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

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The acmes of arborescent lycopod spores (lycospores) of ''Lepidodendron''/''Lepidophloios ''forests in this HST is in keeping with a time of marine transgression and the formation of major areas of peat swamp. Other taxa that show temporary acmes within the zone include ''Cingulizonates loricatus, Radiizonates difformis, Camptotriletes bucculentus ''and ''Grumosisporites varioreticulatus. ''Influxes of inertinitic kerogen may be associated with structural changes in the Coal Measures flora and substrate associated with many intermittent and short-lived flooding events. Towards the top of the ''A. lenisulcata ''zone, pale amorphous organic matter appears, together with the persistent up-hole appearance of ''Laevigatosporites ''spp.
 
The acmes of arborescent lycopod spores (lycospores) of ''Lepidodendron''/''Lepidophloios ''forests in this HST is in keeping with a time of marine transgression and the formation of major areas of peat swamp. Other taxa that show temporary acmes within the zone include ''Cingulizonates loricatus, Radiizonates difformis, Camptotriletes bucculentus ''and ''Grumosisporites varioreticulatus. ''Influxes of inertinitic kerogen may be associated with structural changes in the Coal Measures flora and substrate associated with many intermittent and short-lived flooding events. Towards the top of the ''A. lenisulcata ''zone, pale amorphous organic matter appears, together with the persistent up-hole appearance of ''Laevigatosporites ''spp.
  
=== 7.2 Mid-Langsettian (''C. communis'' zone) ===
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=== 7.2 Mid-Langsettian (C. communis zone) ===
  
In the East Midlands, the Burton Joyce Marine Band is taken as the base of the ''C. communis ''zone. Thick (up to 25 m) low-sinuosity channel sandstones occur towards the base of the zone, e.g. Grenoside Sandstone, Greenmoor Rock and Wingfield Flags, but the overall proportion of sandstone decreases upwards. The degree of downcutting and erosion above the Burton Joyce Marine Band is uncertain. A higher proportion of sandstone occurs in the zone, mostly high-sinuosity channel deposits (facies association A – channel-dominated coastal plain), in the offshore 44/22-1 ([[:File:YGS_CHR_06_ASEQ_FIG_06B.jpg|Figure 6B]]) than in the preceding ''A. lenisulcata ''zone, leading to the interpretation of ''C. communis ''zone as an LST. In 44/22-1 it is 236 m thick. Finer-grained, more argillaceous lithotypes with marine bands and coals become more common in the upper part of the zone (e.g. Kilburn Coal, Low Estheria Marine Band and Silkstone Coal). These may be associated with a diminishing rate of sea-level fall in the later stages of the LST, prior to the onset of renewed sea-level rise during the ensuing TST.
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In the East Midlands, the Burton Joyce Marine Band is taken as the base of the ''C. communis ''zone. Thick (up to 25m) low-sinuosity channel sandstones occur towards the base of the zone, e.g. Grenoside Sandstone, Greenmoor Rock and Wingfield Flags, but the overall proportion of sandstone decreases upwards. The degree of downcutting and erosion above the Burton Joyce Marine Band is uncertain. A higher proportion of sandstone occurs in the zone, mostly high-sinuosity channel deposits (facies association A – channel-dominated coastal plain), in the offshore 44/22-1 ([[:File:YGS_CHR_06_ASEQ_FIG_06B.jpg|Figure 6B]]) than in the preceding ''A. lenisulcata ''zone, leading to the interpretation of ''C. communis ''zone as an LST. In 44/22-1 it is 236m thick. Finer-grained, more argillaceous lithotypes with marine bands and coals become more common in the upper part of the zone (e.g. Kilburn Coal, Low Estheria Marine Band and Silkstone Coal). These may be associated with a diminishing rate of sea-level fall in the later stages of the LST, prior to the onset of renewed sea-level rise during the ensuing TST.
  
 
The ''C. communis ''zone is characterized particularly by persistent and commonly marked local acmes of ''Crassispora kosankei''. This spore type is often particularly associated with coarser-grained sediments of an “alluvial swamp”, rather than a lycospore dominated peatswamp depositional environment. This form of swamp deposition may have characterized the interchannel areas during this time. Other important spore occurrences within this zone include ''Vestispora costata ''and ''V. tortuosa'', as well as the local development of ''Lophotriletes ''sp. A, together with the persistence of common ''Laevigatosporites ''spp.
 
The ''C. communis ''zone is characterized particularly by persistent and commonly marked local acmes of ''Crassispora kosankei''. This spore type is often particularly associated with coarser-grained sediments of an “alluvial swamp”, rather than a lycospore dominated peatswamp depositional environment. This form of swamp deposition may have characterized the interchannel areas during this time. Other important spore occurrences within this zone include ''Vestispora costata ''and ''V. tortuosa'', as well as the local development of ''Lophotriletes ''sp. A, together with the persistence of common ''Laevigatosporites ''spp.
  
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.
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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 100m 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.
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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 70m 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
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bands have been recorded within it up to the MFS of the Vanderbeckei (Clay Cross) Marine Band.
  
 
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) ===
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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.
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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
  
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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large interval of highstand strata deposited immediately above the preceding marine band.
  
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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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
  
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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''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.
  
=== 7.5 Late Duckmantian (L. ''similis-pulchra'' zone) ===
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=== 7.5 Late Duckmantian (L. similis-pulchra zone) ===
  
The base of the Late Duckmantian is taken at the Barnsley (Top Hard) Coal, coincident with a marked seismic break that is inferred to be the initial flooding surface heralding the renewed onset of marine transgression. This zone shows a return to the dominance of argillaceous lithotypes (facies association B), and is interpreted as a TST. The zone is dominated by alternating crevasse-splay mudstone–siltstone units and coal swamp and lacustrine mudstones ([[:File:YGS_CHR_06_ASEQ_FIG_06E.jpg|Figure 6E]], [[:File:YGS_CHR_06_ASEQ_FIG_07B.jpg|Figure 7B]]). Its thickness in 44/27-1 is 258 m.
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The base of the Late Duckmantian is taken at the Barnsley (Top Hard) Coal, coincident with a marked seismic break that is inferred to be the initial flooding surface heralding the renewed onset of marine transgression. This zone shows a return to the dominance of argillaceous lithotypes (facies association B), and is interpreted as a TST. The zone is dominated by alternating crevasse-splay mudstone–siltstone units and coal swamp and lacustrine mudstones ([[:File:YGS_CHR_06_ASEQ_FIG_06E.jpg|Figure 6E]], [[:File:YGS_CHR_06_ASEQ_FIG_07B.jpg|Figure 7B]]). Its thickness in 44/27-1 is 258m.
  
 
The zone contains several coal seams in the lower part and several marine bands in its upper part. Intervals containing low-sinuosity sandstones are locally developed in places (more so in 44/22-1 than 44/27-1 – Figure 6e). Flooding surfaces near the top of the zone are indicated in the East Midlands by the Two Foot, Clowne, Haughton and Sutton marine bands. This transgressive sequence is terminated by the MFS of the Aegiranum (Mansfield) Marine Band, which marks the Westphalian B/C boundary.
 
The zone contains several coal seams in the lower part and several marine bands in its upper part. Intervals containing low-sinuosity sandstones are locally developed in places (more so in 44/22-1 than 44/27-1 – Figure 6e). Flooding surfaces near the top of the zone are indicated in the East Midlands by the Two Foot, Clowne, Haughton and Sutton marine bands. This transgressive sequence is terminated by the MFS of the Aegiranum (Mansfield) Marine Band, which marks the Westphalian B/C boundary.
  
This zone yields palynological assemblages overwhelmingly dominated by lycospores, although this cannot be considered to be diagnostic of the zone, as such spores are ubiquitous throughout the Coal Measures. However, they are accompanied by a consistent prominence of ''Densosporites ''spp. that reflects the major development of mature coal seams within this zone. The zone is also characterized by the evolutionary appearance of taxa that may be associated with large areas of persistent peat swamps; these include ''Vestispora pseudoreticulata, Punctatisporites granifer, Triquitrites ''spp., ''Perotriletes perinatus ''and ''Cristatisporites solaris.''
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This zone yields palynological assemblages overwhelmingly dominated by lycospores, although this cannot be considered to be diagnostic of the zone, as such spores are ubiquitous throughout the Coal Measures. However, they are accompanied by a consistent prominence of ''Densosporites ''spp. that reflects the major development of mature coal seams within this zone. The
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zone is also characterized by the evolutionary appearance of taxa that may be associated with large areas of persistent peat swamps; these include ''Vestispora pseudoreticulata, Punctatisporites granifer, Triquitrites ''spp., ''Perotriletes perinatus ''and ''Cristatisporites solaris.''
  
 
Certain other taxa may not have been able to survive the demise of these very extensive and persistent settled swamp environments. This may have been associated with a decline in the rate of sea-level rise, and the sedimentation rate may have begun to exceed subsidence rates. Various taxa became extinct at the top of this zone, including ''Raistrickia fulva, ''abundant ''Dictyotriletes bireticulatus ''and ''Camptotriletes bucculentus.''
 
Certain other taxa may not have been able to survive the demise of these very extensive and persistent settled swamp environments. This may have been associated with a decline in the rate of sea-level rise, and the sedimentation rate may have begun to exceed subsidence rates. Various taxa became extinct at the top of this zone, including ''Raistrickia fulva, ''abundant ''Dictyotriletes bireticulatus ''and ''Camptotriletes bucculentus.''
  
=== 7.6 Early Bolsovian (U. ''similis-pulchra'' zone) ===
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=== 7.6 Early Bolsovian (U. similis-pulchra zone) ===
  
The widely recognizable Aegiranum Marine Band marks the MFS at the base of the zone, which is interpreted as a HST. The zone is 183 m thick in well 44/27-1. It is characterized by argillaceous lithologies, crevasse-splay and lake deposits, with intervening swamp or lake units. Several coals are present in the lower half of the zone, and marine bands (Edmondia, Main Estheria and Shafton) indicative of flooding events are recognizable higher up in the sequence. Some sandstone bodies, up to 15 m in thickness (facies association B), are locally present (e.g. above the Edmondia Marine Band), although none is evident in the reference well 44/27-1 ([[:File:YGS_CHR_06_ASEQ_FIG_07C.jpg|Figure 7C]]).
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The widely recognizable Aegiranum Marine Band marks the MFS at the base of the zone, which is interpreted as a HST. The zone is 183m thick in well 44/27-1. It is characterized by argillaceous lithologies, crevasse-splay and lake deposits, with intervening swamp or lake units. Several coals are present in the lower half of the zone, and marine bands (Edmondia, Main Estheria and Shafton) indicative of flooding events are recognizable higher up in the sequence. Some sandstone bodies, up to 15m in thickness (facies association B), are locally present (e.g. above the Edmondia Marine Band), although none is evident in the reference well 44/27-1 ([[:File:YGS_CHR_06_ASEQ_FIG_07C.jpg|Figure 7C]]).
  
 
Lycospores, indicative of peatswamp sedimentation, and the alluvial swamp spore ''Crassispora kosankei'', are consistently present throughout this zone. In contrast with the extinctions associated with Vanderbeckei marine incursion, noted above, several spore taxa continue into the ''U. similis-pulchra ''zone. However, the widespread Aegiranum Marine Band must have caused a major disturbance within the basin, severely limiting the area from which swamp taxa could later repopulate the basin. One taxon that became extinct at this horizon was ''Cingulizonates loricatus. ''Spore taxa appearing within this zone include ''Vestispora fenestrata, Torispora securis, Triquitrites bransonii ''and ''Punctatisporites granifer.''
 
Lycospores, indicative of peatswamp sedimentation, and the alluvial swamp spore ''Crassispora kosankei'', are consistently present throughout this zone. In contrast with the extinctions associated with Vanderbeckei marine incursion, noted above, several spore taxa continue into the ''U. similis-pulchra ''zone. However, the widespread Aegiranum Marine Band must have caused a major disturbance within the basin, severely limiting the area from which swamp taxa could later repopulate the basin. One taxon that became extinct at this horizon was ''Cingulizonates loricatus. ''Spore taxa appearing within this zone include ''Vestispora fenestrata, Torispora securis, Triquitrites bransonii ''and ''Punctatisporites granifer.''
  
=== 7.7 Late Bolsovian (''A. phillipsii'' zone) ===
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=== 7.7 Late Bolsovian (A. phillipsii zone) ===
  
A final marine flooding event, the Cambriense Marine Band, occurs at the base of this zone (Calver 1968, Flint et al. 1995, Guion et al. 1995). After this marine band was deposited, uplift and tectonic inversion prevented any further marine incursions (Leeder & Hardman 1990). Grey coal-bearing rocks become much reduced in volume, and redbed sedimentation was more widespread. The zone is interpreted as representing an LST, and is 143 m thick in 44/27-1. It is characterized by a relatively high proportion of sandstone, as base-level fall led to a significant increase in alluvial vigour, with incision and deposition of thick sheets of laterally and vertically linked low- and high-sinuosity channel sandstones up to 30 m thick (facies association A). On shore, widespread sandstones include the Ackworth, Wickersley and Ravenfield rocks. Subordinate claystones and siltstones and several thin, laterally impersistent coal seams are developed. Several such sandstone units are developed in 44/27-1, although in this well none exceed 20 m in thickness ([[:File:YGS_CHR_06_ASEQ_FIG_07D.jpg|Figure 7D]]). Finer-grained crevasse-splay, mouth bar and lake or swamp sedimentation in interchannel areas was comparatively shortlived. Coals developed within this zone are therefore liable to be of a poorer quality with a higher ash content. Interchannel areas were also prone to erosion by the stream avulsion.
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A final marine flooding event, the Cambriense Marine Band, occurs at the base of this zone (Calver 1968, Flint et al. 1995, Guion et al. 1995). After this marine band was deposited, uplift and tectonic inversion prevented any further marine incursions (Leeder & Hardman 1990). Grey coal-bearing rocks become much reduced in volume, and redbed sedimentation was more widespread. The zone is interpreted as representing an LST, and is 143m thick in 44/27-1. It is characterized by a relatively high proportion of sandstone, as base-level fall led to a significant increase in alluvial vigour, with incision and deposition of thick sheets of laterally and vertically linked low- and high-sinuosity channel sandstones up to 30m thick (facies association A). On shore, widespread sandstones include the Ackworth, Wickersley and Ravenfield rocks. Subordinate claystones and siltstones and several thin, laterally impersistent coal seams are developed. Several such sandstone units are developed in 44/27-1, although in this well none exceed 20m in thickness ([[:File:YGS_CHR_06_ASEQ_FIG_07D.jpg|Figure 7D]]). Finer-grained crevasse-splay, mouth bar and lake or swamp sedimentation in interchannel areas was comparatively shortlived. Coals developed within this zone are therefore liable to be of a poorer quality with a higher ash content. Interchannel areas were also prone to erosion by the stream avulsion.
  
 
The alluvial swamp spore ''Crassispora kosankei ''reaches its acme within this zone, reflecting a final period of major alluvial swamp deposition prior to the Late Westphalian climatic change. Lowstand sedimentation, with its reduced accommodation space had a profound effect on the palynofloras. ''Densosporites sphaerotriangularis, Vestispora costata, V. pseudoreticulata, Savitrisporites nux, Dictyotriletes falsus ''and ''Grumosisporites varioreticulatus, ''seen since the Early Westphalian, became extinct within this zone. New taxa appearing within the zone include ''Schopfites dimorphus ''and ''Vestispora laevigata ''(Van Adrichem Boorgaert & Kouwe 1993). Palynological assemblages from this zone commonly contain reworked Early Westphalian or Namurian taxa.
 
The alluvial swamp spore ''Crassispora kosankei ''reaches its acme within this zone, reflecting a final period of major alluvial swamp deposition prior to the Late Westphalian climatic change. Lowstand sedimentation, with its reduced accommodation space had a profound effect on the palynofloras. ''Densosporites sphaerotriangularis, Vestispora costata, V. pseudoreticulata, Savitrisporites nux, Dictyotriletes falsus ''and ''Grumosisporites varioreticulatus, ''seen since the Early Westphalian, became extinct within this zone. New taxa appearing within the zone include ''Schopfites dimorphus ''and ''Vestispora laevigata ''(Van Adrichem Boorgaert & Kouwe 1993). Palynological assemblages from this zone commonly contain reworked Early Westphalian or Namurian taxa.
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=== 7.8 Westphalian D (A. tenuis and A. prolifera zones) ===
 
=== 7.8 Westphalian D (A. tenuis and A. prolifera zones) ===
  
Westphalian D strata are not present in many southern North Sea wells. Where rocks of this age have been penetrated, they tend to be characterized by finer-grained lithologies indicative of an increase in accommodation space. However, this is not always the case and, in some wells, thick low-sinuosity channel sandstones are developed. Well 44/27-1 ([[:File:YGS_CHR_06_ASEQ_FIG_07E.jpg|Figure 7E]]) has several of these sandstone units within an interval of over 250 m of inferred Westphalian D age.
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Westphalian D strata are not present in many southern North Sea wells. Where rocks of this age have been penetrated, they tend to be characterized by finer-grained lithologies indicative of an increase in accommodation space. However, this is not always the case and, in some wells, thick low-sinuosity channel sandstones are developed. Well 44/27-1 ([[:File:YGS_CHR_06_ASEQ_FIG_07E.jpg|Figure 7E]]) has several of these sandstone units within an interval of over 250m of inferred Westphalian D age.
  
 
Tectonic changes associated with the advancing Variscan Front may have been the principal control on sedimentation in the southern North Sea Westphalian D, lifting the swamps above the level that permitted further marine ingress (Leeder & Hardman 1990; [[:File:YGS_CHR_06_ASEQ_FIG_05.jpg|Figure 5]]). Thick sandstones are likely to be a response to localized tectonic uplift outside the basin rather than to sea-level change. In addition, coals are rare in the ''A. tenuis ''zone in the southern North Sea, although again this is not a universal feature of the Westphalian D elsewhere in northwest Europe (e.g. the Newcastle and Halesowen formations of Staffordshire). The Westphalian D rocks of the southern North Sea tend to be reddened, reflecting a climatic change, associated with the formation of Pangaea, that lead to increasing aridity and an overall oxidizing influence. Associated with this environmental change was the evolutionary appearance of several new palynomorph taxa such as ''Thymospora obscura, T. pseudothiesseni, Cadiospora magna ''and an increase in the occurrence of various striate bisaccate taxa that are the dominant palynoflora of the Permian.
 
Tectonic changes associated with the advancing Variscan Front may have been the principal control on sedimentation in the southern North Sea Westphalian D, lifting the swamps above the level that permitted further marine ingress (Leeder & Hardman 1990; [[:File:YGS_CHR_06_ASEQ_FIG_05.jpg|Figure 5]]). Thick sandstones are likely to be a response to localized tectonic uplift outside the basin rather than to sea-level change. In addition, coals are rare in the ''A. tenuis ''zone in the southern North Sea, although again this is not a universal feature of the Westphalian D elsewhere in northwest Europe (e.g. the Newcastle and Halesowen formations of Staffordshire). The Westphalian D rocks of the southern North Sea tend to be reddened, reflecting a climatic change, associated with the formation of Pangaea, that lead to increasing aridity and an overall oxidizing influence. Associated with this environmental change was the evolutionary appearance of several new palynomorph taxa such as ''Thymospora obscura, T. pseudothiesseni, Cadiospora magna ''and an increase in the occurrence of various striate bisaccate taxa that are the dominant palynoflora of the Permian.
  
The ''A. tenuis ''zone of the southern North Sea is tentatively interpreted as representing a TST in the lower part (4306–4225 m in [[:File:YGS_CHR_06_ASEQ_FIG_07E.jpg|Figure 7E]]), with possible LST and HSTs developed subsequently. An LST may be present in 44/27-1 from 4225–4150 m over an interval of prominent low-sinuosity channel sandstones. The succeeding strata, characterized by alternating crevasse-splay and thin low-sinuosity channel sandstones, up to the Saalian unconformity at 4038 m, are interpreted as an HST. As the lateral extent of some of the Westphalian D sandstone bodies becomes better known, a more refined third-order subdivision of the substage into transgressive, highstand and lowstand systems tracts may be possible. Evidence of fourth-order sea-level fluctuation may be suggested in the redbeds by the interdigitation of, on the one hand, originally grey siderite-bearing strata laid down under poor drainage conditions and, on the other, palaesols indicative of good drainage (Besly et al. 1993). In the view of these latter authors, these alternations are attributable to tectonic causes.
+
The ''A. tenuis ''zone of the southern North Sea is tentatively interpreted as representing a TST in the lower part (4306–4225 m in [[:File:YGS_CHR_06_ASEQ_FIG_07E.jpg|Figure 7E]]), with possible LST and HSTs developed subsequently. An LST may be present in 44/27-1 from 4225–4150m over an interval of prominent low-sinuosity channel sandstones. The succeeding strata, characterized by alternating crevasse-splay and thin low-sinuosity channel sandstones, up to the Saalian unconformity at 4038m, are interpreted as an HST. As the lateral extent of some of the Westphalian D sandstone bodies becomes better known, a more refined third-order subdivision of the substage into transgressive, highstand and lowstand systems tracts may be possible. Evidence of fourth-order sea-level fluctuation may be suggested in the redbeds by the interdigitation of, on the one hand, originally grey siderite-bearing strata laid down under poor drainage conditions and, on the other, palaesols indicative of good drainage (Besly et al. 1993). In the view of these latter authors, these alternations are attributable to tectonic causes.
  
 
Strata of the ''A. prolifera ''zone are too poorly preserved or too little known at present off shore to allow the elucidation of sequences. However, considerable thicknesses of strata of this age may be present off shore, possibly containing several third-order systems tracts.
 
Strata of the ''A. prolifera ''zone are too poorly preserved or too little known at present off shore to allow the elucidation of sequences. However, considerable thicknesses of strata of this age may be present off shore, possibly containing several third-order systems tracts.
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Subdivision of the offshore Coal Measures section using the onshore macrofaunal biozone scheme of marine-band faunas will always be partly subjective, when only cuttings samples are available and palynology is the only available biostratigraphical tool. Sequence stratigraphical subdivision off shore is also partly subjective and will be subject to changes and refinements in future studies. Nevertheless, the basinwide macrofaunal biozones provide a good starting point for recognizing systems tracts, as they are generally lithologically discrete. Offshore wells, particularly those penetrating a significant interval thickness and furnished with a good suite of wireline logs, permit large parts of the Coal Measures to be examined in terms of broad lithofacies trends at a scale often not available in the onshore section.
 
Subdivision of the offshore Coal Measures section using the onshore macrofaunal biozone scheme of marine-band faunas will always be partly subjective, when only cuttings samples are available and palynology is the only available biostratigraphical tool. Sequence stratigraphical subdivision off shore is also partly subjective and will be subject to changes and refinements in future studies. Nevertheless, the basinwide macrofaunal biozones provide a good starting point for recognizing systems tracts, as they are generally lithologically discrete. Offshore wells, particularly those penetrating a significant interval thickness and furnished with a good suite of wireline logs, permit large parts of the Coal Measures to be examined in terms of broad lithofacies trends at a scale often not available in the onshore section.
  
Sequence analysis is best achieved where there are good biostratigraphical data and well dated neighbouring offset wells. A combination of terrestrial plant miospore zonal markers and palynofacies influxes can provide reasonably refined dating of the Coal Measures section and give good pointers towards the probable correlation of a particular gamma-ray spike with a named marine band. If a full suite of spectral gamma-ray logs is available, selection of one particular gamma-ray spike may be enhanced using the method of Hollywood & Whorlow (1993). This has been successfully demonstrated by the authors on unpublished wells with spectral gamma-ray logs available in digital format, doing the necessary arithmetical manipulation of a large log file with a digital spreadsheet. Expected interval thickness and wireline-inferred sedimentary facies of particular parts of the section are key parts of the analysis. As a very generalized rule, transgressive deposits are characterized by abundant coals and marine bands. Highstand deposits (if preserved un-eroded by the succeeding lowstand deposits) have common marine bands. Lowstand sediments are characterized by thick, regional, stacked, alluvial channel sandstones (i.e. potential reservoirs).
+
Sequence analysis is best achieved where there are good biostratigraphical data and well dated neighbouring offset wells. A combination of terrestrial plant miospore zonal markers and palynofacies influxes can provide reasonably refined dating of the Coal Measures section and give good pointers towards the probable correlation of a particular gamma-ray spike with a named marine band. If a full suite of spectral gamma-ray logs is available, selection of one particular gamma-ray spike may be enhanced using the method of Hollywood & Whorlow (1993). This has been successfully demonstrated by the authors on unpublished wells with spectral gamma-ray logs available in digital format, doing the necessary arithmetical manipulation of a large log file with a digital spreadsheet. Expected interval thickness and wireline-inferred sedimentary facies of particular parts of the section are key parts of the analysis. As a very generalized rule, transgressive deposits are characterized by abundant coals and marine bands. Highstand deposits (if preserved
 +
 
 +
un-eroded by the succeeding lowstand deposits) have common marine bands. Lowstand sediments are characterized by thick, regional, stacked, alluvial channel sandstones (i.e. potential reservoirs).
  
 
Released wells from The Netherlands and UK sectors permit the study of lateral changes within systems tracts of the Coal Measures over a significant part of the southern North Sea. A north–south transect of the western side of the southern North Sea ([[:File:YGS_CHR_06_ASEQ_FIG_01.jpg|Figure 1]], [[:File:YGS_CHR_06_ASEQ_FIG_08.jpg|Figure 8]], [[:File:YGS_CHR_06_ASEQ_FIG_09.jpg|Figure 9]]) based on the correlation of eight wells selected from UK quadrants 44, 49 and 53, and Dutch quadrants K and P, has been compiled. The correlations demonstrate a significant degree of continuity of interval thicknesses and show lateral facies changes within individual systems tracts.
 
Released wells from The Netherlands and UK sectors permit the study of lateral changes within systems tracts of the Coal Measures over a significant part of the southern North Sea. A north–south transect of the western side of the southern North Sea ([[:File:YGS_CHR_06_ASEQ_FIG_01.jpg|Figure 1]], [[:File:YGS_CHR_06_ASEQ_FIG_08.jpg|Figure 8]], [[:File:YGS_CHR_06_ASEQ_FIG_09.jpg|Figure 9]]) based on the correlation of eight wells selected from UK quadrants 44, 49 and 53, and Dutch quadrants K and P, has been compiled. The correlations demonstrate a significant degree of continuity of interval thicknesses and show lateral facies changes within individual systems tracts.
Line 254: Line 264:
 
There is a generalized north to south decrease in sandstone, e.g. the thick sandstones within the ''A. lenisulcata ''and ''C. communis ''zone recognizable in 44/22-1 and 44/23-4, and in the ''C. communis ''zone in K1-02, are hardly evident in well P/10-1. The sandstone-dominated interval of the upper ''A. modiolaris ''zone HST–LST (Caister–Murdoch sandstone play of Quadrant 44) diminishes in prominence farther south into quadrants 49 (49/1-3) and 53 (53/10-1). This southward reduction in Early Duckmantian sandstone development continues into The Netherlands quadrant P (P10-1 and P13-1). Associated with it, the “coal-break” upper boundary of this unit with the overlying ''L. similis-pulchra ''zone is clear in quadrants 44, 49 and K, but much less so in quadrants 53 and P.
 
There is a generalized north to south decrease in sandstone, e.g. the thick sandstones within the ''A. lenisulcata ''and ''C. communis ''zone recognizable in 44/22-1 and 44/23-4, and in the ''C. communis ''zone in K1-02, are hardly evident in well P/10-1. The sandstone-dominated interval of the upper ''A. modiolaris ''zone HST–LST (Caister–Murdoch sandstone play of Quadrant 44) diminishes in prominence farther south into quadrants 49 (49/1-3) and 53 (53/10-1). This southward reduction in Early Duckmantian sandstone development continues into The Netherlands quadrant P (P10-1 and P13-1). Associated with it, the “coal-break” upper boundary of this unit with the overlying ''L. similis-pulchra ''zone is clear in quadrants 44, 49 and K, but much less so in quadrants 53 and P.
  
Converse to this reduction in net sandstone development, there is an increase in overall thickness of the mudstone-dominated ''L. similis-pulchra ''zone TST southwards, from Quadrant 44 through 49 and into Quadrant P. The base Permian unconformity truncates this interval in K1-02, P10-1 and P13-1, but P13-1 still has a greater thickness of this interval than 44/27-1. This trend may reflect a southward decrease in the development of alluvial channels within this TST that may have significantly reduced this interval thickness in quadrants 44 and 49. The upper Bolsovian, ''A. phillipsii ''zone, LST sandstone unit is clearly developed above a prominent ''c. ''20 m basal sandstone body in quadrants 44 and 49. Farther south, however, these strata have been eroded away at the base Permian unconformity.
+
Converse to this reduction in net sandstone development, there is an increase in overall thickness of the mudstonedominated ''L. similis-pulchra ''zone TST southwards, from Quadrant 44 through 49 and into Quadrant P. The base Permian unconformity truncates this interval in K1-02, P10-1 and P13-1, but P13-1 still has a greater thickness of this interval than 44/27-1. This trend may reflect a southward decrease in the development of alluvial channels within this TST that may have significantly reduced this interval thickness in quadrants 44 and 49. The upper Bolsovian, ''A. phillipsii ''zone, LST sandstone unit is clearly developed above a prominent ''c. ''20m basal sandstone body in quadrants 44 and 49. Farther south, however, these strata have been eroded away at the base Permian unconformity.
  
 
These trends in the sandstone development of the LSTs reflect increasing distance from the northerly sediment source. During periods of HST and TST, with more numerous marine incursions, topographically lower regions in the south of the basin would be flooded first and to greater water depths. There would have been a general northward shallowing in depth of flooding, together with a reduction in salinity towards some areas of peatswamp coal deposition that avoided flooding. Between flooding events, areas of forested peat swamps would be coeval with more continuously waterlogged areas farther south and more areas occupied by lakes.
 
These trends in the sandstone development of the LSTs reflect increasing distance from the northerly sediment source. During periods of HST and TST, with more numerous marine incursions, topographically lower regions in the south of the basin would be flooded first and to greater water depths. There would have been a general northward shallowing in depth of flooding, together with a reduction in salinity towards some areas of peatswamp coal deposition that avoided flooding. Between flooding events, areas of forested peat swamps would be coeval with more continuously waterlogged areas farther south and more areas occupied by lakes.
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Besly, B. M. 1990. Carboniferous. In ''Introduction to the petroleum geology of the North Sea'', K. W. Glennie (ed.), 90–119. London: Geological Society.
 
Besly, B. M. 1990. Carboniferous. In ''Introduction to the petroleum geology of the North Sea'', K. W. Glennie (ed.), 90–119. London: Geological Society.
  
Besly, B. M., S. D. Burley, P. Turner 1993. The late Carboniferous “barren redbed” play of the Silver Pit area, southern North Sea. In ''Petroleum geology of northwest Europe: proceedings of the fourth conference'', J. R. Parker (ed.), 727–40. London: Geological Society.  
+
Besly, B. M., S. D. Burley, P. Turner 1993. The late Carboniferous “barren redbed” play of the Silver Pit area, southern North Sea. In ''Petroleum geology of northwest Europe: proceedings of the fourth conference'', J. R. Parker (ed.), 727–40. London: Geological Society. Butzer, K. W. 1971. ''Recent history of an Ethiopian delta: the Omb River and the level of Lake Rudolf''. Research Paper 136, Department of Geography, University of Chicago.
 
 
Butzer, K. W. 1971. ''Recent history of an Ethiopian delta: the Omb River and the level of Lake Rudolf''. Research Paper 136, Department of Geography, University of Chicago.
 
  
 
Calver, M. A. 1968. Distribution of Westphalian marine faunas in northern England and adjoining areas. ''Proceedings of the Yorkshire Geological Society ''37, 1–72.
 
Calver, M. A. 1968. Distribution of Westphalian marine faunas in northern England and adjoining areas. ''Proceedings of the Yorkshire Geological Society ''37, 1–72.
Line 306: Line 314:
 
Collinson, J. D., C. M. Jones, G. A. Blackbourn, B. M. Besly, G. M. Archard, A. H. McMahon 1993. Carboniferous depositional systems of the southern North Sea. In ''Petroleum geology of northwest Europe: proceedings of the fourth conference'', J. R. Parker (ed.), 677–87. London: Geological Society.
 
Collinson, J. D., C. M. Jones, G. A. Blackbourn, B. M. Besly, G. M. Archard, A. H. McMahon 1993. Carboniferous depositional systems of the southern North Sea. In ''Petroleum geology of northwest Europe: proceedings of the fourth conference'', J. R. Parker (ed.), 677–87. London: Geological Society.
  
Davies, J. H. & A. E. Trueman 1927. A revision of the non-marine lamellibranchs of the Coal Measures and their zonal sequence. ''Geological Society of London, Quarterly Journal ''83, 210–57.  
+
Davies, J. H. & A. E. Trueman 1927. A revision of the non-marine lamellibranchs of the Coal Measures and their zonal sequence. ''Geological Society of London, Quarterly Journal ''83, 210–57. Davies, S. J. & D. McLean 1996. Spectral gamma-ray and palynological characterization of Kinderscoutian marine bands in the Namurian of the Pennine Basin. ''Yorkshire Geological Society, Proceedings ''51, 103–114.
 
 
Davies, S. J. & D. McLean 1996. Spectral gamma-ray and palynological characterization of Kinderscoutian marine bands in the Namurian of the Pennine Basin. ''Yorkshire Geological Society, Proceedings ''51, 103–114.
 
  
Eagar, R. M. C. 1994. Discussion of “Classification of Carboniferous non-marine bivalves: systematics versus stratigraphy” by G. M. Vasey. ''Geological Society of London, Journal ''151, 1030–1033.  
+
Eagar, R. M. C. 1994. Discussion of “Classification of Carboniferous non-marine bivalves: systematics versus stratigraphy” by G. M. Vasey. ''Geological Society of London, Journal ''151, 1030–1033. Elliot, T. 1993. High resolution sequence stratigraphy of clastic basin-fill successions. ''Petroleum Exploration Society of Great Britain, Newsletter'', June 1993, 4.
 
 
Elliot, T. 1993. High resolution sequence stratigraphy of clastic basin-fill successions. ''Petroleum Exploration Society of Great Britain, Newsletter'', June 1993, 4.
 
  
 
Fielding, C. R. 1984. Upper delta plain lacustrine and fluviolacustrine facies from the Westphalian of the Durham coalfield, northeast England. ''Sedimentology ''31, 547–67.
 
Fielding, C. R. 1984. Upper delta plain lacustrine and fluviolacustrine facies from the Westphalian of the Durham coalfield, northeast England. ''Sedimentology ''31, 547–67.
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Guion, P. D. 1984. Crevasse-splay deposits and roof-rock quality in the Threequarters seam (Carboniferous) in the East Midlands coalfield, UK. In ''Sedimentology of coal and coal-bearing sequences'', R. A. Rahmani & R. M. Flores (eds), 291–308. Oxford: Blackwell Scientific.
 
Guion, P. D. 1984. Crevasse-splay deposits and roof-rock quality in the Threequarters seam (Carboniferous) in the East Midlands coalfield, UK. In ''Sedimentology of coal and coal-bearing sequences'', R. A. Rahmani & R. M. Flores (eds), 291–308. Oxford: Blackwell Scientific.
  
Guion, P. D., I. M. Fulton, N. S. Jones 1995. Sedimentary facies of the coal-bearing Westphalian A and B north of the Wales–Brabant high. In ''European coal geology'', M. K. G. Whateley & D. A. Spears (eds), 45–78. Special Publication 82, Geological Society, London.  
+
Guion, P. D., I. M. Fulton, N. S. Jones 1995. Sedimentary facies of the coal-bearing Westphalian A and B north of the Wales–Brabant high. In ''European coal geology'', M. K. G. Whateley & D. A. Spears (eds), 45–78. Special Publication 82, Geological Society, London. Hallsworth, C. R. & J. I. Chisholm 2000. Stratigraphic evolution of provenance characteristics in Westphalian sandstones of the Yorkshire coalfield. ''Yorkshire Geological Society, Proceedings ''53, 43–72. Hampson, G. 1995. Discrimination of regionally extensive coals in the Upper Carboniferous of the Pennine Basin, UK using high resolution sequence stratigraphic concepts. In ''European coal geology'', M. K. G. Whateley & D. A. Spears (eds), 79–97. Special Publication 82, Geological Society, London.
 
 
Hallsworth, C. R. & J. I. Chisholm 2000. Stratigraphic evolution of provenance characteristics in Westphalian sandstones of the Yorkshire coalfield. ''Yorkshire Geological Society, Proceedings ''53, 43–72.  
 
 
 
Hampson, G. 1995. Discrimination of regionally extensive coals in the Upper Carboniferous of the Pennine Basin, UK using high resolution sequence stratigraphic concepts. In ''European coal geology'', M. K. G. Whateley & D. A. Spears (eds), 79–97. Special Publication 82, Geological Society, London.
 
  
 
Harland, W. B., A. V. Cox, C. Llewellyn, A. G. Pickton, A. G. Smith, R. Walters 1982. ''A geological timescale''. Cambridge: Cambridge University Press.
 
Harland, W. B., A. V. Cox, C. Llewellyn, A. G. Pickton, A. G. Smith, R. Walters 1982. ''A geological timescale''. Cambridge: Cambridge University Press.
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Read, W. A. & I. H. Forsyth 1989. Allocycles and autocycles in the upper part of the Limestone Coal Group (Pendleian E1) in the Glasgow–Stirling region of the Midland Valley of Scotland. ''Geological Journal ''24, 121–37.
 
Read, W. A. & I. H. Forsyth 1989. Allocycles and autocycles in the upper part of the Limestone Coal Group (Pendleian E1) in the Glasgow–Stirling region of the Midland Valley of Scotland. ''Geological Journal ''24, 121–37.
  
Rippon, J. H. 1984. The Clowne seam, marine band, and overlying sediments in the Coal Measures (Westphalian B) of north Derbyshire. ''Yorkshire Geological Society, Proceedings ''45, 27–43.  
+
Rippon, J. H. 1984. The Clowne seam, marine band, and overlying sediments in the Coal Measures (Westphalian B) of north Derbyshire. ''Yorkshire Geological Society, Proceedings ''45, 27–43. Ritchie, J. S. & P. Pratsides 1993. The Caister fields, Block 44/23a, UK North Sea. In ''Petroleum geology of northwest Europe: proceedings of the fourth conference'', J. R. Parker (ed.), 759–69. London: Geological Society.
 
 
Ritchie, J. S. & P. Pratsides 1993. The Caister fields, Block 44/23a, UK North Sea. In ''Petroleum geology of northwest Europe: proceedings of the fourth conference'', J. R. Parker (ed.), 759–69. London: Geological Society.
 
  
 
Ritchie, J. S., D. Pilling, S. Hayes 1998. Reservoir development, sequence stratigraphy and geological modelling of Westphalian fluvial reservoirs of the Caister C field, UK southern North Sea. ''Petroleum Geoscience ''4, 203–211.
 
Ritchie, J. S., D. Pilling, S. Hayes 1998. Reservoir development, sequence stratigraphy and geological modelling of Westphalian fluvial reservoirs of the Caister C field, UK southern North Sea. ''Petroleum Geoscience ''4, 203–211.
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Scott, A. C. 1977. A review of the ecology of Upper Carboniferous plant assemblages, with data from Strathclyde. ''Palaeontology ''20, 447–73.
 
Scott, A. C. 1977. A review of the ecology of Upper Carboniferous plant assemblages, with data from Strathclyde. ''Palaeontology ''20, 447–73.
  
Scott, A. C. 1979. The ecology of Coal Measures floras from northern Britain. ''Proceedings of the Geologists’ Association ''90, 97–116.
+
Ross, C. A. 1979. The ecology of Coal Measures floras from northern Britain. ''Proceedings of the Geologists’ Association ''90, 97–116.
  
 
Smith, A. H. V. & M. A. Butterworth 1967. ''Miospores in the coal seams of the Carboniferous of Great Britain''. Special Papers in Palaeontology 1, Palaeontological Association, London.
 
Smith, A. H. V. & M. A. Butterworth 1967. ''Miospores in the coal seams of the Carboniferous of Great Britain''. Special Papers in Palaeontology 1, Palaeontological Association, London.
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==== Description ====
 
==== Description ====
  
Low-sinuosity channel deposits consist mainly of sandstones, locally up to 50–60 m thick, with generally minor fine-grained partings and intraformational conglomerates. These sandstones are typically clean, fine to very coarse-grained and range from well sorted to poorly sorted as grain size increases. Grain size may vary widely within sandstone bodies, and overall upward-fining profiles may be apparent. In places, especially in more distal coastal plain areas, low-sinuosity channel sandstones are entirely fine to very fine grained and well sorted. The base of low-sinuosity channel sandstones is typically erosive and the top sharp. Internally, cross bedding is commonly well developed, with asymptotic sets up to ''c. ''1 m thick forming erosively based cosets 2–5 m thick. Larger planar crossbed sets and downcurrent descending compound cosets (cf. Haszeldine 1983) also occur locally.
+
Low-sinuosity channel deposits consist mainly of sandstones, locally up to 50–60m thick, with generally minor fine-grained partings and intraformational conglomerates. These sandstones are typically clean, fine to very coarse-grained and range from well sorted to poorly sorted as grain size increases. Grain size may vary widely within sandstone bodies, and overall upward-fining profiles may be apparent. In places, especially in more distal coastal plain areas, low-sinuosity channel sandstones are entirely fine to very fine grained and well sorted. The base of low-sinuosity channel sandstones is typically erosive and the top sharp. Internally, cross bedding is commonly well developed, with asymptotic sets up to ''c. ''1m thick forming erosively based cosets 2–5m thick. Larger planar crossbed sets and downcurrent descending compound cosets (cf. Haszeldine 1983) also occur locally.
  
 
==== Interpretation ====
 
==== Interpretation ====
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==== Geometry ====
 
==== Geometry ====
  
Mapping in onshore areas with good well control indicates that facies Cl is typically 20–30 m thick, but locally may reach 50–60 m or more. Lateral continuity is good, with channel sandstones extending for several kilometres perpendicular to palaeoflow and at least several tens of kilometres parallel to palaeoflow. Localized zones of stacked low-sinuosity channel sandstones have resulted from contemporaneous tectonism in places (e.g. Haszeldine 1983, Fielding 1984, Leeder & Hardman 1990, Ritchie & Pratsides 1993), and have major implications for reservoir geometry and prospectivity.
+
Mapping in onshore areas with good well control indicates that facies Cl is typically 20–30m thick, but locally may reach 50– 60m or more. Lateral continuity is good, with channel sandstones extending for several kilometres perpendicular to palaeoflow and at least several tens of kilometres parallel to palaeoflow. Localized zones of stacked low-sinuosity channel sandstones have resulted from contemporaneous tectonism in places (e.g. Haszeldine 1983, Fielding 1984, Leeder & Hardman 1990, Ritchie & Pratsides 1993), and have major implications for reservoir geometry and prospectivity.
  
 
==== Wireline log characteristics ====
 
==== Wireline log characteristics ====
  
Facies Cl is characterized by blocky and consistent gamma-ray and sonic-log responses, with pronounced basal and upper inflexions. Gamma response is typically 30–60&nbsp;API and sonic response 70–80&nbsp;µsft<sup>–1</sup>. Localized fluctuations in log response are attributable mainly to the presence of finer-grained strata towards the top of major depositional units.
+
Facies Cl is characterized by blocky and consistent gamma-ray and sonic-log responses, with pronounced basal and upper inflexions. Gamma response is typically 30–60API and sonic response 70–80µsft–1. Localized fluctuations in log response are attributable mainly to the presence of finer-grained strata towards the top of major depositional units.
  
 
=== Facies Ch – High-sinuosity channel deposits ===
 
=== Facies Ch – High-sinuosity channel deposits ===
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==== Geometry ====
 
==== Geometry ====
  
High-sinuosity channel deposits have been mapped in detail on shore in the UK by us at and around Waverly East opencast site, and previously by BP (Eagle Sandstone reservoir) in the Beckingham oilfield. These studies indicate a maximum thickness of 20–25&nbsp;m and lateral continuity for up to 1&nbsp;km perpendicular to palaeoflow and for several kilometres or tens of kilometres parallel to palaeoflow. Although produced by high-sinuosity channel activity, the deposits preserved consist of many point bar units and commonly form relatively straight channel belts.
+
High-sinuosity channel deposits have been mapped in detail on shore in the UK by us at and around Waverly East opencast site, and previously by BP (Eagle Sandstone reservoir) in the Beckingham oilfield. These studies indicate a maximum thickness of 20–25m and lateral continuity for up to 1km perpendicular to palaeoflow and for several kilometres or tens of kilometres parallel to palaeoflow. Although produced by high-sinuosity channel activity, the deposits preserved consist of many point bar units and commonly form relatively straight channel belts.
  
 
==== Wireline log characteristics ====
 
==== Wireline log characteristics ====
  
Facies Ch is invariably characterized by sharp basal gamma-ray and sonic-log inflexions. However, the upper contact may be either sharp or more gradational, depending upon the lithologies developed. The gamma log response is typically in the range 60–75&nbsp;API reflecting the argillaceous nature of the sandstones. It is typically accompanied by a sonic log response of 80–90&nbsp;µsft<sup>–1</sup>. The main variations in wireline log profiles occur in the upper part of sandstone units where gamma log response and δ''T ''values may increase markedly because of the presence of finer-grained units. δ''T ''values tend to decrease sharply at well cemented or concretionary beds.
+
Facies Ch is invariably characterized by sharp basal gamma-ray and sonic-log inflexions. However, the upper contact may be either sharp or more gradational, depending upon the lithologies developed. The gamma log response is typically in the range 60– 75API reflecting the argillaceous nature of the sandstones. It is typically accompanied by a sonic log response of 80–90µsft<sup>–1</sup>. The main variations in wireline log profiles occur in the upper part of sandstone units where gamma log response and δ''T ''values may increase markedly because of the presence of finer-grained units. δ''T ''values tend to decrease sharply at well cemented or concretionary beds.
  
 
=== Facies Cd – Distributary mouth bar ===
 
=== Facies Cd – Distributary mouth bar ===
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==== Description ====
 
==== Description ====
  
Distributary mouth-bar deposits coarsen upwards overall and comprise an upper sandstone unit overlying a lower mudstone– siltstone unit. The lower mudstone–siltstone unit is typically 10– 15&nbsp;m thick and may contain either a marine or non-marine fauna. Overlying sandstones are well sorted and fairly clean, and they generally increase in grain size upwards from very fine to fine; however, they may locally be coarse and may fine upwards. Mudstones and siltstones are mostly parallel laminated or locally homogenized by bioturbation, particularly at marine horizons. The sandstones tend to be ripple cross laminated near the base and trough cross bedded at higher levels. Distributary mouth-bar units are commonly underlain and overlain by coal seams or rooty horizons.
+
Distributary mouth-bar deposits coarsen upwards overall and comprise an upper sandstone unit overlying a lower mudstone– siltstone unit. The lower mudstone–siltstone unit is typically 10– 15m thick and may contain either a marine or non-marine fauna. Overlying sandstones are well sorted and fairly clean, and they generally increase in grain size upwards from very fine to fine; however, they may locally be coarse and may fine upwards. Mudstones and siltstones are mostly parallel laminated or locally homogenized by bioturbation, particularly at marine horizons. The sandstones tend to be ripple cross laminated near the base and trough cross bedded at higher levels. Distributary mouth-bar units are commonly underlain and overlain by coal seams or rooty horizons.
  
 
==== Interpretation ====
 
==== Interpretation ====
Line 468: Line 466:
 
==== Geometry ====
 
==== Geometry ====
  
Mapping suggests that distributary mouth-bar deposits have a lobate geometry and diameter of ''c. ''10–15&nbsp;km. Thicker, Namurian shallow-water delta-front deposits may, however, be of greater lateral extent.
+
Mapping suggests that distributary mouth-bar deposits have a lobate geometry and diameter of ''c. ''10–15km. Thicker, Namurian shallow-water delta-front deposits may, however, be of greater lateral extent.
  
====Wireline log characteristics====
+
Wireline log characteristics
  
The gamma-log response of basal mudstones and siltstones is in the range 125–>150&nbsp;API and they have a sonic-log response of 90–100&nbsp;µsft<sup>–1</sup>. The gamma log response gradually decreases upwards to about 35–40&nbsp;API in clean distributary mouth-bar sandstones and to 60–70&nbsp;API in more argillaceous deposits. A corresponding upward decrease in δ''T ''values to an average of 80–90&nbsp;µsft<sup>–1</sup> is also observed. The gradational upward increase in gamma-log response from the basal mudstones–siltstones into sandstone and sharp upper inflexion are characteristic of distributary mouth-bar deposits.
+
The gamma-log response of basal mudstones and siltstones is in the range 125–>150API and they have a sonic-log response of 90–100µsft<sup>–1</sup>. The gamma log response gradually decreases upwards to about 35–40API in clean distributary mouth-bar sandstones and to 60–70API in more argillaceous deposits. A corresponding upward decrease in δ''T ''values to an average of 80–90µsft<sup>–1</sup> is also observed. The gradational upward increase in gamma-log response from the basal mudstones–siltstones into sandstone and sharp upper inflexion are characteristic of distributary mouth-bar deposits.
  
 
=== Facies Oc – Crevasse splay ===
 
=== Facies Oc – Crevasse splay ===
Line 478: Line 476:
 
==== Description ====
 
==== Description ====
  
Crevasse-splay deposits are widely developed in the Upper Carboniferous, and almost invariably occur overlying lake or marine-bay deposits (facies Ol) and underlying swamp or flood-plain deposits (facies Os). The predominant lithology is very fine to fine-grained well sorted argillaceous sandstone, typically 2–5&nbsp;m thick, which is closely associated with interbedded siltstones. Sandstones typically have a gradational base and coarsen upwards from parallel laminated mudstones and siltstones; however, they may occasionally be erosively based and fine upwards. Sedimentary structures include convolute and ripple cross lamination, discontinuous parallel lamination and small-scale trough cross bedding. Rootlets are invariably developed in the upper parts of crevasse-splay sequences.
+
Crevasse-splay deposits are widely developed in the Upper Carboniferous, and almost invariably occur overlying lake or marine-bay deposits (facies Ol) and underlying swamp or flood-plain deposits (facies Os). The predominant lithology is very fine to fine-grained well sorted argillaceous sandstone, typically 2–5m thick, which is closely associated with interbedded silt-stones. Sandstones typically have a gradational base and coarsen upwards from parallel laminated mudstones and siltstones; however, they may occasionally be erosively based and fine upwards. Sedimentary structures include convolute and ripple cross lamination, discontinuous parallel lamination and small-scale trough cross bedding. Rootlets are invariably developed in the upper parts of crevasse-splay sequences.
  
 
==== Interpretation ====
 
==== Interpretation ====
Line 486: Line 484:
 
==== Geometry ====
 
==== Geometry ====
  
Mapping of crevasse-splay deposits during this study and previous published studies (Fielding 1984, Guion 1984, Haszeldine 1984) indicates a lobate geometry with major lobes averaging a few metres in thickness and about 5&nbsp;km across. Within these major lobes, individual lobes measuring a few hundred metres across are often discernible in large-scale outcrops (cf. Fielding 1984). Crevasse-splay feeder channels are typically up to 5–6&nbsp;m thick, several hundred metres across, perpendicular to palaeoflow and several kilometres long parallel to palaeoflow.
+
Mapping of crevasse-splay deposits during this study and previous published studies (Fielding 1984, Guion 1984, Haszeldine 1984) indicates a lobate geometry with major lobes averaging a few metres in thickness and about 5km across. Within these major lobes, individual lobes measuring a few hundred metres across are often discernible in large-scale outcrops (cf. Fielding 1984). Crevasse-splay feeder channels are typically up to 5–6m thick, several hundred metres across, perpendicular to palaeoflow and several kilometres long parallel to palaeoflow.
  
 
==== Wireline log characteristics ====
 
==== Wireline log characteristics ====
  
Facies Oc is characterized by very erratic gamma-ray and sonic-log responses, reflecting the variable interbedding of lithologies. Gamma-log response increases from 120–>150&nbsp;API in the underlying facies Ol mudstones–siltstones to 75–105&nbsp;API in argillaceous very fine to fine-grain crevasse-splay sandstones. A corresponding change also occurs in sonic-log response from 90–100 to 80–90&nbsp;µsft<sup>–1</sup>. Crevasse-splay deposits are characterized by gradationally based upwards-decreasing log responses with sharp inflections at the upper contact with overlying coal seams or lake or marine mudstones.
+
Facies Oc is characterized by very erratic gamma-ray and sonic-log responses, reflecting the variable interbedding of lithologies. Gamma-log response increases from 120–>150API in the underlying facies Ol mudstones–siltstones to 75–105API in argillaceous very fine to fine-grain crevasse-splay sandstones. A corresponding change also occurs in sonic-log response from 90–100 to 80–90µsft<sup>–1</sup>. Crevasse-splay deposits are characterized by gradationally based upwards-decreasing log responses with sharp inflections at the upper contact with overlying coal seams or lake or marine mudstones.
  
 
=== Facies Ol – Lake and marine-bay deposits ===
 
=== Facies Ol – Lake and marine-bay deposits ===
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==== Description ====
 
==== Description ====
  
Lake and marine-bay deposits consist essentially of blocky, dark grey to black mudstones and siltstones, which may contain either freshwater or marine faunas. Thin very fine-grained sandstones and nodular horizons are interbedded locally in places. Deposits range in thickness from a few tens of centimetres to exceptionally 15–20&nbsp;m, with an average thickness in the range 2–10&nbsp;m. The main sedimentary structure is parallel lamination; however, convoluting and bioturbation may also occur in places. Lake and marine-bay deposits almost invariably overlie swamp or flood-plain sediments (facies Os) with a sharp contact, and are overlain gradationally by either crevasse splay (facies Oc) or distributary mouth-bar (facies Cd) sediments.
+
Lake and marine-bay deposits consist essentially of blocky, dark grey to black mudstones and siltstones, which may contain either freshwater or marine faunas. Thin very fine-grained sandstones and nodular horizons are interbedded locally in places. Deposits range in thickness from a few tens of centimetres to exceptionally 15–20m, with an average thickness in the range 2–10m. The main sedimentary structure is parallel lamination; however, convoluting and bioturbation may also occur in places. Lake and marine-bay deposits almost invariably overlie swamp or flood-plain sediments (facies Os) with a sharp contact, and are overlain gradationally by either crevasse splay (facies Oc) or distributary mouth-bar (facies Cd) sediments.
  
 
==== Interpretation ====
 
==== Interpretation ====
Line 504: Line 502:
 
==== Geometry ====
 
==== Geometry ====
  
Mapping of lake and marine-bay facies reveals an uninterrupted areal extent of several hundred square kilometres. Thickness variations suggest that the depth of lakes and bays ranged from less than 10&nbsp;m to over 30&nbsp;m in places.
+
Mapping of lake and marine-bay facies reveals an uninterrupted areal extent of several hundred square kilometres. Thickness variations suggest that the depth of lakes and bays ranged from less than 10m to over 30m in places.
  
 
==== Wireline log characteristics ====
 
==== Wireline log characteristics ====
  
Lake and marine-bay mudstones and siltstones are characterized by high gamma-log responses of 120–>150&nbsp;API and δ''T ''values of 90–105&nbsp;µsft<sup>–1</sup>. Marine horizons commonly display particularly high gamma responses, often in excess of 150&nbsp;API. The presence of nodular beds causes a marked reduction in gamma-log response and δ''T ''values.
+
Lake and marine-bay mudstones and siltstones are characterized by high gamma-log responses of 120–>150API and δ''T ''values of 90–105µsft<sup>–1</sup>. Marine horizons commonly display particularly high gamma responses, often in excess of 150 API. The presence of nodular beds causes a marked reduction in gamma-log response and δ''T ''values.
  
 
=== Facies Os – swamp/floodplain deposits ===
 
=== Facies Os – swamp/floodplain deposits ===
Line 514: Line 512:
 
==== Description ====
 
==== Description ====
  
This facies comprises coal seams and associated rootlet-disrupted mudstones, siltstones and fine-grained sandstones. Coal seams are up to 2–3&nbsp;m thick and are typically banded, whereas the associated clastic strata, also on average 2–3&nbsp;m thick, are invariably structureless, because of disturbance by rootlets. Nodules of pyrite or siderite may be developed within all these lithologies. Swamp/floodplain sediments occur mainly at the top of cyclothems overlying channel (facies Cl and Ch), distributary mouth-bar (facies Cd), crevasse-splay (facies Cl) and locally lake or marine bay (facies Ol) deposits.
+
This facies comprises coal seams and associated rootlet-disrupted mudstones, siltstones and fine-grained sandstones. Coal seams are up to 2–3m thick and are typically banded, whereas the associated clastic strata, also on average 2–3m thick, are invariably structureless, because of disturbance by rootlets. Nodules of pyrite or siderite may be developed within all these lithologies. Swamp/floodplain sediments occur mainly at the top of cyclothems overlying channel (facies Cl and Ch), distributary mouth-bar (facies Cd), crevasse-splay (facies Cl) and locally lake or marine bay (facies Ol) deposits.
  
 
==== Interpretation ====
 
==== Interpretation ====
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==== Geometry ====
 
==== Geometry ====
  
Published studies (e.g. McCabe 1984) and mapping by us in the East Midlands indicate a laterally extensive sheet geometry extending over several hundred square kilometres. Total thickness is typically 2–4&nbsp;m, perhaps exceptionally reaching 6m or more in localized areas.
+
Published studies (e.g. McCabe 1984) and mapping by us in the East Midlands indicate a laterally extensive sheet geometry extending over several hundred square kilometres. Total thickness is typically 2–4m, perhaps exceptionally reaching 6m or more in localized areas.
  
 
==== Wireline log response ====
 
==== Wireline log response ====
  
Coal seams exhibit a very distinctive wireline log response characterized by relatively high gamma ray (''c. ''75&nbsp;API) and very high sonic-log values (>110&nbsp;µsft<sup>–1</sup>). Associated floodplain clastic strata typically give gamma log responses of 75–105&nbsp;API, reflecting the predominantly silty lithology and sonic responses of 80–90&nbsp;µsft<sup>–1</sup>. Compared with lithologically similar facies Ol deposits, the relatively low log values are probably attributable to the common occurrence of ironstone nodules in clastic floodplain sediments.
+
Coal seams exhibit a very distinctive wireline log response characterized by relatively high gamma ray (''c. ''75 API) and very high sonic-log values (>110µsft<sup>–1</sup>). Associated floodplain clastic strata typically give gamma log responses of 75–105API, reflecting the predominantly silty lithology and sonic responses of 80– 90µsft<sup>–1</sup>. Compared with lithologically similar facies Ol deposits, the relatively low log values are probably attributable to the common occurrence of ironstone nodules in clastic floodplain sediments.
 
 
[[category:Carboniferous hydrocarbon resources: the southern North Sea and surrounding onshore areas ]]
 

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