CLEANSKIN STRUCTURE, NORTHERN TERRITORY AND QUEENSLAND, AUSTRALIA: EVIDENCE FOR AN IMPACT ORIGIN.

Introduction: The Cleanskin structure is a circular topographic feature straddling the Northern Territory−Queensland border in northern Australia (centre: lat 18°10 S, long 137°56.5 E)[1]. The feature is hosted in weakly deformed early Mesoproterozoic sedimentary rocks (Constance Sandstone, South Nicholson Group) and partly obscured by undeformed early Cretaceous and younger cover. This abstract is based on remote mapping by IPS, brief field reconnaissance by KM, and petrography by PWH. Circular Structure: The central part of the feature comprises a 6–7 km diameter structural dome in which multiple repetitions of strata are separated by circumferential thrust(?) faults. These faults appear to be displaced by E-NE and W-NW oriented faults probably related to regional deformation. A less well defined outer topographic ring averaging about 15 km in diameter is best seen on digital elevation images. Shock Effects: Preliminary field observations in the SE quadrant located shatter cones in sandstone 3.5 km from the obscured centre (Fig. 1), associated with zones of brecciated sandstone and siltstone tens of metres in extent. Thin sections reveal probable low-pressure shock effects in quartz including multiple sets of planar fractures (Fig. 2), commonly with well-developed feather features[2], grain mosaicism, and possible planar deformation features. The combination of well-developed shatter cones with probably petrographic shock effects provide evidence for impact, but the relatively low-level shock indicators and structural relationships suggests that the site is deeply eroded.