Geology

Coochiemudlo is one of the islands in Moreton Bay. Moreton Bay is a large bay on the eastern coast of Australia 19km from Brisbane, Queensland.

The bay extends some 160km from Caloundra in the north almost to Surfers Paradise in the south. It is separated from the Pacific Ocean by a chain of three sand islands. Moreton Island in the north, North Stradbroke Island, and South Stradbroke Island in the south. Moreton Island is protected as Moreton Island National Park. The bay itself contains around 360 islands in total.(please just click on the picture if it isn’t very clear)

The bay is generally shallow and sandy, though a substantive channel is maintained to allow access to the Port of Brisbane for international shipping. The Nerang, Coomera, Logan, Brisbane, Pine and Caboolture rivers all empty into Moreton Bay.

The Tasman Belt forms the main tectonic framework of Eastern Queensland. The belt consists of a series of 5 basins and 5 highs. The belt started to form in the Ordovician period where crustal activity progressed eastwards. There are 7 major tectonic episodes recognized in the geological history of Eastern Queensland:1) Palaeozoic: Initial subsidence, sedimentation. Sediment thickness 6000 metres now of metamorphic character.

2) Mid Devonian: Crustal disruption, uplift, igneous intrusion and metamorphism. Formation of basins and highs. Basins rapidly filled with marine and terrestrial sediment and supported reefal zones on the basin margins.

3) Late Carboniferous-Early Permian: downwarping of basins and rejuvenation of highs, intensive sedimentation.

4) Late Permian – Early Triassic: Major deformation of the belt, vulcanicity.

5) Late Jurassic: deformation of Maryborough Basin, renewed subsidence of the basin and the Capricorn Basin.

6) Late Cretaceous: Deformation of the Maryborough Basin, uplift.

7) Late Cenozoic: Marginal upwarping of the coastal belt, fracturing of the Tertiary surface.

The most significant episode in the geological history was the Mid Devonian to the Early Triassic. This was the cause of the main geomorphology and framework of Eastern Queensland (Royal Society of London, 1976).

The geological evolution of the Coochie area has occurred over the last 400 million years. Between 370 – 290 million years ago, the area around what is now Brisbane was covered by deep oceans and marine sediments were deposited in the area.

From 250 to 150 million years ago volcanoes erupted around this area. The land was above sea-level but the area was covered by rivers and sandstone, shale and coal were deposited from these rivers and swamps. The granites now quarried at Enoggera were formed at this time. Violent volcanic eruptions saw clouds of hot ash flow into existing river valleys forming a distinctive rock-type known as “tuff”. This material is found at the Kangaroo Point cliffs and in much of Brisbane and its distribution can be used to trace the flow of ash away from a volcano to Brisbane’s north.From 60 to 20 million years ago Brisbane was still surrounded by erupting volcanoes, the area had stablised and sedimnents were being deposited in a lake which covered much of the Brisbane area. The original volcanoes have now eroded away, but the igneous cores which remain are now known as the Glasshouse Mountains to the north of Brisbane.

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