School of Earth Sciences - Theses
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ItemThe structural evolution, tectonics and hydrocarbons of the offshore Otway Basin, SE Australia( 2003)The offshore Otway Basin is part of Australia's passive southern margin, in which two separate rift-phases between the Tithonian? and Maastrichtian formed numerous depocentres. The research presented has analyzed and described the structural styles in the offshore Otway Basin and constructed a model of the basin's evolution since the Late Jurassic. The Otway Basin has been divided into four structural zones from north to south. Zone I comprises the onshore area and most of the shelf along the margin. Deep halfgraben developed during the first rift phase with characteristic horst and graben in Palaeozoic basement. To the south, zone I is bound by the Hinge Zone. Structural zone II covers the entire deepwater part of the Otway margin, characterized by a very thick Late Cretaceous section with pervasive Turonian faulting in the east and saucer-shaped depocentres in the west. Large halfgraben controlled deposition of the post-Turonian sedimentation in the eastern Otway Basin. Negative flower structures document strike-slip faulting. Strongly thinned lower laminated continental crust underlies this zone, limited to the south by the Outer Margin Highs. Domino faulting formed halfgraben and less commonly graben in Structural Zone III, the Outer Margin Highs. The base of the Outer Margin High sediments represents a regional decollement surface and domino faulting occurred along a second-generation decollement. Structural zone III is limited to the south by the continent-ocean-boundary with oceanic crust in structural zone IV. In the Shipwreck Trough, halfgraben died out against an accommodation zone which developed into the Shipwreck Fault with strike-slip offset .The regional stress regime indicate sinistral strike-slip movement along this fault zone. In the southwest Shipwreck Trough, four Turonian to early Coniacian syn-rift phases can be distinguished formed through footwall collapse to the north of the Hinge Zone. Differences in the amount of extension in the basin are accommodated along strike-slip faults such as the Shipwreck Fault. Sedimentation rates between 89 and 83 Ma increased whilst extension rates declined. Since approximately 83Ma sedimentation rates declined exponentially in phase with extension rates. Regionally, rapid Turonian extension formed a wide graben system between Antarctica and Australia. With the serpentinization of exhumed mantle peridotite in the Outer Margin Highs during the Coniacian in the east and Turonian in the west, the crustal deformation mechanism changed from mainly pure shear to simple shear along the newly established decollement. Fast spreading since the Mid Eocene caused gravitational collapse of the margin. Changes in heat flow possibly correlate with a change in deformation style in the continental crust. Parallel developments of sedimentation-rate and extension-rate suggest that most of the subsidence was structurally related. The Shipwreck Trough hydrocarbon fairway probably continues south into the Sorrell Basin. Possible stagnant conditions in deeper water offshore across the Hinge Zone might have enhanced the organic content of the Belfast Mudstone creating potential oil source rocks. Deepwater lntra-Paaratte reservoirs are not proven, but the alternating reflective to non-reflective seismic facies might indicate interbedded sands and shales. Large rollover anticlines would make excellent traps in the deepwater Otway Basin.
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ItemLate quaternary rivers and lakes of the Cadell Tilt Block region, Murray Basin, southeastern Australia( 2006)A record of climatic, hydrological and tectonic change spanning the last glacial cycle (-130,000 years) has been obtained from alluvial, aeolian and lacustrine sequences in the Cadell Tilt Block region of the central Murray Basin. Optically stimulated luminescence (OSL) is the principal method of chronological control, with a total of 50 new luminescence ages. Two AMS radiocarbon (^14C) ages are supplementary. Soils are used for relative dating of landforms beyond the range of OSL and ^14C. The result is the largest corpus of late Quaternary ages ever produced for the region. The chronology of the Lake Tyrrell lunette sequence has been revised from previously published interpretations. Beach sediments ~13.5 m above the present lake floor were deposited by Lake Chillingollah, a marine oxygen isotope stage (MIS) 5 (~ 130,000- 75,000 years ago) megalake. The megalake dried because of decreasing winter rainfall and fragmented into a groundwater discharge system. A silty clay dune deflated from the Lake Tyrrell floor ~27,000 years ago ended a long period of pedogenesis and buried evidence for Aboriginal visits to the lakeshore. The earliest evidence for aridification along the Murray River is an episode of riverine source-bordering dune formation in early MIS 4 (~72,000 years ago). The event is a minimum age for the initiation of construction of the Barmah Fan, which accreted in response to uplift of the Cadell Tilt Block. Fan sedimentation on the foot wall close to the fault scarp appears to have accelerated between 65,000 and 45,000 years ago. The Green Gully palaeochannel on the uplifted block was abandoned by the Murray River soon after this period, which culminated in an episode of riverine source-bordering dune formation ~40,000 years ago. The Goulburn River was not defeated by uplift. An older prior stream on the uplifted block, with undatable strong red-brown earth soil profiles along its margins, is not a course of the Goulburn. Instead, the Goulburn River was deflected to the southwest where it developed the Tallygaroopna meander belt ridge. This course had been deflected by ~65,000 years ago. Vertical aggradation of the ancestral Goulburn continued until ~23,000 years ago. Riverine source-bordering dunes were beginning to form again when a clay plug filled the palaeochannel. The Tallygaroopna meander belt ridge is visible beneath the floor of Lake Kanyapella on LIDAR DEM imagery. Downstream it follows the course of Gunbower Creek. Lake Kanyapella is not fault-dammed or fault-controlled because it post-dates formation of the ridge. The lake formed ~34,000 years ago and was sustained by flows from the Tallygaroopna palaeochannel for ~10,000 years. A model of lake formation is proposed based on vertical bedload aggradation. That is, the lake emerged because the Goulburn River had fully-aggraded and could no longer channel its flood flows. This long-term ponding may be of wider palaeohydrological significance. Riverine source-bordering dunes form only at the end of the lacustral period. The Goulburn River avulsed from the meander belt ridge at the end of the Last Glacial Maximum (~18,000 years ago). The Kotupna palaeochannel was rapidly entrenched and back-filled, with riverine source-bordering dunes emplaced along its course in a geological instant. The harsh climate of the LGM was adapted to by the Kow Swamp people who developed robust physical morphologies in response to the cold conditions. Gracilization of the population is related to post-glacial climatic amelioration, which increased gene flow. Robust humans are rare after the LGM. Palaeochannel morphology is not climatically-controlled. Kotupna-type bars were deposited along the Bullatale Creek course of the Murray River in the Holocene, without any concomitant source-bordering dune formation. The Barmah Choke reach of the Murray River is relatively straight because it is a modern avulsion, not an inert Holocene river course. This avulsion happened only ~550 years ago, effectively shutting down the depositional system that constructed the massive Wakool Fan. This event ended a 75,000 year long avulsion sequence.