PRESERVATION POTENTIAL OF AIRFALL VOLCANIC TEPHRAS, TERRESTRIAL SETTINGS/DEVONIAN

Middle to lower Upper Devonian terrestrial strata of the Catskill Mountains region, eastern New York, are estimated to be on the order of 2.4 km-thick, and extend across ~9.5% of the state (50,535 km²/19,512 mi²). These strata are characterized by extensive cover, sometimes rugged terrain, the homogeneous and relatively discontinuous character of fluvial-dominated alluvial plain facies, and a near absence of biostratigraphic data. This makes it difficult to correlate, and determine relative and chronostratigraphic age through the Devonian terrestrial succession.

In recent years, several apparent altered airfall volcanic tephras have been found in the Catskills succession. These layers should yield dateable phenocrysts (e.g., zircons), permitting age dates, and possible correlation with known marine tephras, via phenocryst geochemistry (e.g., apatites, quartz inclusions). In terrestrial settings, reworking may lead to impure tephra layers, but may still yield enough primary volcanic phenocrysts to analyze.

Following explosive volcanic eruptions, tephra material is deposited in all environments downwind of the source. Preservation/stratinomy of primary airfall layers depends on several factors, especially burial soon after deposition, and a lack of post-depositional physical or biological mixing/reworking of tephra material. This is especially so on alluvial plain settings.

A greater preservation potential for airfall tephras in terrestrial settings include: Proximity to airfall source, and larger-scale eruptions; elevated detrital sediment deposition rates, with low surface disturbance; low erosion activity associated with surficial water runoff and wind erosion; and a low degree of biological bioturbation/mixing. Physical and biological mixing due to soil development may also disguise/destroy tephra layers.

Subaqueous deposition in low hydrodynamic settings, with low degrees of reworking and mixing, also enhances preservation potential on a terrestrial alluvial plain. These include: palustrine settings (wetlands, esp. anoxic), common in lower coastal plain settings; and lacustrine settings (ponds, lakes, oxbow lakes). Combined, these include flood plain wetlands and temporary lacustrine bodies proximal to channel levees, where rapid burial may occur during flood events.