PRESENT-DAY SHALLOW-WATER HYDROTHERMAL VENTING ALONG THE EL REQUESÓN FAULT ZONE PROVIDES POSSIBLE ANALOG FOR FORMATION OF PLIOCENE-AGE CHERT DEPOSITS IN BAHÍA CONCEPCIÓN, B.C.S, MÉXICO

Bahía Concepción is an example of the extensional basins and accommodation zones that formed along the Baja California peninsula associated with the opening of the proto-Gulf. Its narrow (5-10 Km), elongate (40 Km) shape results from a half-graben controlled by northwest-southeast trending faults. The most prominent of these faults make up the Concepción fault zone on the peninsula forming the eastern margin of the bay. Several faults cut across Pliocene strata on a trend parallel to the Concepción fault zone. These faults acted as conduits for hydrothermal fluids, resulting in paleo hot spring sites where geothermal fluids rose through, and reacted with, the silica-rich volcanic rocks of the Miocene Comondú Group. These hot, silica-rich fluids may have transformed Pliocene carbonate-rich mudstones and tuffs to Opal-A and low cristobalite, creating the El Mono chert deposits at the base of Peninsula Concepción.

The prominent steep escarpment along much of the western shore of Bahía Concepción suggests that this side of the bay is also bound by faults, referred to here as the El Requesón fault zone. Active intertidal hot springs have been documented in a mangrove stand near Playa Santispac, and in several other locations along these faults. We have extensively studied one area of active hydrothermal venting along an NW-SE trending onshore-offshore fault near Punta Santa Barbara, where geothermal fluids and gas bubbles are being released in the intertidal and shallow subtidal (to 13m water depths) through rocks and soft sediment in a roughly linear trend extending over 750m of coastline. This hydrothermal venting is affecting the ecology of the area. Abundances and diversity appear to be enhanced in rocky habitats, particularly in fish and epifaunal filter-feeding invertebrate assemblages, while abundances and diversities of infaunal animals are lower in areas of active venting through soft sediment. Analyses of the geothermal fluids that are being released at sites along the Requesón fault zone indicate that they are highly enriched in silica. The present-day hydrothermal activity along the Requesón fault zone, particularly in the shallow estuarine setting at Playa Santispac, offer important keys to interpreting the processes along the Concepción fault zone which may have resulted in the formation of the Pliocene El Mono chert deposits.