VOLCANIC ASH BED FORMATION AND CONDENSATION PROCESSES: AN EXAMINATION FROM DEVONIAN STRATIGRAPHIC SEQUENCES

With the recognition that volcanic ashes in sedimentary successions may represent more than one eruptive event, a rush of conceptual alternatives has arrived. A range of post-depositional physical, biological and chemical processes contribute to the preservation, alteration or destruction of primary ash fall layers. One commonly expressed model of ash bed formation is ash condensation/sediment starvation, associated with transgression. However, examination of Devonian K-bentonites show that this is no simple, fits-all answer.

The Lower to Middle Devonian (Lochkovian-Eifelian, ~30 m.y.) succession across the Appalachian basin is comprised of 11 major (“third order”) stratigraphic sequences. In most sequences, a lowstand systems tract (LST) is not recognized. Sequences yield recognizable transgressive, early highstand, and late highstand systems tracts (TST, eHST and lHST, respectively). Of the eleven sequences, only four show significant K-bentonite buildups. Six of them yield no volcanic ash layers, and one sequence features only one.

More than 80 ancient volcanic ashes (K-bentonites) have been found in Lochkovian to Eifelian strata across the basin. Most of these occur in four clusters of 8-15 or more beds. The four major clusters are not explained by a simple model of condensed deposition during TSTs. The oldest cluster (Bald Hill K-bentonites, Lochkovian) is found in the upper part of a thick TST, extending up through the eHST. The second (Sprout Brook K-bentonites, Emsian) occurs in the upper TST of its sequence. The third major cluster, the Tioga Middle Coarse Zone (Eifelian), occurs in the eHST of its sequence. The youngest cluster (Tioga A-G K-bentonites, Eifelian) is found throughout the TST. The second and fourth clusters (+/- the third) occur close below or during the onset of tectonic-induced subsidence in the foreland basin. This could yield a greater condensation effect. However, similar Tioga A-G trends in shelf carbonate and starved-basin clastic successions do not support a strong condensation factor. Subsidence also indicates increased tectonism in the orogenic belt, which may yield greater volcanism, and more ash beds. Other non-clustered K-bentonites (one to a few closely spaced beds) occur variously on sequence-bounding unconformities, or within eHST and even lHST and LST deposits.