DEVONIAN K-BENTONITES, APPALACHIAN FORELAND BASIN: A RELIABLE PROXY FOR ACADIAN VOLCANISM?

Numerous K-bentonite beds occur in Lower to Middle Devonian (Lochkovian-Eifelian) strata of the Appalachian foreland basin, adjacent to the deeply eroded Acadian orogen. The K-bentonites occur predominantly in four clusters of 8-15 beds; a few additional beds occur in parts of the section, and thick stratigraphic intervals feature no K-bentonites. A number of the beds indicate a complex post-depositional history.

K-bentonites may function as time-significant proxies of volcanism during ancient orogenies. However, caution must be used to filter out preservational biases in the sedimentary record. Following deposition of waterlain volcanic ash, numerous physical, biological, diagenetic, and other factors may act upon primary ashfall deposits. Resedimentation, amalgamation, and partial or complete mixing of ash with background sediments smear the primary record of volcanism, as can problems related to diagenesis. This can create a biased perspective of regional volcanic history. A new model of ash bed preservation-destruction in shallow marine settings (e.g., epicontinental seas, foreland basins) permits a more realistic perspective on patterns of ancient volcanism.

Despite the potential for destruction or deception in the primary record of volcanism by post-depositional processes, Lower to Middle Devonian K-bentonites of the Appalachian foreland basin appear to reflect patterns of explosive volcanism during the Acadian Orogeny. The preservation of discrete, altered ash beds in some strata of high physical-biological activity, and their absence in other strata of comparable or little to no physical-biological activity, appears to indicate that K-bentonite distribution through time and space does reflect a plausible, if relatively coarse, history of paleovolcanism.

Four major clusters of K-bentonites indicate major pulses of explosive volcanism in the Acadian Orogen through the Lower to lower Middle Devonian (Lochkovian to Eifelian). These pulses occurred at approximately 417 Ma (Lochkovian-age Bald Hill cluster), 408 Ma (Emsian-age Sprout Brook cluster, and 390 Ma (Eifelian-age Tioga MCZ and Tioga A-G clusters) (dates from Tucker et al., 1998; Roden et al., 1990). They closely correspond to times of reorganization of the foreland, associated with the onset of major Acadian tectonic events.