Editing Westphalian mid-A to mid-C depositional controls, UK Pennine Basin: regional analyses and their relevance to southern North Sea interpretations

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== Westphalian mid-A to mid-C depositional controls, UK Pennine Basin: regional analyses and their relevance to southern North Sea interpretations ==
 
== Westphalian mid-A to mid-C depositional controls, UK Pennine Basin: regional analyses and their relevance to southern North Sea interpretations ==
  
 
[[File:YGS_CHR_07_WEST_FIG_01.jpg|thumbnail|Figure 1 Tectonic framework of the main UK onshore Upper Carboniferous coalfields (based on Rippon 1997).]]
 
[[File:YGS_CHR_07_WEST_FIG_01.jpg|thumbnail|Figure 1 Tectonic framework of the main UK onshore Upper Carboniferous coalfields (based on Rippon 1997).]]
 
[[File:YGS_CHR_07_WEST_FIG_02.jpg|thumbnail|Figure 2 Stratigraphical summary of the UK coal-bearing Westphalian A–C (based on Guion et al. 1995b). Phases 1 to 5 are detailed in Section 1 of the text.]]
 
[[File:YGS_CHR_07_WEST_FIG_02.jpg|thumbnail|Figure 2 Stratigraphical summary of the UK coal-bearing Westphalian A–C (based on Guion et al. 1995b). Phases 1 to 5 are detailed in Section 1 of the text.]]
[[File:YGS_CHR_07_WEST_FIG_03.jpg|thumbnail|Figure 3 Variations in mid-Westphalian A to top Westphalian B palaeo-environments, north–south through eastings 450–460.  
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[[File:YGS_CHR_07_WEST_FIG_03.jpg|thumbnail|Figure 3 Variations in mid-Westphalian A to top Westphalian B palaeo-environments, north–south through eastings 450–460. The vertical axis is the geological succession normalized for regional thickness variations to give a time-equivalent display (see main text): therefore, there is no vertical exaggeration. The horizontal axis is a north–south section through the succession; see Figure 5 for location. The suppression of actual thickness variations allows a more ready assessment of the spatial variations in time-equivalent environments. Note: the unions of coals towards an inferred basin margin northwest of York; the importance of the Top Hard seam group; the general lateral continuity of the named coals; and the persistence of channel belts, evidenced by significant fluvial sandstones, throughout most of the succession. All the named seams are considered to demonstrate some base-level control on their stratigraphical incidence, although their detailed thickness variations and coal types will reflect local control, such as the presence of preceding topographic highs.
 
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[[File:YGS_CHR_07_WEST_FIG_04.jpg|thumbnail|Figure 4 Variations in mid-Westphalian A to top Westphalian B palaeo-environments, north to south through eastings 470– 480. As for [[:File:YGS_CHR_07_WEST_FIG_03.jpg|Figure 3]], the vertical axis is the geological succession normalized for regional thickness variations to give a timeequivalent display (see main text); the horizontal axis is a north–south section through the succession; see [[:File:YGS_CHR_07_WEST_FIG_05.jpg|Figure 5]] for location. Again, the suppression of actual thickness variations allows a more ready assessment of the spatial variations in timeequivalent environments. Note: the unions of coals towards the basin margin southeast of Nottingham; basin-marginal areas where late Westphalian B marine strata occupy a specific condensed succession; the general lateral continuity of the named coals; and the persistence of channel belts, evidenced by significant fluvial sandstones throughout most of the succession. See also the caption for [[:File:YGS_CHR_07_WEST_FIG_03.jpg|Figure 3]].]]
 
[[File:YGS_CHR_07_WEST_FIG_04.jpg|thumbnail|Figure 4 Variations in mid-Westphalian A to top Westphalian B palaeo-environments, north to south through eastings 470– 480. As for [[:File:YGS_CHR_07_WEST_FIG_03.jpg|Figure 3]], the vertical axis is the geological succession normalized for regional thickness variations to give a timeequivalent display (see main text); the horizontal axis is a north–south section through the succession; see [[:File:YGS_CHR_07_WEST_FIG_05.jpg|Figure 5]] for location. Again, the suppression of actual thickness variations allows a more ready assessment of the spatial variations in timeequivalent environments. Note: the unions of coals towards the basin margin southeast of Nottingham; basin-marginal areas where late Westphalian B marine strata occupy a specific condensed succession; the general lateral continuity of the named coals; and the persistence of channel belts, evidenced by significant fluvial sandstones throughout most of the succession. See also the caption for [[:File:YGS_CHR_07_WEST_FIG_03.jpg|Figure 3]].]]
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The Westphalian mid-A to mid-C Coal Measures of the eastern part of the Pennine Basin are very well documented and also serve as useful analogues for stratigraphically equivalent successions in the North Sea. However, there are still conflicting interpretations of the onshore successions, mainly regarding the application of sequence stratigraphy. Sequence stratigraphical interpretation is not straightforward, particularly as major sandbodies occupy nearly all stratigraphical intervals; thus, potential sequence boundaries, defined by erosion surfaces at the bases of channel belts, could lie at virtually all horizons. Sandstone thicknesses and connectivities relate mainly to their location within the evolving basin. Coal and mudstone thicknesses and connectivities are also mainly related to basin location, although the stratigraphical distribution (and some thicknesses) of coal seams indicate control by marine base level. The paper also considers the validity of interpretations relevant to North Sea hydrocarbon fields. The onshore conclusions are derived from study of an extensive dataset across varied basin settings; the more restricted spatial and stratigraphical scope of offshore data significantly limits the interpretation of depositional relationships and trends. The paper’s main conclusions relate to investigative scales, to the achievable inter-basin correlation resolution, to the 3 D connectivity and thickness patterns of coals, mudstones and sandstones (reflecting specific basin subsidence), and to more robust sequence stratigraphical interpretations.
 
The Westphalian mid-A to mid-C Coal Measures of the eastern part of the Pennine Basin are very well documented and also serve as useful analogues for stratigraphically equivalent successions in the North Sea. However, there are still conflicting interpretations of the onshore successions, mainly regarding the application of sequence stratigraphy. Sequence stratigraphical interpretation is not straightforward, particularly as major sandbodies occupy nearly all stratigraphical intervals; thus, potential sequence boundaries, defined by erosion surfaces at the bases of channel belts, could lie at virtually all horizons. Sandstone thicknesses and connectivities relate mainly to their location within the evolving basin. Coal and mudstone thicknesses and connectivities are also mainly related to basin location, although the stratigraphical distribution (and some thicknesses) of coal seams indicate control by marine base level. The paper also considers the validity of interpretations relevant to North Sea hydrocarbon fields. The onshore conclusions are derived from study of an extensive dataset across varied basin settings; the more restricted spatial and stratigraphical scope of offshore data significantly limits the interpretation of depositional relationships and trends. The paper’s main conclusions relate to investigative scales, to the achievable inter-basin correlation resolution, to the 3 D connectivity and thickness patterns of coals, mudstones and sandstones (reflecting specific basin subsidence), and to more robust sequence stratigraphical interpretations.
  
== Introduction ==
 
 
The Westphalian mid-A to mid-C Coal Measures of the eastern part of the Pennine Basin are very well documented, following intensive mining and exploration, and there is an extensive literature. The main aim of the paper is to consider basinwide relationships that can be substantiated from the detail, and then to evaluate their application to other depositional areas, including southern North Sea gas fields. The paper discusses regional correlation and controls on sedimentation for the Pennine and other onshore basins, and attempts to discuss the succession in a sequence stratigraphical context.
 
The Westphalian mid-A to mid-C Coal Measures of the eastern part of the Pennine Basin are very well documented, following intensive mining and exploration, and there is an extensive literature. The main aim of the paper is to consider basinwide relationships that can be substantiated from the detail, and then to evaluate their application to other depositional areas, including southern North Sea gas fields. The paper discusses regional correlation and controls on sedimentation for the Pennine and other onshore basins, and attempts to discuss the succession in a sequence stratigraphical context.
  
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The validity of detailed correlation between the eastern Pennine Basin (Nottinghamshire–Yorkshire coalfield), and related coalfields to the west (Staffordshire, Lancashire, North Wales, etc.) requires careful evaluation. The main marine horizons and the non-marine bivalve zones are readily correlated, but the exact coal-seam equivalence is more difficult to assess. This is partly because the western coalfields are tens to hundreds of kilometres distant from the eastern Pennine Basin, whereas seam splittings and reunions can take place over distances of less than 1km to low tens of kilometres. Also, the western coalfields are themselves separated from one another by outcrops of older and younger strata, large structures and variable depths. Variation in seam nomenclature is both a symptom and a cause of some correlation problems.
 
The validity of detailed correlation between the eastern Pennine Basin (Nottinghamshire–Yorkshire coalfield), and related coalfields to the west (Staffordshire, Lancashire, North Wales, etc.) requires careful evaluation. The main marine horizons and the non-marine bivalve zones are readily correlated, but the exact coal-seam equivalence is more difficult to assess. This is partly because the western coalfields are tens to hundreds of kilometres distant from the eastern Pennine Basin, whereas seam splittings and reunions can take place over distances of less than 1km to low tens of kilometres. Also, the western coalfields are themselves separated from one another by outcrops of older and younger strata, large structures and variable depths. Variation in seam nomenclature is both a symptom and a cause of some correlation problems.
  
Rippon (1997) discussed aspects of correlation, including the value of cross plotting horizons against depths from boreholes. This involves cross plotting a significant stratigraphical range, normally hundreds of metres thick, from two or more locations. Any variation from a 45° plot will (apart from any drilling deviations) have geological significance, and such cross plots may be used for both correlation and the identification of structural and depositional variations. After structural (and any igneous) variations are allowed for, the scatter produced by cross plotting individual features (e.g. a marine band) or packages (e.g. multistorey sandbodies) is mainly a measure of the fluvial influence in the succession. The marine bands plot linearly, reflecting direct base-level control. This simple technique has been used by the author in various UK coalfields for identification of faults, fault throws, and unconformities, given a robust horizon correlation. It is also useful for analyzing various depositional factors, including the correlation of coal seams over considerable distances, and is valuable for inter-coalfield seam correlation across the full Pennine Basin.
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Rippon (1997) discussed aspects of correlation, including the value of cross plotting horizons against depths from boreholes. This involves cross plotting a significant stratigraphical range, normally hundreds of metres thick, from two or more locations. Any variation from a 45°plot will (apart from any drilling deviations) have geological significance, and such cross plots may be used for both correlation and the identification of structural and depositional variations. After structural (and any igneous) variations are allowed for, the scatter produced by cross plotting individual features (e.g. a marine band) or packages (e.g. multistorey sandbodies) is mainly a measure of the fluvial influence in the succession. The marine bands plot linearly, reflecting direct base-level control. This simple technique has been used by the author in various UK coalfields for identification of faults, fault throws, and unconformities, given a robust horizon correlation. It is also useful for analyzing various depositional factors, including the correlation of coal seams over considerable distances, and is valuable for inter-coalfield seam correlation across the full Pennine Basin.
  
[[:File:YGS_CHR_07_WEST_FIG_06.jpg|(Figure 6)]] illustrates a generalized cross plot of boreholes from four coalfields in the western Pennine Basin. The selected bore-holes were cored, and show only minor structural disturbance. The vertical axis is again based on Hennymoor Farm borehole in the eastern Pennine Basin. This acts as a reference against which the other borehole sections are plotted against the horizontal axis. As with [[:File:YGS_CHR_07_WEST_FIG_03.jpg|(Figure 3)]] and [[:File:YGS_CHR_07_WEST_FIG_04.jpg|(Figure 4)]], generalization and small scale are necessary for presentation, but considerable correlative accuracy can be established by more detailed work. Note that time-environment panels, such as illustrated in [[:File:YGS_CHR_07_WEST_FIG_03.jpg|(Figure 3)]] and [[:File:YGS_CHR_07_WEST_FIG_04.jpg|(Figure 4)]], can be constructed for these western coalfields, but their value is reduced because of the data gaps between the individual areas.
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Figure 6 illustrates a generalized cross plot of boreholes from four coalfields in the western Pennine Basin. The selected bore-holes were cored, and show only minor structural disturbance. The vertical axis is again based on Hennymoor Farm borehole in the eastern Pennine Basin. This acts as a reference against which the other borehole sections are plotted against the horizontal axis. As with [[:File:YGS_CHR_07_WEST_FIG_03.jpg|(Figure 3)]] and [[:File:YGS_CHR_07_WEST_FIG_04.jpg|(Figure 4)]], generalization and small scale are necessary for presentation, but considerable correlative accuracy can be established by more detailed work. Note that time-environment panels, such as illustrated in Figures 3 and 4, can be constructed for these western coalfields, but their value is reduced because of the data gaps between the individual areas.
  
 
From the background work reflected in [[:File:YGS_CHR_07_WEST_FIG_06.jpg|Figure 6]], it is concluded that the main marine bands and non-marine bivalve-zone boundaries may be readily matched between the western and eastern coalfields of the Pennine Basin. The main named coal seams of the eastern Pennine Basin can confidently be correlated with the equivalent seams to the west, although structural disturbance is locally a problem. Other general conclusions include the importance of the Top Hard and correlative seams in both the western and eastern coalfields of the Pennine Basin. This may be expected, as these coalfields were originally in depositional continuity, and within the same tectonic setting.
 
From the background work reflected in [[:File:YGS_CHR_07_WEST_FIG_06.jpg|Figure 6]], it is concluded that the main marine bands and non-marine bivalve-zone boundaries may be readily matched between the western and eastern coalfields of the Pennine Basin. The main named coal seams of the eastern Pennine Basin can confidently be correlated with the equivalent seams to the west, although structural disturbance is locally a problem. Other general conclusions include the importance of the Top Hard and correlative seams in both the western and eastern coalfields of the Pennine Basin. This may be expected, as these coalfields were originally in depositional continuity, and within the same tectonic setting.
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A simple sequence-stratigraphical interpretation could view the mid-A to mid-C Westphalian fill of the Pennine Basin as essentially a single depositional system. Variations in fluvial input and erosion, together with the incidence of coals and marine bands, would be dominantly controlled by marine base level, with progressive regional subsidence generating accommodation space. In this interpretation, even if gross regional thickness variations were influenced by subsidence (see Section 2), the general stratigraphical incidence of the main palaeo-environments would be determined mainly by marine base-level variations.
 
A simple sequence-stratigraphical interpretation could view the mid-A to mid-C Westphalian fill of the Pennine Basin as essentially a single depositional system. Variations in fluvial input and erosion, together with the incidence of coals and marine bands, would be dominantly controlled by marine base level, with progressive regional subsidence generating accommodation space. In this interpretation, even if gross regional thickness variations were influenced by subsidence (see Section 2), the general stratigraphical incidence of the main palaeo-environments would be determined mainly by marine base-level variations.
  
However, the regional analysis outlined above indicates that the fluvial systems, at least, did not conform to this single system of basin fill: their stratigraphical incidence, locations and trends, and thickness and connectivity patterns appear to be normally independent of base-level indicators. This is the case even during periods of relatively lower incidences of sandbodies, such as during intervals of regional peat mantling, when some significant channel systems were still present ([[:File:YGS_CHR_07_WEST_FIG_03.jpg|(Figure 3)]] and [[:File:YGS_CHR_07_WEST_FIG_04.jpg|(Figure 4)]]). There are no regional changes comparable to those reported, for example, by Hampson et al. (1999) for the Namurian C to Westphalian A succession in the Ruhr coalfield, where there is a contemporary progression from fluvial to marine delta-front environments, albeit in a somewhat different setting.
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However, the regional analysis outlined above indicates that the fluvial systems, at least, did not conform to this single system of basin fill: their stratigraphical incidence, locations and trends, and thickness and connectivity patterns appear to be normally independent of base-level indicators. This is the case even during periods of relatively lower incidences of sandbodies, such as during intervals of regional peat mantling, when some significant channel systems were still present (Figs 3, 4). There are no regional changes comparable to those reported, for example, by Hampson et al. (1999) for the Namurian C to Westphalian A succession in the Ruhr coalfield, where there is a contemporary progression from fluvial to marine delta-front environments, albeit in a somewhat different setting.
  
 
Rippon (1996, 1999) considered that the preserved sedimentary volume records the interaction of three distinct depositional regimes:* ''Background lithofacies ''represent a near-continuum through the sedimentary pile, comprising fine-grained clastic rocks and coals, of lacustrine and mire environments. There is a general thinning of all these deposits towards the basin margins.
 
Rippon (1996, 1999) considered that the preserved sedimentary volume records the interaction of three distinct depositional regimes:* ''Background lithofacies ''represent a near-continuum through the sedimentary pile, comprising fine-grained clastic rocks and coals, of lacustrine and mire environments. There is a general thinning of all these deposits towards the basin margins.
 
* ''Channel belt lithofacies ''represent near-continuous inflows into and across the basin, irrespective of the background environment; that is, some channel systems became established during times of mainly lacustrine sedimentation, others during times of mainly mire accumulations. Channel-belt pathways do not primarily reflect basin form, but their thicknesses and connectivities do reflect such a control.
 
* ''Channel belt lithofacies ''represent near-continuous inflows into and across the basin, irrespective of the background environment; that is, some channel systems became established during times of mainly lacustrine sedimentation, others during times of mainly mire accumulations. Channel-belt pathways do not primarily reflect basin form, but their thicknesses and connectivities do reflect such a control.
 
* ''Marine incursions ''occurred infrequently, interrupting both of the above regimes. Faunal variations within the marine strata are considered by the author to be more complex than those implied by Calver (1968), who related progressively more marine faunal populations to basin form (see Rippon 1997). Marine-band thicknesses decrease towards the basin margins, but not to the same degree as the lacustrine mud-stones; this is presumably because the marine sediments were deposited over much less time, so that the effect of reduced subsidence at the basin margins is muted. The directions of marine incursions, which remain uncertain, may not necessarily have related to the (very gentle) gross palaeoslopes as indicated by fluvial-channel pathways. These observations suggest that the marine bands represent distinctive events, controlled by eustatic (global) sea-level rise. Detailed field observations suggest that they were typically not in depositional continuity with the lacustrine deposits. It is considered that the onshore UK coalfields, and the southern North Sea areas, were all significantly distant from a fully marine setting. From this brief review, it is concluded that the preserved basin fill does record the interaction of three essentially different depositional regimes. The relative importance of marine base-level and basin subsidence, with respect to these three regimes will now be assessed.
 
* ''Marine incursions ''occurred infrequently, interrupting both of the above regimes. Faunal variations within the marine strata are considered by the author to be more complex than those implied by Calver (1968), who related progressively more marine faunal populations to basin form (see Rippon 1997). Marine-band thicknesses decrease towards the basin margins, but not to the same degree as the lacustrine mud-stones; this is presumably because the marine sediments were deposited over much less time, so that the effect of reduced subsidence at the basin margins is muted. The directions of marine incursions, which remain uncertain, may not necessarily have related to the (very gentle) gross palaeoslopes as indicated by fluvial-channel pathways. These observations suggest that the marine bands represent distinctive events, controlled by eustatic (global) sea-level rise. Detailed field observations suggest that they were typically not in depositional continuity with the lacustrine deposits. It is considered that the onshore UK coalfields, and the southern North Sea areas, were all significantly distant from a fully marine setting. From this brief review, it is concluded that the preserved basin fill does record the interaction of three essentially different depositional regimes. The relative importance of marine base-level and basin subsidence, with respect to these three regimes will now be assessed.
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=== 3.2 Marine base level ===
 
=== 3.2 Marine base level ===
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Guion, P. D. & C. R. Fielding 1988. Westphalian A and B sedimentation in the Pennine Basin, UK. In ''Sedimentation in a synorogenic basin complex: the Upper Carboniferous of northwest Europe'', B. M. Besly & G. Kelling (eds), 153–77. Glasgow: Blackie.
 
Guion, P. D. & C. R. Fielding 1988. Westphalian A and B sedimentation in the Pennine Basin, UK. In ''Sedimentation in a synorogenic basin complex: the Upper Carboniferous of northwest Europe'', B. M. Besly & G. Kelling (eds), 153–77. Glasgow: Blackie.
  
Guion, P. D., I. M. Fulton, N. S. Jones 1995a. 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.  
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Guion, P. D., I. M. Fulton, N. S. Jones 1995a. 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., N. L. Banks, J. H. Rippon 1995b. The Silkstone Rock (Westphalian) from the east Pennines, England: implications for sand body genesis. ''Geological Society of London, Journal ''152, 819–32.
 
 
Guion, P. D., N. L. Banks, J. H. Rippon 1995b. The Silkstone Rock (Westphalian) from the east Pennines, England: implications for sand body genesis. ''Geological Society of London, Journal ''152, 819–32.
 
  
 
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.
 
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.

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