LE3 .A278 2015

2015

Pufahl, Peir

Acadia University

Master of Science

Masters

Geology

Earth & Environmental Sciences

The Neoproterozoic Salitre Formation (ca. 610 Ma) of the Irecê Basin, Brazil, is a 1.2-km-thick succession of peritidal limestone and economic phosphorite that accumulated on an unrimmed epeiric ramp. Lithofacies stacking patterns indicate deposition occurred during a marine transgression punctuated by higher order fluctuations in relative sea level that produced meter-scale, shallowing-upward peritidal cycles. Cycle bases are formed of subtidal, cross-stratified grainstones and hemispheroidal columnar stromatolite reefs. Phosphorite is restricted to the paleocoast where columnar stromatolitic biostromes colonized expansive intertidal flats that prograded over subtidal facies as accommodation filled. Aggradational and then retrogradational stratal stacking of cycles indicates deposition occurred during the transition from lowstand to transgressive conditions. Phosphogenesis was restricted to the shore because stromatolitic biostromes created the necessary solution and surface chemistries for phosphogenesis. Microbial processes associated with stromatolitic growth actively store, release, and concentrate phosphate to promote authigenic francolite precipitation. Also important were the sealing effects of interbedded, fine-grained storm layers. Energetic subtidal environments where stromatolitic patch reefs developed promoted recycling of P back to seawater, preventing phosphogenesis. Petrographic and stable isotopic data (C, O) indicate that subsequent hydrothermal alteration resulted in pervasive dolomitization and remobilization of P to precipitate secondary phosphatic minerals and produce economic phosphorite. These results suggest that the benthic P-cycle in the Neoproterozoic was more complex than previously surmised and emphasize the multifaceted significance of microbial processes.

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https://scholar.acadiau.ca/islandora/object/theses:353