K-BENTONITES, ASH BED PRESERVATION, AND IMPLICATIONS FOR LOWER TO MIDDLE DEVONIAN VOLCANISM, EASTERN UNITED STATES

Lochkovian to Eifelian marine rocks in the Appalachian Basin feature numerous thin clay beds (K-bentonites) that represent ancient volcanic ashes. Deposited in the Appalachian foreland basin adjacent to the deeply eroded Acadian orogen, they provide a significant window into the timing and nature of Devonian volcanism and tectonism in the orogenic belt.

Time-space distribution of Lochkovian to Eifelian K-bentonites shows a distinct pattern of clustering of multiple beds, a few scattered beds, and thick intervals with no K-bentonites. Four clusters of seven to fifteen individual, closely-spaced layers occur in the middle Lochkovian (Bald Hill K-bentonites, ca. 418 Ma, Kalkberg-New Scotland fms.), upper Pragian or lower Emsian (Sprout Brook K-bentonites, ca. 408 Ma, Esopus Fm.) and lower Eifelian (two clusters, the Tioga Middle Coarse Zone and Tioga A-G K-bentonites, ca. 390 Ma, Onondaga Fm.).

A few additional K-bentonite layers occur locally to basinwide through Emsian to Eifelian strata. No K-bentonites are noted through much of the succession, however. This poses a question of whether the presence/absence of K-bentonites is a function of volcanic history in the adjacent orogenic belt, or largely a result of preservational biases in the marine foreland basin, where processes modify the record of primary volcanism.

The distribution of airborne volcanic ash is related to volcanic and atmospheric processes, and geography. Post-depositional processes (e.g., sedimentation rate, physical and biological processes), however, modify the record of primary waterlain ashfall. These factors may lead to preservation of primary volcanic ash beds, resedimentation, or partial to complete mixing of ash with background sediments.

Examination of a spectrum of facies through time appears to indicate that, though filtered through various preservational biases, the middle Lochkovian, upper Pragian-lower Emsian, and lower Eifelian were peak intervals of volcanism in eastern North America.