Applications of tonsteins to coal geology: some examples from western United States

Abstract Tonsteins are thin, distinctive, widespread clay bands or partings that are associated with coal beds. They are now generally accepted as the product of alteration of volcanic ash, mostly as distal air-falls from violent eruptions of siliceous material. In tonsteins, glass predominated in the original ash, but was usually accompanied by phenocrysts of quartz, zircon, magnetite/ilmenite, feldspar, and other minerals. The glass and some of the phenocrysts altered to kaolinite, less commonly to smectite. Tonstein studies can yield geologic information of great variety and significance. However, many tonstein occurrences are overlooked in the field. Most important, this means that the potential value of tonstein studies goes largely unrecognized and undeveloped. Following are some examples of untested ideas as well as some applications that merit further development. Where available, examples are given, mainly from western North America. Tonsteins as marker beds: Assuming that tonstein represent air-fall volcanic material deposited within a short time (days to months), each one is of the same age everywhere and distal occurrences may be laterally extensive. Recognition of their widespread character has lead to their empirical use in coalbed correlation for half a century (Williamson, 1970). Their use as marker beds has much more potential. Tonsteins have been observed to enter the top of a coal, pass diagonally through it, and out the bottom. This allows recognition of changes in the geographic position of coal formation through time. Similarly, multiple tonsteins in a given coal divide it into exact time slices regardless of changes in thickness or petrographic composition (Ryer et al., 1980). There is some chance that tonsteins may serve as marker beds on an intercontinental or even global scale. Eruptions of magnitudes vastly greater than any in historic times have been documented in the geologic record. Extrapolating from modern examples, one can speculate on the likely record of such larger events, although the interpretation will be complicated by plate tectonics and changes in wind patterns as well the improbability of simultaneous coal-forming environments (to preserve tonsteins) in widely separated locations. Efforts are currently underway to correlate Carboniferous tonsteins in Britain and continental Europa with those in the eastern United States. Tonstein as absolute (numerical) time horizons: Recent technological improvements in radiometric dating provide great opportunities (Kunk and Sutter, 1984). Precision is available to less than a million years in the Carboniferous; step-wise heating permits evaluation of sample alteration and laser-microprobe techniques can produce age measurements from single small grains. The latter vastly reduces the sample size required so that even cores and cuttings from bore holes are potentially datable. The small sample size also permits distinctions of detrital contaminants or other grains not related to the age of the volcanic event. Coals, with their well-preserved phenocryst-bearing tonsteins, could become the most datable part of the sedimentary record along with bentonites in marine shales. Furthermore, increased availability of precisely dated coal beds should lead to precise radiometric ages for palynological zones, thereby increasing the usefulness of palynology for time determination. Tonsteins in interpretation of paleodepositional environments: Beneath any given ash is the land surface that existed at the time of eruption. The nature of the contact between the resulting tonsteins and subjacent coal should reflect the character of that surface, including topographic irregularities, type of vegetation, as well as the presence of open water. Lateral variations in these should record the distribution of sub-environments; that is, local paleogeoraphy. Evidence of early compactional effects may indicate the water content and physical properties of the substrate. In similar fashion, the re-establishment of vegetation may be recorded in nature of the contact and composition of the coal immediately above a tonstein. Lateral changes may reflect differences in the time or kinds of plants involved in resumption of peat accumulation. On a larger scale, differences in plants might reflect climatic changes related to world-scale eruptions. Tonsteins in coalification, diagenesis, and coal quality: Alteration of the volcanic material involves substantial loss of chemical components in solution — mainly Si, Na, Mg, Ca, and Fe — as well as a host of minor and trace elements. Where do they go? Presumably these elements pass from the ash parting through the adjacent peat and are totally removed from the system. However, to my knowledge, there are no studies of the possible effects tonsteins have on the composition of overlying or underlying coal. In some cases, it is clear that elements have been added to tonsteins but without clear indication of the source, timing, or pathways involved. For example, some tonsteins are impregnated with or partly replace by siderite; where unaltered feldspars are preserved, this suggests early diagenesis (Triplehorn et al., 1984). Aluminum phosphate minerals of the crandallite group are becoming more frequently recognized and sometimes almost totally replace kaolinite. The source, timing, and pathway for the phosphorous is currently being investigated, as well as those for Sr, Ba, rare earths and other elements (depending on the mineral variety present). The inorganic part of mineral matter in coal is generally interpreted as detrital sediment washed into the coal-forming environment, or as later chemical precipitates introduced by ground or diagenetic water. Wind transport, including but not restricted to volcanic ash, should also be considered. Such material may be thoroughly mixed with the organic matrix so that it is not recognizable in a discrete mineral-rich layer and certainly not recognizable as a tonstein in core or outcrop. Potential for advances in coal geology: Tonsteins in coals have the potential to provide better time control than the fossils in marine rocks, to permit detailed analysis of paleoenvironments of deposition, and to increase understanding of the composition of coals and the coalification process.