INVESTIGATION OF DIAGENESIS OF PHOSPHATIC SEDIMENTS IN THE ORDOVICIAN REEDSVILLE FORMATION NEAR THE ALLEGHENY FRONT IN PENDLETON COUNTY, WEST VIRGINIA

The Ordovician Reedsville Formation contains an upward transition from carbonates to clastics deposited in a shelf to nearshore setting. It represents the lower part of the Taconian clastic sequence in areas west of the Shenandoah Valley. The pervasive Orthorhynchula community occurs near the top of the Reedsville Formation. Phosphatic material occurs as lingulid brachiopods and nodules in two horizons of siltstones and fine sandstones at the top of the Orthorhynchula zone near the Allegheny Front in Pendleton County, West Virginia. Although phosphatic sediments are common in the sedimentary record, phosphatic sediments in the Ordovician rocks of the Appalachians are comparatively rare. Lingulid brachiopod fragments commonly 1 to 10 mm in length and nodules commonly 1 to 15 mm in diameter are locally abundant. Powder X-ray diffraction indicates the siltstones are rich in quartz with minor amounts of microcline, illite, and chlorite. Petrographic analysis indicates that phosphate nodules are fecal pellets consisting of crushed lingulid fragments with a matrix of quartz and clay. EDS analysis and chemical mapping show impurities of Na, Si, Fe and Sr in many phosphatic grains. Cathodoluminescence (CL) petrography indicates nodules show primarily orange luminescence while lingulid fragments vary in color from dark brown to orange. Darker regions are attributed to Fe in the apatite structure quenching luminescence. Approximately 99% of quartz grains show blue luminescence. Na, Si, Fe and Sr substitute in the apatite structure and these impurities along with CL petrography indicate that the original biogenic hydroxyapatite has undergone extensive diagenesis; however the nodules are better preserved than the lingulid fragments. Although most investigations of phosphatic sedimentary rocks have been on those of the late Paleozoic and Tertiary, investigations of early Paleozoic phosphatic rocks can provide insight regarding long term geologic details of the phosphate cycle and the geochemical stability of sedimentary phosphate.