Regional Stratigraphy and Sedimentology of Phosphatic Event Beds and Associated Strata in the San Gregorio Formation (Upper Oligocene-Lower Miocene), Baja California Sur, Mexico

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PI's will integrate stratigraphic, sedimentologic and petrologic approaches in an investigation of the phosphate-rich San Gregorio Formation (Upper Oligocene- Lower Miocene), a widespread and economically-important unit in Baja California Sur, Mexico. Working collaboratively with Mexican investigators, we aim to clarify the roles of paleogeography and event deposition in the origin of phosphorites and associated strata. Initial field observations indicate that the San Gregorio Formation is event stratified. Our findings suggest that transportation of many phosphatic sediments was genetically- linked to explosive volcanic episodes; furthermore, deposition of phosphatic event strata was exclusively associated with short- duration intervals of infaunal bioturbation. Phosphatic grain types vary widely, but are consistently unimodal within individual beds; we wish to understand the systematic stratigraphic and regional variability in phosphatic grain type and to decipher what these variations signify about the evolution of source areas for allochthonous phosphorites. Petrologic studies of phosphatic grains, including the employment of backscatter SEM on polished thin sections, will permit correlations between observed trends in phosphatic grain type and regional sedimentary paleogeography within the San Gregorio Formation and will further document the association of microbes and other biota in the origin of phosphatic grains.