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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

The Black Death of a Carbonate Ramp: Poisoning of the Phosphoria Platform and Its Relevance to Source Rock Depositional Models

Christopher F. Cassle1; Previous HitSvenTop Egenhoff1

(1) Geosciences Department, Colorado State University, Fort Collins, CO.

The Permian Phosphoria Formation (late Leonardian to Guadalupian) in the northwestern US consists of anomalously large accumulations of phosphorite, organic-rich siltstones and mudstones, carbonates, and chert, with up to 32.9% TOC locally. Deposition occurred along a westward-deepening ramp within the epicontinental Phosphoria Sea along the northwestern margin of Pangea.

The Phosphoria Formation depositional transect exhibits carbonates with sulphate intercalations in supratidal settings, ooids and other pack- to grainstones in shallow subtidal settings, and intercalations of coarse storm beds and carbonate mudstones in deeper shelf positions. Sea-level fluctuations controlled these facies belts as they moved up and down the ramp profile. However, during two distinct transgressional phases the carbonate system was abruptly shut down and dominated by phosphate deposition, characterized by coarse-grained phosphates in shallow-water, grading into finer-grained phosphatic mudstones towards the distal ramp.

The catastrophic demise of the Phosphoria carbonate was likely triggered by the influx of large quantities of TOC-rich, oxygen-depleted waters during transgressive phases. The arid environment and the likely negative water budget of the shallow Phosphoria Sea promoted oceanic waters to be constantly drawn onto the craton. While the enormous amount of TOC partially degraded within this semi-enclosed basin, huge quantities of phosphate was liberated and concentrated within the sediment on the basin floor, as well as replacing the underlying carbonates. With the onset of regressive conditions the influx of TOC and phosphate came to a halt, and carbonate deposition was re-established.

The accumulation of widely traceable phosphate and organic-rich sediments is restricted to a critical moment in Earth history in the Late Permian. During this post ice-age time oceans are thought to be completely stratified and anoxic, preserving large quantities of organic material. During transgressions these TOC-enriched deep-ocean waters were flushed into the Phosphoria basin where they caused a devastating death of the carbonate ramp system and the genesis of the richest phosphate deposit on Earth.