EOLIAN DUST AND THE ORIGIN OF DEVONIAN CHERT IN EASTERN NORTH AMERICA

Early and early Middle Devonian aridity across North America was accompanied by deposition of copious amounts of chert. In the central Appalachian basin, chert deposition occurred for more than 15 My from the early Lochkovian into the Eifelian (Helderberg Group, the Oriskany Group, and Huntersville Chert). Members of the Helderberg Group contain decimeter-scale interbedded chert and silty limestone. The overlying Oriskany Group consists of the Shriver Chert and the Oriskany Sandstone (SS), a mature quartz arenite cemented by calcite, chert, or quartz. In a few areas, a limestone facies containing matrix-supported rounded quartz sand replaces the sandstone. The Huntersville chert is composed of the following lithotypes: (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 (Sheppard and Heald, 1984). All lithotypes in the Huntersville contain some dolomite and detrital silt-size quartz.

At the time of deposition of Devonian siliceous strata, the Appalachian Basin was approximately 30 degrees south latitude, analogous to modern-day belts of aridity, high pressure, and eolian environments. An eolian provenance has been suggested for the quartz in the Oriskany SS (Grabau, 1932, 1940; Cecil et al., 1991); the quartz sand was either delivered by wind, or ergs were reworked during sea-level rise, analogous to conditions in the Persian Gulf where dust and sand are blown into modern carbonate environments. Thus, the silica supply to Early and early Middle Devonian depositional sea ways in the Appalachian Basin and across North America can be explained by temporal and spatial variations in eolian dust deposition in a hot-arid paleoclimate. The stratigraphy and petrology of the enormous amount of chert in Early and early Middle Devonian marine strata indicate diagenesis of chemically reactive quartz dust, which supplied silica for chert precursor gels and residual particles that are equivalent in size to α-quartz crystallites in chert.