Neville Exon

Drilling reveals climatic consequences of Tasmanian Gateway Opening

One of the great stories of geoscience is how Gondwana broke up and the other southern continents drifted northward from Antarctica, which led to major changes in global climate. The recent drilling of Ocean Drilling Project (ODP) Leg 189 addressed in detail what happened as Australia drifted away from Antarctica and the Tasmanian Gateway opened. The drifting contributed to the change in global climate, from relatively warm early Cenozoic “greenhouse” conditions to late Cenozoic “icehouse” conditions. It isolated Antarctica from warm gyral surface currents from the north and provided the critical deepwater conduits that eventually led to ocean conveyor circulation between the Atlantic and Pacific Oceans.

The geological development of the passive margins of the Exmouth Plateau off northwest Australia

Abstract The Exmouth Plateau is a very large sunken continental block off Australias Northwest Shelf, whose culmination lies 800 m below sea level. It is elongated in a northeasterly direction, and bounded by abyssal plains on three sides. Its broad tectonic framework is controlled by Mesozoic rifting and transform faulting, which accompanied the breakup of Australia and “Greater India”, after which the plateau sank to bathyal depths. The pre-breakup sequence consists of more than 5000 m of Palaeozoic and early Mesozoic shallow marine to terrestrial detrital sediments. The post-breakup sequence consists of about 2000 m of late Mesozoic and Cainozoic marine sediments, mainly carbonates. The northern margin came into existence in the Callovian (155 m.y. ago), when a continental fragment separated from it and moved off to the northwest. An early phase of rifting gave rise to Triassic—Jurassic intermediate and acid volcanics (213-192 m.y. ago), which overlie a thick Triassic paralic sequence. Steady subsidence north of an east—west hinge line allowed several thousand metres of early and middle Jurassic carbonates and coal measures to accumulate before breakup. Breakup occurred along a series of rifted and sheared segments, and the margin is further complicated by northeast-trending Callovian horsts and grabens. The horsts were planed off in late Jurassic and early Cretaceous times, and the whole margin was covered by a few hundred metres of late Cretaceous and Cainozoic pelagic carbonate as it sank steadily to its present average depth of 2000–2500 m. The northeast-trending western margin formed by rifting in the Neocomian (120–125 m.y. ago), as “Greater India” moved off to the northwest. Callovian normal faults parallel the margin. A thick Triassic paralic sequence is unconformably overlain by thin late Jurassic and younger marine sequences, indicating that the area was high in the early and middle Jurassic. Thin late Cretaceous and Cainozoic pelagic carbonates cover the margin, which now lies more than 2000 m below sea level. The northwest-trending southern margin formed by shearing in the Neocomian, at the same time as the western margin. It is cut by northeast-trending normal faults, which formed in the late Triassic and Callovian, and is paralleled by Neocomian and later normal faults. A thick Triassic paralic sequence is unconformably overlain by a thick late Jurassic and Neocomian delta, suggesting that the area was high in the early and middle Jurassic, but a depocentre before and afterwards. There was thermal uplift of more than 1000 m during the Neocomian period of shearing, and igneous intrusions buttressed the margin. Later normal faulting lowered the outermost margin, and turned the uplift into a marginal anticline trending northwest. The anticline had sunk beneath the sea by late in the Cretaceous, and thereafter this margin was covered by a thin sequence of pelagic carbonate, which now lies beneath more than 1500 m of water.

Kenn Plateau off northeast Australia: a continental fragment in the southwest Pacific jigsaw

The submarine Kenn Plateau, with an area of about 140 000 km2, lies some 400 km east of central Queensland beyond the Marion Plateau. It is one of several thinned continental fragments east of Australia that were once part of Australia, and it originally fitted south of the Marion Plateau and as far south as Brisbane. It is cut into smaller blocks by east- and northeast-trending faults, with thinly sedimented basement highs separated by basins containing several kilometres of sediment. In the Cretaceous precursor of the Kenn Plateau, Late Triassic to Late Cretaceous basins probably rested unconformably on Palaeozoic to Triassic rocks of the New England Fold Belt. Rift volcanism was common on the northern plateau and was probably of Early Cretaceous age. Late Cretaceous extension and breakup were followed by Paleocene drifting, and the Kenn Plateau moved to the northeast, rotated 30° anticlockwise and left space that was filled by Tasman Basin oceanic basalts. During these events, siliciclastic sediments poured into the basins from the continental mainland and from locally eroding highs. After a regional Late Paleocene to Early Eocene unconformity, siliciclastic sedimentation resumed in proximal areas. In deep water, radiolarian chalks were widely deposited until biosiliceous sediment accumulation ended at the regional Late Eocene to Early Oligocene unconformity, and warming surface waters led to accumulation of pure biogenic carbonates. Calcarenite formed in shallow water on the margins of the subsiding plateau from the Middle Eocene onward. Some seismic profiles show Middle to Late Eocene compression related to New Caledonian obduction to the east. Hotspots formed parts of two volcanic chains on or near the plateau as it moved northward: Late Eocene and younger volcanics of the Tasmantid chain in the west, and Late Oligocene and younger volcanics of the Lord Howe chain in the east. As the volcanoes subsided, they were fringed by reefs, some of which have persisted until the present day. Other reefs have not kept up with subsidence, so guyots formed. The plateau has subsided 2000 m or more since breakup and is now subject solely to pelagic carbonate sedimentation.

Paleoceanographic Evolution of the Tasmanian Seaway and its Climatic Implications

The opening or closing of major oceanic gateways or seaways through plate tectonics can significantly change surface and/or deep ocean circulation. This clearly has led to fundamental changes in the Earths environmental system, global climate, and marine and terrestrial biogeography. Ocean circulation changes resulting from gateway development can significantly affect global heat transfer, and thus climate. However, these climatic effects should be considered within an Earth System context involving a variety of integrated environmental feedbacks. The opening of the Tasmanian Gateway during the Eocene-Oligocene transition (∼33.5 Ma), and later Seaway expansion, led to critical changes in Southern Hemisphere Ocean circulation resulting from the development of the Antarctic Circumpolar Current (ACC). Gateway opening initiated thermal isolation of Antarctica leading to the crossing of a major global climatic threshold and to significant Antarctic ice expansion. Much of this climate change resulted, not from circulation changes alone, but through environmental feedback mechanisms associated with ice expansion and cooling. These included increased albedo, ice sheet elevation, atmospheric changes, increased Southern Ocean productivity, and intensification of thermohaline circulation leading to expansion of deep cold waters. Cooling of the deep ocean and the continents also likely led to decreased atmospheric greenhouse gases CO 2 and CH 4 that, in turn, contributed to pronounced cooling in the earliest Oligocene. Antarctic circumpolar circulation continued to strengthen during the Oligocene through early Neogene in response to further Seaway expansion, increasing thermal isolation of Antarctica and related development of the Antarctic System. Results of ODP Leg 189 have confirmed that the initial main opening of the Tasmanian Gateway at the Eocene/Oligocene boundary (∼33.5 Ma) coincided with major ice growth and cooling on Antarctica, as reflected by oceanic isotopic and biotic trends. This continues to implicate gateway opening as fundamental in triggering the shift in the Earth System at the beginning of the Oligocene. Changes in the paleobiogeography of planktonic microfossil assemblages in the SW Pacific, and in shallower marine groups of the New Zealand Platform, are critical for reconstructing paleoceanographic changes resulting from Tasmanian Gateway evolution. The distribution of late Eocene planktonic microfossil assemblages in the SW Pacific, and of shallower marine taxa on the New Zealand Platform, are consistent with the influence of a broad, warm subtropical gyre from the north that extended to high southern latitudes.

Geology and petroleum potential of the Fairway Basin in the Tasman Sea

The Fairway Basin is a large, generally north – south-trending, sediment-filled structure in water 1500 – 3000 m deep, on the eastern slope of the Lord Howe Rise in the Tasman Sea, and is partly within Australian jurisdiction. It was poorly known until a few years ago, when seismic profiling and piston coring cruises were carried out. The basin, about 1100 km long and 120 – 200 km wide, can be divided into three segments—north, central and south—that trend northwest, north and north-northwest, respectively. All three segments probably formed by thinning of continental crust during breakup of Lord Howe Rise and surrounding aseismic continental ridges in the Late Cretaceous and Paleocene. Normal faulting, large inputs of terrigenous sediment and subsidence to bathyal marine depths occurred during that time. A period of compression, perhaps related to overthrusting on New Caledonia, occurred in the Eocene, leading to uplift (and in parts, erosion) of northern Lord Howe Rise, and reversal of faulting in the basin. By the Oligocene, the area was again in bathyal depths, and pelagic ooze and some turbidites accumulated. The basinal sequence is generally 2000 – 4000 m thick, with 1200 – 3200 m of Cretaceous to Eocene sediment concentrated in depocentres, capped by 500 – 800 m of Oligocene and younger sediment. In the depocentres, numerous sedimentary diapirs pierce sedimentary sequences. The sedimentary diapirs appear to be fed by Cretaceous muds deposited during rifting. Often, these diapirs are overlain by faults extending to the seafloor, and hummocky bathymetry is possibly caused by fluid escape. The overall geology suggests that the Fairway Basin may be a large frontier hydrocarbon province. Seismic profiles display a bottom-simulating reflector above many depocentres, 500 – 700 m below the seafloor. The bottom-simulating reflector has positive polarity, which could result from a diagenetic phase transformation, a thin gas hydrate layer with a sharp top, or from the sharp base of a gas layer (probably beneath gas hydrates). Standard piston cores taken above diapirs and apparent fluid-escape features have recovered little gas. Other than drilling, the next steps in assessing petroleum potential are to clearly document fluid-escape structures, and to sample any fluids emitted for hydrocarbons.

Geomorphology and evolution of the gigantic Murray canyons on the Australian southern margin

The Murray canyons are a group of deeply incised submarine canyons on a steep 400 km section of the continental slope off Kangaroo Island, South Australia. Some of the canyons are amongst the largest on Earth. The canyons, some 80 km long, descend from the shelf edge to the abyssal plain 5200 m deep. Sprigg Canyon, the deepest and one of the largest, has walls 2 km high. The thalwegs of the larger canyons are concave in profile, steepest on the upper continental slope (15 – 30°), with about 4° gradient on the mid slope, then level out on the lower slope to merge with the 1° continental rise. Between canyons, the continental slope is slightly convex to linear with a gradient of about 5 – 6°. Canyon walls commonly slope at 15 – 22°. The passive continental margin narrows to 65 km at the Murray canyons and links the Bight and Otway Basins. West-northwest-trending Jurassic – Cretaceous rift structures control the irregular shape of the central canyons. At the western end, large box canyons, 1 km deep, are incised into thick sediments of the Ceduna Sub-basin. Formed by headscarp erosion, some of these canyons have coalesced by canyon capture. The upper parts of most canyons are cut into Cretaceous sediments and in some places are floored by basement rocks. Large deep-water holes, spaced about 5 km apart and up to several hundred metres deep, along the outlet channels of the larger and steeper canyons were probably gouged by turbidity currents resulting from major slope failures at the shelf edge, but may be sites of fluid discharges. Quaternary turbidites were deposited on the abyssal plain more than 100 km from the foot of slope. Canyon downcutting has been episodic since the latest Cretaceous, with peak activity since the Oligocene due to strong glacioeustatic fluctuations and cycles. Canyon development occurred during lowstands and early in transgressions when sediment input at the shelf edge was usually highest. The timing of canyon development is linked to major unconformities in adjacent basins, with downcutting events recorded or inferred during the Early Paleocene, Middle Eocene, Early Oligocene, Oligocene/Miocene transition (ca 24 Ma), Middle Miocene (ca 14 Ma) and latest Miocene – Pleistocene. The early phases involved siliciclastic sediments only, while post-Early Eocene canyon cutting was dominated by biogenic carbonates generated on the shelf and upper continental slope. The Murray River dumped its sediment load directly into Sprigg Canyon during extreme lowstands of the Late Pleistocene when the Lacepede Shelf was mostly dry land.

Miocene volcanic seamounts on northern Lord Howe Rise: lithology, age and origin

Multibeam sonar swath‐mapping has revealed small submarine volcanic cones on the northeastern Lord Howe Rise, a submerged ribbon continent. Two such cones, aligned north‐northwest and 120 km apart, were dredged at 23–24°S. Water depth is about 1150 m nearby: the southern cone rises to 750 m and the northern to 900 m. Volcanic rocks dredged from the cones are predominantly highly altered hyalodastites with minor basalt. The clasts are mostly intensely altered vesicular brownish glass with lesser basalt, set in zeolitic, clayey, micritic, or ferruginous cement. Lavas and hyalodastites contain altered phenocrysts of olivine and plagioclase, and fresh clinopyroxene. The latter have compositions between acmite and Ti‐augite, matching clinopyroxene phenocrysts in undersaturated intraplate basanitic mafic lavas. Interbedded micrites in the volcaniclastics represent calcareous ooze that was deposited with (or later than) the volcanic pile. Foraminifers indicate that the micrite is dominantly Early Miocene (ca 16 Ma), and that the ooze was deposited in cool water. Upper Miocene to Pliocene micrites, presumed to be later infillings, all contain warm‐water forms. This evidence strongly suggests that both cones formed in pelagic depths in the Early Miocene. Ferromanganese crusts from the two cones are up to 7 cm thick and similar physically, but different chemically. The average growth rate is 4 mm/106 y. Copper, nickel, and cobalt content are relatively high in the north, but copper does not exceed 0.08 wt%, nickel 0.65%, and cobalt 0.25%. The Mn:Fe ratio is high in the south (average 13.7) suggesting strong hydrothermal influence. Such small volcanic cones related to intraplate hot‐spot‐type volcanism may occur in extensive fields on the Lord Howe Rise, like those off southern Tasmania. On the Lord Howe Rise, the known small volcanic cones coincide with broad gravity highs in areas of shallow continental basement. The highs are aligned parallel to the Lord Howe Rise margins, and it is probable that the Miocene volcanism is related to jostling on older faults. Other fields of small volcanic seamounts may occur on the Lord Howe Rise on such fractured gravity highs.

Transect sismique continu entre l'arc des Nouvelles-Hébrides et la marge orientale de l'Australie : programme FAUST (French Australian Seismic Transect)

Abstract Within the framework of the joint FAUST (French Australian Seismic: Transect) program, the Australian research vessel Rig-Seismic recorded a series of multichannel seismic profiles between the New Hebrides Arc and the Australian Margin. The very high quality of these profiles allows the examination of the continental or oceanic basement over the whole area surveyed. In basins beneath the eastern margin of the northern Lord Howe Rise and the New Caledonia Basin, where the crust is thinner, the Moho discontinuity is present at about 8.5 seconds two-way time (TWT) (about 12–13 km) on the profiles processed onboard. The data collected suggest that the sedimentary section within the basins includes syn-rift Cretaceous sediments that are not yet definitively identified. The Upper Eocene-Middle Oligocene subduction of the Australian plate beneath the Pacific plate is highlighted by thrust faults concentrated along the Lord Howe, Norfolk and Fairway Ridges. This episode probably corresponds to the period of convergent tectonism associated with the Upper Eocene obduction of the New Caledonian ophiolites.

Nature and origin of the submarine Albany canyons off southwest Australia

The Albany canyons complex off southwest Australia extends 700 km from Cape Leeuwin to east of Esperance. The submarine canyons head on the uppermost continental slope and extend from there up to 90 km offshore, to the lowermost slope and onto the abyssal plain. Distributaries have transported shelf carbonate grains at least 150 km onto the abyssal plain. The largest canyons have cut down 1500 – 2000 m in places. In general, on the upper slope they have cut down into harder, older rocks: canyon walls are steep, canyon axes slope at up to 20°, and ancient structures control their orientation. On the lower slope the canyons generally have not eroded down into harder rocks: canyon walls are less steep, canyon axis slopes are lower, and the canyons are generally oriented downslope. The canyons have exposed Jurassic and younger sedimentary rocks: their nature, canyon morphology and information from seismic reflection profiles have helped us build an understanding of canyon history. Floodplain deposition rather than erosion occurred during Australia – Antarctic rifting in the Late Jurassic, so river canyons (possible precursors of marine canyons) were unlikely to have been cut. A transition from non-marine to shallow-marine sedimentation characterised Early Cretaceous deposition in the slowly developing rift of the Australo-Antarctic Gulf. Gradients were low and canyon cutting unlikely. Deep river canyons were probably cut during uplift and erosion immediately before the Santonian breakup from Antarctica and their paths probably controlled later marine canyons. Only with the onset of rapid sea-floor spreading and subsidence in the Middle Eocene (ca 43 Ma) did gradients steepen and major marine canyon cutting become possible. The major sea-level fall at the Middle/Late Eocene boundary (ca 40 Ma) may perhaps have accelerated canyon formation. Carbonate sedimentation started to replace siliciclastic sedimentation in the late Middle Eocene, and became completely dominant in the Oligocene as the Antarctic Circumpolar Current started to scour the outer shelf of fines. However, carbonate grains displaced from the outer shelf could continue to cut the canyons, largely during periods of low sea level.

Southeast Australia: A Cenozoic Continental Margin Dominated by Mass Transport

The Southeast Australian continental margin extends for 1,500 km northward from Bass Strait to the Great Barrier Reef. Mass transport dominates the continental slope, which stretches from the shelf break around 150 m depth to the abyssal plain around 4,500 m depth. The continental slope has average slopes of 2.8–8.5° and extends seaward from the shelf break an average distance of 50 km. Margin structure results from Late Cretaceous rifting, producing exposed fault blocks and igneous complexes on the lower slope, and an overlying sediment wedge around 0.5 km thick, centered at the shelf break. Recent collection of multibeam echosounding and high-resolution seismic data provide a detailed view of mass-transport features over a 900 km length of the margin. The features are mostly slab slides, box canyons, and linear canyons. They are ubiquitous along the steep rifted margin, but absent in regions of gentler slopes such as submarine plateaus and failed rift arms. Submarine landslides range in scale from hundreds of small slides of <0.5 km3 volume, up to the largest documented slide of 20 km3. However, potential future slide masses of basement blocks up to 105 km3 have been identified. Cores that penetrated the basal-slide surface show variable sediment accumulation, since the mass-movement event, but four penetrations show accumulations of <2 m, and one of <0.6 m. At current accumulation rates, these data indicate that many landslides occurred less than 25 ka, with some as recent as 6 ka. Mass movements appear to follow a pattern of box canyon development exploiting structural trends in pre-rift and syn-rift strata, until the canyon head intersects the toe of the Tertiary sediment wedge. Once this occurs, sediment creep, faulting and failure of the wedge toe migrates up slope, finally reaching the upper slope and Quaternary deltaic depocenters.