PLIOCENE RAMP FACIES AT EL MANGLE, BAJA CALIFORNIA SUR: SEQUENCE STRATIGRAPHY IN THE GULF OF CALIFORNIA

Coastal development in the Gulf of California during the late Pliocene is revealed by a graded sequence of terrestrial and marine facies integrated into a single ramp structure at El Mangle, 28 km north of Loreto in Baja California Sur, Mexico. Although best exposed at El Mangle, several distinctive units may be traced laterally for 8 km across Ensenada El Mangle parallel to the shoreface of the Gulf of California. Onshore to offshore, the sedimentary succession includes: proximal red clays, hydrothermally altered sediments rich in opalized debris from salt-tolerant land plants, conglomerate beds with an intertidal marine biota, and a distal limetone dominated by pectens and echinoids. An upper Pliocene position broadly within the Piacenzian Stage is indicated by the echinoid Clypeaster marquerensis. A more precise correlation may be provided by nannofossils from the opal bed near the bottom of the succession. With a seaward inclination of 6^o, the entire ramp sequence sits unconformably on a 300-m wide shelf eroded in Miocene andesite that forms the underlying basement. The sideways "v-shaped" profile of conglomerate beds (with clasts derived from andesite parent rocks), defines a flooding surface with a classic transgressive-regressive pathway. The volcanic uplands that fringe the western margins of the basin at El Mangle rise steeply with a maximum 44% gradient immediately above the paleoshoreline. In stark contast to tectonically altered landscapes of coeval age immediately north or south, the Pliocene ramp sequence in Ensenada El Mangle is remarkable for its pristine condition.