Geology of the Dookie Region

Geological History:

The following information for the Dookie Land Management Group has been summarised by Steve Ryan from the following references:

• Douglas, J.G. and Ferguson, J.A. (eds). (1988). Geology of Victoria. Geological Society of Australia, Victoria Division, Melbourne.

• Laing A.C.M., O'Shea P.J. and Tickell S.J. (1977). Explanatory notes on the Wangaratta 1:250 000 geological map. Geological Survey of Victoria Report. 1977/4.

• Tickell, S.J. (1989). Dookie 1:100 000 Map Geological Report. Geological Survey of Victoria Report. 87.

The oldest rocks in the Dookie area are of Cambrian age. These rocks occur as a sequence of sediments, basalts and intruded igneous rocks. The rocks were formed by igneous activity that probably occurred in an oceanic environment, explaining the presence of igneous (gabbro, basalts) and marine sedimentary (chert, siltstone, sandstone, conglomerates) rocks existing together. By the end of the Cambrian period (around 500 million years) the volcanic activity ceased.

From this time to the end of the Early Devonian (approximately 387 million years) a thick sequence of rocks, sandstones, siltstones, shales, mudstones and rare conglomerates were deposited under relatively deep oceanic conditions. These rocks occasionally contain marine fossils.

In the Middle to Late Devonian (approximately 387 - 360 million years) the older rocks underwent folding and faulting during a major tectonic event known as the Tabberabberan Orogeny. This event formed the Mount Wellington Fault Zone. Igneous rocks, such as the granites outcropping around Pendle Hill and in the Lake Rowan district, were also intruded during this time.

A major gap in the geological history of the Dookie area occurs between the Carboniferous period (360 million years) and the Tertiary (65 million years old). This gap represents erosion and the deposition of sediments away from the area, such as the Permian (between approximately 286 - 248 million years) glacial deposits outcropping around Wilby.

In the Early Tertiary period, the Murray Basin attained its present form and stream activity has continued largely uninterrupted to the present day. Before the deposition of the Shepparton Formation in this area volcanic lava extruded. The lava at Cosgrove is unique to the region and is probably related to similar volcanic rocks in central New South Wales.

The oldest Murray Basin sediments outcropping in Dookie are the clays, silts and sands of the Shepparton Formation (3 million - 25,000 years old). The sediments were deposited by pre existing rivers and streams that meandered on the Riverine Plain. The Coonambidgal Formation represents silts and sands deposited along present day rivers from 25,000 - 5,000 years ago.

The rivers and streams of the current Murray region are still active, eroding and depositing sediments in this area and elsewhere.

Stratigraphy:

The local geology of the Dookie area consist mainly of alluvium, with Cambrian and Ordovician age rocks occurring in the Dookie hills. The rocks of the Dookie hills are a part of the Mount Wellington Fault Zone, one of the major geological features cross cutting Victoria. A summary of the rock types observed in the Dookie area, from oldest to youngest is outlined below.

CAMBRIAN ROCKS (approximately 500 - 550 million years old)

Several types of Cambrian age rocks exist around Dookie, these include:

• Intrusive igneous rocks are part of a sill (a sheet like body of igneous rock that has formed beneath the Earth's surface) which run in an east west direction. The igneous rocks are known as Gabbro. They are coarse-medium grained and are generally dark green to grey in colour. These rocks occur in the hills immediately north of Dookie, on Mount Major, the Ascot Hills and the hills around Borinda.

• Extrusive igneous rocks (rocks that flowed onto the Earth's surface during igneous/volcanic activity). These rocks are similar to basalts and are fine grained and dark green-grey in colour. The basalts outcrop in the hills around Borinda, in the Ascot Hills and on the north side of Mt. Major.

• Sedimentary rocks - chert, black siltstone, sandstone and conglomerates which were formed in a marine environment. These rocks are found extensively around Mt. Major and nearby hills, and in small outcrops to the northeast and northwest of the Dookie township.

LOWER ORDOVICIAN ROCKS (approximately 465 - 500 million years old)

Lower Ordovician age rocks are sandstone, mudstone and black shales which were deposited in an ocean environment. The shales are interbedded with quartz sandstones that occur as a bed 10 to 40 cm thick. The black shales contain fossils known as graptolites and therefore an approximate age can be determined. The Ordovician rocks outcrop south of Mt. Major and immediately west of the Dookie Agricultural College.

SILURIAN - DEVONIAN ROCKS (approximately 387 - 440 million years old)

The Silurian - Devonian rocks are marine sedimentary rocks consisting of sandstone, siltstone, mudstone and conglomerate. The rocks outcrop poorly and no fossils have been found.

The rocks are best observed in road cuttings where sandstones, thin mudstones, siltstones and rare pebbly conglomerate are exposed. These sedimentary rocks occur southwest of the Dookie Agricultural College, north and south of the Midland Highway and around the junction of the Midland Highway with the Dookie-Nalinga Road.

TERTIARY AGE ROCKS (approximately 6 to 7 million years old)

A Tertiary lava, of volcanic origin, occurs west of Cosgrove and is exposed in several quarries. The quarries extract and crush the lava for road construction.

QUATERNARY AGE ROCKS/SEDIMENTS

These rocks and sediments were deposited in a flood plain environment and over much of the pre-existing topography to form the present day Riverine Plain. The Quaternary sediments are made up of the following:

Shepparton Formation (approximately 2 million to 25 000 years before the present)

The Shepparton Formation consists of silty clay and clay with less sand. These sediments were formed by processes similar to those existing in the region today. As rivers and streams meandered they would deposit sand close to their banks and during times of flood, clay and silt were deposited out onto the flood plain. Soils can be directly related to the position of the streams. Soils closest to the original watercourse (stream/river) are sandier and become more clayey as they move away from this watercourse.

Lunette Deposits (probably Pleistocene age between 1.8 million to 10 000 years before the present)

Lunettes are crescent shaped dunes which form on the eastern or leeward side of pre-existing lakes. There are two types of lunettes: those made of sand and those made of clay. In the Dookie area clay lunettes exist. They formed when lakes dried out for at least part of the year. During this time, the clay soil of the lake bed disperses, cracks and is blown to the edge of the lake. The clay lunettes in this area occur immediately east of the Dookie Agricultural College and can be observed from Thomas Road.

Coonambidgal Formation ( 25 000 - 5 000 years before the present)

In the Dookie area this formation represents a narrow belt of light grey or brown silt, clay and/or sand deposited by the Broken River where it has cut into the Shepparton Formation. Soils on these sediments are only lightly developed.

Colluvium (Pleistocene - Recent)

Colluvium are aprons of rock material (gravel and clay) that build up on the slopes and edges of hills. Colluvium on the hills that expose Cambrian age rocks is made up of a rich red brown clay and gravel. The Ordovician and Silurian - Devonian hills have small colluvial aprons made up of clay with gravel occurring on the upper slopes.