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Geology Group Diary (33)

Started by johnd, July 12, 2018, 03:19:59 pm

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The Geology Group met at 10.30am on Wednesday 11 July at Merlin's Bridge village hall. This month's topic was...
The Permian Period lasted from 290 to 245 Ma which brought to an end the Palaeozoic era. The period was terminated by a major catastrophic event known as the Permian Mass Extinction when 95% of all animals and plants became extinct.  Invertebrate groups such as trilobites, goniatites, rugose corals and many brachiopods became extinct. On land many amphibians and reptiles died out to be replaced by the early dinosaurs in the succeeding Triassic Period (245-208). A combination of various factors may have brought about the great extinction. Here are some of the possible events. 1.Asteroid impact; dust and rock fragments from the collision of an asteroid 100 km in diameter would blot out sunlight for years. However, it is difficult to find evidence of such craters since the earth's surface has changed considerably since Permian times. 2.Formation of Pangaea; this super continent emerged during the Permian causing a eustatic (world wide) lowering of sea level and marine regression over the continental shelves thus destroying shallow water marine habitats. Polar glaciations would also contribute to lowering sea level. 3.Vulcanism; the vast Permian basalt lava flows in Siberia would have been produced by powerful eruptions that would emit CO2 and SO2 in enormous quantities causing climate change and the destruction of vegetation and consequent death of animal life.  4.Climatic events; Global warming due to enhanced greenhouse effect would cause increased aridity (continental climate) and salinity (high evaporation), severe decline in oxygen levels and the presence of excess carbon dioxide, methane and hydrogen sulphide in the oceans would poison marine life.
The Permo-Triassic Environment.
[The Permo-Triassic rocks are often referred to as the New Red Sandstone]
As the Rheic Ocean closed and the Variscan Mountains were formed, the supercontinent of Pangaea developed as the existing continents collided. Britain was located between 20° and 30°N on the north west side of the Tethys Ocean. At the beginning of Permian times Britain was an arid upland area where barchan sand dunes accumulated under the influence of the north east trade winds. Fault bounded basins such as the Vale of Eden received great thicknesses of aeolian (wind blown) sediment at this time. In the late Permian the marine transgression of the Zechstein Sea covered much of NE England and the North Sea area. The sea initially covered the desert landscape with muddy sediments forming the Marl Slate, which is a bituminous shale containing fish remains. Next the Magnesian limestone was laid down and this now forms a well defined escarpment stretching from Sunderland to Teesdale. This limestone is formed of the mineral dolomite or calcium magnesium carbonate. It contains many marine fossils such as brachiopods, gastropods and bivalves. A concretionary form of the Magnesian limestone known as the 'cannonball limestone' is well developed in the Roker area of Sunderland. As rapid evaporation occurred in the hot arid climate within the Zechstein Sea, a sequence of repeated cycles of desiccation occurred in which salts were precipitated in order of increasing solubility. Starting with the least soluble calcium carbonate CaCO3,then gypsum CaSO4, halite NaCl, magnesium and potassium salts (most soluble). The fluctuating level of the Zechstein Sea would account for the repeated cycles of desiccation. In the 1970s the Boulby mine near Staithes (Cleveland) was opened in order to extract potassium chloride (KCl)  and halite (NaCl) from up to 1500 metres below the surface. The potash is required for agricultural fertilizer; rock salt is also produced and both minerals are exported via Teesside. North Sea oil and gas reservoirs are found in Permian sandstones trapped by impervious salt domes. The natural gas derived from underlying Coal Measures, migrated upwards through the Permian sands.
The Triassic is so called from its threefold division in Germany, but in Britain the middle division (Muschelkalk) is missing. Here arid conditions continued throughout the period, beginning with the Bunter Sandstone (Buntsandstein) and Pebble Beds, now part of the Sherwood Sandstone Group. Braided rivers and flash flooding transported coarse sands and gravels from the Variscan highlands to produce cross stratified sandstones, pebble beds and conglomerates that are characteristic of the Lower Triassic strata. Examples of the pebble beds can be seen in Staffordshire on Cannock Chase, High Shutt near Alton Towers and in Park Hall Country Park. The Budleigh Salterton Pebble beds in South Devon also show evidence of braided rivers and wind faceted 3 sided driekanter pebbles on desert surfaces. The overlying Keuper Marl now called the Mercia Mudstones comprise red-brown clays up to 1000 metres thick in the English Midlands and they contain considerable deposits of halite and gypsum. These were laid down in shallow playa lakes as evaporites. Note that rock salt was extensively worked around Northwich and Droitwich and gypsum was mined in the Newark area. At the end of the Triassic, the Rhaetic marine transgression heralded the onset of the Jurassic period as the shallow marginal waters of the advancing Tethys Ocean covered the desert plains and playa lakes. The Rhaetic bone bed represents a condensed deposit of fish teeth, scales, and bones, coprolites from aquatic reptiles and early dinosaur bones that may be the result of mass mortality due to salinity changes or simply the sweepings of  the advancing seas.
Commercially the Sherwood Sandstone Group is a major aquifer providing water for both domestic and industrial users throughout the Midlands. Where it is sandwiched between the underlying impervious Upper Carboniferous strata and the overlying Mercia mudstones, the porous and permeable Sherwood sandstone ( up to 600 metres thick) can yield up to 125 litres per second from a vast underground reservoir.  Coarse grained sandstone is both highly permeable and porous since there are spaces between the grains which allow water to pass through and also be stored.
"To see the world in a grain of sand.........(and) hold infinity in the palm of your hand." William Blake
John Downes


6 more images of Permo Triassic.