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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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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