UPPER FREEPORT ROOF SHALES, APPALACHIAN BASIN

Roof shales of the Upper Freeport (UFR) cover 175,000 sq km in PA, OH, MD and WV, and represent lakes bounded to the north and southeast by cratonic and orogenic uplands. Fresh water origin of these shales is supported by both fossil content and geochemistry. Fossils include widespread conchostracans (Palaeolimnadiopsis), paleolimulids (Euproops danae), and bivalves (Anthraconaia pruvosti ). High ratios of organic carbon to pyrite sulfur are consistent with sulfate-limited, low salinity depositional conditions. However, rhythmic laminae are found in 6 outcrops from central PA to MD, and one, numerically analysed, in WV exhibited lunar periodicities, which suggests connection to the open sea. The Appalachian Basin was apparently connected with the more open Illinois Basin by sags in the Cincinnati Arch; marine shales in central KY are equivalent to the fresh UFR in OH & WV. Because the paleoequator paralleled the orogen, rivers supplied plentiful fresh water to keep the basin northeast of KY fresh enough to explain the fossil and geochemical data. The presence of tidal laminites present a dilemma because an opening broad enough to allow tidal flux 650 km from eastern KY would also allow entry of noticeable marine water into the lake basin. Possible modern analogues (lower Amazon) would require broad-mouthed funnel-shaped estuaries which decrease rapidly in width with distance landward. To maintain a tidal range in an estuary 20 m deep would require a 30-fold decrease in estuary width over a distance of 650 km. If fresh water influx intense enough to keep a relatively wide basin effectively fresh is considered unlikely, then the possibility of wind-driven seiches in a more enclosed fresh water body (perhaps analogous to the modern Baltic) will have to be considered. A 20-m deep, 650 km long fresh-water body would have a wind-driven seich with a period of 26 hours which could result in laminite deposits.