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Variation in Skeletal Carbonate Abundance and Preservation under Upwelling Conditions: a Case Study in the Permian Phosphoria Formation

Madeline Marshall
Department of Geophysical Sciences, University of Chicago, Chicago, IL, USA
[email protected]


Marine systems that experience coastal upwelling are characterized by elevated nutrient supply, increased biological productivity, greater rain of organic matter, and dysoxia to anoxia at the seafloor. These conditions produce distinctive facies of organic-rich mudstone, phosphorite, and chert along a gradient from core to margin of upwelling cells in both ancient and modern seas. I will investigate patterns of abundance and preservational state of macroinvertebrate and fish skeletal remains within the Permian Phosphoria Formation of southeastern Idaho. Skeletal patterns will be evaluated in a sequence-stratigraphic context to assess the additional role of hiatuses (sediment starvation and erosion) in the concentration and preservation of biogenic particles.

These Permian phosphatic deposits are world-renowned for their mining production, and are well-exposed in repeated sections due to post-Paleozoic folding and faulting. This will be the first detailed, parasequence-scale sequence stratigraphic interpretation of the western Phosphoria Basin, and one of the few studies to investigate bioclast preservation as a function of upwelling conditions.

Preliminary research indicates that skeletal remains will be most abundant in dysoxic facies along upwelling cell margins, where nutrient supply favors animal populations but anoxic events are sporadic rather than persistent and can enhance postmortem preservation. Stratigraphically significant surfaces will likely have higher skeletal abundances owing to low dilution of bioclastic input, but retarded burial might yield poorer preservational states of those remains. Results from this study will provide new insights into spatiotemporal trends in bioclastic fabrics, including the lateral extent and thickness of moldic intervals and associated phosphatic and organic-rich sediments.

AAPG Search and Discovery Article #90199 © 2014 AAPG Foundation 2014 Grants-in-Aid Projects