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Exploration and Evaluation Techniques |
1 Department of Earth Sciences, University of Sheffield, Sheffield S1 3JD, UK
2 US Geological Survey, Reston, VA 22092, USA
Tonsteins are thin mudstone beds composed of kaolinite that formed from the alteration of volcanic ash in coal-forming environments. In the Coal Measures of Britain, tonsteins have been recognized as volcanic in origin based on bedform, volcanic textures, field relationships with other volcanics, distinctive volcanic minerals and an igneous geochemistry. The sharp contacts, together with a contrasting composition with adjacent sediments and lateral continuity, are diagnostic of an altered air-fall volcanic ash. Passage into less altered volcanics and into a K-bentonite have both been shown in Britain. A more direct determination of volcanic origin is possible from the identification of volcanic minerals, whole-rock geochemistry and analyses of silicate melt inclusions in volcanic minerals such as ß quartz. Once a volcanic origin is established the stratigraphic value of a tonstein is enhanced. Immobile trace elements in tonsteins also provide information on the tectono-magmatic setting of the parent volcano. Two tonstein groups are identified in the Coal Measures of Britain, one formed from basic (basaltic) ash of local origin and the other from acid (rhyolite or rhyodacite) ash, probably associated with distant Plinean or ultraplinean eruptions at a destructive plate margin to the south or southeast.
