Depositional architecture and history of the late Permian Broughton, Pheasants Nest and Erins Vale formations, Southern Sydney Basin, New South Wales, Australia

Abstract

This thesis concerns a sedimentological study of the Late Permian Broughton, Pheasants Nest and Erins Vale Formations, southern Sydney Basin, New South Wales, aimed at producing a palaeoenvironmental interpretation of these sequences. It is based on facies analysis of borecore data, vertical and lateral profiling of outcrop sequences, including architectural element analysis, and an integration of the results of these studies with previous work. The sequences studied begin with the interdigitating lower offshore silt of the upper Berry Siltstone and lower offshore sand-belt of the lower Broughton Formation, including progradational volcanic shoreface sequences. Sedimentation occurred in the 'Broughton Seaway' between the western cratonic landmass and a north-northeast oriented volcanic barrier in the east. River-derived meltwater underflows, massflows, tidally-driven longshore currents and migration of large sandwaves into the lower offshore sand-belt were the major depositional processes. The upper offshore sand-sheets of the middle Broughton Formation represent a widespread development of time-transgressive, above storm wave base, aggradational sedimentation. The progradational sandskirts of volcanic alluvial fans in the south (Jamberoo Sandstone Member) constructed a delta which was of a mixed type between a pyroclastic and alluvial fan delta. These kinds of deltas probably served as significant sources and depositional conduits for the offshore sediments which were dispersed mainly through wave action in a storm-dominated setting._ Submarine volcanic flows and associated shallow marine environments, and emerged volcanic islands with woodlands were coevally present in the southeast during the deposition of the Broughton Formation. The upper Broughton Formation is generally marked by a northeastward diachronous progradation of the southern shoreline, which progressively filled up the Broughton Seaway. Progradational evolution of the deltaic to barred shoreline, reflecting waning influence of the Gerringong volcanic barriers, was contemporaneous with the upper deltaic and distributary coastal zone sedimentation of the Pheasants Nest Formation. Actually, the periglacial braided subaerial fans and subaerial volcanism modelled for the lower and middle Pheasants Nest Formation greatly contributed to a northward progradation of the shoreline and retreat of the Broughton Seaway. With additional contributions from the western craton, a centripetal sediment transport pattern toward a northeast-oriented depocentre resulted. This meant development of a northeasterly flowing longitudinal drainage system. Physiographically, most of the former Broughton Seaway had now transformed into a broad valley between the volcanic and cratonic hinterlands. During the upper Pheasants Nest Formation time, extensive development of a flat, alluvial plain with an axial drainage pattern, low-lying coastal swamps and networks of high sinuosity, single channel or multiple channel anastomosing river systems is postulated. A shallow epeiric Erins Vale sea was caused by transgression across the topographically low-lying northeastern to northern alluvial plains of the Pheasants Nest Formation . The lower-middle Erins Vale Formation represents shoreface-offshore conditions in the east and north, including an oxygen-deficient, lower offshore regime (Kulnura Marine Tongue) - and shoreline fades of a transgressive barrier system in the west and south. Deposition during the upper Erins Vale Formation occurred through fan delta progradation from the western craton, which passed upward into the subaerial fan of the Marangaroo Conglomerate; a central and southern regressive barrier system and an energetic shoreface sand in the east which passed upward into the Wilton Formation. The upper Nowra Sandstone and lower Berry Siltstone form a transgressive systems tract. A sustained tectonic loading producing protracted subsidence was apparently the primary cause of basin subsidence. The upper Berry Siltstone and lower to middle Broughton Formation represent an overall highstand systems tract with portions of the upper Berry Siltstone and the lowest Broughton Formation showing interludes of minor transgressive pulses. This reflected a more stabilized base-level situation, following a reduction of basin subsidence rate. The upper Broughton Formation and lower-middle Pheasants Nest Formation constitute a lowstand systems tract, including early regressive phases represented by the upper Broughton Formation. It was related to the development of a foreswell on the basin margin and subsidence in the depocentre caused by the propagation of compressional energy from the orogen. The upper Pheasants Nest Formation indicates an early transgressive stage and the lower Erins Vale Formation, including the Kulnura Marine Tongue, is a transgressive systems tract. The middle Erins Vale Formation represents a highstand systems tract. Subsequently, the early regressive phases represented by the upper Erins Vale Formation, the lowstand wedge of the basal Wilton Formation and the Marangaroo Conglomerate constituted a lowstand systems tract. They developed as a result of erosion of a fores well produced on the cratonic margin. This study provides evidence in support of the Currarong Orogen. The Offshore Uplift is probably a preserved portion of the Currarong Orogen and the Newcastle Sub-basin was a likely depocentre for the Late Permian sequences. A periglacial climate with seasonal freezing and thawing had significant influence on sedimentation. Episodic meltwater discharge mobilized large volumes of volcaniclastic materials in spring (as lahars at times of large floods and/ or volcanic eruptions) and associated ice floes enroute to the sea. The major variables for the studied sequences in the southern Sydney Basin are constrained in this thesis, and comparative studies identified modern and ancient global examples that are broadly similar to the Late Permian sequences.