Cottons Breccia of King Island, Tasmania: Glacial or non-glacial, Cryogenian or Ediacaran?

The Cottons Breccia of King Island, Tasmania, is a 100-m-thick carbonate-clast diamictite traditionally interpreted as the product of a Cryogenian or Ediacaran glaciation. It was recently reinterpreted as a mass-flow deposit, unrelated to glaciation, within an active rift basin. We reaffirm the glacial–periglacial interpretation on the basis of sedimentary facies, internal facies relations, clast lithology and isotopic composition, clast fabric analysis, and consistent stratigraphic position beneath a typical post-glacial cap dolostone. The Cumberland Creek Dolostone closely resembles basal Ediacaran cap dolostones world-wide in terms of colour, texture, sedimentary structures and isotopic characteristics. Because it was deposited above storm wave-base, differences in δ13C between closely adjacent sections suggest diachronous deposition during post-glacial marine transgression of a basin with steep local topography. This is compatible with an active rift basin in which glacigenic diamictites of the end-Cryogenian (Marinoan) pan-glacial episode were lodged.