Guy R. Holdgate

Origin and Timing of the Miocene-Pliocene Unconformity in Southeast Australia

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 < 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 ∼ 10 Ma) sediments underlie the unconformity and earliest Pliocene (∼ 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.

Cenozoic stratigraphic succession in southeastern Australia

Strata of Cenozoic age occur around the southern margin of Australia as thin and discontinuous outcrops, interpolated and fleshed out by economic exploration onshore and offshore. The neritic strata fall into four sequences or allostratigraphic packages of (I) Paleocene — Early Eocene, (II) Middle Eocene — Early Oligocene, (III) Late Oligocene — Middle Miocene, and (IV) Late Miocene — Holocene age: a four‐part pattern that can be seen also in the flanking pelagic and terrestrial realms including regolith deep weathering. Problems of correlation and age determination (predominantly biostratigraphic) have included biogeographical constraints (endemism in neritic molluscs and terrestrial palynomorphs, mid‐latitude assemblages in calcareous plankton), and slow progress in magnetostratigraphy and chemostratigraphy. Sequence I largely repeats the Cretaceous siliciclastic‐coal, marginal‐marine facies (carbonate‐poor, with marine and non‐marine palynomorphs and agglutinated foraminifers) punctuated by marine ingressions with microfaunas and sparse macrofaunas. Sequence II contains the first carbonates in the region since the Palaeozoic and the most extensive coals of the Cenozoic anywhere. Sequence III contains the most extensive neritic carbonates and the last major coals. Sequence IV is more strongly siliciclastic than the two preceding. Each of these four second‐order entities (107 years duration) comprises third‐order packages each with an unconformity and marine transgression. These packages hold true right along the southern Australian margin in the sense that the hiatuses and transgression do not display significant diachroneity at the relevant time‐scales (105–106 years). Recognised, delimited and correlated independently of the putatively global Exxon sequences, they are remarkably consistent with the latter, thereby providing a significant regional test. There are two widespread emphases on southern Australian geohistory and biohistory: (i) to regard the regional story as part of the global story of accreting continents, an expiring Tethys, and an episodically cooling planet; and (ii) a somewhat contrary emphasis, with the region being a special case of rapid longitudinal motion towards the equator. Both emphases are plausible with the former being the more heuristic. The stratigraphic record is strongly punctuated, the four sequences being separated by both tectonic and climatic events. Thus: the sequence I/II gap involved extensive plate‐tectonic reorganisation and a new spreading regime from ca 43 Ma, coevally with early growth of Antarctic ice; in the II/III gap, deformation in marginal basins is coeval with a global low in cooling, large ice sheet and falling sea‐level to ca 30 Ma; and the III/IV gap is marked by widespread cessation or contraction of stratal accumulation and withdrawal of thermophilic taxa coevally with the major expansion at ca 14 Ma of the Antarctic ice sheet, onset of intense canyon cutting, and plate‐wide basin inversion.

Tectonic significance of the Lambert graben, East Antarctica: Reconstructing the Gondwanan rift

Antarcticas Lambert graben, Australias North West Shelf, and the eastern Indian Peninsula all host thick, fault-bounded Permian-Triassic successions. These terranes were adjacent to each other in Gondwana. The Lambert graben intersects the modern coastline, strikes oblique to shelf architecture, and has a geophysical signature that can be traced >1000 km inland. Vitrinite reflectance data from the graben margins record Permian-Triassic infill. Australias North West Shelf is the relict of an intracontinental Carboniferous-Permian rift that was infilled during the Permian-Triassic then driven to oceanic completion during Jurassic-Cretaceous Gondwana breakup. This rift was compartmentalized over length scales of ∼650 km, corresponding to accommodation zones, margin-normal geophysical lineaments, and long-lived crustal weaknesses. In eastern India, similar compartmentalization is marked by extensive coal-bearing graben systems. Gondwana reconstructions indicate that the Lambert graben corresponds to the orientation and length scale of Carboniferous-Permian rift compartmentalization. The Lambert graben represents an accommodation zone of a wide intracontinental rift that extended from Australias North West Shelf, between India and Antarctica, to southern Africa. This rift collected Gondwanas thick Permian-Triassic sedimentary blanket and rich alluvial coal deposits.

The Pliocene climatic and environmental evolution of southeastern Australia: evidence from the marine and terrestrial realm

Abstract During the Pliocene the global climate fluctuated markedly with the expansion and contraction of the Northern and Southern Hemisphere ice sheets. The signals of this change are well preserved in the thick (up to 1 km) Seaspray Group cool-water carbonate sediments in the Gippsland region and associated thin terrestrial deposits in southeastern Australia. This study uses seismic, facies, foraminiferal proxy data and palaeobotanical data to chart the Pliocene climate and environmental change in the marine and terrestrial realms of southeastern Australia. Complex submarine canyoning occurred at the shelf/upper bathyal transition during the Pliocene in Gippsland. Low-energy pelagic marl (wackestone/packstone) characterise canyon and inter-canyon environments in the earliest Pliocene, depositing plankton oozes with interbedded calciturbidites. From upper Early Pliocene to Late Pliocene time high-energy limestone (grainstone) facies infilled these submarine canyons associated with progradation of the succession from outer to middle shelf palaeoenvironments. Plankton proxy data suggest cool conditions in the basal part of Early Pliocene. Relatively stable warmer marine conditions prevailed throughout most of the Early Pliocene, corresponding to a period of globally low δ18O values in the oceans associated with minor Antarctic ice sheet expansion. From middle to Late Pliocene time marked fluctuations in the abundance of cool and warmer water plankton taxa occurred, corresponding to a time of global marine δ18O fluctuations and generally heavier δ18O values associated with the expansion of the Antarctic ice and Northern Hemisphere ice sheets. Upwelling is interpreted to have occurred throughout much of the warmer Early Pliocene, caused by a more northerly (compared to today) positioned and weaker Subtropical Front. Upwelling was prevalent in the outer shelf to upper slope facies at the ‘palaeo’ Bass Canyon and the Subtropical Front migrated northwards to Gippsland during Late Pliocene glacial periods. Terrestrial palaeobotanical records indicate a shift from widespread Araucarian forests and rainforest, including ‘tropical’ taxa now extinct in the region, to a landscape by the end of the Late Pliocene similar to that of the present day with a mosaic of Eucalyptus–Acacia–Casuarinaceae sclerophyllous forests and open vegetation, with local areas of Nothofagus-dominated cool temperate rainforests. Palaeobotanical proxy data indicate that regional climate oscillated between warm–wet and cool–dry phases, with an overall cooling–drying trend through the Pliocene. Earliest Pliocene climates in southeast Australia were warm–wet with a summer rainfall peak (mean annual temperature, MAT, 2–4°C higher than present, mean annual precipitation, MAP, 50–70% higher than present), whereas terminal Late Pliocene climates were drier–cooler with a winter rainfall peak (MAT 0–2°C higher than present, MAP 0–30% higher than present).

NEOGENE TECTONICS IN SE AUSTRALIA: IMPLICATIONS FOR PETROLEUM SYSTEMS

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.

No mountains to snow on: major post-Eocene uplift of the East Victoria Highlands; evidence from Cenozoic deposits

Since the idea of the Pliocene Kosciusko Uplift in the Southeastern Highlands of Australia was first introduced, there has been considerable debate about the validity of this Cenozoic uplift event. Until the mid 1990s, most researchers argued that most highland relief was present by the Cretaceous. Since the late 1990s, there has been a paradigm shift that extensive young Cenozoic uplift created much of the high relief. In this paper, we synthesise Cenozoic stratigraphic and structural data from the East Victoria Highlands to assess the timing and origin of uplift. New high-resolution radar topography data indicate extensive east-northeast- and northeast-trending vertical and horizontal fault block displacement of the Cenozoic volcanic and sedimentary paleovalley infill. We suggest that regional uplift and exhumation of the East Victoria Highlands took place along these faults, initiated during the Late Eocene to Early Oligocene, and movements continue to the present day. By a combination of block faulting and epeirogenic uplift the divide migrated 40 km north reaching the present position by Pliocene time. Paleocurrent, lateral stream and magnetic basaltic valley flow directions indicate northward paleoflow directions for many of the Eocene – Oligocene valleys even those south of the present divide. Paleovalleys close to the Gippsland Basin show southward flow directions. The uplift that began in the Eocene causing valley cut and infill, eroded an Early Cenozoic paleoplain surface. Remnants of Late Eocene to Oligocene ligneous sediments are preserved as sub-basaltic, lowland valley, fluvio-lacustrinal sediment on this surface. Three large low-gradient paleodrainage systems that begin south of the present divide flowed north over 100 km to the Murray Basin where they are overlain by younger sediments. In contrast, paleodrainage systems flowing south from the present coastal escarpment to Gippsland, were shorter and steeper. The similarities of palynofacies of this infill to the adjacent basins suggest the valleys were low-relief/low-altitude paleodrainage systems that extended over the East Highlands. Based on our palynology results from Mt Hotham (present-day height 1800 m), previous macrofossil estimates of 800 m maximum Eocene relief could be overestimates of the paleo-height. Due to Cenozoic uplift, the strata (where present) are preserved as hilltop deposits and flows tilted away from the present divide. The Late Eocene to Pliocene uplift is probably primarily responsible for the topographic relief of the present East Victoria Highlands, is of the order of several hundreds of metres to a kilometre, and commenced in the Late Eocene at a divide closer to the Gippsland Basin than at present. This uplift continues to the present day as shown by the active seismicity in the area.

Plio-Pleistocene tectonics and eustasy in the Gippsland Basin, southeast Australia: evidence from magnetic imagery and marine geological data

The Pliocene and Pleistocene sediments of the Gippsland shelf are dominated by mixed carbonates and siliciclastics. From a detailed stratigraphic study that combines conventional marine geology techniques with magnetic imagery, the Late Neogene tectonic and eustatic history can be interpreted and correlated to the onshore section. Stratigraphic analyses of eight oil and gasfield foundation bores drilled to 150 m below the seabed revealed three principal facies types: (i) Facies A is fine‐grained limestone and limey marl deeper than 50 m below the seabed, of Late Pliocene age (nannofossil zones CN11–12); (ii) Facies B is a fine‐coarse pebble quartz‐carbonate sand that occurs 10–50 m below the seabed in the inner shelf, grading down into Facies A in wells in the outer shelf, and is of Early‐Middle Pleistocene age (nannofossil subzones CN13a-14b: ca 1.95–0.26 Ma); and (iii) discontinuous horizons of Facies C composed of carbonate‐poor carbonaceous and micaceous fine quartz sand occurring 10–50 m below the seabed. The sparse benthic foraminifers in Facies C are inner shelf or Gippsland (euryhaline) Lakes forms. Holocene sands dominate the upper 1.5–2.5 m of the Gippsland shelf and disconformably overlie cemented limestones with aragonite dissolution, indicating previous exposure to meteoric water. Nannofossil dating of the limestones indicates ages within subzone CN14b (dated between ca 0.26 and 0.47 Ma). Airborne magnetic imaging across the Gippsland shelf and onshore provides details of buried magnetic palaeoriver channels and barrier systems. The river systems trend south‐southeast from the Snowy, Tambo, Mitchell, Avon, Macalister and Latrobe Rivers across the shelf. Sparker seismic surveys show the magnetic palaeochannels as seismic ‘smudges’ 20–40 m below the seabed. They appear to correspond to Facies C lenses (i.e. are Early to Middle Pleistocene features). Magnetic palaeobarrier systems trending south‐southwest in the inner shelf and onshore beneath the Gippsland Lakes are orientated 15° different to the modern Ninety Mile Beach barrier trend. Offshore, they correlate stratigraphically to progradation packages of Facies B. Analysis of bore data in the adjacent onshore Gippsland Lakes suggests that a Pliocene barrier sequence 100–120 m below surface is overlain by fluvial sand‐gravel and lacustrine mud facies. The ferruginous sandstone beds resemble offshore Facies C, and are located where magnetic palaeoriver channel systems occur, implying Early to Middle Pleistocene ages. Presence of the estuarine bivalve Anadara trapezia in the upper lacustrine mud facies suggests that the Gippsland Lakes/Ninety Mile Beach‐type barriers developed over the past 0.2 million years. Further inland, magnetic river channels that cut across present‐day uplifted structures, such as the Baragwanath Anticline, suggest that onshore Gippsland uplift continued into the Middle Pleistocene.

A review of Tertiary brown coal deposits in Australia : Their depositional factors and eustatic correlations

The paleogeographical setting, sequence stratigraphy, and timing for six Tertiary brown coal deposits along the southern seaboard of Australia indicate a concentration of major coal-forming phases to periods of significant coastal onlap coupled with frequent sea level oscillations.

The palaeogeographic and palaeoenvironmental evolution of a Palaeogene mixed carbonate–siliciclastic cool-water succession in the Otway Basin, Southeast Australia

Abstract The Otway Basin in southeast Australia contains a thick sequence of Cenozoic shelfal carbonates and siliciclastics that preserve signals relating to the progressive opening of the Southern Ocean since the Paleogene. This multidisciplinary study integrates outcrop and subsurface well data from over 100 wells and bores throughout the Otway Basin with micropalaeontological analyses to constrain the age and palaeoenvironments of the Nirranda Group (Late Eocene to Middle Oligocene) and the Heytesbury Group (Late Oligocene to mid Miocene). These data were used to deduce the Late Eocene to Late Oligocene palaeogeographical evolution of the area. During the Late Eocene paralic high energy siliciclastic shoreline to shelf facies dominated the region, deepening southwards where mid to outer shelf conditions preserved high energy sandy carbonate facies. Above the Eocene–Oligocene boundary low energy inner to mid shelfal silt and muddy sand persisted to the north, deepening southward to carbonate-dominated low energy outer shelf to bathyal marls. The change from siliciclastics to carbonates at the Eocene–Oligocene boundary in the Otway Basin may relate to regional tectonics. In the Early Oligocene, high energy inner to outer shelf sand bodies formed in front of marine to inner shelf mudstone facies; the sand units are likely to have been influenced by strong local longshore drift and ocean swells that increased as the Southern Ocean widened to create a larger fetch. In the eastern half of the basin, later in the Early Oligocene, mixed paralic to inner shelf siliciclastic and carbonate facies were deposited passing to inner to mid shelf marl and mudstone and outer shelf to bathyal marls basinward. During this time, low to high energy shelfal calcarenite, chalk and marl dominated the westerly edge of the basin. The contrast in facies from west and east in the basin is inferred to be due to contrasting terrigenous input, environmental energy and ramp/shelf geometry. By Late Oligocene times (the Clifton Formation) the Otway Basin was dominated by high energy carbonate facies deposited in mid to outer shelf palaeoenvironments. The base of the Clifton Formation preserves a shift in facies and foraminiferal faunas that correlates to the major sea level fall at the Early–Late Oligocene boundary. This sea level fall is related to a major ice advance in Antarctica that corresponds to mid-Oligocene unconformities globally. The switch from low to high energy facies across the Early–Late Oligocene boundary in the Otway Basin suggests that the Southern Ocean swells experienced by the modern Otway coast were well established by Middle Oligocene time, evidence of the strengthening ‘proto‘ Antarctic swell regime. By Early Miocene times, with final deepening of the Drake Passage, the Antarctic Circum-Polar Current formed and predominantly inner to outer shelf marls and limestones were deposited in the Otway Basin.

Cenozoic fault control on ‘deep lead’ palaeoriver systems, Central Highlands, Victoria

Basaltic eruptions across the Central Highlands of Victoria have sealed in-place Early to middle Cenozoic palaeodrainage systems (also known as deep leads). The basal gravels of the deep leads have been mined extensively in the past for their rich placer-gold deposits. Detailed mapping of the distribution of all palaeorivers has been carried out using drilling results and modern aeromagnetic/radiometric surveys. The palaeochannel isopachs (including basalt and sediment) do not thicken in a modern downvalley direction. Instead, deeper depressions alternate with shallower areas. The variations in thickness, and parts of the palaeochannel courses, are controlled by a series of east-northeast-trending basement highs. The basement highs are caused by a set of east-northeast-trending (Otway Basin-style) faults visible on radar shuttle imagery in the Central Highlands. They have not previously been recognised in regional geological mapping. Most published fault trends are north – south oriented, parallel to the strike of the Palaeozoic basement rocks. Exceptions occur at Ballarat where there is an orthogonal east-northeast set mapped in underground quartz reef workings that show right-lateral strike-slip movements. The east-northeast faults show half-graben block-style rotational movement on basement, creating north- and south-facing fault scarps along the horst ridges. Where palaeochannels overlie the grabens, valleys broadened, infill thickens, and locally drainage directions may change. When the drainage cuts through the horsts, steeper incised valleys result, and this is where, in the historical past, some gold leads were ‘lost’. The initial timing of the block movement pre-dates at least the Early Oligocene to Late Miocene ages of the basal palaeovalley sediments, as shown by revised palynological dating. In places, the modern drainage divide coincides with east-northeast-trending faults. In the Ballarat area, an earlier divide accentuated by the aeromagnetic palaeodrainage mapping occurs up to 25 km south and appears to pre-date the earliest basalt flows at around 7.0 Ma. This evidence suggests the divide can change position through time by differential movements along east-northeast faults and transferral of maximum uplift to adjacent blocks.