THE DOWN RANGE GEOMORPHOLOGY OF MAPCIS (MASSIVE AUSTRALIAN PRECAMBRIAN/CAMBRIAN IMPACT STRUCTURE)

Background This is the fourth in a series of GSA presentations on MAPCIS. In November of 2007 a 2000km diameter ring was discovered using Google Earth. In 2008 and 2009, the general geomorphology of an oblique impact was described, possible coeval impacts were found and the age was ascertained to be ~545ma. As might be expected from an oblique impact, much of the ejecta and energy are directed down range to form unique and fascinating geomorphology.

Methods Remote imaging and 1: 250,000 GA geology maps defined the surface features. Mining company maps, results from scientific drill cores, magnetic anomaly maps and magnetic intensity maps established a picture of the subsurface, deep structures. These maps were made into overlays to compare and contrast the surface and subsurface geomorphology against models of oblique impacts and other known oblique impact sites.

Results The directly downrange sector is defined by a 57 degree angle opening to the SSW from the impact center to the points where it passes through 100km wide gap between the Petermann Ranges and the Eastern Musgraves. The mountain gap has a signature in both structure (radial ridges/faults) and metamorphic changes. Large pseudotachylite deposits find their furthest distance from the impact within the angle. Significant commercial sulfide deposits of Ni-Cu and PGE’s in the Musgraves are confined to a defined region within the down range angle. Thick non- glacially associated diamictites of early Cambrian age can be found further down range in the Officer Basin. Several circular magnetic anomalies suggest secondary impacts and a SSW ejecta debris flow which continues for over 900km. The lack of a known Acraman Impact 590mya, ejecta layer within the angle completes the picture.

Conclusions The observed geomorphology is consistent with a large oblique impact with a trajectory from NNE to SSW centered at 25°32'55.66"S 131°23'21.50"E dated ~545mya.