Editing A sequence-stratigraphy scheme of the Late Carboniferous, southern North Sea, Anglo-Dutch sector

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The important Early Langsettian palynomorph marker ''Radiizonates aligerens ''became extinct within this zone, possibly reflecting the regional basinwide change to channel incision and major sandstone deposition of the following lowstand deposits, conditions that the donor plant could not tolerate. However, this palynomorph can occur in large numbers in the earlier parts of the zone and could be reworked into the overlying erosive HST and LST sandstones for more than 100 m above.
 
The important Early Langsettian palynomorph marker ''Radiizonates aligerens ''became extinct within this zone, possibly reflecting the regional basinwide change to channel incision and major sandstone deposition of the following lowstand deposits, conditions that the donor plant could not tolerate. However, this palynomorph can occur in large numbers in the earlier parts of the zone and could be reworked into the overlying erosive HST and LST sandstones for more than 100 m above.
  
=== 7.3 Late Langsettian (Lower ''A. modiolaris'' zone) ===
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=== 7.3 Late Langsettian (Lower A. modiolaris zone) ===
  
 
Above the basal flooding surface, taken at the Tupton Coal, this zone is characterized by channel sandstones of low and high sinuosity, such as the Penistone Flags and the Parkgate Rock equivalents. It also contains significant intervals of finer-grained crevasse-splay and swamp and lake strata, with coals ([[:File:YGS_CHR_06_ASEQ_FIG06C.jpg|Figure 6C]]). This zone has been designated a TST; it is 70 m thick in well 44/22-1. Many important coals also occur in the uppermost part of the zone, the Parkgate–Piper, Deep Hard, Flockton and Joan, possibly representing flooding surfaces, although no marine bands have been recorded within it up to the MFS of the Vanderbeckei (Clay Cross) Marine Band.
 
Above the basal flooding surface, taken at the Tupton Coal, this zone is characterized by channel sandstones of low and high sinuosity, such as the Penistone Flags and the Parkgate Rock equivalents. It also contains significant intervals of finer-grained crevasse-splay and swamp and lake strata, with coals ([[:File:YGS_CHR_06_ASEQ_FIG06C.jpg|Figure 6C]]). This zone has been designated a TST; it is 70 m thick in well 44/22-1. Many important coals also occur in the uppermost part of the zone, the Parkgate–Piper, Deep Hard, Flockton and Joan, possibly representing flooding surfaces, although no marine bands have been recorded within it up to the MFS of the Vanderbeckei (Clay Cross) Marine Band.
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This interval is characterized by common ''Lycospora ''spp., with reduced abundances of ''Crassispora kosankei ''and ''Laevigatosporites ''spp. ''Krauselisporites ''spp., ''Punctatisporites nitidus ''and ''P. limbatus ''are particularly prominent within this zone, which may be a direct response to the transgressive depositional regime. Several other spore groups also dominate lake clay-stones and associated coal swamps. For example, ''Schulzospora rara ''is a very distinctive form, which reaches its acme in the uppermost part of the Langsettian and then terminates within the Joan Coal and coeval equivalents at the Westphalian A/B boundary. In South Wales, this form is commonly observed associated with coals at three distinct horizons. In the 49 Quadrant it has been observed commonly at two distinct horizons. Farther north in Quadrant 44 and onshore UK it is only widely observed within the uppermost horizon (Joan Coal). Even farther north, towards Tyneside and the Scottish coalfields, this taxon is rare or absent (see [[:File:YGS_CHR_06_ASEQ_FIG_10.jpg|Figure 10]]). A possible interpretation of this distribution may be that ''S. rara ''was associated with coastal brackish swamps and that its decreasing frequency northwards records the brackish edge of marine incursion that progressively diminished up palaeoslope within this TST.
 
This interval is characterized by common ''Lycospora ''spp., with reduced abundances of ''Crassispora kosankei ''and ''Laevigatosporites ''spp. ''Krauselisporites ''spp., ''Punctatisporites nitidus ''and ''P. limbatus ''are particularly prominent within this zone, which may be a direct response to the transgressive depositional regime. Several other spore groups also dominate lake clay-stones and associated coal swamps. For example, ''Schulzospora rara ''is a very distinctive form, which reaches its acme in the uppermost part of the Langsettian and then terminates within the Joan Coal and coeval equivalents at the Westphalian A/B boundary. In South Wales, this form is commonly observed associated with coals at three distinct horizons. In the 49 Quadrant it has been observed commonly at two distinct horizons. Farther north in Quadrant 44 and onshore UK it is only widely observed within the uppermost horizon (Joan Coal). Even farther north, towards Tyneside and the Scottish coalfields, this taxon is rare or absent (see [[:File:YGS_CHR_06_ASEQ_FIG_10.jpg|Figure 10]]). A possible interpretation of this distribution may be that ''S. rara ''was associated with coastal brackish swamps and that its decreasing frequency northwards records the brackish edge of marine incursion that progressively diminished up palaeoslope within this TST.
  
=== 7.4 Early Duckmantian (Upper ''A. modiolaris'' zone) ===
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=== 7.4 Early Duckmantian (Upper A. modiolaris zone) ===
  
The base of this zone is taken at the Vanderbeckei Marine Band. Sedimentary sequences within this zone are distinctly arenaceous in character, with a dominance of coarse clastic lithologies comprising low-sinuosity sandstone bodies up to 35 m thick (facies association A), commonly best developed within the upper part of the zone. Intervening lithofacies comprise mouth-bar and crevasse-splay units ([[:File:YGS_CHR_06_ASEQ_FIG_06D.jpg|Figure 6D]], [[:File:YGS_CHR_06_ASEQ_FIG_07A.jpg|Figure 7A]]). In contrast to the offshore area, the upper part of the ''A. modiolaris ''zone, in the central Pennine Basin, mostly comprises argillaceous strata, although more sandstone-dominated sequences are locally present (Parkgate–Deep Hard rocks). This zone is considered to be an HST overlain erosively by an LST. Apart from the basal MFS, marine bands are not a feature of this zone, as would be expected during lowstand sedimentation. Off shore, the highstand deposits may be equivalent to the informally named Lower Caister Sand, the unconformably overlying lowstand deposits equivalent to the Caister upper and lower Main Sand (''sensu ''Ritchie & Pratsides 1993, Ritchie et al. 1998). Its thickness is 122 m in 44/22-1 and 96 m in 44/27-1.
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The base of this zone is taken at the Vanderbeckei Marine Band. Sedimentary sequences within this zone are distinctly arenaceous in character, with a dominance of coarse clastic lithologies comprising low-sinuosity sandstone bodies up to 35m thick (facies association A), commonly best developed within the upper part of the zone. Intervening lithofacies comprise mouth-bar and crevasse-splay units ([[:File:YGS_CHR_06_ASEQ_FIG_06D.jpg|Figure 6D]], [[:File:YGS_CHR_06_ASEQ_FIG_07A.jpg|Figure 7A]]). In contrast to the offshore area, the upper part of the ''A. modiolaris ''zone, in the central Pennine Basin, mostly comprises argillaceous strata, although more sandstone-dominated sequences are locally present (Parkgate–Deep Hard rocks). This zone is considered to be an HST overlain erosively by an LST. Apart from the basal MFS, marine bands are not a feature of this zone, as would be expected during lowstand sedimentation. Off shore, the highstand deposits may be equivalent to the informally named Lower Caister Sand, the unconformably overlying lowstand deposits equivalent to the Caister upper and lower Main Sand (''sensu ''Ritchie & Pratsides 1993, Ritchie et al. 1998). Its thickness is 122m in 44/22-1 and 96m in 44/27-1.
  
Following the deposition of the Vanderbeckei Marine Band, a reduced rate of sea-level rise, followed by a rapid fall in base level, caused a reduction in the rate of formation of accommodation space and markedly increased (low- and high-sinuosity) channel sandstone sedimentation. Evidence of incision, erosion and reworking over this zone is provided by the occurrence of reworked Langsettian spores, and rarer Namurian spores. Some more sandstone-prone wells may yield consistent occurrences of ''Radiizonates aligerens ''from this interval, a Langsettian marker, some 100 m and more above the base of the Duckmantian. In other instances the coarse sandstone units yield only very poor palynomorph assemblages, because of the high energy of deposition. Incision during sandstone deposition may have removed a large interval of highstand strata deposited immediately above the preceding marine band.
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Following the deposition of the Vanderbeckei Marine Band, a reduced rate of sea-level rise, followed by a rapid fall in base level, caused a reduction in the rate of formation of accommodation space and markedly increased (low- and high-sinuosity) channel sandstone sedimentation. Evidence of incision, erosion and reworking over this zone is provided by the occurrence of reworked Langsettian spores, and rarer Namurian spores. Some more sandstone-prone wells may yield consistent occurrences of ''Radiizonates aligerens ''from this interval, a Langsettian marker, some 100m and more above the base of the Duckmantian. In other instances the coarse sandstone units yield only very poor palynomorph assemblages, because of the high energy of deposition. Incision during sandstone deposition may have removed a
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large interval of highstand strata deposited immediately above the preceding marine band.
  
 
The spore ''Crassispora kosankei ''is prominent in this zone, commonly occurring in very great abundance. This is in keeping with the alluvial swamp depositional environment that has been associated with this arborescent lycopod spore. This can be compared with its abundance in the previous ''C. communis ''LST, although in the latter it is less prominent than here in the Early Duckmantian. ''Laevigatosporites ''spp. are, like the ''C. communis ''zone, also in abundance within the Upper ''A. modiolaris ''zone, reflecting the waterlogged ephemeral depositional environment. Another spore taxon that is prominent within this zone and the ''C. communis ''zone is ''Vestispora costata'', which may, like ''C. kosankei, ''come from a plant that favoured an environment of alluvial swamps and levee depositional environments. The important Duckmantian and younger palynological marker taxa ''Endosporites globiformis ''and ''Dictyotriletes bireticulatus ''make their appearance within this zone.
 
The spore ''Crassispora kosankei ''is prominent in this zone, commonly occurring in very great abundance. This is in keeping with the alluvial swamp depositional environment that has been associated with this arborescent lycopod spore. This can be compared with its abundance in the previous ''C. communis ''LST, although in the latter it is less prominent than here in the Early Duckmantian. ''Laevigatosporites ''spp. are, like the ''C. communis ''zone, also in abundance within the Upper ''A. modiolaris ''zone, reflecting the waterlogged ephemeral depositional environment. Another spore taxon that is prominent within this zone and the ''C. communis ''zone is ''Vestispora costata'', which may, like ''C. kosankei, ''come from a plant that favoured an environment of alluvial swamps and levee depositional environments. The important Duckmantian and younger palynological marker taxa ''Endosporites globiformis ''and ''Dictyotriletes bireticulatus ''make their appearance within this zone.

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