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| Geology
of the Kimberley, Australia |
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| The geologic
history of the Kimberley has been turbulent,
from volcanic eruptions to major extinction
events to ice ages.
The Kimberley is flanked by two ancient mountain ranges or orogens: the King Leopold Orogen on the west and the Halls Creek Orogen on the east. Detailed mapping, geophysical and geochemical studies, and precise isotopic dating of zircon crystals show that the orogens formed 2500
to 1800 million years ago (Ma) from the accretion of three terranes each with different geologic histories, magmatic activity, and tectonic settings. Sands and silts, once thought to be continuous across the orogens, were deposited, deformed, and metamorphosed at different times in each terrane. About 1820 Ma, the Kimberley Craton collided with the rest of the North Australian Cration. Basaltic
lava, Carson Volcanics, extruded 1800
Ma covering an area of
250,000
sq km. In the northern Kimberley, the Charnley,
Drysdale, Mitchell, Prince Regent, and other
rivers cut deep gorges and waterfalls in
sandstone and volcanic rocks. The sandstone
units were intruded 1790
Ma by thick sheets (sills) of mafic magma,
Hart Dolerite. Uplift and erosion produced
the open valleys and flat-topped black hills
of the Kimberley plateau. The dolerite forms
black hills with blocky boulders and bouldery outcrops in the
floors of narrow sandstone gorges in the
northwestern Kimberley. Around 515
million years ago (Ma), there was a huge
outpouring of lava, Antrim basalts, covering
400,000
sq km of the Kimberley and Northern Territory,
nearly all, of which, has been eroded. In
the western Kimberley, the Lennard and Fitzroy
Rivers cut through limestone of the Napier
Range forming the Windjana
and Geikie
Gorges, respectively. Steep, red cliffs
of limestone line these gorges exposing
a 350-375 Ma Devonian reef. The reef
extends for 220 miles along the northern
margin of the Canning Basin and once extended
630 miles further to the north in the Bonaparte
Gulf. Today, the reef winds through the
countryside 50-100 m above the plain much
as it did when it stood above the ocean
floor in the Devonian. In the Late Carboniferous
and Early Permian (280 to 320 Ma), ice
sheets several kilometers thick covered
all or most of Australia. Water at the base
of the ice dissolved the limestone forming
karst landforms like Mimbi caves and Tunnel
Creek. In Purnululu
National Park (Bungle Bungle), sandstone
forms beehive mounds encased in a skin of
alternating orange and black bands. At the
edge of the Great Sandy Desert lies the
world's second
largest meteorite crater from which meteorite fragments have
been recovered. Wolfe
Creek meteorite crater was identified
in 1947 by geologist Frank
Reeves during an aerial survey. One-third
of the world's supply of diamonds comes
from the Argyle
Diamond Mine. Argyle also produces most
of the world's gem-quality pink diamonds. |
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