The Oligocene-Miocene San Gregorio Formation of Baja California Sur, Mexico: An Early Record of Coastal Upwelling Along the Eastern Pacific Margin

GRIMM, K. A., M. LEDESMA, and R. E. GARRISON, University of California, Santa Cruz, CA, and C. FONSECA, Universidad Autonoma de Baja California Sur, La Paz, B.C.S., Mexico

The Oligocene-Miocene San Gregorio Formation (Tsg) is a lithologically diverse package of siltstones, biosiliceous sediments, tuffs, and phosphorites. In the northern part of the study area (near La Purisima), the Tsg is richly biosiliceous, organic rich, and finely laminated; in many respects, the Tsg resembles the Hames member of the Monterey Formation in the Salinas basin of California. In the south, the Tsg is thinner, detritus rich, and contains minable phosphorite reserves. Allochthonous phosphatic beds are prominent throughout the formation; many of these deposits apparently imported burrowing crustacea into anoxic paleoenvironments. We conclude that iterative episodes of turbulent transport of well-sorted (peloidal and coated grain) phosphatic sediment commonly resulted in dep sitional amalgamation of discrete event beds; varying degrees of postdepositional homogenization by infauna resulted in crudely stratified to massive (and economically minable) phosphorite beds.

We regard the association of organic-rich and finely laminated biosiliceous sediments with phosphorites as a primary index of sedimentation in an upwelling environment. In addition, we observe thick, cross-stratified accumulations of barnacle coquina; we attribute this low diversity deposit to cool-water deposition on a carbonate shoal. The distinctive occurrence of cool water carbonates at near tropical paleolatitudes suggests the ecologically limiting effect of cold upwelled waters.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)