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Unconformity near
Harrogate, North Yorkshire
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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.
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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.
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 Desert dune sands containing large
scale cross-bedding are well displayed in Field House Farm and Quarrington
Quarry both in 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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Potash and salt, Permian evaporates, are mined here.
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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.
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Permo-Triassic
boundary (250 Ma)
reconstruction of the Western
Hemisphere, modified
from Torsvik& Cocks (2004, fig. 11), also
showing the flood basalts in Siberia and China.
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From “European geography
in a global context from the Vendian to the end
of the Palaeozoic”
L. R. M. COCKS1 & T. H.
TORSVIK
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