Sobhan, Abdul Mayeen Nazre
Description
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...[Show more] 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.
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