page13 permian triassic

Northern England as a desert and tropical seas. Permian and Triassic times. 296 to 208 Ma.

*Near Harrogate, North Yorkshire*	By the early Permian, all the world's continents had converged, becoming one huge landmass called Pangaea. During late Carboniferous-early Permian times uplift coupled with a worldwide fall in sea-level led to erosion of up to 1.300 km. of Carboniferous sediments. Early Permian deposits in Northern England, therefore, rest unconformably on Carboniferous rocks. As the British Isles drifted northwards from the equator the climate became hot and arid. During the early Permian higher ground, located in the present day North Sea and the Pennines, underwent continued erosion while low lying areas became deserts of wind-blown dune sands and conglomerates deposited by flash floods.
Later, parts of Britain were periodically invaded by a sea known as the Zechstein which extended to Germany. The Zechstein Sea flooded in and evaporated in four major cycles. It was this sea which laid down the Marl Slate, famous for its fossil fish, followed by beds of Magnesian Limestone (dolomite) which are quarried in many parts of Durham. High rates of evaporation at various levels of the upper Permian resulted in deposits of gypsum, halite (salt) and potash (carnallite KMgCl3.6H2O). Some of these minerals are mined at Boulby and also at Kirkby Thore near Penrith. During the Triassic period the Zechstein sea had retreated and the climate had become a little wetter. Lack of fossils makes the Permian-Triassic boundary uncertain in Northern England. The horizon is characterised by a succession of red marls (calcareous mudstones) deposited on coastal flats, followed by the Sherwood Sandstone (formerly Bunter Sandstone). The sandstones were fed by rivers mainly from the south.	*Cross-bedded desert sand dunes, Field House Farm, Durham*
	Towards the end of the Triassic, high rates of evaporation returned and low lying areas such as Cheshire and north east Yorkshire became sabkha environments. (A sabka is a wide area of coastal flats bordering the sea; the name comes from such an area on certain parts of the coast of Arabia). Periodic flooding caused by spring tides and strong on-shore winds followed by intense evaporation results in the precipitation of carbonate-sulphate and halite deposits. It was in this type of environment that the Mercia Mudstone Group (formerly Keuper Marl) was deposited. A major marine incursion which deposited sediments of the Penarth Group (formerly the Rhaetic beds) marked the end of the Triassic period and heralded the beginning of the Jurassic.

Desert dune sands containing large scale cross-bedding are well displayed in Field House Farm and Quarrington Quarry, Durham, where the Yellow Sands underlie the "Marl Slate" famous for its fossil fish as well as on the coast around Cullercoats harbour and below Tynemouth Priory. Between Frenchman's Bay (where there is a geological information board) and Marsden Bay (on the coast near South Shields) Permian rocks viz. the Yellow Sands, Marl Slate and overlying Magnesian Limestone are well displayed. The cliffs on the coast between Hartlepool and Tynemouth consist mainly of Magnesian Limestone. Reef limestone is exposed on the coast at Black Halls rocks and on the Tunstall Hills (SSSI) south of Sunderland.
Exposures of Triassic rocks are sparse. Some can be seen by the River Leven west of Hutton Rudby and also on the beach at Seaton Carew.

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