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Geology and geomorphology

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.

Geomorphology

A view of the Victoria River District
A view of the Victoria River District 

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

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.

Volcanic eruptions

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.

Long period of erosion

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.

Recent deposition

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.