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Silty marine mudrocks represent one of the most volumetrically important sedimentary rock types, but their genesis remains very poorly understood. It is widely believed that silt and siltstone associated with dominantly calcareous and dolomitic strata is often eolian in origin, but very little documentary evidence has been published in support of this belief.
The Permian Phosphoria Formation provides an intriguing case study of possible windblown silt deposited in marine environments. Preliminary work suggests that organic-rich siltstone facies of the Meade Peak Member represent wind-transported suspension deposits, accumulated during sea level lowstands to transgressions (Carroll et al., 1998). This implies that the long accepted models for the genesis of one of the world's largest phosphate accumulations need to be re-evaluated, to consider the impact of airborne Fe influx on productivity.
If previous climate simulations suggesting zonal wind circulation are correct, time-equivalent eolian-derived siltstone facies should fine in a downwind direction, to the south. In contrast, monsoonal circulation might result in a more equivocal regional distribution of grain sizes. We are measuring grain-size distributions from siltstone and other silty facies at a number of widely distributed sites, correlated based on previous biostratigraphic studies and on outcrop and well-log gamma-ray spectrometry.
We are also using petrographic, microprobe, SEM, and SEM-CL analyses will to document textural evidence for silt origin. Oxygen isotopic analyses of different quartz grain size fractions are being used to constrain their temperature of crystallization. A large, regionally distributed sample set will permit an assessment of the relative importance of reworked versus primary quartz.
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Location map showing Phosphoria Formation paleogeography during deposition of the Meade Peak Member (after Maughan, 1984))
Cross-section of the Meade Peak Member, based on subsurface well logs. Note onlap toward the east (modified from Stephens and Carroll, 1999).
Colin Walling is working on his Ph.D. dissertation on the Phosphoria Formation. He previously completed his M.S. on the same topic. We are collaborating with John Valley and Mike Spicuzza of the UW Stable Isotope Laboratory.
Financial support provided by the National Science Foundation.
page created December 5, 2001