Variations in Phosphatic Grain Types and Facies Variations in the Oliog-Miocene San Gregoria Formation of Baja California Sur, Mexico

LEDESMA, MARIA, KURT GRIMM, and BOB GARRISON, University of California, Santa Cruz, CA

Sediments from the San Gregoria Formation, including phosphorites, were deposited during a period of active volcanism in a fore-arc basinal setting. Three major types of phosphatic grains are present in the phosphorites: multiple coated grains, concentrically coated grains with multi-generations of phosphate accumulation around a nucleus and commonly bored, possibly by bacteria, single-coated grains with one generation of phosphatization surrounding a detrital nucleus, and non-nucleated peloidal grains that also represent single events of phosphatization. Phosphatized microstructures of probable microbial origin have been identified by electron microscopy in the multiple coated grains, suggesting possible involvement in the phosphatization process.

In the northern sections of the study area, transport and deposition of phosphatic grainstones into an oxygen-poor basin occurred via turbidity currents. Here, phosphorites occur interbedded with deep-water porcelanites, cherts, and thinly laminated organic-rich mudrocks; the dominant type of phosphatic grain is the multiple coated variety, suggesting original formation on a sediment-starved outer shelf or isolated bank prior to redeposition. In the south, phosphorites are also allochthonous, but here they are associated with more detrital-rich, shallow-water lithologies (including coquinas and tidal laminites). Dominant phosphatic grains are single-coated and non-nucleated peloids, suggesting fewer episodes of phosphatization. In contrast to the northern occurrences, these southern p osphorites appear to have been redeposited on a shallow shelf.

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