Year

2023

Degree Name

Bachelor of Science (Honours)

Department

School of Earth, Atmospheric and Life Sciences

Advisor(s)

Allen Nutman

Abstract

The Upper Permian sedimentary successions of the southern Sydney Basin have been the subject of several stratigraphic, sedimentological, and petrographic studies and more recently several studies dating the sedimentary succession using U-Pb zircon and 40Ar-39Ar geochronology. In this study the Broughton Formation of the Lower Permian, part of the upper Shoalhaven Group succeeding the Talaterang Group and underneath the Illawarra Coal Measures was studied. The Broughton Formation is part of a Permian retroarc depositional basin within the Southern Sydney Basin (SSB) which is unconformable on the Lachlan Fold belt to south and west, and in tectonic contact with the New England Fold Belt to the north and east. The chronology of the Upper Permian of the SSB is well constrained. However, the chronology of the Lower Permian of the SSB, including the Broughton Formation, is less well constrained. Thus, there is debate over the timing and absolute length of several glacial and interglacial cycles. The project aimed to establish the sediment provenance and depositional age range of the Broughton Formation and thereby potentially contribute to resolving the end of P3 Permian glaciation and the beginning of the following interglacial period. Furthermore, the Hunter-Bowen Orogeny, a major tectonic event occurred prior to the Late Permian, resulting in changes in the geological setting which should be reflected in the detrital profile of the SSB and more specifically the Broughton Formation. As such, identifying the geodynamic changes that occurred during this period will identify the provenance of sediment. Using sedimentary logging field observations, petrography, whole rock geochemistry and U-Pb zircon dating methods the project confirmed that shallow marine volcaniclastic sandstone units are characteristic of the Broughton Formation. Additionally, evidence for cold water/ glacial conditions are found throughout the formation, by way of iv dropstones and glendonites occurrences. The volcaniclastic sediment provenance identified was likely derived from proximal, intermediate to mafic, latite Members of the Broughton Formation. U-Pb zircon geochronology identified secondary sources of detritus that originated from the LFB to the south and southwest and proximal volcanic provinces. Geochemical analysis suggested the detritus was immature and undergoing a weak degree of weathering at the time of deposition, suggesting rapid depositional processes at work. This supported by the geochronological analysis, deriving a critical result, by demonstrating that the Broughton Formation was deposited over a very short time at about 265-263 Ma (2 million years). The Broughton Formation input was largest at its base and followed by an increase volume of detritus derived from the Lachlan Fold Belt towards the top of the formation, as volcanism in the SSB basin waned. The timing of the most active mafic magmatism in the SSB (~275-260 Ma) falls within a quiet time for magmatic activity in the NEO, which was concentrated in the early Permian and then in the latest Permian - Triassic. Geochemical discrimination diagrams indicated the backarc basin affinity of the Broughton Formation volcanism. This suggests a slab roll back, extensional phase during which volcanism occurred. This was further supported by sudden increase in mafic volcanic detritus, aggradational / progradational depositional conditions, and no evidence of volcanic material from the NEO. The deposition period of the Broughton Formation represented the period of activation and deactivation of the extensionally influenced, Southern Sydney retroarc basin magmatism, and the resulting rapid volcanic detrital depositional phase. The new geochronology also confirms that the P3 glacial stage continued until after 263 Ma.

FoR codes (2020)

370599 Geology not elsewhere classified

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.