Devonian‐Carboniferous stratigraphy of the southern Hastings Block, New England Orogen, eastern Australia

Abstract

Devonian and Carboniferous rocks within the Hastings Block represent an arc‐related basin fill located, out of place, on the outboard margin of the subduction complex and juxtaposed along strike from the fore‐arc basin of the southern New England Orogen. The stratigraphic sequence, structural style and history of rocks in the southern part of the Hastings Block differs from that in the northern part of the block. The stratigraphy of the southern part of the Hastings Block consists of a succession of nine formations (eight new) ranging in age from Late Devonian (Frasnian) to Late Carboniferous (Namurian). A further five formations (three new) ranging in age from Siluro‐Devonian to Late Devonian (Famennian) are confined to fault‐bounded blocks. Three of the latter are probably correlative with parts of the continuous Frasnian‐Namurian succession. The Port Macquarie Block is considered part of the southern Hastings Block because it contains two closely comparable formations. Late Devonian formations include the sequential Bitter Ground Volcanics, Birdwood Formation (both Frasnian in age) and the interdigitating Rollans Road/Cowangara Formations (Famennian‐earliest Tournaisian), together with the fault‐bounded Ellenborough Volcanics, a probable equivalent to the Birdwood Formation, and the Famennian Wallibree Formation and the undated and possibly equivalent Touchwood Formation. These are characterized by the association of basaltic to andesitic volcanics with deep‐water turbidites and radiolarian‐bearing sedimentary rocks. Whilst there is an overall similarity with coeval units within the Tamworth Belt of the southern New England Orogen, no detailed lithological correlations are possible between correlative formations in the two areas. The Early Carboniferous Kindee Conglomerate, Nevann Siltstone, Pappinbarra Formation and Hyndmans Creek Formation, alternating proximal to distal deep‐water turbidites with minor ash‐fall tuffs, have an andesitic provenance similar to that of the northern part of the Hastings Block but different from equivalent parts of the Tamworth Belt, which changed from andesitic to dominantly dacitic provenance at the Devonian‐Carboniferous boundary. Following a possible hiatus, and a change to a dacitic provenance, the Mingaletta Formation (Namurian) was deposited in shallow marine to continental environments; it is closely comparable lithologically and faunally to equivalent units in the northern Hastings Block and Myall Block of the Tamworth Belt. Differences between the northern and southern parts of the Hastings Block are explained by variations in location within the same sedimentary basin. Comparison with the Tamworth Belt suggests that whilst sequences in the two areas were deposited in arc‐related settings, conditions of deposition and the nature and timing of geological processes, particularly the composition of volcanics and processes of shallowing upwards, varied between the two regions until the Namurian. This suggests possible separation of the two regions prior to the Namurian.