Ken J. Woolfe

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3–4 °C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

What is a fluvial levee

Abstract Fluvial levees are elevated partitions between channels and floodplains. Because of their character and position, levees may provide critical controls on, and insights into, geomorphic processes that determine the distribution of water and sediment within river systems. Few studies have analysed the character, distribution, sedimentology and processes that form levees in modern depositional environments. Characterisation of levee deposits from the Mississippi River continues to form the basis for most levee interpretations from the rock record. This discussion paper assesses the reliability of interpretations of levee deposits in numerous examples from the rock record, and their associated inferences for river style. This uncertainty reflects the lack of definitive sedimentological attributes for levee deposits, their limited preservation potential, and the fact that levee identification in the rock record is inhibited by the reliance on geometric descriptors or indirect associations between channel and floodplain facies. Given these concerns, it is suggested that geomorphologists and sedimentologists need to recognise the limitations of our present knowledge of levees, and work towards a more systematic understanding of these significant fluvial landforms in the full spectrum of modern (and ancient) river settings.

Lowstand rivers need not incise the shelf: An example from the Great Barrier Reef, Australia, with implications for sequence stratigraphic models

A key tenet of many sequence stratigraphic models is that sea-level lowering causes widespread fluvial incision of the subaerially exposed continental shelf, and that river-borne terrigenoclastic sediments bypass the lowstand shoreline via canyons to the continental slope and basin floor. Consequently the occurrence of incised channels is considered a fundamental criterion for the recognition of sequence boundaries in ancient shelf successions. Contrary to this, we argue that rivers may not necessarily incise during glacio-eustatic lowstands if they flow out onto a coastal plain flanked by a broad, low-angle shelf. On the Great Barrier Reef shelf, fluvial incision did not occur during the last glacial maximum (LGM), but instead, subaerial accommodation was created and infilled as contemporary rivers graded to the “LGM-bayline.” Incision was restricted to the lowstand shelf break, where canyons of limited extent formed by nickpoint retreat.

Terrigenous sedimentation and coral reef growth: a conceptual framework

Abstract The role of terrigenous (and non-framework carbonate) sedimentation has not been prominent in models of coral reef growth and evolution. We derive and discuss a semi-quantitative model which relates coral reef growth to sedimentation. The model is independent of coral biology and is based upon the relative net rates of framework and non-framework sediment accumulation and/or removal. The model might enable some forecasting of long-term responses to changes in sedimentation regime and other environmental factors. The occurrence of turbid-zone reefs is successfully explained in regions of very high turbidity but with little or no net sediment accumulation. Potential future use of the model may include aiding prediction of the effects of other geological and oceanographic factors on the growth or demise of coralline communities.

Patterns of mixed siliciclastic–carbonate sedimentation adjacent to a large dry-tropics river on the central Great Barrier Reef shelf, Australia

The Great Barrier Reef represents the largest modern example of a mixed siliciclastic‐carbonate system. The Burdekin River is the largest source of terrigenous sediment to the lagoon and is therefore an ideal location to investigate regional patterns of mixed sedimentation. Sediments become coarser grained and more poorly sorted away from the protection of eastern headlands, with mud accumulation focused in localised ‘hot spots‘ in the eastern portion of embayments protected from southeast trade winds. The middle shelf has a variable facies distribution but is dominated by coarse carbonate sand. North of Bowling Green Bay, modern coarse carbonate sand and relict quartzose sand occur. Shore‐normal compositional changes show Ca‐enrichment and Al‐dilution seawards towards the reef, and shore‐parallel trends show Al‐dilution westwards (across bays) along a Ca‐depleted mixing line. Intermediate siliciclastic‐carbonate sediment compositions occur on the middle shelf due to the abundance of relict terrigenous sand, a pattern that is less developed on the narrow northern Great Barrier Reef shelf. Rates of sediment deposition from seismic evidence and radiochemical tracers suggest that despite the magnitude of riverine input, 80–90% of the Burdekin‐derived sediment is effectively captured in Bowling Green Bay. Over millennial time‐scales, stratigraphic controls suggest that sediment is being preferentially accreted back to the coast.

Distribution of riverine sediment chemistry on the shelf, slope and rise of the Gulf of Papua

The tectonically active islands of the Indo-Pacific Archipelago deliver much sediment to the ocean margins. In the Gulf of Papua on the south coast of Papua New Guinea (PNG), the chemical composition of surface sediment from grab samples indicates that Fly River muds are dispersed to the north and east, where they are joined by sediment plumes from the other large rivers along the south coast of PNG. This is the likely source of terrestrial sediment on the Papuan Plateau and the northern Coral Sea Abyssal Plain. The sediment is transported through submarine troughs and canyons offshore, far to the east of the riverine inputs. Immediately south and 30–50 km offshore from the Fly and Purari deltas is a platform of algal and reef carbonate materials, containing little or no terrestrial surface sediment.

Drilling for Antarctic Cenozoic Climate and Tectonic History at Cape Roberts, southwestern Ross Sea

Since the early 1990s, the Cape Roberts Project has been working to document, through sediment drilling, the proximal record of Antarctic ice sheet history and climate for the southwestern part of the Ross Sea, Antarctica. It has also investigated the tectonic history of the Transantarctic Mountain range of the West Antarctic Rift system and its links to the separation of Australasia from Antarctica over the past 55 million years. The recovered sedimentary sequences and the information they contain provide a record of environmental conditions when global temperatures were similar to those proposed for the end of this century, a test for models of ice and climate behavior, and a better understanding of the tectonic evolution of the region.

Composition and textural variability along the 10 m isobath, Great Barrier Reef: evidence for pervasive northward sediment transport*†

Previous workers have proposed that northward‐directed bedload transport dominates the inner shelf of the Great Barrier Reef lagoon. Results from a sediment sampling survey along the 10 m isobath between Bowen and Cape York reveal a series of northward trends of increasing sediment maturity and demonstrate pervasive north‐directed sediment transport interacting with a succession of sediment (fluvial) sources. South of the Tully River, the occurrence of limited compositional variability indicates significant mixing on the inner shelf. However, further north the data are highly variable, suggesting that sediment inputs from individual rivers may be retained relatively close to source. This may be related to a greater sediment trapping efficiency within northern embayments and/or by lower net rates of along‐shelf transport.

“Basic” entropy grouping of laser-derived grain-size data: an example from the Great Barrier Reef

Abstract Rapid acquisition and management of high quality grain-size data was facilitated by establishing a Dynamic Data Exchange (DDE) link between a laser particle sizer (Malvern Mastersizer X) and a spreadsheet program (EXCEL for WINDOWS). The resultant data set was grouped, using “ENTROPY”, an entropy analysis program, to delimit grain-size facies for samples collected from a section of the Great Barrier Reef shelf. ENTROPY accepts ASCII data and can be used to group frequency data from a range of applications.

Deep-marine seabed erosion and gravel lags in the northwestern Rockall Trough, North Atlantic Ocean

A zone of active seabed erosion has been identified in the northwestern Rockall Trough using seismic reflection profiles and cores. The region from George Bligh Bank to Rockall Bank has been subject to vigorous bottom-current activity, for at least the last 35 Ma. Bottom currents originate from southward flowing North Atlantic Deep Water, in water depths of 500–2000 m. Acoustic character mapping reveals a zone of erosion extending over 8500 km2 along the northwestern margins of the trough. The erosion surface is characterized by an acoustically reflective seabed, with occasional parallel to transparent reflectors. These are the result of strong bottom-currents exposing the underlying volcanic basement, drift sequences and fan sediments. The erosion surface is covered by a <10 m veneer of Quaternary sediment. The majority of the basin consists of well-stratified, parallel Quaternary drift and hemipelagite sequences. Along the western margin of the trough, these sediments form broad-sheeted drifts. Eocene sediments adjacent to George Bligh Bank have been exposed by strong bottom-currents for the last 35 Ma, compared with the flanks of Rockall Bank, where sedimentation was intermittent. Core sampling from George Bligh Bank and Rockall Bank recovered Quaternary gravelly–sandy muds interpreted as gravel lags, and muddy sandy contourites, overlying early–mid-Eocene aged sediments. The gravels represent the influence of a strongest North Atlantic Deep Water flow, winnowing coarse sediment into ‘lags’, commonly preserved within sandy contourite sequences.