The Great Australian Bight (GAB) is an extensive W-E striking continental margin basin that drifted northwards during the Cenozoic following rifting and separation from Antarctica in the mid/late Cretaceous. Seafloor spreading accelerated in the mid-Eocene and was associated with local volcanism. The mid-Eocene succession of the GAB is conspicuously mounded and separates a dominantly siliciclastic succession below from a fully marine carbonate succession above. The mounded succession was penecontemporaneous with major changes in global climate, oceanographic conditions and tectonic re-organization in the region, and thus may hold important clues as to the palaeo-environmental changes associated with these changes. The mid Eocene has so far only been described locally or in passing, usually by studies focused on either the siliciclastics below or the carbonates above. It was therefore chosen as a major focus point for the research project reported herein. Exploration activity in the GAB has been limited despite the presence of a working petroleum system and large target structures, but industry interest has increased over the past few years leading to 3D seismic surveys being acquired in the GAB. The focus for exploration is the Cretaceous succession beneath the relatively thin Cenozoic cover, which however, is still important in terms of shallow hazards and as overburden to the anticipated productive sections. As is often the case, the new 3D seismic data shows many overburden features in great detail and thus affords new insights to be gained that improve our understanding of the post-rift evolution of the marginThis thesis expands upon and reinterprets a pre-existing sequence framework in the Cenozoic GAB based from ODP Leg 182 results. A vast database of 2D and 3D seismic surveys has been integrated with exploration wells and borehole data and several surfaces have been calibrated to borehole and well constraints, then mapped to the maximum lateral extent across the available dataset. Surface mapping provided new insight into sequence deposition and palaeoenvironmental settings. Structure maps and thickness maps highlight key depocentre locations and trends over the Cenozoic GAB as well as stacked mass debris aprons. The newly discovered sequences raise new questions regarding trigger mechanisms in a-seismic areas and feed into industry geohazard perception models. The base surface of the Cenozoic framework hosts a plethora of mounded features across shelf and basinal section. All mounds within the dataset have been mapped. A set a bryozoan reef mounds have been interpreted lying parallel to the margin as linear complexes over 500 km. They coincide with the underlying siliciclastic delta clinoform breakpoints and provide insight into the changing palaeoenvironment at the 43 Ma mark, cessation of siliciclastics and regional marine transgression. Further mound mapping aided by 3D attribute extractions along the base Cenozoic unconformity led to the interpretation of a series of enigmatic igneous-based mounded features. The discoveries have been included in a comparative study, comparing all mounded features (igneous or carbonate) and contrasting their individual characteristics of geometry, seismic facies, dimension in order to understand mound origin and emplacement. A new grouping of mounds in the GAB has been established, the origin and emplacement mechanisms of which contribute to the global knowledge base.

The Chatham Islands are located in the SW Pacific Ocean, approximately 850 km to the east of the South Island of New Zealand. This small group of islands is situated near the eastern margin of the Chatham Rise, an elongated section of submerged continental crust that represents part of the Late Paleozoic-Mesozoic Gondwana accretionary margin. The location and much of the geology of the Chatham Islands are attributed to intra-plate basaltic volcanism, initiated during the Late Cretaceous, in association with development of a failed rifting system to the south of the Chatham Rise. Despite the volcanic nature of much of the geology, the majority of the Cenozoic sedimentary stratigraphic record on the islands comprises non-tropical skeletal carbonate deposits whose deposition was often coeval with submarine volcanics and volcaniclastic deposits. This has resulted in complex stratigraphic relationships, with the volcanic geology exerting a strong influence on the geometry and distribution of the carbonate deposits. These limestones, despite some general field descriptions, have been little studied and are especially poorly understood from a petrographic and diagenetic perspective. The carbonate geology in detail comprises eleven discrete limestone units of Late Cretaceous through to Pleistocene age which were studied during two consecutive field expeditions over the summers of 2005 and 2006. These limestone occurrences are best exposed in scattered coastal outcrops where they form prominent rugged bluffs. While many of the younger (Oligocene to Pliocene) outcrops comprise of poorly exposed, thin and eroded limestone remnants (it;5 m thick), older (Late Paleocene to Early Oligocene) exposures can be up to 100 m in thickness. The character of these limestones is highly variable. In outcrop they display a broad range of textures and skeletal compositions, often exhibit cross-bedding, display differing degrees of porosity occlusion by cementation, and may include rare silicified horizons and evidence of hardground formation. Petrographically the limestones are skeletal grainstones and packstones with a typical compositional makeup of about 70% skeletal material, 10% siliciclasts, and 20% cement/matrix. Localised increases in siliciclastics occur where the carbonates are diluted by locally-derived volcaniclastics. The spectrum of skeletal assemblages identified within the Chatham Island limestones is diverse and appears in many cases to be comparable to the bryozoan dominant types common in mainland New Zealand and mid-latitude Australian cool-water carbonates in general. However, some key departures from the expected cool-water carbonate skeletal makeup have been identified in this study. The occurrence of stromatolitic algal mats in Late Cretaceous and Early Eocene carbonate deposits indicates not cool-temperate, but certainly warm-temperate paleoclimatic conditions. A change to cool-temperate conditions is recorded in the limestone flora/fauna from the mid-Late Miocene times following the development and later northward movement of the Subtropical Front. An uncharacteristic mix of shallow-shelf (bryozoans) and deeper water fauna (planktic foraminifera), together with their highly fragmented and abraded nature, is indicative of the likely remobilisation and redistribution of carbonate, primarily during episodic storm events. The Chatham Islands limestones formed within the relative tectonic stability of an oceanic island setting, which was conducive to ongoing carbonate accumulation throughout much of the Cenozoic. This contrasts markedly with other mainland New Zealand shelf carbonates which formed over sporadic and short-lived geological periods, experiencing greater degrees of burial cementation controlled by a relatively more active tectonic setting. As a consequence of the tectonically stable setting, the Chatham Islands limestones have experienced little burial and exhibit a paucity of burial cementation effects. They remain commonly soft and friable. Detailed petrographic investigations have shown the limestones are variably cemented by rare uneven acicular spar fringes, poorly to well-developed syntaxial rim cements about echinoderm fragments, and equant/blocky microsparite. Staining of thin sections and cathodoluminescence petrography show these spar cement generations are non-ferroan and their very dull- to non-luminescent nature supports precipitation from Mn-poor oxygenated waters, likely of an either meteoric or combined marine/shallow burial origin. Micrite is the dominant intra- and inter-particle pore fill and occurs both as a microbioclastic matrix and as precipitated homogenous and/or micropeloidal cement. The rare fringing cements often seen in association with homogenous and/or micropeloidal micrite may be indicative of true early marine (seafloor) cement precipitation and localised hardground development. An interesting feature of the geology of the Chatham Islands is the occurrence of carbonate material within sedimentary dykes. The locations of the dykes are in association with volcanic and volcaniclastic deposits. Similarities between dyke characteristics at Red Bluff on Chatham Island with mainland occurrences from East Coast and Canterbury Basins (North and South Islands, respectively) on mainland New Zealand have been recognised. They show complex structures including sidewall striations, internal flow structures as revealed by grain sorting, and extra-clast inclusions of previous fill lithologies which are characteristic of carbonate injection. This is in contrast to other dykes which are known to be of a passive fill origin. Multiple phases of carbonate sediment injection can be recognised by crosscutting relationships enabling the determination of a parasequence of events. Possible injection mechanisms are most likely associated with sediment overloading or hydrothermal pressurisation associated with emplacement of submarine volcanics. The Chatham Islands provide an exciting example of a geologically unique and complex non-tropical carbonate depositional setting. The production of carbonates is controlled by volcanic and volcaniclastic sediment input with the types of carbonate deposits and water depth variations related to thermal uplift/subsidence in association with global eustatic sealevel and temperature changes associated with development of Southern Ocean water fronts from the Late Cretaceous-Cenozoic. Carbonate deposition on the Chatham Islands is considered to relate to a rather variable and small scale oceanic, high energy, cool-water carbonate ramp setting whose geometry was continually evolving/changing as a consequence of periodic volcanic episodes.