Geochemical fingerprinting of volcanic ashes from the Fort Peck Lake Area, Montana, USA

Abstract

Within the lower Paleocene Tullock Formation that is exposed in northeastern Montana (USA), an alternation of fluvial deposits with lignite coal seams is observed. In these coals different tephras are interbedded. In order to test if the coal seams are lateral continuous features and thus formed by allogenic processes, a tephrostratigraphic framework is established for the upper part of the Tullock Formation. This framework is based on the major element composition of titanite grains and the physical properties of the tephras. Additionally, the structures within the tephras are assessed and the geochemical data is compared with previous studies on the provenance of the tephras. Geochemical fingerprinting of titanite grains shows that certain tephra layers can be identified and distinguished. However, a number of the titanites from the different tephra beds appear to be similar in major element composition. Nevertheless, the application of multidimensional scaling to total elemental composition of the titanite grains reveals that also the more subtle differences can be distinguished. It is possible to establish a regional tephrostratigraphic framework in which some coal seams can even be correlated over ~80 kilometers by combining the elemental data with observed physical properties of the tephras in the field. Moreover comparing the geochemical data with existing geochemical data indicates that volcanic ashes preserved in coal #5, #7 and #9 (X and W coal zones) originate from an andesitic magma while coal #10 and coal #11 (V and U coal zones) are dominantly preserving ashes that originate from a more rhyolitic magma. This might imply that the provenance of the tephras is from two different sources or that they record fractional crystallization within a single magma chamber from an andesitic magma evolving to a more rhyolitic magma. However these interpretations on the provenance of the tephras are hypothetical and uncertain and need more specific research. The tephrostratigraphic correlation pattern demonstrates independently that coal #7 (X-coal) and #9 (W-coal) can be correlated for over 60 kilometers and #10 (V-coal) and coal #11 (U-coal) can be correlated for over 80 kilometers. The lateral extensiveness of these lignites support the hypothesis that allocycles and not autocycles were controlling coal formation. Yet to fully demonstrate which type of allocycle is responsible, exact dating of the tephra layers is necessary and a depositional model on how the allocycle controls coal formation should be further formulated and tested.