THE EL-CIEN FORMATION OF SOUTHERN BAJA-CALIFORNIA, MEXICO - STRATIGRAPHIC PRECISIONS

The Fl Cien Formation is exposed west and northwest of La Pat, in the southern Baja California Peninsula, Mexico. The unit has a thickness of about 125-175 m. It is composed of marine clastic sedimentary rocks. The formation consists of two members. The basal San Juan Member is mainly composed of tuffaceous and often weakly phosphatic mudstones, phosphatic silt- and sandstones, conglomerates and tuff. In addition discrete layers of granular phosphorite are intercalated. One of these beds is exploited. The sequence of strata reflects multiple relative sea level fluctuations and depositional environments extending from the open shelf to the coast. The upper Cerro Colorado Member consists of sandstones, tuffaceous sandstones and conglomerates. This regressive sequence was mainly deposited in a coastal setting. Sedimentation was controlled by the balance between a relative rise of sea level and high terrigenous influx.