EL MONO CHERT; A COASTAL BASIN MEMBER OF THE PLIOCENE INFIERNO FORMATION, BCS, MÉXICO

A well-bedded, 14-meter thick chert unit assigned to the Infierno Formation is located near the southeast corner of Bahía Concepción at the base of the Concepción Peninsula in Baja California Sur, México. Previously undescribed, the age of this chert is restricted by fossils that date conformably overlaying limestones as Late Pliocene in age. The chert beds, as well as the rest of the associated sequence, were deposited in an interconnected set of a small basins flooded from Bahía Concepción, forming limestone beds. A shallow-water environment for the chert is indicated by the occurrence of fossil mangrove roots preserved in life position along the margin of one basin and the abundant presence of the ichnofossil Ophiomorpha on a laterally extensive horizon within the chert body. Mangrove roots in living position in chert were identified by as belonging to the genus of black mangrove, Avicennia.

Stratigraphic bracketing of the chert unit by its underlying subaerial fan-deposit conglomerate and its overlying fossiliferous limestone confirms a likely water depth between intertidal and 10 m. XRD and microscopic analyses of the chert suggest that a portion of the silica was deposited originally as a particularly silica rich tuff. Evidence of paleo hydrothermal activity along the basin cutting faults of the Concepción Peninsula indicates that hot fluids rose through, and reacted with, the underlying silica-rich volcanic rocks of the Comondú Formation. Silica dissolved from these rocks was deposited in the overlying Infierno Formation units, eventually transforming most of the volcanic glass, and all the original carbonates, to opal-A and low cristobalite. All the processes associated with the formation of the Mono chert are presently active along the El Requesón fault zone that delineates the western margin of Bahía Concepción.