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GEOLOGY: Rocks and Periods

Permian Period (ca.299Ma – 251Ma)

The addition of a Permian System of rocks to the history of the Earth was proposed in 1841 by eminent geologist Roderick Impey Murchison (1792-1871) after performing geological surveys in the Perm region of Russia where beds of this age are well represented.

Permian Column

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

Earth movements during the preceding Carboniferous Period gradually raised the land’s surface across much of the UK in an episode of mountain-building dubbed the Hercynian Orogeny. This event raised the Harz Mountains in Germany and, further north, the Mercia Highlands which once extended between Devon and the Wash. Former areas of well-vegetated tropical delta-marsh, within which the commercially important Coal Measures developed were buried as a hot arid desert advanced across the area. It was under these conditions that rocks of Permian age began to be deposited within the Tees Valley between 299 and 251 million years ago.

Lower Permian

The lowest, and hence oldest, beds in the Permian succession locally are a mixture of dune-bedded sandstones and coarse breccias that accumulated upon the undulating Carboniferous land surface. Occasional wind-polished rocks, known as ventifacts, can be found amongst the deposits, which are comparable to those accumulating in the Sahara today. A lack of fossils perhaps highlights the harsh conditions which existed during the sediment’s emplacement.

wave washed platform

Permian Magnesian Limestone foreshore as seen at Hartlepool Headland.

Upper Permian

Further mountain-building (orogenic) activity to the south caused the land surface to buckle and fold forming a broad inland basin. To the north and east, a communication developed with the Zechstein Sea, which rapidly transgressed across North East England to occupy the former desert plain. Further subsidence meant that this new arm of the sea reached depths approaching 200 metres further east, though locally the area was close to a shoreline. This marginal environment was colonised by coral reefs, stromatolites, and a rich fauna of other marine creatures. Their remains combined with a restricted input of fine sediment blown from the nearby desert to produce beds of limestone.

This reef environment was to be short-lived however, as a new period of uplift caused the English Zechstein to become cut off from the main water body. Conditions rapidly deteriorated in the isolated sea as evaporation of its diminishing waters concentrated their mineral content. During the final phases a sabkha zone developed, comprising hypersaline lagoons, pools of hot mud, and glittering beds of evaporites stretching across the desert.

The Upper Permian is typified by five such incursions, followed by evaporation, of the English Zechstein (cycles EZ1-5) with the later episodes never attaining great depth. The resulting strata comprise various limestones and mudstones with intervening beds of evaporites. The latter became important commercially during the late 1800s when salt (halite)extraction on Teesside constituted the beginnings of today’s modern chemical industry. Later, anhydrite was mined in great quantities around Billingham, and potash (sylvinite) along with rock salt is still extracted from deep mines, over a kilometre below the surface, at Boulby Mine, near Staithes.

fossil fish

Fossil fish from the Permian Period

Fossil content within the Permian succession diminishes the higher up one looks, and this is not simply an effect of the harsh conditions locally, but is reflected within the fossil record worldwide. During what has become known as the Permian Mass Extinction, some 95% of all marine species died out never to return, with a lesser, though not insubstantial, number of terrestrial creatures joining them. The event is billed by geologists as the greatest extinction so far suffered by life on Earth. Life’s tenacity, however, never fails to amaze, and the survivors of this catastrophe would, over the next 40 million years or so, adapt and radiate into niches vacated by many of their predecessors to produce a whole new era of life on Earth – the Mesozoic Era.

At-a-glance information:

Lower Permian strata (offshore Rotliegendes) are virtually absent in our area when high ground formed during the Variscan Orogeny was reduced by erosion to a desert peneplain with depressions. Our first Late Permian beds were laid down when the Zechstein Sea flooded this surface.

Permian evaporites became important commercially during the late 1800s when salt extraction on Teesside constituted the beginning of the modern chemical industry. Later, anhydrite was mined at Billingham, and potash (sylvinite) along with rock salt is still extracted from the deep mine at Boulby Mine, near Staithes.



By the eminent geologist Roderick Impey Murchison (1792-1871) in 1841 after visiting Perm, Russia where beds of this age are found.

Blakey

Geography of the Earth ca.260million years ago



Saharan (10° to 20°)

Hot and dry

Worldwide, Great Britain is part of the super continent of Pangaea formed at the end of the Carboniferous. Its rifting apart would have important influences on our local geology.

Cyclic, crustal depression and uplift with some worldwide influences (eustatic sea level changes), periodically cutting off the Zechstein inland sea from the open ocean.

Stony & sandy deserts and sabkhas. Our area is on the western margin of the Zechstein Sea that was infilled repetitively with evaporite deposits and with dolomitic limestone, reef and back reef deposits around its margins.

The Permian strata rest unconformably on the Carboniferous with, usually, shallow dips and are subject to reactivation of pre-Permian fractures probably in Tertiary times.

The local stratigraphic succession is shown in the table below.

Stratigraphy of Permian Strata

Stratigraphy of Permian Strata



Dolomitic shale (‘Marl Slate’) and limestone with stromatolitic reefs, turbidites and collapse breccias.

Fish, bivalves and brachiopods, often stunted.

Undulating eastern slopes of the Pennines and the County Durham limestone plateau




Underlies most of the area with major changes in facies eastwards as evaporates form the bulk of the sequence.

Forms an extensive area west and north of Darlington and Stockton and extending to the coast at Hartlepool, mostly under glacial and post-glacial cover.

Some good exposures in old quarries, especially near and north of Piercebridge. Working quarry at Hart. Exposures on the coast at Hartlepool and in Crimdon Dene.



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