An unconformity is present close to the Miocene-Pliocene boundary in the onshore and nearshore portions of the Otway, Port Phillip-Torquay, and Gippsland basins of southeast Australia. The unconformity is angular (generally Ͻ 1-5؇ angularity), with the underlying Miocene units having been deformed (gentle folding and reverse faulting) and eroded prior to deposition of the Pliocene succession. The unconformity also marks a change from Oligocene-Miocene deposition of cool-water carbonate sediments and brown coal-bearing successions to the accumulation of more siliciclastic-rich sediments in Pliocene time. The Miocene-Pliocene boundary therefore represents an interval of significant regional uplift in the southeast Australian basins. The amount of section removed is greatest around the Otway and Strzelecki ranges in Victoria, where up to a kilometer of section may have been removed. In most onshore sections of the Victorian basins hundreds of meters of section have been eroded. In distal offshore locations the boundary becomes conformable. The timing of uplift and erosion is best constrained in the Otway and Port Phillip basins, where late Miocene (N16 ϳ 10 Ma) sediments underlie the unconformity and earliest Pliocene (ϳ 5 Ma) sediments overlie it. This timing coincides with a change in the dynamics of the Australian plate, beginning at around 12 Ma. Southeast Australia is currently under a NW-SE compressional regime, and this has probably persisted since the late Miocene. In the basins (as opposed to the basement-dominated highland areas), the late Miocene uplift event is more significant than later Pliocene-Recent uplift.
The influence of Neogene tectonics in the SE Australian basins has generally been underestimated in the petroleum exploration literature. However, onshore stratigraphic and offshore seismic data indicates that significant deformation and exhumation (up to one km or more) has occurred during the late Tertiary-Quaternary. This tectonism coincides with a change in the dynamics of the Australian plate, beginning at around 12 Ma, resulting in a WNW–ESE compressional regime which has continued to the present day.Significant late Miocene tectonism is indicated by a regional angular unconformity at around the Mio-Pliocene boundary in the onshore and nearshore successions of the SE Australian basins.Evidence of on going Pliocene- Quaternary tectonism is widespread in all of the SE Australian basins. Late Tertiary tectonism has produced structures in the offshore SE Australian basins which have been favourable targets for petroleum accumulation (e.g. Nerita structure, Torquay Sub-basin; Cormorant structure, Bass Basin). In the offshore Gippsland Basin, most of the oil- and gas-bearing structures have grown during Oligocene-Recent time. Some Gippsland Basin structures were largely produced prior to the mid- Miocene, while others have a younger structural history. In areas of intense late Tertiary exhumation and uplift (e.g. proximal to the Otway and Strzelecki Ranges), burial/maturation models of petroleum generation may be significantly affected by Neogene uplift.Many structures produced by late Miocene-Pliocene deformation are dry. These relatively young structures may post-date the major maturation episodes, with the post-structure history of the basins dominated by exhumation and cooling.
Macquarie Island is composed of a complete section of oceanic crust that formed in a slow-spreading mid-ocean ridge 2.0 to 3.5 km below sea level. Vitriclastic facies preserved on the island have both pyroclastic and hyaloclastic characteristics. Monomict hyaloclastic breccia facies are widespread across the island and are predominantly composed of near-primitive (~7.9 wt% MgO) subalkaline/ transitional (~0.7 wt% K 2 O) sideromelane shards and crystalline basalt clasts with low vesicularity (LV, < 15% vesicles). Breccias are thick bedded and structureless with matrix-supported angular pillow fragments, bomb-sized fluidal mini-pillows, and globular glass lapilli. Clasts are lithologically similar to interbedded pillow basalts and laterally grade into fine-grained sandstone facies. These sandstones are normal-graded, well-laminated, thin bedded, and interstratified with red pelagic mudstone. Lithofacies associations indicate that the hyaloclastic breccias were formed proximal to a source vent via quench-fragmentation, and subsequently reworked by ocean-bottom currents into distal epiclastic sandstone facies. During eruption, co-genetic pillow lava and hypabyssal intrusions mingled with the breccia, forming fluidal peperite. Rare polymict pyroclastic facies only occur in the highest stratigraphic levels and are mostly composed of highly vesicular (HV, 15-50% vesicles) sideromelane shards and crystalline basalt clasts with alkaline (~1.0 wt% K 2 O) fractionated (~6.8% MgO) compositions. Minor lithic grains are composed of subalkaline (~0.7 wt% K 2 O) to very highly alkaline (~1.7 wt% K 2 O) LV sideromelane shards, and amphibole-bearing diabase. The pyroclastic facies contains medium to thick beds of lapilli-tuff that exhibit both reverse and normal grading, diffuse lamination, and planar-grain fabric. These beds are locally overlain by thin fine-grained tuff beds entirely composed of cuspate to very thin elongate bubble-wall shards. These characteristics indicate that explosive deep-marine eruptions produced high-density coarse-grained gravity flows that were covered by slower suspension settle-out of delicate bubble-wall shards. Stratigraphic relationships suggest that explosive eruptions started during the waning stages of more alkaline volcanism along the proto-Macquarie spreading center.
Phosphates are present on the surface of the Mio-Pliocene unconformity in the Otway, Port Phillip and Gippsland basins of south-east Australia. The phosphates occur as lenticular lag deposits and include reworked phosphatic intraclasts, vertebrate bone and teeth. In situ phosphatized burrows are also found in sediments of Late Miocene and Early Pliocene age. The phosphatic intraclasts on the unconformity are interpreted as reworked phosphatized burrows derived from latest Miocene sediments (6 to 5 Ma). The phosphatization of these intraclasts is temporally related to the unconformity. The timing of phosphogenesis coincides with a period of transgression across the south-east Australian margin following Late Miocene uplift. This transgression is responsible for initial marine erosion of the underlying Miocene sequence, creation of a period of very slow sedimentation that was favourable to phosphate formation and subsequent deposition of the latest Miocene through to Pliocene sediments. The continental weathering of the uplifted highlands adjacent to the sedimentary basins, global phosphorus enrichment in the Late Miocene oceans and localized upwelling may all have contributed to phosphatization in south-eastern Australia.
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