Lower Palaeozoic unconformities in Tasmania

Abstract

Lower Palaeozoic unconformities are now known in some 29 exposures in Tasmania which are broadly equivalent to the Tyennan Unconformity of Browne.However, the time interval is found to vary and it is clear that different orogenic pulses are involved, all belonging to a general period of orogeny.The Tyennan Orogeny as defined by Browne is anomalous and requires re-definition.It is therefore proposed that the name Tyennan Orogeny be retained to mean the general orogenic period of the Cambrian as exposed in the 'ryenna Valley where the Ordovician Junee Group rests on pre-Dundas strata.An unconformity between the Cambrian Dundas Group and pre-Dundas rocks is defined as the Stichtan Unconformity.An unconformity between the Junee Group and the Dundas Group is defined as the J ukesian Unconformity.The Dundas Group has a eugeosynclinal facies with rhythmic recurrence of coarse conglomerates, breccias and greywackei:l, which xeflect at least six orogenic pulses (as yet unnamed), all later than the Stichtan Movement and earlier than the .Tukesian Movement.All eight orogenic pulses occur within the time interval of the Tyennan Unconformity, which includes the upper half of the Cambrian Period and perhaps some earlier time.It is uncertain whether Dundas sediments were ever deposited over the regions now exposing the Tyennan Unconformity.The Jukesian regression was the most widespread emergence to be recorded in Tasmania between the Lower Cambrian and Middle Devonian Epochs.