02888nas a2200181 4500008004100000245014600041210006900187300001200256490000800268520225600276653002002532653001302552653001702565653002102582653001702603100002502620856006102645 2004 eng d00aK-bentonites, Volcanic Ash Preservation, and Implications for Lower to Middle Devonian Volcanism in the Acadian Orogen, Eastern North America0 aKbentonites Volcanic Ash Preservation and Implications for Lower a474-4890 v1163 a

Lower to Middle Devonian marine strata in the Appalachian foreland basin feature up to 80 or more thin K-bentonites that represent ancient volcanic ashes. The time vs. space distribution of K-bentonites through the Lochkovian to Eifelian Stages (representing ∼30 m.y.) shows a distinct pattern of clustered multiple beds, several scattered beds, and thick intervals with no K-bentonites. Four clusters of 7 to 15 individual, closely spaced layers occur in the middle Lochkovian (Bald Hill K-bentonites, Kalkberg–New Scotland Formations), late Pragian or early Emsian (Sprout Brook K-bentonites, Esopus Formation) and early Eifelian (two clusters, the Tioga middle coarse zone and Tioga A–G K-bentonites, Onondaga Formation).

Detailed examination of these Devonian K-bentonites shows that in many cases they do not represent a single eruption. Multilayered beds, fossil layers within beds, authigenic minerals (e.g., glauconite and phosphate nodules), subjacent hardgrounds, and an irregular distribution of beds through space and time raise questions about the depositional history and preservation potential of volcanic ash in marine environments and the degree to which the beds represent a primary record of volcanism. These and other lines of evidence indicate that postdepositional physical, biological, and geochemical processes (e.g., sedimentation rate, event, and background physical processes, burrowing) have modified the primary record of these water-laid ash-fall events. These factors may lead to preservation of primary ash deposits or to their resedimentation and/or partial to complete mixing with background sediments.

The preservation potential, and resulting distribution, of the Devonian K-bentonites can be analyzed across a spectrum of preservational magnafacies. In this paper I present a model of ash preservation; the model incorporates environmentally related physical, biological, and chemical processes active in epicontinental seas and marine foreland basins. Conclusions based on the model indicate that the middle Lochkovian, early Emsian, and early Eifelian were times of peak volcanic activity in eastern North America, related to times of increased tectonism in the Acadian orogen.

10aAcadian Orogeny10aDevonian10aK-bentonites10amarine processes10aPreservation1 aVer Straeten, C., A. uhttp://gsabulletin.gsapubs.org/content/116/3-4/474.short