Gigantic Ordovician volcanic ash fall in North America and Europe: Biological, tectonomagmatic, and event-stratigraphic significance: Comments and Replies COMMENT Donald R. Chesnut, Jr. Kentucky Geological Survey, University of Kentucky, Lexington, Kentucky 40506-0107 Huff et al. 's (1992) paper on the Ordovician bentonites has broad implications in several fields of geology. Their work should help make the Millbrig-Big Bentonite event a valuable stratigraphic "golden spike" across two ancient continents. However, I am still unclear about the dispersal pattern of the ash and the location of the volcanic source of the ash. In my studies of Pennsylvanian tonsteins of eastern North America, I have attempted to determine the dispersal mechanism and volcanic source for a late Paleozoic (311 Ma) ash fall (Chesnut, 1985). I assumed that widespread dispersal of large quantities of ash is controlled by upper-level winds (upper troposphere), such as up- per convection cell winds (e.g., Hadley and Ferrel) and the jet streams. Jet streams are generally not of sufficient breadth to deposit a wide ash fall, but upper convection cell winds are especially at- tractive for dispersal of ash along broad areas. On the basis of the original ash distribution (Huff et al., 1992, Fig. 4) plotted on the Middle Ordovician plate reconstruction of Scotese and McKerrow (1991), eastern North America and Balto- scandia would have been influenced by the Southern Hemisphere Hadley and Ferrel convection cells (Fig. 1). These convection cells move seasonally, such that eastern North America was within the influence of the Hadley cell in the Southern Hemisphere summer and perhaps within the influence of the Ferrel cell during the southern winter. Baltoscandia may have been dominated by the Ferrel cell during both seasons. The upper Hadley cell winds are generally more predictable than the winds of the other convection cells because the Hadley cell is the primary convection cell generated by solar heating (insolation). In fact, the strong Hadley cell controls to a large degree the position of the other convection cells. Figure 1. Wind patterns in latitudes where Middle Ordovician North Amer- ica and Baltoscandia were located. Open arrows indicate upper-level winds. The upper-level Hadley winds in the Southern Hemisphere gen- erally come from the northeast, and the upper-level Ferrel winds come from the southwest (Fig. 1). Thus, an ashfall would be expected to be elongated northeast-southwest when influenced by either of these two cells. However, the Middle Ordovician ash-fall pattern of the Millbrig-Big Bentonite beds is almost perpendicular to the ex- pected trend (Huff et al., 1992, Fig. 4). Even more perplexing is my attempt to place a single volcanic source somewhere between the two continents which would provide wind-blown ash dispersal pat- terns matching the Millbrig-Big Bentonite beds. If these beds belong to a single ash fall from a single volcanic source, as Huff et al. proposed, a different dispersal mechanism would be required. In this case, they proposed that ultraplinian, mushroom-shaped dispersal patterns overrode the prevailing wind patterns (Huff et al., 1992, p. 876). This would require a mushroom-shaped ash cloud several thousand kilometres in diameter, which is one or two orders of magnitude greater than that described by Carey and Sparks (1986). There are several possible solutions to this ash-dispersal prob- lem. Perhaps the plate reconstructions of Scotese and McKerrow (1991) are wrong, and the plates should be rotated to match the wind patterns. Or perhaps the plates are mislocated and should actually be repositioned northward where Northern Hemisphere Hadley cell wind directions match the ash-fall pattern without rotating the plates. Neither of these two alternatives seems likely, on the basis of the large amount of data compiled by Scotese and McKerrow to arrive at their reconstruction. Perhaps, then, the reconstructed ash-fall patterns reflect preservation rather than the actual Middle Ordovi- cian dispersal pattern, which may have been oriented differently. The reliability of this evidence may be critical to Huff et al.'s dis- persal interpretation. Perhaps numerous volcanoes were scattered across a broad region, and the ash-fall patterns reflect these scattered sources. How- ever, the tight radiometric and geochemical constraints Huff et al. (1992) reported seem to indicate a single volcanic source. Perhaps there were unusual perturbations in the wind patterns caused by unusually strong orogenic effects, storms, or monsoons; yet these effects would not be expected over such a large region. If the evi- dence presented by Huff et al. (1992) is true, then an ultraplinian, mushroom-cloud dispersal mechanism, which at first seems implau- sible at this scale, is also correct. The prospect that these beds represent a truly monumental volcanic eruption on a scale never before recognized should incite further studies that have wide-rang- ing implications, including volcanically induced greenhouse effects, geochemical alteration of sea water, and dispersal patterns for as- teroid impacts. But first this single-volcano-dispersal scenario must be tested further. My concern is the reliability of the reconstructed ash-fall pattern (Huff et al., 1992, Fig. 4). What is Huff et al.'s evidence that the reconstructed pattern reflects a true ash-fall pattern rather than merely ash-fall preservation? REFERENCES CITED Carey, S., and Sparks, R.S.J., 1986, Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns: Bulletin of Volca- nology, v. 48, p. 109-125. Chesnut, D.R., Jr., 1985, Source of the volcanic ash deposit (flint clay) in the Fire Clay coal of the Appalachian Basin: Congrès International de Stratigraphie et de Géologie du Carbonifère, 10th, Madrid, 1983, Compte Rendu, v. 1, p. 145-154. Huff, W.D., Bergstrom, S.M., and Kolata, D.R., 1992, Gigantic Ordovician volcanic ash fall in North America and Europe: Biological, tectono- GEOLOGY, April 1993 381