CLIMATE AND SEA-LEVEL CHANGES

Although the Sydney Basin at this time was situated in the region of the South Pole the climate was mild enough to support luxuriant growth. The lush vegetation formed extensive thick peat mires over millions of years. These were later transformed into coal seams. The episode of vast coal-forming forests ended with a massive extinction of life throughout the world. This extinction event, 250 million years ago, coincided with a large fall in sea level and a period when no deposits were added to the Sydney Basin. (Click on picture for more examples) As the sea level rose once more, sediments that had been eroded from the New England Fold Belt were again transported to the Central Coast via the Gunnedah and Sydney Basin. They accumulated in streams, estuaries, beaches, deltas and tidal inlets. We can see this variety of environments in the many different layers in the sedimentary rocks of the Terrigal Formation (also known as the Narrabeen Group) that are exposed along the Central Coast. Large rounded pebbles and cobbles in conglomerate rocks indicate that they were transported by high energy streams probably flowing from mountains to the north. When the streams slowed down as they entered the coastal plain they dropped their load of pebbles and sand, forming conglomerates and coarse sandstones typical of this area. (click on image to see more) (click on image to see more) Fine-grained siltstones and mudstones formed in low energy estuaries and coastal lagoons like those along the Central Coast today. They often contain burrows, and fossils of fish, plants, insects and amphibians. This photograph shows the fossilised wing of a Clatrotitan, a giant insect related to dragonflies. It was found in Triassic sediments at Brookvale. The sandstones often formed on beaches, on sand barriers and tidal deltas at the seaward end of an estuary. These sandstones also contain trace fossils such as reptile and amphibian tracks and crayfish burrows. In 1997 there was a rare find of a large fossil of an amphibian in rocks from a local quarry at Kincumber. (see newspaper article) During the formation of the Sydney Basin sea levels were generally falling and there are numerous fossils of fish, insects and amphibians that seem to have been caught high and dry, stranded in cut-off meanders in streams or estuaries. In 1986 there was a large find of fish fossils at Somersby. (see newspaper article)