Product Specifications Coverage: Partial coverage, predominantly in northern Australia, along major transport routes, and other selected areas. About 1000 maps have been published to date. Currency: Ranges from 1968 to 2006. Coordinates: Geographical and UTM. Datum: AGD66, new edition WGS84; AHD. Projection: Universal Transverse Mercator UTM. Medium: Paper, flat copies only.

22-2/D51-16/4-1 Vertical scale: 300

22-2/J55-1/6-6 Vertical scale: 1000

Examination of dredge hauls taken from the Rowley Terrace, northwest Australia, has revealed the occurrence of several Mesozoic nannofossil-bearing levels. Dating has been satisfactorily achieved even though some key species important for subdividing Mesozoic rocks elsewhere are either missing or have a different stratigraphic significance, in northwest Australia.

No abstract available

A series of regional seismic reflection lines has been recorded, which link three key exploration wells on the northern Exmouth Plateau and Rowley Terrace to the lower continental slope; this slope has also been extensively dredged. This study primarily examines these new data, combined with all existing seismic, well and dredge data for some 200 000 km 2 of the northwest Australian margin, to come up with an improved structural and stratigraphic interpretation, and hence a better understanding of geological history. The study area lies between the shallow North West Shelf and the Argo Abyssal Plain and is underlain by continental crust. Water depths increase slowly seaward from 500 to 3000 m and then rapidly to 5500 m at the edge of the abyssal plain. The northern Exmouth Plateau and Rowley Terrace margin is underlain by thinned continental crust, and contains 5000-10 000 m of Permian and younger sediments. The sequences we analyse here are the top 1000 m of Upper Triassic sedimentary rocks, 1000-2000 m of Jurassic sedimentary and volcanic rocks, 500-1 000 m of Cretaceous sedimentary rocks and 500-2000 m of Cainozoic sedimentary rocks. Triassic sediments were laid down as a fluvio-deltaic blanket across this part of Gondwana, which had continental crust extending over the area presently occupied by the Argo Abyssal Plain. Coral reefs existed near the open ocean, but we have no unequivocal seismic evidence of them except on the Wombat Plateau. Toward the end of the Triassic a period of faulting on the Exmouth Plateau led to the formation of the F unconformity, but the Rowley Terrace remained undisturbed. In the Early Jurassic, a thick sequence of marine clastics with some carbonates was deposited, which, after a regression, was overlain unconformably near the sea by a thick Middle Jurassic coal measure sequence. At the end of the Middle Jurassic, there was a period of thermal uplift, normal and strike-slip faulting, volcanism and erosion over a zone within 100- 150 km of the future abyssal plain, making the widespread E unconformity particularly angular in this area. This tectonism culminated in breakup in the Callovian-Oxfordian, and the "Argo Landmass " drifted north-westward leaving oceanic crust behind. Continental volcanics are interpreted to cover about 25 000 km2 of the northwestern Rowley Terrace below the E unconformity. An Upper Jurassic to Berriasian deltaic sequence was commonly confined to structural lows, suggesting that rapid subsidence of the margin commenced later, accompanied by a marine transgression that led to the formation of the Valanginian D unconformity. Rapid subsidence of oceanic crust led to the collapse of the continental margin along faults in a zone 30-40 km wide, with the continent-ocean boundary positioned somewhat shallower than the abyssal plain, and about 10-20 km seaward of the magnetic anomaly that previous authors used to locate it. Maximum vertical displacement of the Mesozoic sequences across this collapsed zone is now about 2000 m. No major tectonic events occurred on the continental margin after the Valanginian, with steady subsidence leading to a general deepening of environments. Continental movement northward led to an increasing proportion of biogenic carbonate in the sediments. Much of the area was in bathyal depths by the Paleocene, but thereafter increased carbonate production and oceanward progradation led to a shallowing of water with time in many areas.

We report major and trace-e lement compositions of 14 basalts and dolerites dredged from seven sites along the outer rifted margin of northwestern Australia. Lavas from the margin of the Scott Plateau-Rowley Terrace, of interpreted Callovian-Oxfordian age, are evolved basalts and ferrobasalts with Zr/ Nb from 5-17, and LREE-enriched REE patterns [(La/ Yb)N from 3.4 to 8], and are T-MORB transitional to more P-MORB from the SW Indian Ridge and the Red Sea-Afar rift zone. They are significantly different from the unusually depleted N-MORB drilled less than 100 km farther west on the Argo Abyssal Plain at ODP Site 765. T- to P-MORB were also dredged at the foot of the southwestern corner of the Exmouth Plateau. This dredge site is only 80 km from ODP Site 766, which also yielded N-MORB. Basalts very similar to those drilled at ODP Site 766 were also dredged at a site only 20 km east of Site 766. The evolved, mainly ferrobasaltic nature of the Rowley Terrace-Scott Plateau margin basalts, their T- to P-MORB compositional signatures, and their close spatial association with strongly depleted N-MORB resemble in many ways the lava pile formed during the last 4 m.y. along the southern Red Sea-Gulf of Aden-Asal-Afar region. Although the latter region is associated with the effects of the long-lived (~40 m.y.) Afar plume, a plume origin for the northwest Australian margin basalts is less easily demonstrated. Problems with the plume hypothesis for this region include (1) the remarkably rapid change from plume-influenced MORB along the Rowley Terrace margin to depleted N-MORB at the foot of this margin at the eastern end of the Argo Abyssal Plain, requiring a sudden switch-off of the hypothetical plum; (2) the apparent recurrence of this same pattern of T-MORB to depleted N-MORB basalts 400 km farther south on the same margin some 20-25 m.y. later (Valanginian); and (3) the occurrence of perhaps more convincingly plume-related basalts that constitute the Wallaby Plateau, which formed as part of interpreted plume head eruptions of the Kerguelen plume around 115 Ma, and were apparently erupted onto Cuvier Abyssal Plain oceanic crust only probably 30-40 m.y. old.

22-2/F52-8/3-4

22-2/F52-4/2-6 Contour interval: 25

Titles in this newsletter: Lead-isotope-based stratigraphic correlations and ages of Proterozoic sediment-hosted Pb-Zn deposits in the Mount Isa Inlier Potential for magnetostratigraphy as a correlation tool in the Late Permian coal measures of eastern Australian basins New geological and geochronological constraints on volcanogenic massive sulphide prospectivity near Halls Creek (WA) Revision of Late Triassic biostratigraphy of the North West Shelf Permian-Carboniferous magmatism in north Queensland: a new perspective Mineralisation potential of the granites of the Cape York Peninsula Batholith, Cape Weymouth, Coen, and Ebagoola 1:250 000 Sheet areas Implications of Pb-isotope data for tectonostratigraphic correlations in the Proterozoic of central Australia Benthic chamber technology and the sea-floor of Port Phillip Bay, Melbourne Landscape evolution in the East Kimberley region, Western Australia Mapping large-scale hydrothermal systems using coincident magnetic and gamma-ray spectrometric anomalies The 'Keith-Kilkenny Lineament': fault or fiction? A milestone in geomagnetic processing Plate tectonics of the Christmas Island region, northeast Indian Ocean