Untitled Document
Submarine plateaus off northeast Australia
Kinta Hoffmann1, Neville Exon2,
Patrick Quilty3, and Claire Findlay4
1 Geological Survey of Queensland, 80 Meiers Rd,
Indooroopilly, QLD 4068
2 Research School of Earth Sciences, Australian National University, Canberra,
ACT 0200.
3 School of Earth Sciences, University of Tasmania, Hobart, TAS 7000;
4 Canberra, ACT
Bathymetric map of offshore
northeast Australia showing the Kenn Plateau, Mellish Rise (north
and south segments), and Louisiade Plateau with the ship's tracks,
dredge and gravity core sites from GA274 cruise. Legend: dredge
sites (blue triangles), gravity core sites (red circles), ship's
track (solid black line), seismic lines (dashed black line), and
ocean drilling sites (numbered blue circles).
Mellish Rise and adjacent deep-water plateaus of northeast Australia:
new evidence for continental basement from Cenozoic micropalaeontology
and sedimentary geology. In: Blevin, J.E., Bradshaw, B.E. and Uruski,
C. (Eds), Eastern Australasian Basins Symposium III, Petroleum Exploration
Society of Australia, Special Publication, 317-323.
Widespread rifting and seafloor spreading replaced the compressional
regime off eastern Australia from around 120Ma, in the Early Cretaceous.
The magnetic anomalies in the newly formed oceanic crust indicate that
spreading commenced in the south and migrated progressively northwards
carving off large ribbon-like microcontinents. Gaina et al (1998) postulated
from geophysical data that the Mellish Rise and other seafloor highs
off northeast Australia might be microcontinental fragments but no geological
evidence was available.
Two scientific cruises using R.V. Southern Surveyor (GA270 and GA274)
obtained core and dredge samples, and seismic and bathymetric data, from
the Kenn Plateau and Mellish Rise, and the Louisiade Plateau. A diverse
suite of sedimentary rocks includes shallow and deep-water carbonates,
siliciclastics, volcanogenic facies, deep-water siliceous lithologies,
and some seawater precipitates. The carbonates have provided age dates
from foraminiferal and calcareous nannofossil assemblages, which range
in age from Paleocene to present day, and prevailing climate and palaeo-water
have also been determined.
Some carbonates and siliciclastics contain quartz and mica grains and metamorphic
lithic fragments indicating a continental provenance. Dredge sites located
along the recently acquired seismic profiles indicate that the continental-derived
lithologies correspond to isostatically buoyant basement blocks that have
a uniform chaotic seismic reflection. The elevation of the basement blocks
relative to oceanic crust in the intervening deep basins, coupled with
the petrological properties of the grains and lithics, suggest that continental
basement underlies the seafloor highs. The best evidence comes from a Late
Eocene sandy glauconitic calcarenite on the southern Mellish Rise, which
incorporates angular quartz grains. The age implies that deposition occurred
subsequent to drifting and hence reflects a local source rather than contamination
by terrestrial material from the continental shelf.