HOLOCENE SEDIMENTARY STRUCTURES IN GULF OF CALIFORNIA ANOXIC MARGIN BASINS: ALFONSO, LA PAZ AND PESCADERO BASINS, GULF OF CALIFORNIA, MEXICO

Box cores, multicores and gravity cores collected during cruises in the Gulf of California from 1994 to 2001 have revealed a variety of primary and secondary sedimentary structures that provide information regarding changes in climate and oceanography of the region. Alfonso Basin and La Paz Basin on the western side and Pescadero Basin on the east side of the Gulf are margin basins which have sills or shoreward slopes in the Oxygen Minimum Zone and preserve primary varves whose physical and associated geochemical characteristics yield information on Holocene climate and oceanographic changes in the Gulf over the past 7000-10,000 years Before Present.

The preservation and contrast in laminations are intimately related to bottow water oxygen content. Below 0.2 ml/L laminae remain sharply defined. Increasing oxygen content permits initial microbioturbation and finally complete macro-benthic bioturnbation and complete homogenizqation of the sediments. Power spectra computed for the various parameters show decadal, centennial and millenial periods.

Strongly defined periods include 150, 200, 300, 900 and 1500 years. These cycles can be related to changes in productivity/dissolution processes in the water column.

Variations in preservation of laminae reflect the position of the various basins with respect to the mjor upwelling areas on the eastern margin of the Gulf. Compoistion also indicates a shift from opaline silica dominance on the east side to carbonate dominance in the biogenic sediments of the west side. Production of organic matter and subsequent bacterial action produce changes in solution rates on each side of the Gulf.