DEVONIAN TERRESTRIAL VERTISOLS IN THE CATSKILL MOUNTAINS, NY: A FINGERPRINT OF EXPLOSIVE ACADIAN VOLCANISM?

Forty years ago, only a small number of altered airfall volcanic tephra layers (“ash” beds, K-bentonites) were known from Devonian sedimentary rocks in the eastern U.S. Today more than 100 discrete layers are documented. These outline an apparent broad brush history of explosive plinian-type volcanic activity during the Acadian orogeny. However, adjacent to an active magmatic arc, why are not hundreds or thousands of tephras documented from the Devonian sedimentary succession?

Explosive eruptions of siliceous magmas eject tephra high into the atmosphere, where it is transported and settles out over all depositional environments downwind. However, preservation of a primary, single-event airfall tephra bed, its modification, or its destruction through mixing with background sediments, is dependent on multiple physical, biological and chemical factors active in depositional environments.

Paleosol studies in the Middle to Late Devonian terrestrial succession in the Catskill Mountains, New York, indicate a pattern of closely-spaced vertical variations in the types of fossil soils. Vertisols feature deformational structures associated with the expansion and shrinking of smectitic clays due to wetting and drying. These alternate with inceptisols, entisols and alfisols, all of which lacked smectite clays and show no similar deformation.

If smectite clays in vertisols were derived from the weathering of igneous rocks exposed in the Acadian orogen, they would occur continuously through the succession – but they don’t. One plausible explanation for the anomalous short term presence/absence trends of smectitic clays is episodic input of airborne volcanic ash into the sedimentary system, and subsequent devitrification of volcanic glass to smectite. In terrestrial settings, discrete, primary ashfall layers would generally be extensively eroded and reworked (except in wetlands, oxbow lakes), and/or be incorporated and mixed into paleosols by soil forming processes.

The sporadic occurrence of smectite-rich vertisols in terrestrial strata of the Acadian foreland appears to represent a “fingerprint” of much more abundant explosive volcanic activity in the Acadian hinterland than is recorded by discrete, altered tephra beds.