain disperse terrigenous grains or relics of a siliciclastic matrix, indicating that the limestones were formed by replacement. The yellowish and reddish sandstones and greywackes are less abundant. Some beds display an erosive base and internal grading. Nearly all rocks in the LPF exhibit varying degrees of replacement by silica minerals and calcite. The occurrence of Globuligerina oxfordiana, Nannoconus sp., and Saccocoma arachnoidea suggest an Upper Jurassic (possibly until earliest Cretaceous) age. In addition, the LPF is intersected by trachybasaltic porphyric dikes which are similar to a 97.4±2.5 Ma trachytic body exposed ~7 km south of Timbabichi. A similar age is inferred for the dikes intersecting the LPF. Near the LPF type locality Cretaceous granitic rocks occur in Punta Botella. However, the contact between the LPF and the granite is not exposed. The emplacement of the granite probably is responsible for the strong replacement by calcite and silica in the LPF. The LPF reflects tranquil deposition in a deep water depositional setting, interrupted sporadically by high energy events, such as turbidites. Jurassic sedimentary and metasedimentary rocks exposed in Vizcaíno and on Santa Margarita Island were deposited close to a magmatic island arc. The LPF possibly formed in a backarc basin, similar to the "Middle" Cretaceous La Olvidada nappe in northern Baja California.