EOLIAN DUST AND THE ORIGIN OF DEVONIAN CHERTS IN THE APPALACHIAN BASIN, USA

Highly siliceous strata were deposited across much of North America when aridity prevailed during the 20 Ma period that spans the Early Devonian and early Middle Devonian. In the Appalachian basin, these siliceous strata include the Shriver Chert (basal unit), Oriskany Sandstone (middle unit), and Huntersville Chert (upper unit), and their stratigraphic equivalents from southwestern Virginia to central New York. The Shriver Chert predominantly consists of decimeter-scale interbedded chert and silty limestone. The overlying Oriskany Sandstone, which is one of the more prolific oil and gas reservoirs in the basin, is a calcite-cemented highly mature quartzarenite that in places grades into limestone containing matrix-supported, rounded quartz sand grains. The Huntersville Chert grades from impure chert in West Virginia into the cherty Onondaga Limestone in Pennsylvania and New York. Previous workers have described seven lithologies in the Huntersville as follows: (1) clean chert, (2) chert with organic material, (3) spicular chert, (4) dolomitic chert, (5) glauconitic chert, (6) silty argillaceous chert, and (7) dolomitic silty argillaceous chert. All lithologies in the Huntersville contain some dolomite and silt-size (quartz) detritus.

At the time of deposition of Devonian siliceous strata, the Appalachian basin was located at approximately 30 degrees south latitude, in a belt of aridity, analogous to modern-day belts of aridity, high pressure, and eolian environments. An eolian provenance has been suggested previously for the quartz in the Oriskany Sandstone; the quartz sand either was delivered to marine environments by wind, or ergs were reworked during sea-level rise, analogous to modern conditions in the Persian Gulf where dust and sand are blown into carbonate environments. Thus, the silica supply to Early Devonian and early Middle Devonian depositional systems in the Appalachian Basin can be explained by temporal and spatial variations in eolian processes in a hot-arid paleoclimate. The chert stratigraphy and petrology is consistent with diagenesis of chemically reactive quartzose dust, i.e., dust delivered by winds was the predominant source of quartz silt, soluble silica, and residual particles of a-quartz in the cherts.