Abstract

There are, broadly speaking, in the Devonian of Victoria, two provinces of differing sedimentary, igneous and tectonic history. In the central Victorian Province marine sedimentation was essentially continuous from Silurian into Lower Devonian times. In the eastern Victorian Province, on the other hand, marine sedimentation was successively interrupted in late Silurian times by orogenesis (Bowning orogeny) followed by widespread granitic emplacement, rapid de-roofing of the granitic intrusions, and a new cycle of rapid non-marine sedimentation, essentially acid volcanism with but minor marine incursions. This was followed later by block faulting and planation prior to the widespread late Emsian marine incursion that was connected with accumulation of the Buchan Group.

No sedimentary or igneous record is available for Givetian and possibly late Eifelian and early Frasnian times, during which interval an acme of deformation, the Tabberabberan orogeny, with associated intrusions of essentially meridionally aligned dyke swarms, affected the entire state. In eastern and western Victoria this deformation was followed by and, we may presume, connected with the rapid erosion and accumulation of vast thicknesses of non-marine Upper Devonian sediments over uneven surfaces. In central Victoria however, there was little sedimentation; vast, essentially acid, volcanic outpourings took place, in part at least from ring structures, to be intruded subsequently by their parent magmas at some time close to the Devono-Carboniferous boundary.

Devonian rocks in Tasmania lie at the southern end of the Tasman geosynclinal zone. The Skalian to early Siegenian upper part of the Eldon Group of western Tasmania is thought to have been deposited on a rather shallow sea floor which slowly deepened, with turbidity currents occurring. These beds, and probably also the co-eval (?) off-shore deeper-water Mathinna Beds of northeastern Tasmania, were folded prior to the deposition of the Emsian to Eifelian Spero Bay Group that crops out on the southwest coast. The Spero Bay Group itself was folded prior to the deposition of the (probably) Givetian Eugenana Beds, cave deposits which are still horizontal. Thus, as in the case of the Tabberabberan Orogeny in Victoria, folding probably began in the Siegenian and was completed during or prior to the Givetian. Three main pulses have been suggested in Tasmania. Granitic rocks with radiometric ages ranging from 375 to 340 my. (late Siegenian to late Tournaisian) intrude the folded Devonian sediments.