Robert M. Carter

New evidence for episodic post-glacial sea-level rise, central Great Barrier Reef, Australia

Abstract We present an extensive database of 364 radiocarbon dates from coastal and marine sediments of the central Great Barrier Reef (GBR) shelf, of which 110 are previously unpublished. The elevation data have been reduced to a common datum (Australian Height Datum, AHD) and the various sources of error have been assessed. Using modern lithological and biological relationships with sea level, the elements of the radiocarbon database have been converted into sea-level indicators. The upper bound of the assembled dataset corresponds to a best-estimate sea-level curve, and the full dataset provides a narrow envelope for sea-level rise on the GBR shelf for the last 11–12 kyr (not including hydro-isostatic crustal flexing). The envelope is consistent with episodic rise in post-glacial sea levels. The rising post-glacial sea level probably included Stillstands (or minor falls), at ca. −45 m AHD (at ca. 10.5 kyr B.P.), −5 m (7.8 kyr B.P.), −2 m (ca. 6 kyr B.P.) and +1.7 m (5.5 kyr B.P.). There is evidence for a significant fall in sea level between stillstands at −11 m (8.5 kyr B.P.) and −17 m (8.2 kyr B.P.). Stillstand durations apparently ranged between The vertical spread in the derived sea-level data is very wide. The use of shell material for dating seems unreliable and prone to large and unpredictable errors. Data from bulk mangrove muds appear reliable for determination of ancient sea level, but may at times result in sea level being placed up to 4 m below the true level. In-situ biogenic carbonates such as preserved oyster beds and coral micro-atolls are the most reliable indicators of sea-level position, while deposits of mangrove mud give a useful first-order approximation of ancient sea levels. Caution should be used in drawing ‘sea-level curves’ from few data points. We conclude that the post-11–12 kyr B.P. relative rise in sea level was episodic on the central GBR continental margin. More data are required to define clearly sea-level change up to ca. −20 m at 9 kyr B.P.

Regional sediment recycling in the abyssal Southwest Pacific Ocean

An active plate boundary with a high sediment output, three major submarine channels, and the world9s largest deep western boundary current (DWBC) make up an extensive recycling system along the 4500 km continental margin, east of New Zealand. Seismic reflection, sedimentary, and oceanographic data demonstrate that detritus from the rising mountains of the New Zealand plate boundary is transferred to the Southwest Pacific abyssal floor by turbidity currents flowing along Solander (>450 km long), Bounty (950 km), and Hikurangi (1400 km) channels. These conduits discharge directly into the DWBC, which transports material north to form a series of sediment drifts. The northernmost drift, containing sediment from Hikurangi Channel and eroded drifts to the south, is now subducting into Kermadec Trench. Geochemical data suggest that sediment is recycled through the mantle to re-emerge in the arc volcanic rocks. Thus one cycle is completed and a new one begins.

Post-breakup stratigraphy of the Kaikoura Synthem (Cretaceous-Cenozoic), continental margin, southeastern New Zealand

The stratigraphy and structure of the Kaikoura Synthem (Cretaceous—Cenozoic) is synthesised from available seismic records and well reports on the continental margin of southeastern South Island. Four seismic sequences are distinguished, corresponding to lithostratigraphic groups that can also be recognised nearby on land: Matakea Group (Cretaceous), Onekakara Group (late Cretaceous—Oligocene), Kekenodon Group (Late Oligocene—Miocene), and Otakou Group (Miocene—Recent). These sequences achieve maximum thickness within three sedimentary basins underlying the continental shelf (Canterbury and Foveaux Basins) and slope (Great South Basin). The Great South Basin is separated from the Canterbury and Foveaux Basins across the WaipounamouFaultSystem, anortheasterly oriented zone of faults which controlled rifting and Matakea Group sedimentation along the western edge of a Cretaceous aulacogen now manifest as the Bounty Trough. The Canterbury andFoveaux Basins contain transgressive sequences (Onekakara Group) tha...

Sedimentary cyclicity in the marine Pliocene-Pleistocene of the Wanganui basin (New Zealand): Sequence stratigraphic motifs characteristic of the past 2.5 m.y.

Earth’s climatic history since 2.5 Ma has been controlled by Milankovitch variations in the planetary orbit, comprising alternate periods of glaciation and interglaciation with a dominant frequency of 41 000 yr. Concomitantly, eustatic sea level has fluctuated 70 to 130 m, causing rapid transgressions and regressions of the shoreline across the world’s continental shelves. The resulting sedimentary record is cyclothemic, each cyclothem corresponding to a single climate and sea-level cycle. The Wanganui basin, New Zealand, contains a 2-km-thick, almost complete, composite record since isotope stage 100 (ca. 2.5 Ma) in the form of 47 superposed cyclothems of shelf origin. Each cyclothem corresponds to an unconformity-bound stratigraphic sequence, and typically contains a transgressive systems tract, sometimes a mid-cycle shell bed, a highstand systems tract, and sometimes a regressive systems tract. No advantage accrues from using transgressive-regressive units rather than cyclothems and/or sequences in description of the succession. Six basic sequence motifs represent deposition in locations between the shoreline and offshore shelf, i.e., the Hawera, Birdgrove, Turakina, Seafield, Castlecliff, and Rangitikei motifs. A seventh, the Nukumaru motif (which includes dominant coquina limestone), represents deposition in shallow-water areas of reduced terrigenous sediment on the flank of the basin. The sequence motifs represented in any section change systematically in sympathy with basin-scale changes in subsidence and sediment supply. In contrast with the 41000 year length of individual glacioeustatic sequences, these basin-wide tectonic cycles have a periodicity of many hundreds of thousands to a few million years, i.e., that of third- or fourth-order sequences of the Exxon type. This, coupled with the restriction of strongly cyclothemic sediments to geological periods of known glacio-eustasy (PermianCarboniferous, Pliocene-Pleistocene), suggests that tectonic subsidence cycles rather than glacio-eustasy are the driving forces behind the development of the third- and fourth-order unconformity-bound sequences that are reported to occur throughout the stratigraphic record.

Evolution of the sedimentary system beneath the deep Pacific inflow off eastern New Zealand

Results from Ocean Drilling Program sites 1121–1124 show the Eastern New Zealand Oceanic Sedimentary System (ENZOSS) evolved in response to: (1) the inception of the circum-Antarctic circulation, (2) orbital and non-orbital regulation of the global thermohaline flow, and (3) development of the New Zealand plate boundary. ENZOSS began in the early Oligocene following opening of the Tasmanian gateway and inception of the ancestral Antarctic Circumpolar Current (ACC) and SW Pacific Deep Western Boundary Current (DWBC). Widespread erosion, marked by the Marshall Paraconformity, was followed by extensive drift formation in the late Oligocene–early Miocene. Alternating nannofossil chalk and nannofossil-rich mud deposited in response to 41-kyr orbital regulation of the abyssal circulation, with the mudstones representing times of increased inflow of corrosive southern-source waters. Drift deposition at the deepest sites was interrupted by bouts of erosion coincident with Mi1–5 isotopic events signifying expansions of the East Antarctic Ice Sheet and enhanced bottom water formation. By late Miocene times, the basic ENZOSS was established. South of Bounty Trough, the energetic ACC instigated an erosional/low depositional regime. To the north, where the DWBC prevailed, orbitally regulated drift deposition continued. Increased convergence at the New Zealand plate boundary enhanced the terrigenous supply, but little of this sediment reached the deep ENZOSS as the three main sediment conduits – Solander, Bounty and Hikurangi channels – had not fully developed. The Plio–Pleistocene heralded a change from a carbonate- to terrigenous-dominant supply caused by interception of the DWBC by the three channels (∼1.6 Ma for Bounty and Hikurangi, time of Solander interception unknown). The Solander and Bounty fans, and Hikurangi Fan-drift systems formed, and drifts downstream of those systems, received terrigenous detritus. Supply increased with accelerating uplift along the plate boundary, but delivery to the DWBC was regulated by eustatic fluctuations of sea level. Times of maximum supply to all three channels was during glacial lowstands whereas the supply either ceased (Bounty, Solander), or reduced (Hikurangi) in highstands. In glacial times, sediment was entrained by a DWBC invigorated by an increased input of Antarctic bottom water. The ACC also accelerated under strengthened glacial winds. Thus, glacials were times of optimum sediment supply to ENZOSS depocentres where depositional rates were 2–3 times more than interglacial rates.

Sediment flux across the Great Barrier Reef Shelf to the Queensland Trough over the last 300 ky

The continental margin off northeast Australia, comprising the Great Barrier Reef (GBR) platform and Queensland Trough, is the largest tropical mixed siliciclastic /carbonate depositional system in existence. We describe a suite of 35 piston cores and two Ocean Drilling Program (ODP) sites from a 130 £ 240 km rectangular area of the Queensland Trough, the slope and basin setting east of the central GBR platform. Oxygen isotope records, physical property (magnetic susceptibility and greyscale) logs, analyses of bulk carbonate content and radiocarbon ages at these locations are used to construct a high resolution stratigraphy. This information is used to quantify mass accumulation rates (MARs) for siliciclastic and carbonate sediments accumulating in the Queensland Trough over the last 31,000 years. For the slope, highest MARs of siliciclastic sediment occur during transgression (1.0 Million Tonnes per year; MT yr 21 ), and lowest MARs of siliciclastic (,0.1 MT yr 21 ) and carbonate (0.2 MT yr 21 ) sediment occur during sea level lowstand. Carbonate MARs are similar to siliciclastic MARs for transgression and highstand (1.1‐1.4 MT yr 21 ). In contrast, for the basin, MARs of siliciclastic (0‐0.1 MT yr 21 ) and carbonate sediment (0.2‐0.4 MT yr 21 ) are continuously low, and within a factor of two, for lowstand, transgression, and highstand. Generic models for carbonate margins predict that maximum and minimum carbonate MARs on the slope will occur during highstand and lowstand, respectively. Conversely, most models for siliciclastic margins suggest maximum and minimum siliciclastic MARs will occur during lowstand and transgression, respectively. Although carbonate MARs in the Queensland Trough are similar to those predicted for carbonate depositional systems, siliciclastic MARs are the opposite. Given uniform siliciclastic MARs in the basin through time, we conclude that terrigenous material is stored on the shelf during sea level lowstand, and released to the slope during transgression as wave driven currents transport shelf sediment offshore. q 2000 Elsevier Science B.V. All rights reserved.

Marshall Paraconformity: a mid-Oligocene record of inception of the Antarctic circumpolar current and coeval glacio-eustatic lowstand?

Abstract The sedimentary fill of the Canterbury Basin, New Zealand, is the product of a long-term (80 Ma), tectonically controlled relative sea-level cycle with a megasequence geometry analogous to the sequence stratigraphic model of Vail (Am. Assoc. Petrol. Geol. Stud. Geol No. 27, 1, 1–10, 1987). The condensed section of the megasequence, resolvable in detail in outcrop and on seismic profiles, comprises a basin-wide pelagic to hemipelagic limestone interval. A regional mid-Oligocene unconformity, the Marshall Paraconformity, lies within the limestone interval onshore and correlates with hiatuses in at least two, and possibly three, offshore exploration wells and with a temporary lithological change from limestone to quartz sand at a fourth. Strontium isotopic age estimates confirm that a 2–4 Ma hiatus is associated with onshore outcrops of the Marshall Paraconformity (between ∼32 and 29 Ma), which correlates with the opening of the Pacific sector of the Southern Ocean and the postulated mid-Oligocene sea-level fall of Haq et al. (Science235, 1156–1167, 1987; Spec. Publ. Soc. Econ. Paleonotol. Mineral. No. 42, 71–108, 1988). Lowering of base level, coupled with cooling and enhancement of current activity, may have caused the temporary cessation of limestone deposition and a regional hiatus. This hypothesis reconciles the apparently contradictory palaeogeographical evidence for a regional highstand. The Marshall Paraconformity may exemplify the signature by which similar glacio-eustatic events can be recognized in offshore platform facies.

Evolution of Pliocene to Recent abyssal sediment waves on Bounty Channel levees, New Zealand

Abstract Levees bordering Bounty Channel 900 km east of New Zealand accommodate a 400 m-thick sequence (maximum) of sediment waves that have formed since Pliocene times. These bedforms, with amplitudes of 2–17 m and wavelengths of 0.6–6 km occur in 4100–4900 m of water and were formed by turbidity currents, as indicated by their restriction to levee backslopes, the frequent occurrence of turbidites in cores and the preferential but not exclusive development of waves on the left-bank levee in accord with the Southern Hemisphere coriolis deflection. The wave field was instigated in the Late Pliocene when glacially lowered sea level allowed rivers draining the Southern Alps of South Island to discharge directly into Bounty Channel and its attendant canyons. The field grew vertically through the coalescence of small waves into larger bedforms that continually migrated across and up levee backslopes at an average rate of 5.6m/100 yrs. Wave growth decreased into the Late Pleistocene probably in response to progressive containment of turbidity currents as the relief of Bounty Channel increased to 200 m or more. The glacial periods of wave growth were interrupted by interglacial interludes of quiescence when the field was draped mainly by pelagic calcareous ooze.

New Evidence for the Holocene Sea-Level High from the Inner Shelf, Central Great Barrier Reef, Australia

Radiocarbon dates from fossil oyster beds of intertidal origin on Magnetic Island, north Queensland indicate that the local Holocene maximum of relative sea-level was attained no later than 5660 +/- 50 B.P. (conventional uncorrected age) and remained at 1.6-1.7 m above modern levels until 4040 +/- 50 B.P. Given the tectonic stability of the area, this implies that eustatic sea-level remained at its Holocene peak for at least ca. 1600 yr. The new high-precision sea-level data indicate sea levels 1-5 m higher than those of the same age inferred from buried mangrove deposits on the inner shelf in north Queensland. Uncertainties in deriving relative sea-level from such mangrove deposits may be a significant source of error in worldwide attempts to distinguish the eustatic and crustal warping components off relative sea-level change, especially in the tropics.

Episodic post‐glacial sea‐level rise and the sedimentary evolution of a tropical continental embayment (Cleveland Bay, Great Barrier Reef shelf, Australia)

Cleveland Bay is a 400 km2 landlocked tropical embayment located at 19° S and 146° 55´E The bay is protected from the dominant southeasterly tradewind by Cape Cleveland, but lies open to northerly and northeasterly weather and to the effects of occasional tropical cyclones. Water‐motion within the bay is dominated by the effects of refracted southeasterly‐generated waves (mostly 0.5–1.2 m high, 4–6 s period) and by semi‐diurnal tidal currents, which reach speeds of 15–30 cm/s during spring tides. Residual circulation within the bay is anticlockwise and results in preferential sediment accumulation on the eastern side. The bay contains three main Holocene stratigraphic units (A‐C) which rest on weathered Late Pleistocene clay. The Pleistocene land surface is planar, dips seawards at 0.8 m/km and is incised by a major complex of fluvial and tidal channels. Seismic unit C encompasses cross‐bedded or draped fill of the channel system. Seismic unit B, occurring laterally to C, comprises massive grey mud with m...