Comparison of Low-pressure Shock-metamorphic Effects in Quartz from Barringer Crater, Arizona, and Kentland Dome, Indiana

Introduction: The recognition of low-pressure shock-metamorphic effects in quartz is of increasing importance in studies attempting to verify an impact origin for highly eroded structures [1], recognize sedi-mentary-target impacts characterized by a wide range of shock-pressure regimes, and identify distal impact-ejecta layers or impact-tsunamiites where high-pressure indicators may be rare or highly diluted. Low-pressure effects in quartz, if occurring in isolation without other supporting impact indicators, cannot normally be used as diagnostic evidence of shock metamorphism [2]. However, this lack of utility may be in part due to a dearth of studies systematically documenting these effects at verified impact sites. Our study is a petrographic and universal stage (U-stage) microscope comparison of low-pressure shock-metamorphic effects in quartz from a proximal ejecta lithic clast of the Lower Permian Coconino Sandstone at the ~0.05 Ma, 1.2-km-diameter Barringer (Meteor) Crater, AZ, and from the Middle Ordovician St. Peter Sandstone exposed on the flank of the ~4-km-diameter central uplift of the <97 Ma, 13-km-diameter Kentland Dome structure, IN. Numerous classic studies verify an impact origin for Barringer Crater (see recent summary in [3]) and multiple lines of evidence support a similar origin for Kentland Dome [4–7]. Shock-Metamorphic Effects: Coconino Sandstone, Barringer Crater. Un-shocked Coconino Sandstone (sample MET-1-2) consists of a fine-to medium-grained, moderately well-sorted, rounded to well-rounded quartz arenite containing ~4% chert, ~1% feldspar, and ~1% accessory mineral framework grains, and ~20 vol% porosity. Quartz is characterized by sharp to slightly undulatory extinction under cross-polarizing light. The examined shocked sample (MET-1-4) matches the previously defined shock class 1b [8], which is recognized as weakly shocked sandstone that lacks remnant porosity and contains abundant grain comminution and fracturing (Fig. 1), common radial grain-contact concussion fractures (Fig. 2), and little or no intergranular high-pressure quartz polymorphs such as those defining the higher shock classes [8]. Rare planar microstructures are present, consisting of planar fractures (PFs) and poorly developed, incipient, non-decorated planar deformation features (PDFs), which have indexed Miller crystallographic orientations [9] equivalent to c(0001) , } 3 1 10 { , and } 2 1 10 { (Fig. 2). About 50% of the quartz grains display marked grain mosaicism that is distinct from undulatory extinction in the unshocked quartz. This observation is qualitatively supported by comparison of U-stage-measured optic-axis spread within individual quartz grains from both unshocked and shocked samples [10] (Fig. 3). St. Peter Sandstone, Kentland Dome. Unshocked St. Peter Sandstone (sample ST PETE-2) consists …