GEOCHEMICAL AND PHYSICAL EVIDENCE INDICATING COMPLEX DEPOSITIONAL AND ERUPTIVE HISTORIES FOR DEVONIAN K-BENTONITES OF THE APPALACHIAN FORELAND BASIN
Analyses of Lower to Middle Devonian K-bentonites in New York, Pennsylvania, and Virginia reveal physical and geochemical characteristics that support modern ideas that a more complex depositional history is recorded in many individual beds. Trace element geochemistry, petrographic thin sections, polished slabs of indurated K-bentonites, and detailed field observations show that an individual K-bentonite bed contains an abundance of information concerning its depositional history. Geochemical inconsistencies between apatite phenocrysts from multiple, individual K-bentonite beds show more than one geochemical fingerprint within a single K-bentonite bed. This suggests that these supposed single event beds may contain volcanic material from two or more eruptive events. Minute irregularities in crystal orientation and alternating coarse and fine grained layering seen in petrographic thin sections of K-bentonites suggest fluctuations in the depositional environment where changing turbulence (and biological activity) reworked and redeposited material on the sea floor. Fossil horizons located within the same K-bentonite bed across the basin suggest that the bed records colonization events by marine fauna during deposition of the K-bentonite. In addition, variations in bedding showing multiple, distinguishable layers within a K-bentonite bed can be observed in some Devonian K-bentonite beds. By examining geochemical and physical characteristics of these K-bentonite beds, it is observed that many Devonian K-bentonite beds do not represent the altered remains of single eruptive events.
A K-bentonite either (1) contains volcanic material from a single eruptive event, (2) is the result multiple eruptive events, and is expressed as a subtle, multiple-layered K-bentonite bed, or (3) represents the mixing of ashfall material from a few to many eruptive events where no distinction between the events can be made. Distinctive physical features within K-bentonite layer samples and/or contradictory geochemical fingerprints of phenocrysts from subdivisions of the same bed support the interpretation that more than one eruptive event may have taken place.