A Shallow Bedded Chert In The Gulf of California

J. Ledesma-Vazquez, C. E. Amaro-Franco, R. Berry, and M. E. Johnson

A well-bedded, 15 meter thick Pliocene chert unit is located in the Gulf of California near the southeast corner of Bahia Concepcion, Baja California Sur, Mexico. The chert, as-well-as the rest of the related sequence, was deposited in a group of very shallow, fault controlled basins. The shallow depth of the basins has been supported by fossil mangroves in living positions in the chert and the abundant presence of the ichnofossil Ophiomorpha throughout the sequence. Overlying limestone and underlying reddish fanglomerate also demonstrate the shallow (maximum 10 meters), possibly intertidal depositional environment for the sequence. XRD and microscopic analyses of the chert suggest that the silica was originally deposited as a particularly silica rich tuff, with inor biogenic components. Basin cutting faults acted as conduits transporting silica enriched hot water, of magmatic origin, that eventually transformed most of the silica to opal-A and low cristobalite. Atomic absorption spectrometry showed high concentrations of Fe, Mg, Ca, and Na which support a volcanic setting for the origin of the chert beds. The faulting, magmatic silica rich waters, and volcanic origin for the chert beds are all associated with the tectonic opening of the Gulf of California.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California