By Robert Karfs, NT Department of Infrastructure,
Planning and Environment. Taken from Managing for healthy
country in the VRD, published by Tropical Savannas CRC, Darwin
2000.
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A view of the Victoria River District
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Most of the Northern Territory's Victoria River District lies
less than 300 metres above sea level. About half the area is
rugged, with considerable outcrop or with shallow, sandy soils. The
remainder is characterised by gently undulating erosional plains
formed by the weathering of underlying rock, flat lateritic
surfaces, and alluvial plains associated with the coast and major
streams. The main geomorphological divisions include the Sturt
Plateau, Victoria River plains and benches, Victoria River plateau,
Victoria River basin, and the Cambridge Gulf lowlands.
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Geomorphic divisions of the Victoria River District
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Geology
The district is underlain by strongly folded, metamorphic
basement rock believed to be around 1.5 billion years old. A series
of almost horizontal sedimentary rock overlays this basement and is
known as the Sturt Block. Jasper Gorge Sandstone is part of this
block that was deposited, uplifted and eroded until about 700
million years ago. After that time, the climate grew colder and
glaciers covered the region.
A period of extensive volcanic activity began about 600 million
years ago, during which time a vast amount of basalt was extruded
forming the major part of what is now called the Antrim Plateau
Volcanics. These volcanics and the Sturt Block sedimentary rocks
form the distinctive mesas and plateaux that are so characteristic
of the VRD.
A long period of erosion then extended until about 100 million
years ago. This was followed by the incursion of a large shallow
inland sea over most of northern Australia. During this time, a
blanket of marine shales, sandstones and mudstones was deposited.
These sediments are known as the Mullaman Beds and have been dated
at about 95 million years old. A much wetter climate around 30-65
million years ago is thought to have resulted in the formation of a
deeply weathered and very flat lateritic peneplain; the end result
of long-term erosion that persisted until about 15-20 million years
ago.
Dissection of this landscape then appears to have been initiated
by uplift that began the present day cycle of erosion. Drainage
systems of that time actively eroded the major part of the VRD,
exposing older resistant rocks as mesas, plateaux and mildly folded
hills. The only broad area of the lateritic surface remaining
occurs in the east and south of the district and is known as the
Sturt Plateau. The clay soils plains of this area probably formed
in areas which were inland swamps or lakes and which escaped
laterisation.
Alluvial deposition along major streams has occurred over more
recent times. Periods of erosion and deposition occurred during
times of sea level fluctuation from about 1.8 million years ago.
The sea level dropped around 5,000 years ago and large areas of
alluvial floodplain are now above deposition level. On the Sturt
Plateau, fluvial deposition is confined to salt water lakes (such
as Lake Nongra), shallow swamps and braided river channels.
