Woodleigh impact structure, Australia: Shock petrography and geochemical studies

Abstract

Abstract-The large, complex Woodleigh structure in the Carnarvon basin of Western Australia has recently been added to the terrestrial impact crater record. Many aspects of this structure are, however, still uncertain. This work provides a detailed petrographic assessment of a suite of representative drill core samples from the borehole Woodleigh 1 that penetrated uplifted basement rocks of the central part of this structure. Fundamental rock and mineral deformation data and high-precision chemical data, including results of PGE and oxygen isotopic analysis, are presented. The sampled interval displays likely impact-produced macrodeformation in the form of fracturing and breccia veining at the microscopic scale. Contrary to earlier reports that these breccias represent pseudotachylite (friction melt) or even shock/shear-produced pseudotachylitic melt breccia cannot be confirmed due to pervasive post-impact alteration. Abundant planar deformation features (PDFs) in quartz, in addition to diaplectic glass and partial isotropization, are the main shock deformation effects observed, confirming that Woodleigh is of impact origin. Over the investigated depth interval, the statistics of quartz grains with a variable number of sets of PDFs does not change significantly, and the patterns of crystallographic orientations of PDFs in randomly selected quartz grains does not indicate a change in absolute shock pressure with depth either. The value of oxygen isotopes for the recognition of meteoritic contamination, as proposed by earlier Woodleigh workers, is critically assessed. Neither INA nor PGE analyses of our samples support the presence of a meteoritic component within this basement section, as had been claimed in earlier work.

Characterization and significance of shocked quartz from the Woodleigh mpact structure, Western Australia

“…3a-3b), up to 4 sets in rare cases but predominantly 1-2 sets. Universal stage orientation measurements of these features are described in Koeberl et al (2001) and Reimold et al (2003). We also note that quartz often occurs as mineral veins, the quartz crystals within the veins displaying ubiquitous PDFs (Fig.…”

“…Microstructures preserved within quartz grains in W-1 were examined using optical microscopy, acid etching, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) to confirm the presence and nature of shock deformation features (Gratz et al 1996;Montanari and Koeberl 2000). These are complimentary to the U-stage studies of Koeberl et al (2001) and Reimold et al (2003).…”

“…3a-3d). These shock features have been analyzed and described in more detail by Reimold et al (2003) but include brecciation, kink bands in biotite, planar deformation features, and lowered birefringence in quartz, feldspar, and diaplectic quartz. The rare, rounded, gneissose and granitic clasts in the W-2A conglomerate reveal planar lamellae, presumably shock deformation features, in quartz.…”

Shock‐metamorphic petrography and microRaman spectroscopy of quartz in upper impactite interval, ICDP drill core LB‐07A, Bosumtwi impact crater, Ghana

“…Similar PDF distributions are reported from such impact sites as West Clearwater Lake (Engelhardt et al 1968; indexed and replotted in Grieve et al 1996), multiple Canadian craters, including also West Clearwater Lake data (both type C and D planar features) (Robertson et al 1968), Ries (Engelhardt and Bertsch 1969; indexed and replotted in Grieve et al 1996), North American Cretaceous-Tertiary boundary localities (Bohor et al 1987;Grieve and Alexopoulos 1988; indexed and replotted in Grieve et al 1996), Gosses Bluff (Fig. 23 of Grieve et al 1996), Manson (Short and Gold 1996), and Woodleigh (i.e., lower half of the Woodleigh 1 core, Reimold et al 2003).…”

Impacts and Mass Extinctions Revisited

“…5). Accord− ing to Renne et al (2002: 247), its size is poorly constrained and subject of an ongoing debate (between 40 and 120 km), while the age "could have been much older than mid−Devo− nian" (see also Hough et al 2003;Reimold et al 2003;Glikson et al 2005;Uysal et al 2005). …”