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Triassic (251Ma- 200Ma)

Triassic Column

Column showing divisions in the Triassic System of the Tees Valley.

The Triassic Period was named in 1841 by German geologist Friedrich August von Alberti (1795-1878), after a suite of rocks which occur between those of the Permian and Jurassic. This succession was further subdivided into a three-fold sequence of Bunter (or Buntsandstein), Muschelkalk, and Keuper Marl.

The episode commenced around 251 million years ago and lasted c.50 million years to be succeeded by the Jurassic Period. The Triassic marks the beginnings of a recovery after an unprecedented mass extinction affecting up to 95% of all marine life on Earth, and c.70% of land-dwelling forms. Such a fundamental change in fauna marks an important transition between Palaeozoic (Ancient life) and Mesozoic (Middle life) Eras of life on Earth. Forms such as trilobites became extinct, whereas others such as modern corals, marine reptiles and the Palaeozoic Ceratites and Goniatites evolved and expanded.

Blakey

Geography of the Earthe ca.250million years ago

During the Triassic the geography of North-western Europe differed greatly to that existing today. At this time, all of the landmasses of the Earth had agglomorated into a single continent named Pangaea. An episode of mountain building, known as the Variscan Orogeny, locally raised parts of the Earth’s crust to form a mountain chain extending between North Devon and The Wash. The so-called Mercia Highland was a fundamental influence, controlling the nature of the rocks deposited during the Triassic.

Triassic deposits have their type locality in Germany and Austria where thick marine deposits indicate the presence of an epicontinental sea in the German Basin with poor connections with the southerly Tethys Sea. Deposits in the UK are generally continental sabhka – a hot arid coastal environment – or shallow water deposits. Rain falling on the Mercia Highlands washed eroded debris down the slopes of the mountains onto a broad flat tropical plain with a shallow sea to the east.

The Lower Triassic deposits found in the Tees Valley do not correlate well with the German type locality and hence have been redefined to suite the local representatives of these rock units. Here the episode is represented by the Sherwood Sandstone Group, a suite of yellow and red sandstones deposited primarily in shallow water. The Middle Triassic, represented by the Muschelkalk of Germany, is missing in Britain as an episode of erosion ensued, represented by an unconformity know as the Hardegsen Disconformity which extends into the modern North Sea Basin. The Upper Triassic locally is represented by the Mercia Mudstone Group, a suite of red-brown and blue-grey mudstones with occasional halite and gypsum deposits. All of these deposits are unfossiliferous demonstrating the harsh conditions locally at this time.

Triassic rocks crop out at very few localities in the Tees Valley making any exposure all the more valuable. The Triassic succession is mostly obscured by the Tees Estuary on the coast, and thick glacial deposits inland. However, an example of Sherwood Sandstone can be seen at Foxton Bridge, and exposures of Mercia Mudstone can be viewed alongside the River Leven downstream of Hutton Rudby.

Sherwood Sandstone Group

washed platform

Exposures of Sherwood Sandstone near Foxton Bridge on the River Leven

During the Late Permian and Early Triassic the Tees Valley constituted an arid coastal plain. It was subject to rapid deposition of sediment carried by flash floods descending from the Mercia Highlands, an upland area extending from North Devon eastwards through Norfolk. Adjacent to the base of the uplands, thick pebble beds developed when descending water dumped the heavier material it carried as it flowed onto flatter ground. Lighter particles of sediment, sand and mud, were spread in great fans away from the uplands across the flat plain, and it is these that make up the Sherwood Sandstone Group in our region. Alternating beds of yellow or red sandstone and thin mudstones exhibiting desiccation-cracks typify the rock unit, which underwent various episodes of both deposition and erosion. It has a distinct lack of fossils, partly because of the harsh environment but also because life on earth appeared to be still recovering in numbers from the Permian Mass Extinction. An erosion surface (unconformity) marks the top of the Sherwood Sandstone Group.

Mercia Mudstone Group

small exposure

Small exposure of blue-grey Mercia Mudstone alongside Stainsby Beck

The next episode in the area’s development saw a shallow sea advance across the coastal plain as an intermittent marine connection opened to the southeast. Circulation of water within this sea was restricted and it frequently cut off from the main water body. When it was isolated there was evaporation which then concentrated the dissolved salts. Saline lagoons, pools of hot mud, and glittering beds of salt developed. As a result, the Mercia Mudstone Group is comprised of beds of dark red-brown or blue-grey mudstone, frequently mottled. The Mercia Mudstone Group is a product of fine-grained material washed or blown into the sea from the flat plain. The mudstones alternate with occasional evaporite beds including halite, gypsum, and anhydrite deposited from evaporating sea-water. The uppermost beds in this sequence have been dubbed the Tea Green Marls.

Penarth Group

The close of the Triassic locally saw the Tethys Sea readvance across the saline Triassic plain. The Penarth Group of rocks (also known as the Rhaetic) were deposited over a peroid of c.4 million years. The highly poisonous saline surface, over which the sea advanced, combined with restricted circulation of its waters, produced considerable variations in salinity and oxygen content at first. The lack of oxygen formed the black sulphurous shales typical of the Westbury Beds in the lower part. In this layer a bone bed indicates mass mortality of many creatures, this is believed to have been caused by algal blooms.

As the sea deepened marine creatures of many kinds began to flourish. Consequently, higher in the Penarth Group succession, and hence later in time, lie the brown and green mudstones, known as the Cotham Beds, containing the fossils of shelly creatures, indicating life beginning to establish itself locally in the improving waters.



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