Knowledge Of Deserts and Playa Lakes – The Permian and Triassic Periods.
Peter Toghill’s Chapter 9 shows that these Periods lasted from 290 to 245 Ma and from 245 to 208 Ma respectively. In Britain, the rocks of these periods are dominated by red sandstones and the two systems together are often referred to as the New Red Sandstone in contrast to the Old Red Sandstone of earlier Devonian period. These two continental sequences are separated by the marine deltaic sequences of the Carboniferous, but in north-east Scotland they often succeed each other without any intervening Carboniferous and the boundary is then difficult to establish. The term Permian was introduced by Murchison in 1841, based on his studies around the city of Perm in Russia and the Triassic is based on a three-fold division of rocks in Germany set up by Alberti in 1834. However, the desert sandstones of Britain contrast with the marine sequences found in the present-day Mediterranean and Alpine areas which were part of the Tethys Ocean area (see Peter Toghill’s Chapter 10 below). Permian and Triassic fossils are either rare in Britain or, as in the dolomitic and saline Magnesium Limestone of north-east England, are difficult to relate to those of true marine areas. Animals and plants throughout the world were affected by a remarkable mass extinction at the end of the Permian which brought the Palaeozoic Era to an end. Many have heard of the destruction of the dinosaurs at the end of the Cretaceous at 65 Ma, but the Permian mass extinction, 245 Ma, was far greater, with 95% of known species having become extinct. Another mass extinction at the end of the Triassic saw the disappearance of 80% of the species then existing. Some small mammal-like reptiles survived this second mass extinction by burrowing in the ground, but had to wait another 180 million years to evolve into true mammals. The dinosaurs ruled as land vertebrates during the Mesozoic Era. Geologists now recognise a number of lesser mass extinctions in the last 500 million years.
As to the reasons for these extinctions, a number of suggestions have been made. Many relate to possible changes in the salinity of the seas and to the dominant aridity of the planet as Pangaea came together. Again the formation of Pangaea would have reduced plate tectonic activity at former ocean ridges while their reduction in sea levels world-wide and an associated reduction in shallow-water habitats while the existence of only one land-mass and increased competition for food might have been another factor. Again, the unusually large amounts of volcanic activity in the Permian caused great thicknesses of Permian basalt lavas in Siberia. Yet again such activity would have produced great quantities sulphur dioxide which would have caused acid rain, which could have destroyed vegetation and might have increased the acidity water bodies, including seas and oceans. Furthermore, collision with an asteroid which pierced the earth’s crust and penetrated molten rocks within the mantle, could have released pressure, thinned the crust over a hot spot, and thus produced volcanic activity deleterious to life in general. Peter Coghill notes at this point that the Earth is believed by some to be due for a twentieth century mass extinction by asteroid collision in October 2028, while we have actual evidence for a very large crater in Southern Mexico which may be of late Cretaceous age, and thus may have been the cause of the dinosaurs’ extinction.
However Peter Toghill provides a summary of Permian and Triassic Britain which is as follows. A harsh desert climate affected Britain during these 80 million years, only relieved by a maritime spell in the late Permian, and even this produced mainly saline rocks. Early Permian volcanic activity in south-west England and Southern Scotland was followed by the deposition of coarse Breccias in wadis and in scree slopes on the flanks of the intensely weathering Variscan highlands. These Breccias passed laterally into the Dune fields which covered vast areas of Britain, particularly in basins between the eroding highlands. These appear to have been formed in a Sahara-like desert at around 20-25 degrees North and deposited by a predominantly easterly wind. The dune sandstones pass east in the North Sea basin into red aeolian and coarse sandstones, and evaporites of the Rotliegendes. These are the source rocks for North Sea natural gas covered by later Permian evaporites, and the basin in which they were formed was probably below sea level. The invasion of the North Sea and Irish Sea basins by the saline Zechstein and Bakevellia Seas in the late Permian produced thick dolomitic carbonates and evaporites up to 1000m thick in the North Sea and north-east England and to a lesser extent in north-west England and the Irish Sea basin. These evaporites (halite and anhydrite mainly) were produced on sabkha salt flats around the edges of the Zechstein Sea, which at times dried up completely to produce potassium salts in the shimmering Playa lake basins surrounded by wind blasted desert landscapes. Five cycles of carbonates (Magnesium Limestone) followed by evaporites occur in north-east England and the North Sea. Between cycles, the Zechstein Sea was replenished from the north over barriers which were periodically flooded giving a ‘sea’ up to 300m deep in the subsiding North Sea basin. This cyclical flooding could have been caused by the end of the glaciation around the Gondwanaland south pole.
The Zechstein Sea had disappeared by the end of the Permian Period to be replaced in the Triassic by a subsiding North Sea basin and other fault-bounded rifts (grabens) in which thick continental deposits accumulated in the form of fluvial sandstones and playa deposits. Subsistence may have been as much as 2.5 km in some Triassic basins with corresponding uplift in the surrounding blocks or horsts. The early Triassic was dominated by a river system flowing from the south which deposited thick pebble beds and these deposits are followed by fluvial sandstones deposited by ephemeral rivers in a desert landscape. In the mid-Triassic a probable marine incursion from the south affected the English Midlands and the increased humidity produced new vegetation and a relatively abundant vertebrate fauna. During the later Triassic the whole of Britain became low desert plains over which thick carbonate-rich marls were formed in the playa lake basins with thick evaporites, mainly halite. These marls and marginal conglomerates blanked the desert highlands and sediments while rich vertebrate faunas fell into fissures in the dried out landscape. At the end of the Triassic, the Rhaetic marine transgression spread shallow waters of the marginal Tethys Ocean over the deserts of Britain to herald the start of the Jurassic Period. Britain, with its desert sediments is not the best place for the study of Permian and Triassic fossils, but elsewhere in the world the biggest mass extinction of all time, at the end of the Permian, caused major changes in flora and fauna with 95% of all species dying out, and marking the end of the Palaeozoic Era. The Triassic Period saw the start of the Mesozoic Era with its new reptile and early mammal groups and the fast evolving ammonites, although these were affected by another mass extinction at the end of the Triassic.