Australia: The Land Where Time Began
Tethys Ocean - Jurassic-Cretaceous
The Jurassic Coast
The Jura Mountains were part of the seafloor of Tethys that were thrust up during the final stages of mountain building as the ocean finally closed. The sediment record has shown that there were a series of banks and basins along the northern part of Tethys, just the type of topography that is ideal for inducing the upwelling of cool, nutrient-rich, waters that lead to a surge in primary production in the surface waters. The author3 believes there would be other rock outcrops beneath the sunflower fields and vineyards.
Some of the best Jurassic outcrops from southern Tethys are found in North Africa and the Middle East. The author3 suggests that rocks near Lyme Regis and Charmouth best represent Early Jurassic life. This site was made famous by Mary Anning, a fossil collector from Dorset in the first half of the 19th century, having found many fossils including near complete skeletons of marine reptiles. Many of the animals found in this region are marine reptiles that were top predators, including big-headed pliosaurs, plesiosaurs and saltwater crocodiles. Some reached 12 m in length and features they had in common are strong jaws and many sharp teeth. The ichthyosaurs, that arose in the Triassic, are suggested by the author3 to have possibly been the most highly evolved and by the stage of their evolution in the part of the Jurassic represented by the fossils at this site had stopped laying amniotic eggs, instead giving birth to live young, as has been shown by a fossil that has been found that was a mother ichthyosaur in the process of giving birth when she died.
Among the fossils found by Mary Anning were many ammonites and flying reptiles, insects and dinosaur bones, animals that are believed to have been trapped in the lime mud of the shallow coastal seas and lagoons. The stage of the Jurassic all the Lyme Regis rocks are from has been named the Liassic.
At another coastal fossil site, Osmington Mills demonstrates an environmental change from sands and mud that are river-fed of a shore face and shelf that were pounded by the waves to water that was free of sediment and in which pristine calcium carbonate was deposited as tiny spherical sand grains that were deposited in a warm agitated lagoon. These oolite sediments are characteristic of limestone of Jurassic age from other parts of Europe. They have also been found where similar deposition conditions prevailed in other sites and from other times.
There are also many trace fossils (ichnofossils), tracks, trails, burrows and resting places of animals living on and below the sediment surface, the animals not usually being found, though a few sea urchins (echinoids) have been found in their burrows.
At Kimmeridge Bay (Kimmeridgian Stage) fossils of bivalves and ammonite have been found that were flattened in layers of laminated dark-grey shales. There was a large volume of organic material in these muds that were well-compacted to become mudstone. When these deposits were buried deeply enough the organic material was 'cooked' to produce very large volumes of oil. A subtle change in environmental conditions occurred near the end of the Jurassic that led to widespread parts of the seafloor stagnating. The author3 suggests this was coupled with very high levels of phytoplankton at the surface, the combination leading to preservation of the organic matter in the sediment. These conditions soon changed, the next layers deposited in the region being rich in lime and packed with shells. Once buried this sediment was cemented to form grey-white limestone, Portland Stone. Some of the largest known ammonites were found in Portland stone, such as Titanites, that had a shell up 60 cm in diameter. Tentacles would have extended from the open living chamber.
Near the outermost part of the Lulworth Cove, also on the Jurassic Coast, have fossils of a later part of the Jurassic. These Jurassic rocks were uplifted, and tilted to almost vertical, during movements of Earth that were related to a later stage in the closing of the Tethys Ocean. Portland Stone, that is very hard, forms the seaward line of cliffs, and overlying the Portland Stone is Purbeck limestone, that is almost as hard. A narrow breach has been opened by the relentless battering of waves that is probably along a lineament in the rocks that had been weakened by fractures associated with their uplift and later cannibalised by a small stream that drained to the coast. Lulworth Cove was formed by the sea carving out a bay, that is almost perfectly circular, from the younger, softer rocks behind the cliff face that were deposited in the Cretaceous.
The sea level was rising worldwide during the Purbeck, but locally it dropped, a series of islands forming along the coast of Dorset, surrounded by saline lagoons on the rim of Tethys. The rocks have preserved fossil soils and a luxuriant tropical forest composed of Giant cypress, monkey-puzzle trees, cycads and ferns. An almost complete record of a Jurassic forest known in the world that extends from Mupe Bay near Lulworth to Portland and Weymouth has been preserved in the rocks. On the outer ledges at Lulworth there is evidence of the sea returning and flooding giant stumps by what became a shallow, saline lagoon, and they were covered by a layer of algae (cyanobacteria, cyanophytes, blue-green-green algae) that secreted lime in layers to form donut stromatolites.
In Texas the El Capitan fossil reef from the Permian, just prior to the mass extinction event gives some idea of the numbers and diversity of species to be found on reefs from this time.
The author3 describes drilling through the KT boundary and the 'gentle transition' of species across it in the cores from the ocean floor. They first encountered the black, organic-rich sediments at about 850 m below the seafloor. There were many layers, some of which were thick and others thin, and intercalated between these layers were layers of sediment of a lighter greyish or greenish colour, the layers extending over 200 m of section, that dated to between 100-85 Ma, based on microfossil species that had been recovered from the sediment. This 'black shale episode' needed to be explained.
The author3 suggests there are 2 main black shale episodes that are known, both from the Middle Cretaceous, and both appear to occur in almost every part of Tethys Ocean. Evidence of such black shale episodes have been found beneath the Mediterranean, the Black Sea, the Central Atlantic and the Caribbean, as well as much of the North Atlantic and South Atlantic of the present. Extensions of the Tethys Ocean extended along these north-south rifts, at the beginning of opening of these nascent oceans.
According to the author3 some of the best localities for finding terrestrial outcrops of black shale events of Mid-Cretaceous age are the Marche and Umbrian areas of Italy, though in this case the shale beds are replaced by layers of chert, a hard, quartz rock. Chert is also found in chalk cliffs and beach pebbles, such as in Britain. White limestone bands separate bands of black organic-rich chert, the black bands have been found to span exactly the same time periods as the black shale bands from beneath the ocean floor found in cores from the DSDP and ODP boreholes. The author3 has found that at places where the Tethys covered continental surfaces as its level was rising the same episodes of black shale deposition indicated a cycling of organic-rich/organic-poor conditions. These have been found in Ukraine on the Black Sea coast, through Italy, Sicily, France, Spain, the southwest US and Mexico. The distribution of black shales on the southern margin of the Tethys Ocean have been found from north-western Australia through the Middle East, Morocco, Algeria and Tunisia, to Venezuela in the far west.
The author3 suggests it is necessary to understand the fundamental controls of the large-scale supply of organic matter to the seafloor and its preservation within the sediment, to explain the widespread burial of organic carbon at this time. Primary biological productivity and recycling is the first. The second is the mechanisms involved in ocean stirring. In the oceans of the present neither excessive supply nor the easy preservation is the norm.
The author3 suggests that the rapid growth in the Cretaceous of the new ocean contributed to the Black Death that occurred throughout the Tethys world. Large submarine mountain chains are formed at the spreading centres of new oceans as the newly formed ocean floor bulges up. Oceans around the world are forced to rise as the great bulk of these mountain ranges displace huge volumes of water, the rising water spills over the margins of adjacent landmasses. As these new oceans formed sea levels continued to rise for millions of years, reaching levels suggested by the author3 to be 300 m higher than at present, the highest they reached for the past billion years. A new zenith was reached by Tethys, as well as by its peripheral seas, with 82 % of the Earth’s surface being below water compared to 67 % of the present.
In the Late Cretaceous Europe was mostly submerged, an arm of Tethys spread across North Africa as the Trans-Saharan Seaway, and the North American continent was flooded by the Mowry Seaway. In the warm shallow waters of these marginal seas, as well as in the deep ocean, the skeletons of the many microscopic life forms in the plankton drifted to the seafloor to accumulate as soft chalk sediments. There are dark bands of flint in these chalk rocks that are found from the Anglo-Paris Basin to North Africa and from Kansas to the Crimean Peninsula. They all show a fundamental rhythm of climate change in the past that the author3 says is linked to the ‘slow ticking of an astronomical clock’.
Stow, Dorrik, 2010, Vanished Ocean; How Tethys Reshaped the World, Oxford University Press.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|