Editing Carboniferous and Permian rocks between Tynemouth and Seaton Sluice - an excursion

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[[File:YGS_NORTROCK_FIG_08_4.jpg|thumbnail|Figure 8.4 Lenticular bed geometries in the Table Rocks Sandstone (Locality 3). Photo: B.R. Turner.]]
 
[[File:YGS_NORTROCK_FIG_08_4.jpg|thumbnail|Figure 8.4 Lenticular bed geometries in the Table Rocks Sandstone (Locality 3). Photo: B.R. Turner.]]
  
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== 8 The Carboniferous and Permian rocks between Tynemouth and Seaton Sluice ==
  
By '''Brian Turner''' University of Durham, with a contribution on the Permian by '''Denys Smith''' GEOPERM & University of Durham
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By Brian Turner University of Durham, with a contribution on the Permian by Denys Smith GEOPERM & University of Durham
  
 
== Purpose ==
 
== Purpose ==
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== Maps ==
 
== Maps ==
  
O.S. 1:50 000 Sheet 88 Tyneside & Durham; B.G.S. 1:50 000 Sheet 15 Tynemouth (solid).
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O.S. 1:50 000 Sheet 88 Tyneside & Durham; B.G.S. 1:50 000 Sheet 15 Tynemouth (solid).
  
 
== Geological background ==
 
== Geological background ==
  
The area between Tynemouth and Seaton Sluice ([[:File:YGS_NORTROCK_FIG_08_1.jpg|Figure 8.1]]) lies at the southern end of the Northumberland coastal plain. It is an area of low, relatively flat ground covered by a veneer of glacial '''till''', modified by late glacial and post-glacial '''solifluction'''. As a result, the underlying solid rocks of Upper Carboniferous and Permian age are exposed only in quarries, cliff faces, rocky headlands and on wave cut platforms.
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The area between Tynemouth and Seaton Sluice ([[:File:YGS_NORTROCK_FIG_08_1.jpg|Figure 8.1]]) lies at the southern end of the Northumberland coastal plain. It is an area of low, relatively flat ground covered by a veneer of glacial till, modified by late glacial and post-glacial solifluction. As a result, the underlying solid rocks of Upper Carboniferous and Permian age are exposed only in quarries, cliff faces, rocky headlands and on wave cut platforms.
  
This stretch of coast provides one of the best exposed sequences of Westphalian B Coal Measures anywhere in Britain. The succession consists of about 115 m of shale, mudstone, siltstone and sandstone arranged in vertically stacked coarsening-upward sequences capped by a '''seatearth''' and coal seam, interbedded with a number of prominent channel and distributary mouthbar sandbodies, each one named after their outcrop locality ([[:File:YGS_NORTROCK_FIG_08_2.jpg|Figure 8.2]]). Typical coarsening-upward coal-bearing '''facies''' sequences consist of (from bottom to top): (1) black shale containing fish and plant debris plus fresh-water '''bivalves'''; (2) rhythmically banded dark grey mudstone and siltstone, passing upwards into fine-grained sheet sandstone, containing burrows, ripple '''cross-lamination''' and some small-scale trough cross-bedding; (3) ironstone-bearing seatearth with small rootlets and scattered ''Stigmaria; ''and (4) coal. '''Trace fossils''' and plant fragments are common, but body fossils are generally confined to the fresh-water mussel bands above coal seams. Ironstone bands and nodules occur in the shales and mudstones, some of them showing '''cone-in-cone''' structure.
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This stretch of coast provides one of the best exposed sequences of Westphalian B Coal Measures anywhere in Britain. The succession consists of about 115 m of shale, mudstone, siltstone and sandstone arranged in vertically stacked coarsening-upward sequences capped by a seatearth and coal seam, interbedded with a number of prominent channel and distributary mouthbar sandbodies, each one named after their outcrop locality ([[:File:YGS_NORTROCK_FIG_08_2.jpg|Figure 8.2]]). Typical coarsening-upward coal-bearing facies sequences consist of (from bottom to top): (1) black shale containing fish and plant debris plus fresh-water bivalves; (2) rhythmically banded dark grey mudstone and siltstone, passing upwards into fine-grained sheet sandstone, containing burrows, ripple cross-lamination and some small-scale trough cross-bedding; (3) ironstone-bearing seatearth with small rootlets and scattered ''Stigmaria; ''and (4) coal. Trace fossils and plant fragments are common, but body fossils are generally confined to the fresh-water mussel bands above coal seams. Ironstone bands and nodules occur in the shales and mudstones, some of them showing cone-in-cone structure.
  
The coarsening-upward coal-bearing facies sequences result from the gradual infilling of shallow interdistributary bays and lakes by shoal-water '''lacustrine''' delta complexes originating from overbank flooding and '''crevassing''' along numerous distributary channels which drained a southerly sloping, low relief coastal alluvial plain remote from open marine influences. The black fossiliferous shales were deposited on the anoxic bottom of unstratified fresh to brackish water lakes, followed by mudstones, siltstones and fine sandstones as the crevasse-splay delta system prograded into the lakes. As the lakes filled with sediment the surface was colonized by vegetation, giving rise to peat swamps and ultimately, ''in situ ''coal, following compactional subsidence, transgression (mussel bands) and burial. The presence of '''faults''' and soft, weak shales and mudstones has caused extensive collapse of cliff faces, and the rapid retreat of the coastline due to marine erosion.
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The coarsening-upward coal-bearing facies sequences result from the gradual infilling of shallow interdistributary bays and lakes by shoal-water lacustrine delta complexes originating from overbank flooding and crevassing along numerous distributary channels which drained a southerly sloping, low relief coastal alluvial plain remote from open marine influences. The black fossiliferous shales were deposited on the anoxic bottom of unstratified fresh to brackish water lakes, followed by mudstones, siltstones and fine sandstones as the crevasse-splay delta system prograded into the lakes. As the lakes filled with sediment the surface was colonized by vegetation, giving rise to peat swamps and ultimately, ''in situ ''coal, following compactional subsidence, transgression (mussel bands) and burial. The presence of faults and soft, weak shales and mudstones has caused extensive collapse of cliff faces, and the rapid retreat of the coastline due to marine erosion.
  
At the end of the Carboniferous Period the newly-deposited strata were uplifted and gently folded and faulted during the '''Variscan''' earth movements and were subjected to perhaps 40 '''Ma''' of subaerial erosion as the region gradually drifted northwards out of the equatorial belt into the tropics. At least 400 m of Coal Measures were eroded from southeast Northumberland during this phase and a mature desert peneplain was established before the oldest Permian deposits — the 260–255 Ma wind-blown Yellow Sands — were preserved in west-southwest–east-northeast ridges or '''draa'''. The desert was flooded and the ridges inundated some 255 Ma ago when the almost landlocked Zechstein Sea was formed; this sea was subsequently filled with a thick cyclic sequence of marine '''carbonate''' sediments and '''evaporites''' during the last 5–7 Ma of the Permian Period. These cyclic rocks form a continuous cover in coastal districts south of the Tyne ([[:File:YGS_NORTROCK_FIG_00_1.jpg|Figure 1]]) and in the undersea area off the Northumbrian coast, but are preserved on land in southeast Northumberland in only a few isolated '''outliers''' in each of which the sequence is thin and incomplete. Two of these outliers, at Tynemouth and Cullercoats, are included in this excursion.
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At the end of the Carboniferous Period the newly-deposited strata were uplifted and gently folded and faulted during the Variscan earth movements and were subjected to perhaps 40 Ma of subaerial erosion as the region gradually drifted northwards out of the equatorial belt into the tropics. At least 400 m of Coal Measures were eroded from southeast Northumberland during this phase and a mature desert peneplain was established before the oldest Permian deposits — the 260–255 Ma wind-blown Yellow Sands — were preserved in westsouthwest–east-northeast ridges or draa. The desert was flooded and the ridges inundated some 255 Ma ago when the almost landlocked Zechstein Sea was formed; this sea was subsequently filled with a thick cyclic sequence of marine carbonate sediments and evaporites during the last 5–7 Ma of the Permian Period. These cyclic rocks form a continuous cover in coastal districts south of the Tyne ([[:File:YGS_NORTROCK_FIG_00_1.jpg|Figure 1]]) and in the undersea area off the Northumbrian coast, but are preserved on land in southeast Northumberland in only a few isolated outliers in each of which the sequence is thin and incomplete. Two of these outliers, at Tynemouth and Cullercoats, are included in this excursion.
  
 
== Excursion details ==
 
== Excursion details ==
  
=== Locality 1, Tynemouth Cliff [NZ 374 694] ===
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=== Locality 1, Tynemouth Cliff [NZ 374 694] ===
  
Tynemouth Cliff [NZ 374 694], not at high tide nor for the infirm; binoculars helpful. Park in ticket car park [NZ 371 694] opposite The Gibralter Rock, and take narrow road and paved footpath south and east for c.550 m to the landward end of North Pier; steps here afford access northwards to the boulder-strewn and very uneven rock platform, which can be followed (with care) around the headland and back to the car park via Short Sands and a paved and stepped path up the bay head. The round trip takes up to hours.
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Tynemouth Cliff [NZ 374 694], not at high tide nor for the infirm; binoculars helpful. Park in ticket car park [NZ 371 694] opposite The Gibralter Rock, and take narrow road and paved footpath south and east for c.550 m to the landward end of North Pier; steps here afford access northwards to the boulder-strewn and very uneven rock platform, which can be followed (with care) around the headland and back to the car park via Short Sands and a paved and stepped path up the bay head. The round trip takes up to 21/2 hours.
  
The spectacular coarse dark red mottling of the otherwise buff-coloured Carboniferous (Coal Measures) sandstone that forms the rock platform and lower part of the cliff immediately north of North Pier, was caused by desert weathering in early Permian time. The scattered boulders of Scandinavian '''schist''' and '''gneiss''' are thought to have been brought to Tynemouth by ice during the Pleistocene Period. Many of the other boulders are ships' ballast. The 3–4 m wide west–east subvertical '''basaltic''' Tynemouth '''Dyke''' cuts the sandstone of the shore platform ([[:File:YGS_NORTROCK_FIG_08_3.jpg|Figure 8.3]]) and is a member of the 58 Ma (Palaeocene) Mull Swarm. Conspicuously cross-bedded Yellow Sands (c.9 m) '''unconformably''' overlie the Coal Measures sandstone in the adjoining cliff. Continue northwards c.130 m past the buttresses and pipe, pausing at intervals thereafter to examine a markedly scoured erosion surface that divides the Coal Measures sandstone into two unequal parts ([[:File:YGS_NORTROCK_FIG_08_1.jpg|Figure 8.1]], bottom left inset). The upper of these locally contains, at its base, a striking variegated breccia of angular to subrounded fragments of red and purple mudstone, siltstone and ironstone in a mainly buff-coloured sandstone matrix; this '''breccia''' is thought to be the remains of muddy sediments that were formerly associated with the Metal (= ?Top Grey or Top Five-Quarter) Coal and were ripped up and redeposited by turbulent floodwaters. An exceptionally complex large pouch of mauve and red, fine-grained, laminated sandstone at the cliff foot north of the pipe may be a load cast, caused by liquefaction of the sand (causing 'quick' conditions) soon after it was deposited; a sand volcano may have existed here but was later eroded away.
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The spectacular coarse dark red mottling of the otherwise buff-coloured Carboniferous (Coal Measures) sandstone that forms the rock platform and lower part of the cliff immediately north of North Pier, was caused by desert weathering in early Permian time. The scattered boulders of Scandinavian schist and gneiss are thought to have been brought to Tynemouth by ice during the Pleistocene Period. Many of the other boulders are ships' ballast. The 3–4 m wide west–east subvertical basaltic Tynemouth Dyke cuts the sandstone of the shore platform ([[:File:YGS_NORTROCK_FIG_08_3.jpg|Figure 8.3]]) and is a member of the 58 Ma (Palaeocene) Mull Swarm. Conspicuously cross-bedded Yellow Sands (c.9 m) unconformably overlie the Coal Measures sandstone in the adjoining cliff. Continue northwards c.130 m past the buttresses and pipe, pausing at intervals thereafter to examine a markedly scoured erosion surface that divides the Coal Measures sandstone into two unequal parts ([[:File:YGS_NORTROCK_FIG_08_1.jpg|Figure 8.1]], bottom left inset). The upper of these locally contains, at its base, a striking variegated breccia of angular to subrounded fragments of red and purple mudstone, siltstone and ironstone in a mainly buff-coloured sandstone matrix; this breccia is thought to be the remains of muddy sediments that were formerly associated with the Metal (= ?Top Grey or Top Five-Quarter) Coal and were ripped up and redeposited by turbulent floodwaters. An exceptionally complex large pouch of mauve and red, fine-grained, laminated sandstone at the cliff foot north of the pipe may be a load cast, caused by liquefaction of the sand (causing 'quick' conditions) soon after it was deposited; a sand volcano may have existed here but was later eroded away.
  
North of the buttresses the full succession in the cliff is visible, and comprises late Permian Magnesian Limestone in addition to the Yellow Sands and Coal Measures; most of the sequence is inaccessible or dangerously steep and scaling should not be attempted. The upper part of the sequence here is similar to that at the equivalent but more accessible exposure at Trow Point (Excursion 13, Locality 1), 3 km to the south-southeast, but the disturbed (resedimented) beds at the top of the Raisby Formation are here represented partly by a highly unusual debris-flow. This deposit passes southwards in about 30 m from the northeast corner of the headland into a rock composed of tabular to subrounded '''clasts''' of Raisby Formation '''dolostone''' in a 30–50% matrix of wind-abraded coarse''' quartz''' grains and, after a further 30 m, into a '''conglomerate'''-like vuggy rock composed mainly of Raisby Formation dolostone clasts in a sparse sand matrix; look for large fallen blocks to examine in detail. Clues to the origin of these unusual rocks come from Claxheugh Rock and Downhill Quarry, both in western Sunderland, where massive downslope submarine slides at the end of Raisby Formation time created canyons that in both places (and presumably others too) cut down through the Marl Slate into the Yellow Sands. Most of the Permian units may be traced westwards along the north face of the headland as they rise very gently to crop across the Castle foreditch; in spring and summer the niches marking the positions of the Marl Slate and the residue of the Hartlepool '''Anhydrite''' furnish favoured nesting sites for seabirds, especially kittiwakes.
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North of the buttresses the full succession in the cliff is visible, and comprises late Permian Magnesian Limestone in addition to the Yellow Sands and Coal Measures; most of the sequence is inaccessible or dangerously steep and scaling should not be attempted. The upper part of the sequence here is similar to that at the equivalent but more accessible exposure at Trow Point (Excursion 13, Locality 1), 3 km to the south-southeast, but the disturbed (resedimented) beds at the top of the Raisby Formation are here represented partly by a highly unusual debris-flow. This deposit passes southwards in about 30 m from the northeast corner of the headland into a rock composed of tabular to subrounded clasts of Raisby Formation dolostone in a 3050% matrix of wind-abraded coarse quartz grains and, after a further 30 m, into a conglomerate-like vuggy rock composed mainly of Raisby Formation dolostone clasts in a sparse sand matrix; look for large fallen blocks to examine in detail. Clues to the origin of these unusual rocks come from Claxheugh Rock and Downhill Quarry, both in western Sunderland, where massive downslope submarine slides at the end of Raisby Formation time created canyons that in both places (and presumably others too) cut down through the Marl Slate into the Yellow Sands. Most of the Permian units may be traced westwards along the north face of the headland as they rise very gently to crop across the Castle foreditch; in spring and summer the niches marking the positions of the Marl Slate and the residue of the Hartlepool Anhydrite furnish favoured nesting sites for seabirds, especially kittiwakes.
  
=== Locality 2, Cullercoats Bay [NZ 365 714] ===
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=== Locality 2, Cullercoats Bay [NZ 365 714] ===
  
Access is by slipway off the promenade road at each end of the bay. The point at the southern end of Cullercoats Bay lies immediately north of the Ninety Fathom Fault ([[:File:YGS_NORTROCK_FIG_08_1.jpg|Figure 8.1]]) which is part of a major east–west trending fault system defining the southern margin of the Northumberland Basin; this is an asymmetric, extensional '''half-graben''' that became filled with over 4 km of sediment during Carboniferous time. The Ninety Fathom Fault is best exposed on the south side of the point where it '''downthrows''' strata to the north by about 150 m, bringing Upper Carboniferous Coal Measure shales and mudstones associated with the Hutton Coal seam against Permian Yellow Sands. Mineralized fractures and joints in the Yellow Sands, close to the fault, bind the rock together, enabling it to resist erosion by the sea; the unmineralized sandstone between the '''joints''' has been eroded by the sea to form caves, good examples of which can be seen at the base of the cliff at the back of the bay. The fault plane at the surface '''dips''' about 35° northwards. On its south side several small folds, with their axes perpendicular to the fault plane, together with the northerly dip of the beds, have created basin and dome structures in the centre of which is the Top Hutton seam and overlying sandstone. North of the fault a faulted '''syncline plunges''' seawards. Some 30 m of Permian Yellow Sands, of probable late Lower Permian age, are well exposed at the back of the bay where the structural and sedimentological features of the unit can be examined in detail except at high tide.
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Access is by slipway off the promenade road at each end of the bay. The point at the southern end of Cullercoats Bay lies immediately north of the Ninety Fathom Fault ([[:File:YGS_NORTROCK_FIG_08_1.jpg|Figure 8.1]]) which is part of a major east–west trending fault system defining the southern margin of the Northumberland Basin; this is an asymmetric, extensional half-graben that became filled with over 4 km of sediment during Carboniferous time. The Ninety Fathom Fault is best exposed on the south side of the point where it downthrows strata to the north by about 150 m, bringing Upper Carboniferous Coal Measure shales and mudstones associated with the Hutton Coal seam against Permian Yellow Sands. Mineralized fractures and joints in the Yellow Sands, close to the fault, bind the rock together, enabling it to resist erosion by the sea; the unmineralized sandstone between the joints has been eroded by the sea to form caves, good examples of which can be seen at the base of the cliff at the back of the bay. The fault plane at the surface dips about 35° northwards. On its south side several small folds, with their axes perpendicular to the fault plane, together with the northerly dip of the beds, have created basin and dome structures in the centre of which is the Top Hutton seam and overlying sandstone. North of the fault a faulted syncline plunges seawards. Some 30 m of Permian Yellow Sands, of probable late Lower Permian age, are well exposed at the back of the bay where the structural and sedimentological features of the unit can be examined in detail except at high tide.
  
The Yellow Sands contain mainly medium to coarse, well rounded and well sorted grains, poorly cemented by a patchy '''calcite''' cement, except near the Ninety Fathom Fault where it was locally firmly cemented by '''baryte''' derived from fluids moving up along the fault plane in the underlying Coal Measures. The sandstone is rather friable at outcrop and contains small calcite-rich nodules, and '''cataclastic veinlets''' of granular quartz close to the Ninety Fathom Fault. At depth the sandstone is grey owing to the presence of '''pyrite''' which oxidizes near the surface to '''limonite''', giving the sands their distinctive yellow colour. The Yellow Sands are interpreted as a formation of sinuous-crested dunes superimposed on about eleven parallel ridges of sand, trending west-southwest to east-northeast. The ridges were deposited on an easterly sloping peneplain of Coal Measure rocks, separated and underlain by thin discontinuous interdune breccias. The preserved parts of the dunes contain large-scale tabular and wedge shaped cross-bedding deposited by bidirectional seasonal winds blowing towards the south and northwest.
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The Yellow Sands contain mainly medium to coarse, well rounded and well sorted grains, poorly cemented by a patchy calcite cement, except near the Ninety Fathom Fault where it was locally firmly cemented by baryte derived from fluids moving up along the fault plane in the underlying Coal Measures. The sandstone is rather friable at outcrop and contains small calcite-rich nodules, and cataclastic veinlets of granular quartz close to the Ninety Fathom Fault. At depth the sandstone is grey owing to the presence of pyrite which oxidizes near the surface to limonite, giving the sands their distinctive yellow colour. The Yellow Sands are interpreted as a formation of sinuous-crested dunes superimposed on about eleven parallel ridges of sand, trending west-southwest to east-northeast. The ridges were deposited on an easterly sloping peneplain of Coal Measure rocks, separated and underlain by thin discontinuous interdune breccias. The preserved parts of the dunes contain large-scale tabular and wedge shaped cross-bedding deposited by bidirectional seasonal winds blowing towards the south and northwest.
  
The Yellow Sands are sharply overlain by the Marl Slate which is a dark grey, laminated, bituminous '''dolomitic''' shale or shaley dolomite up to 1.5 m thick. This is well exposed at low tide on the foreshore just inside the south pier where it occurs in the nose of a syncline. The Marl Slate, which at Cullercoats has yielded an extensive fish fauna of early Upper Permian age and rarer land-derived plants, passes seawards beneath the pier under a ledge of pale yellow bedded dolomite that forms the centre of the syncline. The Marl Slate is overlain by the Raisby Formation which consists of a lower part of limestone and an upper part of predominantly dolomitized limestone. Disturbance of strata at the top of the Raisby Formation, seen on the foreshore at Cullercoats, is attributed to the effects of submarine slumping and sliding that took place on an easterly facing basin margin slope at the end of Raisby Formation time.
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The Yellow Sands are sharply overlain by the Marl Slate which is a dark grey, laminated, bituminous dolomitic shale or shaley dolomite up to 1.5 m thick. This is well exposed at low tide on the foreshore just inside the south pier where it occurs in the nose of a syncline. The Marl Slate, which at Cullercoats has yielded an extensive fish fauna of early Upper Permian age and rarer land-derived plants, passes seawards beneath the pier under a ledge of pale yellow bedded dolomite that forms the centre of the syncline. The Marl Slate is overlain by the Raisby Formation which consists of a lower part of limestone and an upper part of predominantly dolomitized limestone. Disturbance of strata at the top of the Raisby Formation, seen on the foreshore at Cullercoats, is attributed to the effects of submarine slumping and sliding that took place on an easterly facing basin margin slope at the end of Raisby Formation time.
  
The Marl Slate was deposited in a stratified, partly anoxic sea up to 200 m deep soon after the initial transgression of the Upper Permian Zechstein Sea. Evidence of thinning and wedging out of the Marl Slate against Lower Permian sand dunes, suggests the development of shallow water conditions and ?interdunal lagoons. The Zechstein Sea quickly submerged the desert landscape concomitant with a change to more open marine conditions, and the Raisby Formation here was deposited under moderate water depths on an easterly facing carbonate ramp.
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The Marl Slate was deposited in a stratified, partly anoxic sea up to 200 m deep soon after the initial transgression of the Upper Permian Zechstein Sea. Evidence of thinning and wedging out of the Marl Slate against Lower Permian sand dunes, suggests the development of shallow water conditions and ?interdunal lagoons. The Zechstein Sea quickly submerged the desert landscape concomitant with a change to more open marine conditions, and the Raisby Formation here was deposited under moderate water depths on an easterly facing carbonate ramp.
  
 
=== Locality 3, Table Rocks [NZ 364 722] ===
 
=== Locality 3, Table Rocks [NZ 364 722] ===

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