South Australian Basin Research Articles

Clay minerals in Southeast Indian Ocean sediments, transport mechanisms and depositional environments

The clay mineralogy of the sediments of the Southeast Indian Ocean and Southwest Pacific Basin has been analysed by using chemical analyses and X-ray diffraction on 245 samples. The Wilkes Land sector of Antarctica supplies two different assemblages of clays to the South Indian Basin. The assemblage from western Wilkes Land is characterised by iron-bearing mica, amphibole and montmorillonite, while the eastern part is characterised by alkali felspar, quartz, muscovite and interstratified smectite. Both areas are the source of plagioclase, and the mineralogy is believed to reflect the change from a granitic to mafic shield from East to West Wilkes Land. The smectites are probably reworked from Tertiary strata on the continental shelf and rise. Minor kaolin is associated with the montmorillonite. Chlorite is derived from Victoria Land metasediments and transported by the west-flowing bottom waters to the Adélie continental rise. Abundant iron-bearing illite from western Wilkes Land must have been deposited on the continental shelf before the formation of the icecap. Eolian processes are believed to be the main supplier of kaolin, smectite and quartz to the South Australian basin. South Australia is the source area for abundant interstratified smectite. New Zealand supplies a dominantly mechanically weathered assemblage of muscovite, chlorite, amphibole and plagioclase to the surrounding basins. This assemblage is probably glacial eroded rock flour from South Island.

Erosion of deep-sea sediments in the Southern Ocean between longitudes 70°E and 190°E and contrasts in manganese nodule development

Using our previously published results, we present trend surface maps of surface sediment age, and the thickness of sediment deposited during the last 0.7 m.y. for the area between the Kerguelen Plateau, Broken Ridge, Australia, New Zealand, and Antarctica. Four separate centers of widespread deep-sea sediment erosion have developed in response to high bottom-water velocities. Two of these, centered on the South Tasman Basin and the western end of the South Australian Basin, respectively, areassociated with extensive manganese nodule development. The manganese nodule fields are almost entirely developed on sediment which averages at least 0.5 m.y. in age. In contrast, the southern and eastern flanks of the Kerguelen Plateau, the Ross Sea and the Campbell Plateau have very limited manganese nodule deposits, despite abundant evidence of widespread active erosion. It is concluded that sediment erosion is a necessary but insufficient requirement for extensive manganese nodule development in the area, and that an important role is also played by submarine volcanism.

Spreading of Antarctic Bottom Water and its effects on the floor of the Indian Ocean inferred from bottom-water potential temperature, turbidity, and sea-floor photography

Data on bottom-water potential temperature, turbidity and current indications show that in the Southern Ocean west of the Kerguelen Plateau, Antarctic Bottom Water (AABW) of Weddell Sea origin spreads northwards from the Atlantic—Indian Basin in two directions: (1) AABW enters the Agulhas Basin through relatively deep areas in the Mid-Indian Ridge at 20–25°E and possibly at 35°E, and flows northwards into the Mozambique Basin as far as its northern limits; (2) a more easterly spreading path extends from the Atlantic—Indian Basin through the Crozet into the Madagascar, Mascarene, Somali and Arabian Basins. The passage in the western branch of the Indian Ridge for the AABW spreading from the Crozet into the Madagascar Basin appears to be at 29-26°S and 60–64°E. East of the Kerguelen Plateau in the South Indian Basin, the bottom water formed mainly along the Adélie Coast and Ross Sea travels west towards the Kerguelen Plateau and then parallel to it. This water finally flows eastwards hugging the Southeast Indian Ridge. Significant deviations from this general circulation pattern occur due to local topographic effects. Some AABW in the South Indian Basin exits through a passage at 120–125°E in the region of the Australian—Antarctic discordance in the Southeast Indian Ridge and enters the South Australian Basin and subsequently the Wharton Basin. This passage is clearly indicated by the northward extension of a cold, bottom-water tongue as shown by the temperature distribution in the region; the bottom-water effects in the passage are reflected in the high turbidity and current lineations on the sea floor. In the Southern Ocean basins, bottom-water turbidity is generally high, reflecting in part the strong bottom-water activity. The effects of AABW circulation on the sea floor—in the form of well-developed small- or large-scale current ripples and erosional/depositional features, manganese-nodule formations, and unconformities and reworking of sediments observed in cores — are also marked in these basins. Even though the AABW in the Wharton Basin is cold, its spreading effects on the sea floor are minimal in this basin in contrast to the basins west of the Mid-Indian Ridge at comparable latitudes.

Regional deep-sea dynamic processes recorded by late Cenozoic sediments of the southeastern Indian Ocean

A large collection of USNS Eltanin deep-sea sedimentary cores and bottom photographs from the southeast Indian Ocean between long 70°E and 120°E, and between Antarctica and lat 30°S, were analyzed. Cores from the crest and flanks of the mid-ocean ridge are mostly late Quaternary in age, with only rare breaks in sedimentation. In great contrast, flanking this zone in deep basins immediately to the south of the ridge in the South Indian Basin and in a broad zone in the western sector of South Australian Basin, there are areas where bottom currents have systematically eroded or inhibited deposition of sediments ranging in age from Quaternary to Pliocene, and occasionally middle Tertiary. This regional deep-basin erosion extends northward between Broken Ridge and the Naturaliste Plateau to the Wharton Basin where sediments as old as Late Cretaceous are exposed. As indicated by disconformities, ocean-floor characteristics, and seismic-profile data, much of the shallower, north-trending Kerguelen Plateau has also undergone widespread erosion by bottom currents. The erosional disconformities in the deep basins have been created by general increase in velocities of Antarctic Bottom Water during the last 2.5 m.y., apparently with major separate pulses during the Brunhes epoch (t = 0.69 m.y. to present) and part of the Matuyama epoch (t = 2.43 to 0.69 m.y.). Extensive areas of manganese nodules have developed in conjunction with this bottom-current activity, most spectacularly as a vast pavement in the northwestern sector of the South Australian Basin. This feature, which we name the “Southeast Indian Ocean Manganese Pavement,” is approximately 106 km2 in area. The available evidence indicates long-term major erosion by the eastward-flowing Circumpolar Current across the central and southern parts of the Kerguelen Plateau. In the deep basins, high-velocity Antarctic Bottom Water flows eastward through the northern sector of the South Indian Basin, with important northward flow crossing the mid-ocean ridge at 110°E and 120°E into the South Australian Basin. This northward branch traverses the western sector of the South Australian Basin, the Southeast Indian Ocean Manganese Pavement, and then flows between Broken Ridge and Naturaliste Plateau into the Wharton Basin. Major Cenozoic to Late Cretaceous hiatuses in the Wharton Basin revealed by deep-sea drilling suggest that northward-flowing bottom water through this conduit has been a very long term feature.

Deep-Sea Erosion and Manganese Nodule Development in the Southeast Indian Ocean

Features exhibited by a large number of sea floor photographs together with the dating of 187 sediment cores from the southeast Indian Ocean have revealed extensive manganese nodule development and sediment erosion in deep basinal areas. The most extensive nodule field, with an area of 10(6) square kilometers, occurs in the northwestern sector of the South Australian Basin and is named the Southeast Indian Ocean Manganese Pavement. The crests and flanks of the adjacent mid-ocean ridge are, in contrast, free of nodules and marked by much less dynamic bottom water conditions.