The Ash-Fall Pattern of the Fire Clay Tonstein, Central Appalachian Basin: Paleogeographic and Plate Tectonic Implications

Abstract

Thickness data for the Fire Clay tonstein bed (Middle Pennsylvania, Breathitt Formation) of Kentucky and its correlatives in West Virginia, Virginia, and Tennessee show a moderately complex ash-fall pattern across the central Appalachian basin. Within the tonstein bed are four or possibly more distinct graded units, which may represent multiple ash-fall events. The mineralogical and chemical signatures for all tonstein beds are similar and suggest a single magmatic source. The absence of coal or non-ash-fall detrital bands between the individual graded units demonstrates that the events occurred in rapid succession. The ash-fall deposits are elongated and thin generally from southwest to northeast. The longest and thickest part of the deposits lies along the southeastern side of the basin, where the deposit's southwestern end is up to 12 in. thick. The areal distribution indicates ash deposition on unstable surfaces and probably changes in wind patterns. Paleogeographic interpretation suggest that the Fire Clay ash fall or falls were deposited from a western source near the paleoequator. This source is thought to be on the Yucatan block, which was involved in collisional tectonics and eastward-directed subduction during the Middle Pennsylvanian. The block contained a volcanic chain from which the deposits may have originated.more » This composite ash-fall bed, which became the Fire Clay tonstein, formed the most important isochron in the Middle Pennsylvanian of North America.« less