Marine geology of the continental shelf off southeast Australia

The continental shelf of southeast Australia between Sugarloaf Point and Gabo Island, New South Wales, varies in width from 72 km east of Newcastle to 17 km east of Montague Island. Over much of this area an inner shelf zone (less than about 60 m water depth), a middle shelf zone (60-130 m), and an outer shelf zone (more than 130 m) can be distinguished by morphology and sediment type. Three groups of terraces are present, shallower than 60 m, between 70 and 110 m, and between 110 and 140 m. The shelf break lies between 130 and 170 m. There is a correlation between the depth of the shelf break and the depth of basement. It is likely that the morphology of the outer shelf is related to morphological variations in basement surface, which is the little-modified Atlantic-type rift scar produced during sea-floor spreading in the Cretaceous. The declivity of the upper continental slope is usually less than 2Â° to the north of Jervis Bay, but to the south it progressively increases so that slopes steeper than 10Â° are common. Ten submarine canyons or canyon-like features dissect the slope to the south of Jervis Bay. A sequence of sediments above acoustic basement (S2), thickening eastwards from the midshelf position, is shown in seismic profiles. This sediment wedge is divided into two parts by an unconformity (SI); the lower part is transgressive on S2, and the upper is progradative on SI, and is best seen offshore from the Sydney Basin. Twice as much sediment occurs on the narrow shelf south of Jervis Bay as on the wider shelf to the north. Basement highs crossing the shelf have been identified southeast of Sugarloaf Point and northeast of Jervis Bay; basement lows occur off Newcastle and off Disaster Bay. The probable axis of eastward tilt of the continental shelf in the mid-shelf position has been identified as a basement ridge or step. Tilting and the development of a sediment wedge have probably continued since the Late Cretaceous. Surface sediments are dominantly terrigenous in water depths shallower than 60 m; eastwards, carbonate components become increasingly dominant with increasing depth. Kaolinitic and chloritic muds occur as discrete zones in the mid-shelf area. Nearly all the shelf sediments are sands, with minor quartz or calcareous gravel occupying the inner and outer shelf areas. Fine to very fine-grained sands have a distribution similar to that of the muds. An unstable amphibole-pyroxene-epidote heavymineral suite occurs on the outer shelf, and a stable tourmaline-rutile-zircon suite predominates in water depths less than 60 m. The north-to-south variation in heavy minerals is similar to that in onshore localities, where ilmenite decreases and rutile/zircon increase from south to north. All sediments on the shelf are considered to be relict, with the possible exception of the muds which may be accumulating at present. Sediments between 15 and 60 m were probably deposited during the Holocene transgression; the mid-shelf sands and muds possibly represent a mixed barrier facies, and the outer shelf carbonates probably date back to the end-Wisconsin sea-level low. Sedimentation on the continental shelf at the present day is therefore limited to the zone 0-15 m, and possibly to the muds of the middle shelf. The principal controls on geochemical differentiation in the shelf sediments are depositional environment, provenance, and effluent input. Nickel, iron, cobalt, and cadmium occur predominantly in the middle and outer shelf sediments; carbonate, phosphate, strontium, and arsenic increase with depth; and copper, lead, and zinc occur principally in the mid-shelf muddy sands. Manganese is unusually deficient in east Australian shelf sediments. Zinc, arsenic, cobalt, and cadmium occur in concentrations considered high when compared with standard rocks and sediments from other continental shelves. This is a result of high input together with minimum

Tags: AU-NSW, Bulletin, Earth Sciences, GA Publication, Published_External, geology, marine, marine survey
Organization: Geoscience Australia Data