ABSTRACT: During the Late Jurassic (Tithonian), a worldwide phosphorogenic episode is recorded from deposits occurring along the western margin of continental masses. Mesozoic outcrop studies suggest the origin of bedded neritic and deep-water phosphatic limestones in selected remnants of the San Pedro del Gallo terrane southwest of the Walper Megashear in northeastern Mexico were dominated by tectonics along a transform plate boundary. Three lower Oxfordian to upper Hauterivian tectonically isolated remnants of the San Pedro del Gallo terrane are examined, concentrating on the phosporite bearing La Caja Formation (lower Kimmeridgian to upper Tithonian). The primary phosphorus source for both the neritic and deep-water bedded phosphatic limestones within the La Caja Formation seems to have been upwelling of nutrient-rich water, followed by degradation of organic matter associated with biologic communities. Based on biostratigraphic evidence and geochemical fingerprinting, one suite of bedded phosphate stratiforms within the La Caja Formation is interpreted to have originally developed on a tectonically unstable shelf margin at neritic depths, subsequently redeposited out in the basin as turbidites, and became intercalated with outer bathyal to upper abyssal carbonate and siliciclastic sediments. Included within these deep-water facies of the LaCaja Formation is an outer bathyal to upper abyssal second suite of bedded phosphatic limestones (BPL). The formation of neritic phosphatic sediments is reasonably well understood by most carbonate and phosporite workers; however, the La Caja BPL represent the deepest reported paleobathymetric occurrence of this lithotype. The history of bedded phosphorite formation within the San Pedro del Gallo backarc basin reflects changes in paleocurrents and shifts in zones of nutrient-rich upwelling due to changes in basin configuration, and is a significant constraint on interpretations of continental paleogeography in the Northern Hemisphere during the Late Jurassic.


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