Australia: The Land Where Time Began
Flooding of the Continents when Sea Levels were High
The chalk cliffs on almost every continent are recognisable as being of Cretaceous age, the name actually being from the Latin word for chalk ‘creta’. Chalk is a fine-grained pure limestone the outer surface of which is often soft and crumbly.
The extent of the Tethys at its peak is indicated by the chalk deposits of Cretaceous age that are present in many parts of the world. The hills of chalk are gentle and rolling but the author3 describes chalk soil as ‘scruffy’ and harsh, covered with sparse vegetation. Though the rock crumbles to the touch, cliff lines of such rock are steep, sharp and dramatic. It remains cool to the touch even when it is exposed to strong, harsh sunlight in very hot locations and is often speckled with irregular dark nodules of flint. Half-rounded flint pebbles are common on beaches of the present, though the uplands of chalk may he been completely gone, having been dissolved and swept out to sea.
Chalk from the central Tethys can be found on the axis of the Mediterranean countries – Crete, Corsica, Calabria, Sicily, as well as a number of the Greek islands. The author3 describes it as thick, very fine-grained and monotonous. Under a scanning electron microscope each layer of chalk can be seen to be composed of coccolith platelets of calcium carbonate with a diameter of 1 micron.
Chalk is present from the Carpathians to beyond the Black Sea, south through the Middle East and across North Africa. It also spread over what had previously been dry land of the continents that after years of persistent erosion had become flat and low-lying. The Tethys had spread from Libya and Tunisia across Africa to Nigeria and into the South Atlantic that was rapidly growing. From the Gulf of Mexico it spread to the north through Texas, Alabama, and Colorado, linking with the Mowry Sea further to the north, and from there into the Arctic Ocean. During the Jurassic modern style tropical coral reefs had taken over but were replaced in the Late Cretaceous by rudist bivalves, an unusual type of mollusc that had a bottom shell that was cone-shaped and an upper shell that was lid-like. Very large numbers of these molluscs grew together and appear to have temporarily smothered the hexacorals as they competed for space. Further north Tethys’ chalk and rudist reefs were replaced on the seafloor by sandier shorelines and Arctic clays.
The Anglo-Paris Basin is one of the classic northern European regions that were covered by these chalk seas. There are high white cliffs on both sides of the English Channel (or La Manche), the Cliffs of Dover and the Seven Sisters of Kent and Les Falaises d’Etretat in northern France. The North and South Downs of England are of chalk rock. They disappear beneath the North Sea but are seen again in Denmark. Wherever they are present they are very similar throughout the area covered by the Tethys. The author3 suggests they are even more distinctive for the number of black flints present in bands that are more or less regularly spaced.
In the plankton of the oceans coccoliths, that had calcite shells and diatoms that had clear silica glass shells, had come to dominate. There were also many organisms in the zooplankton, such as foraminifers, calpionellids and silicoflagellates. The white ooze covering the seafloor that resulted from the constant rain of the shells of these small organisms that had died, or been eaten by the zooplankton, was formed of this skeletal debris. Diatoms and silicoflagellates make their ornate skeletal enclosures of the opal type of silica, and when there is no living tissue in contact with it as the carapace is buried in the sediment it soon becomes chemically unstable. This opaline silica dissolves in the pore water of the sediment, and when the chemical conditions have change enough it precipitates out around a nucleus to form flint nodules that are of irregular shapes and formed of quartz, the ultrastable form of silica. The crystal lattice of the quartz nodules has impurities that impart a distinctive colour to it, black if the impurities are organic matter, brown or yellow hues if it is iron.
The stability and climate of the seas at that time can be indicated by the regular banding of the chalk-flint bands. The author3 suggests the banding was probably caused by an original periodicity in the flux of diatoms to the bed of the ocean, this in turn being driven by long-term cycles in the climate that were related to periodic changes in the Earth’s orbit around the Sun. Milan Milankovitch, a Serbian mathematician demonstrated that there were subtle changes in 3 different aspects of the Earth’s orbit:
· There is a variation of 6 % over 100,000 years in the degree of eccentricity in the Earth’s elliptical orbit;
· There a variation of a few degrees in the tilt of the Earth’s axis over 41,000 years;
· The Earth’s wobbles as it moves along its orbit, in a fashion similar to a top, over a period of 22,000 years.
Slight, though significant, differences in global temperatures result from the complicated interaction between these different cycles. The Milankovitch cyclicity has been used to explain the alternating periods of the phases of glacials and interglacials of the last ice age. It has since been implicated by some in other cycles, such as mass extinction events.
Stow, Dorrik, 2010, Vanished Ocean; How Tethys Reshaped the World, Oxford University Press.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|