The stratigraphy of the Kanawha Formation in West Virginia has been confused by regional miscorrelations of many units. To resolve these inconsistencies, this report has: (1) revised and defined three widely distributed marine units as the Betsie, Dingess, and Winifrede Shale Members of the Kanawha Formation (Middle Pennsylvanian); (2) extended the name “Fire Clay” into West Virginia from Kentucky for a coal bed regionally identified by its flint clay (tonstein) parting and miscorrelated in different areas of West Virginia as the older Hernshaw coal bed or the younger Chilton coal bed; and (3) reestablished the stratigraphic positions of several key coal beds that have been regionally miscorrelated from their type areas. A stratigraphic section parallel to depositional strike, from the Kanawha River Valley in central West Virginia to the Tug Fork of the Big Sandy River in southwestern West Virginia, shows the correlation and continuity of marine members and coal beds of the middle part of the Kanawha Formation.

The Middle Pennsylvanian Fire Clay tonstein, mostly kaolinite and minor accessory minerals, is an altered and lithified volcanic ash preserved as a thin, isochronous layer associated with the Fire Clay coal bed. Seven samples of the tonstein, taken along a 300-km traverse of the central Appalachian basin, contain cogenetic phenocrysts and trapped silicate-melt inclusions of a rhyolitic magma. The phenocrysts include beta-form quartz, apatite, zircon, sanidine, pyroxene, amphibole, monazite, garnet, biotite, and various sulfides. An inherited component of the zircons (determined from U-Pb isotope analyses) provides evidence that the source of the Fire Clay ash was Middle Proterozoic (Grenvillian) continental crust inboard of the active North American margin. 40 Ar/ 39 Ar plateau ages of seven sanidine samples from the tonstein have a mean age of 310.9 ± 0.8 Ma, which suggests that it is the product of a single, large-volume, high-silica, rhyolitic eruption possibly associated with one of the Hercynian granitic plutons in the Piedmont. Biostratigraphic analyses correlate the Fire Clay coal bed with a position just below the top of the Trace Creek Member of the Atoka Formation in the North American Midcontinent and near the Westphalian B-C boundary in western Europe.

40 Ar/ 39 Ar plateau age spectra of seven sanidine samples from the Fire Clay tonstein (Middle Pennsylvanian), collected along a 300-km traverse in the Appalachian basin, range from 310.3 to 311.4 Ma. All plateau ages agree, within the limits of analytical precision, with their respective total gas ages. This agreement, together with the reproducibility between samples, suggests the analyzed samples did not contain any significant contaminant feldspar. The mean of these seven plateau ages, 310.9 ± 0.8 Ma, is interpreted to represent a precise numerical estimate of time of eruption and deposition of this tonstein and the coal bed in which it is found. The lack of any discernible difference between the age of two samples of the Fire Clay tonstein collected from east of the Pine Mountain thrust fault, along with the age of five samples from west of this fault, suggests that the Fire Clay tonstein has been reliably correlated with a tonstein on the Cumberland overthrust sheet. This correlation, together with the age data presented in this paper, indicates that the Pine Mountain thrust fault must be younger than the 310.9-Ma age obtained for the Fire Clay tonstein. The Fire Clay tonstein is biostratigraphically correlated with the Trace Creek Shale Member of the Atoka Formation in the Midcontinent of North America and with a position near the Westphalian B-C boundary in Western Europe. Our age of 310.9 ± 0.8 Ma for the Westphalian B-C boundary represents a well-constrained point, useful for the numerical refinement of the geologic time scale.

Volcanic ash that falls into marine settings commonly alters to smectitic deposits known as bentonites, the volcanic origin of which has been recognized for many decades. However, volcanic ash falling into nonmarine coal-forming environments generally alters to kaolinitic claystones called tonsteins, and these beds have only recently been universally accepted as being volcanic in origin. The recognition of tonsteins as altered volcanic ash is based on mineralogy, texture, radiometric age, and field relations. Tonsteins occur on almost every continent, but are best known from Europe and North America. Their geologic range is coincident with that of coal-forming environments; i.e., from Devonian to Holocene. The coal-forming environment is well suited for preservation of thin air-fall deposits because it features low depositional energy, topographic depression, rapidity of burial by organic matter, and lack of detrital input due to the baffling effect of plant growth. Volcanic ashes deposited within or beneath peat beds are strongly affected by humic and fulvic acids generated from organic matter. This acidic, organic-rich, highly leaching environment is partly responsible for the alteration of volcanic glass and mineral phases into kaolinite by first-order (solution-precipitation) reactions. Bed thickness also affects ash alteration, resulting in a vertical zonation of clay mineralogy in thick beds. In addition, voluminous ash falls can have an important effect on the biological and hydrological regimes of the peat swamp. Most distal tonsteins contain a restricted suite of primary volcanic minerals, such as euhedral beta-quartz paramorphs and water-clear quartz splinters (both with glass inclusions), sanidine, idiomorphic zircon, biotite, rutile, ilmenite and magnetite, apatite, allanite, and other accessory minerals specific to a silicic magma source. Textural features indicating an volcanic air-fall origin include bimodal size distribution of components, “graupen,” accretionary lapilli, altered glass bubble junctions, and aerodynamically shaped altered glass lapilli. Radiometric dating of primary minerals in tonsteins shows that they are coeval with the stratigraphic ages of enclosing rocks. Tonstein field relations indicate an volcanic air-fall origin because they are thin, widespread, continuous layers, with sharply bounded upper and lower contacts, that often pass beyond the bounds of the swamp and are occasionally penetrated by stumps in growth position. The volcanic air-fall origin of tonsteins predicates their usefulness in many geologic studies. Because they are isochronous, tonsteins can be used to vertically zone coal beds and thus provide controls for geochemical sampling, organic petrography studies, and mine planning. Regional correlations of nonmarine strata can be made with tonsteins, and intercontinental correlations may be possible. Furthermore, the presence of clay-free volcanic-ash layers in coal beds may indicate a raised-bog origin for the peat swamp. Radiometric dating of primary volcanic minerals in tonsteins allows age determination of coal beds and the calibration of palynomorphic zones. Multiple tonsteins in thick coal beds may be useful for studying the style and history of explosive volcanism.