The Carboniferous Period

Australia: The Land Where Time Began

The Carboniferous Period 360-286 mya

The Carboniferous Period saw great changes in the arrangement of landmasses on the Earth. During this period Australia was on the outer margin of Gondwana. When the Tethys Sea opened up, the part of Gondwana occupied by Australia was swung down south into the Antarctic latitudes. By the time it reached its southernmost point half the Australian landmass, still attached to Gondwana, was covered by continental ice sheets. As the climate cooled, the flourishing life in the wetter areas around the water bodies began to decline, becoming impoverished, as did the sea bordering Australia. On land and in the sea a typical polar pattern of low diversity of life evolved. On the land the vegetation around the margins of the ice sheets was of a low-growing tundra type.

The South Pole was situated on Gondwana, and glaciation and the formation of permanent ice caps had profound effects on the southernmost parts of Gondwana. Climatic fluctuations are believed to have characterised the Carboniferous with the glacial and interglacial phases alternating.

Life was still confined mostly to the swamps and areas where the watertable was high, hillsides and dry areas remaining unvegetated. Around the world there are believed to have been vast areas of bare rock, gravel and sand, probably looking much like the land before life spread from the sea. Giant Clubmosses and Giant Horsetails still dominated equatorial swamps where life proliferated. There were many insects and other arthropods that had moved on to the land. The vertebrates were mainly the Lungfish, Amphibians and early Reptiles. In the southern parts of Gondwana the Giant Clubmoss Flora could not thrive in the cooling conditions was gradually replaced by tundra vegetation around the ice sheets towards the end of the period.

Australia

As the Carboniferous opened, Australia hadn't changed position much from its location at the close of the Devonian, but it had been rotated slightly, and had started to move south. Over the next 74 million years of the Carboniferous it travelled from the equatorial region into high latitudes. The north-south axis of the continent was at 90^o to that of the present continent by the Late Carboniferous, being affected by winter darkness that lasted unbroken for months and becoming intensely cold. Glaciers began to form on the high country and glaciers are believed to have spread until a permanent ice cap had formed that, at its peak, seems to have covered more than half of Gondwana.

In the Yarrol and New England provinces major tectonic activity resulted from the interaction of the stabilised land of the continent and that of the proto-Pacific Plate. Increased volcanic activity, deformation of rock strata , igneous rock intrusion combined with uplift of the area to increase the area of stabilised land. The Sydney Basin, Galilee Basin and Bowen Basins resulted from all this activity, the depressions being originally complexes of trough-like rifts that were created by the tearing strains of interplate movements, volcanoes being associated with some of the rifts. The troughs were flooded by marine incursions and most of them were filled by the sediments derived from eroding land and the volcanic material, the basins being formed by those that weren't filled by the sediments and where subsidence continued.

In an area of northwestern New South Wales and southwestern Queensland, a zone of crustal weakness developed in a previously stabilised part of the crust, forming a depression that became the Cooper Basin that was occupied by swamps in the Permian, the algae deposited kerogen from which the natural gas, light oils and condensates of the area were produced.

In central Australia the Alice Springs Orogeny continued the uplift of the mountainous country in the area of the MacDonnell Ranges.

As Australia continued moving south and the climate cooled further the remaining Giant Clubmosses declined even further. The result was that no coal formed in Australia in the Carboniferous, though it did in the Northern Hemisphere. The tundra conditions around the margins of the ice sheets prevailed through to the early part of the Permian. On the tundra the vegetation was an impoverished Rhacopteris Flora. Ice sheets are believed to have covered more than half of the surface of Australia, exterminating life over the area.

Among the fauna in the Early Carboniferous were Corals, Brachiopods, Echinoderms and many Crinoids, Bryozoans, Molluscs and Foraminifera. Trilobites were declining, but many types of fish and sharks flourished, though they gradually became increasingly impoverished throughout the Period, only a few species surviving. Endemism increased as a high latitude biota became established. The only insect that has been found in fossil deposits from this period is one that was found associated with a fossilised Fairy Shrimp in glacial sediments in Tasmania. So life was not completely extinguished, some managing to survive in the most extreme conditions as they do at the present.

In Australia the flora and fauna became impoverished as the ice age intensified and the climate deteriorated. By the close of the Late Carboniferous the only vegetation surrounding the ice sheets was of a low-growing, low diversity, tundra type of pro-Gymnosperms. The cold seas contained an assemblage of high latitude, low diversity of marine animals.