A RECORD OF EXPLOSIVE ACADIAN VOLCANISM PRESERVED IN THE APPALACHIAN FORELAND BASIN

Unroofing of the Appalachians has largely destroyed the record of Devonian volcanism. Two other primary sources of data are available to reconstruct Acadian magmatism: the plutonic rocks of the orogen, and tuffs and K-bentonites preserved in the adjoining Appalachian foreland basin fill (NY, PA, MD, VA, WV, OH). Over 80 altered tephra layers, a record of plinian-type volcanism, are recognized in the Lower to lower Middle Devonian (ca. 30 million years, Lochkovian – Eifelian stages) foreland basin fill. Additional beds are reported from higher strata. The distribution of these ancient tephras through space and time in the foreland basin fill provide relatively high resolution data on the timing, nature, and tectonic setting of the volcanism.

Interpretation of the volcanic record from the tuffs and K-bentonites is not straight-forward and simple, however. Many beds show a complex depositional and/or volcanic history of amalgamation, reworking, condensation, etc. In addition, an unknown amount of the volcanic record is undecipherable due to mixing of ash layers with background sediments by various stratinomic (physical, biological and chemical) processes.

The following observations can be made at present. Four clusters of volcanic beds (8-15 or more beds in each cluster) occur in the Lochkovian (417 Ma, Bald Hill K-bentonites; BH), lower Emsian (408 Ma, Sprout Brook K-bentonites; SB), lower Eifelian (391 Ma, Tioga Middle Coarse Zone; MCZ) and middle Eifelian rocks (390 Ma, Tioga A-G K-bentonites; TAG). Additional single K-bentonites occur scattered through parts of the sedimentary succession. Two of the clusters occur widely across the basin (BH, TAG); the other two are restricted to the northern (SB) or southern parts (MCZ) of the basin, apparently indicative of shifting volcanic centers over time along the orogen. Three of the clusters of volcanics occur at or near the onset of significant deepening events in the foreland basin (BH, SB, TAG). Two, and possibly all three of those are closely associated with load-induced flexural subsidence of the foreland and the initial introduction of thick clastic packages into the basin (SB, TAG, +/- BH), indicative of major pulses of tectonism and orogenic uplift. The MCZ cluster marks the onset of regional subsidence and clastic influx in the south prior to a more basinwide event.