Date of Award
Master of Science (MS)
C. ver Straeten
Detailed analysis of Lower to Middle Devonian K-bentonites in the Appalachian Basin reveals physical and geochemical characteristics within supposed single eruptive event deposits that indicate a more complex depositional history is recorded in many of these volcanic beds. Variations in bedding showing multiple coarse and fine-grained layers within a K-bentonite layer can be seen in some Devonian K-bentonite beds. Microscopic irregularities show variations in the orientation of volcanic grains, and subtle phenocryst layers within a K-bentonite suggest changes in depositional environment where changing water and wave regimes rework and redeposit material on the sea floor. The presence of fossil layers within the same K-bentonite across the basin suggests that the bed records colonization by marine fauna during its deposition. In addition, geochemical, inconsistencies within the same layer show evidence that these layers may record more than one eruptive event in a single layer. Based on the analytical methods used in this study many K-bentonite beds are shown to record a more complex preservational and eruptive history than previously thought. By examining physical, petrologic, and elemental geochemical characteristics of these beds, it is seen that many Devonian K-bentonite beds do not in all cases represent the altered remains of single eruptive events.
K-bentonites either (1) contain volcanic material from a single eruptive event, (2) are the result multiple eruptive events, expressed as a subtle, multiple-layered K-bentonite, or (3) a mixing of volcanic material from a few to many ashfall events where no distinguishable boundary can be seen. The presence of distinctive physical features within layer samples and/or inconsistent trace elemental abundances within phenocrysts from subdivisions of the same bed support the interpretation that more than one eruptive event may have taken place. Data collected in this study strongly supports modern ideas that many Devonian K-bentonites contain more significant volcanic information than previously thought.
Benedict, Lucas, "Complexity of Devonian K-bentonites in the Appalachian foreland basin: geochemical and physical evidence supporting multi-layered K-bentonite horizons" (2004). Geology Theses and Dissertations. 11.