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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=== 3.3 Basin subsidence ===
 
=== 3.3 Basin subsidence ===
  
Basin subsidence is considered to control the overall patterns of thickness variations (and therefore connectivities) of all the deposits, but not the channel-belt pathways, which were mainly influenced by upstream and gross palaeoslope factors (Rippon 1996). Individual sandstones, coal beds, lacustrine deposits and marine bands are typically thicker in the more rapidly subsiding parts of the Pennine Basin, where enhanced subsidence allowed greater accumulations with minimum contemporary erosion. Towards the basin margins the full succession is thinner, but thicker sandstones and coals can occur, representing condensed composites of individual beds. Contemporary erosion is common both within the sandbodies and at their bases. Basin marginal areas are particularly instructive where they are not coal rich. In some areas, for example in parts of Lincolnshire (National Grid Square SK86) much of the succession between the Low Estheria Band and the Vanderbeckei Marine Band (see [[:File:YGS_CHR_07_WEST_FIG_03.jpg|(Figure 3)]], [[:File:YGS_CHR_07_WEST_FIG_04.jpg|(Figure 4)]]) consists of sandstone, reflecting the existence of long-lived channel belts throughout this interval. Other areas are known where an abnormally high proportion of marine or near-marine strata (including the Clowne, Haughton, Sutton, and Aegiranum marine bands) occur in the succession between the Clowne Seam and early Westphalian C strata (e.g. in parts of Lincolnshire, SK88). It may be argued for such locations that significant sandstones (e.g. the Oaks Rock, see [[:File:YGS_CHR_07_WEST_FIG_04.jpg|(Figure 4)]]) must therefore be coeval with the marine strata. However, the relationship between the Oaks Rock and coeval strata is considered to be complex, involving marine-band attenuation against topographic highs (e.g. Sutton Marine Band) and erosion in channel-belt areas (e.g. Haughton and Sutton marine bands). There is also a possibility of non-deposition of the marine phase in places, with the channel belt prograding into marine waters (see Section 2.2.3). It is concluded that some channel systems did prograde into essentially marine environments in the Pennine Basin area, but only in exceptional cases, reflecting the rarity of the marine invasions themselves.
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Basin subsidence is considered to control the overall patterns of thickness variations (and therefore connectivities) of all the deposits, but not the channel-belt pathways, which were mainly influenced by upstream and gross palaeoslope factors (Rippon 1996). Individual sandstones, coal beds, lacustrine deposits and marine bands are typically thicker in the more rapidly subsiding parts of the Pennine Basin, where enhanced subsidence allowed greater accumulations with minimum contemporary erosion. Towards the basin margins the full succession is thinner, but thicker sandstones and coals can occur, representing condensed composites of individual beds. Contemporary erosion is common both within the sandbodies and at their bases. Basin marginal areas are particularly instructive where they are not coal rich. In some areas, for example in parts of Lincolnshire (National Grid Square SK86) much of the succession between the Low Estheria Band and the Vanderbeckei Marine Band (see Figs 3, 4) consists of sandstone, reflecting the existence of long-lived channel belts throughout this interval. Other areas are known where an abnormally high proportion of marine or near-marine strata (including the Clowne, Haughton, Sutton, and Aegiranum marine bands) occur in the succession between the Clowne Seam and early Westphalian C strata (e.g. in parts of Lincolnshire, SK88). It may be argued for such locations that significant sandstones (e.g. the Oaks Rock, see Fig. 4) must therefore be coeval with the marine strata. However, the relationship between the Oaks Rock and coeval strata is considered to be complex, involving marine-band attenuation against topographic highs (e.g. Sutton Marine Band) and erosion in channel-belt areas (e.g. Haughton and Sutton marine bands). There is also a possibility of non-deposition of the marine phase in places, with the channel belt prograding into marine waters (see Section 2.2.3). It is concluded that some channel systems did prograde into essentially marine environments in the Pennine Basin area, but only in exceptional cases, reflecting the rarity of the marine invasions themselves.
  
Some onshore coalfields include areas that were not in basin settings. In the UK the largest of these is the southern part of the English Northeast coalfield, in County Durham. In the Durham area, Westphalian deposition took place across the concealed easterly extension of the Alston Block ([[:File:YGS_CHR_07_WEST_FIG_01.jpg|(Figure 1)]]), a positive structural unit during Carboniferous times. The succession here is relatively condensed throughout, compared with other areas of the Pennine Basin. There was also a different dominant sandstone provenance, with sediment derived mainly from the northeast (see for example Guion & Fielding 1988). The combination of a more proximal channel-source area (Rippon 1996) with a relatively condensed succession has led to a significantly greater proportion of sandstone. Furthermore, the lower subsidence rates have led to more closely spaced erosion surfaces within and beneath the sandbodies.
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Some onshore coalfields include areas that were not in basin settings. In the UK the largest of these is the southern part of the English Northeast coalfield, in County Durham. In the Durham area, Westphalian deposition took place across the concealed easterly extension of the Alston Block (Fig. 1), a positive structural unit during Carboniferous times. The succession here is relatively condensed throughout, compared with other areas of the Pennine Basin. There was also a different dominant sandstone provenance, with sediment derived mainly from the northeast (see for example Guion & Fielding 1988). The combination of a more proximal channel-source area (Rippon 1996) with a relatively condensed succession has led to a significantly greater proportion of sandstone. Furthermore, the lower subsidence rates have led to more closely spaced erosion surfaces within and beneath the sandbodies.
  
 
It is also possible to have a sand-poor structurally defined basin because it lies away from main channel-belt pathways. Some parts of the Pennine Basin itself are relatively sand poor, and others relatively sand rich. The organization of the distributive networks that supplied sand into the region would have been an important factor in determining these differences in sand content. The illustration of these points is beyond the scope of this paper, but Rippon (1996) presented a schematic representation of sand-prone and sand-poor areas.
 
It is also possible to have a sand-poor structurally defined basin because it lies away from main channel-belt pathways. Some parts of the Pennine Basin itself are relatively sand poor, and others relatively sand rich. The organization of the distributive networks that supplied sand into the region would have been an important factor in determining these differences in sand content. The illustration of these points is beyond the scope of this paper, but Rippon (1996) presented a schematic representation of sand-prone and sand-poor areas.

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