--> ABSTRACT: Barite and Phosphate in Devonian Slaven Chert: Example of Deposition in an Oxygen Minimum Zone, by Karen K. Graber and Henry Chafetz; #91030 (2010)

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Barite and Phosphate in Devonian Slaven Chert: Example of Deposition in an Oxygen Minimum Zone

Karen K. Graber, Henry Chafetz

Bedded barite and phosphate within the Slaven Chert in East Northumberland Canyon, Toquima Range, Nye County, Nevada, formed within an oxygen minimum zone during Late Devonian. The East Northumberland Canyon deposits are a part of the Roberts Mountains allochthon, which was overthrusted onto eastern carbonate shelf deposits by the Late Devonian-Early Mississippian Antler orogeny.

Petrographic and stratigraphic analyses of the barite and chert beds indicate they formed as sedimentary and early diagenetic precipitates. The barite occurs as layers of fine crystals and/or spherulites. The finely crystalline beds lack matrix chert and probably formed at and/or above the sediment-water interface. In contrast, radiating spherulites have radiolarian fossils, phosphate, and silt grains within them, and are most commonly associated with matrix chert. These data suggest that the spherulites grew within the sediment as an early diagenetic mineral.

High organic-matter concentrations (up to 6%), well-preserved microfossil skeletal hash, and phosphate in barite and chert deposits indicate that the rocks formed in an anoxic environment. REE values of phosphate from the Slaven Chert resemble patterns of phosphorites formed in zones of coastal upwelling at low latitudes versus phosphate formed in non-upwelling areas. These facts suggest the barite, chert, and phosphate formed within an oxygen minimum zone associated with coastal upwelling currents.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.