Tectonic implications of some field relations of the Adelaidean Cooee Dolerite,Tasmania

Complex contact relations between thin sills of the Adelaidean Cooee Dolerite and the enclosing flysch sediments (Burnie Formation) indicate emplacement of the sills into substantially uncompacted quasi‐liquid sediments very shortly after deposition. The sills are consequently coeval with the sediments rather than synchronous with the first phase of folding of the sediments (the Penguin Orogeny).

The K/Ar date on the Cooee Dolerite, here revised as 725 ± 35 m.y. (J. Richards, pers. comm.), very probably provides an estimate of the age of the sediments, rather than the age of the orogeny. Age relationships of the Cooee Dolerite, its host sediments and their deformation proposed hitherto can no longer be regarded as settled. The Burnie Formation may be part of a passive continental margin to the Rocky Cape Block, which was deformed during the Cambrian as a result of subduction beneath the active margin of the Tyennan Block.