The Cottons Breccia (Ediacaran) and its tectonostratigraphic context within the Grassy Group, King Island, Australia: A rift-related gravity slump deposit

Abstract

The Cottons Breccia is part of the upper Neoproterozoic (Ediacaran: 586–572 Ma) Grassy Group, exposed on King Island, Tasmania. This formation has been widely interpreted as an unambiguous tillite, in support of tectono-climatic models that invoke catastrophic latest Neoproterozoic global glaciation. However, dissenting opinions interpret the Cottons Breccia as a tectonic breccia. In this paper,we reviewthe tectonostratigraphy of the Cottons Breccia, and the enclosing Grassy Group. We find no unambiguous evidence for a direct glacial origin, as postulated by some early workers. An interpretation of the Cottons Breccia as a deposit within half-graben in the shoulder regions of an actively developing latest Neoproterozoic volcanic rifted margin is consistent with all the available sedimentological, structural, geochronological, and tectonic evidence.