PLANAR LAMELLAR FEATURES IN DEVONIAN VOLCANIC QUARTZ GRAINS: THE RESULT OF SHOCK DEFORMATION?

There has been much debate as to whether volcanism and its associated pressures can produce shocked quartz grains. Known lower and middle Devonian bentonite ash layers from the Kalkberg and Onondaga Formations at Cherry Valley, NY were sampled and compared using stereoscopic, petrographic, and x-ray diffraction analyses. Samples from both the Kalkberg and Onondaga displayed quartz particles of volcanic origin, possessing geometric straight edges, sub-angular corners, highly reflective surfaces, and water-clear transparency. These grains are easily distinguishable from reworked detrital quartz grains, which are sub-rounded to rounded in shape, discolored, and frosted. Less than ten percent of the grains of volcanic origin have closely spaced internal parallel lamellae of single and multiple sets, extending throughout much of the grain, and creating optical discontinuities. Multiple planar features may rule out Böhm lamellae and suggest a shock origin due to explosive volcanism. Since these planar features are present in known bentonites throughout the Kalkberg and Onondaga, and it is unlikely that multiple impact events were occurring in lockstep with volcanic eruptions, it is evident that the quartz grains with observed lamellar features are of volcanic origin. However, doubt remains as to whether or not these features are the result of shock deformation.