Ediacaran ice-rafting and coeval asteroid impact, South Australia: insights into the terminal Proterozoic environment

Abstract

Isolated quartzose pebbles, clusters of quartz granules, orthogonal aggregates of poorly sorted quartzose coarse sand, and ovoid pellets (≤2 mm long) of quartz silt occur in hemipelagic marine mudstone of the mid-Ediacaran Bunyeroo Formation exposed in the Adelaide Geosyncline (Adelaide Rift Complex), and ovoid pellets of quartz silt in cores of the correlative marine Dey Dey Mudstone from deep drillholes in the Officer Basin, South Australia. This detritus is interpreted respectively as dropstones, dumps, and frozen aggregates dispersed by sea ice possibly of seasonal origin, and till pellets transported by glacial ice. The ice-rafted material in the Bunyeroo Formation only has been found <10 m stratigraphically below and above a horizon of dacitic ejecta related to the 90 km diameter Acraman impact structure in the Mesoproterozoic Gawler Range Volcanics 300 km to the west. Furthermore, till pellets have been identified 4.4 to 68 m below distal Acraman ejecta in the Dey Dey Mudstone >500 km northwest of the impact site. The Acraman impact took place at a low paleolatitude (∼12.5°) and would have adversely affected the global environment. The stratigraphic observations imply, however, that the impact occurred during, but did not trigger, a cold interval marked by sea ice and glacial ice, although the temporal relationship with Ediacaran glaciations elsewhere in Australia and on other continents is unclear. Release from the combined environmental stresses of a frigid, glacial climate near sea-level and a major impact in low latitudes may have been a factor influencing subsequent Ediacaran biotic evolution.

Key Words:

• South Australia
• Adelaide Geosyncline
• Officer Basin
• Ediacaran
• ice rafting
• glaciation
• impacts
• evolution

Acknowledgements

We thank Peter Haines, Nick Lemon, Richard Jenkins, Tanya Young, Kath Grey and Andrew Hill for discussions, Phil Schmidt for providing data used in Figure 5, and Wayne Mussared and Ian Pontifex for preparing thin-sections. DMMcK and LJW thank Primary Industry and Resources South Australia for financial support. LJW acknowledges an Australian Postgraduate Award and an AAPG Grant-in-Aid. Malcolm Walter and Peter Haines provided helpful reviews. This paper is a contribution to IGCP 512 on Neoproterozoic Ice Ages, and is TRaX Record #66.