Volcanic Tephra Bed Formation and Condensation Processes: A Review and Examination from Devonian Stratigraphic Sequences

The formation and preservation of airfall volcanogenic tephras is the result of various interacting factors. Explanations of the depositional history of individual tephra beds lie along a spectrum between two end-member models of a primary single event and a complex depositional history. Nonrecognition of the history of a tephra bed may lead to errors in geochronology, correlation, and interpretation of the record of explosive volcanism in sedimentary successions. Increased understanding of sequence stratigraphic principles and condensation processes favors the model of tephras with a complex depositional history. This is supported by studies that indicate many tephras in the rock record result from amalgamation and/or reworking in the depositional environment. A prediction of increased tephra bed formation/preservation within transgressive systems tracts due to condensation processes is tested through 11 Lochkovian to Eifelian (Lower to lower Middle Devonian) depositional sequences. The character and distribution of over 80 Devonian K-bentonites and tuffs in the Appalachian Basin (eastern United States) indicates a complex history of tephra deposition and preservation. Of 11 sequences, four have numerous tephra beds, one sequence yields one tephra bed, and six sequences yield no tephra beds. In total, 38 individual beds cumulatively occur within transgressive systems tracts, 39 in early highstand systems tracts, one in late highstand systems tracts, and one in lowstand systems tracts, noting that lowstands are poorly preserved or not recognized along the outcrop belt. This indicates that airfall tephras are concentrated in the transgressive and early highstand systems tracts, not just in condensed intervals of transgressive systems tracts; they do not consistently occur through all sequences. A simple sequence stratigraphic condensation model for the deposition and preservation of volcanic tephras is not applicable.