Editing Carboniferous miospore biostratigraphy of the North Sea

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The correlation of lithostratigraphical units between well sections is the fundamental palynostratigraphic methodology in hydrocarbon exploration. Claims that palynology provides only low-resolution biostratigraphy in the offshore Carboniferous (e.g. to stage level; Leeder et al. 1990b) are now seen as untenable (see [[:File:YGS_CHR_02_CARB_FIG_01.jpg|Figure 1]]). Correlation of miospore biozones now equals or exceeds the level of resolution achieved by most other Carboniferous fossil groups, with the notable exception of the ammonoids. Further, the framework provided by the biozones allows more detailed intra-biozonal correlations to be made. These may be based upon relatively short-lived palaeoclimatic or facies signals within the palynological data, or upon data from other disciplines such as geochemistry or wireline log analysis (Leeder et al. 1990b, Besly et al. 1993, Davies & McLean 1996, Pearce et al. 2005). Detailed analysis of closely sampled sections may allow the recognition of palynologically distinctive mud-stone or coal units. Correlation of these has provided the basis for detailed subdivision and correlation of Westphalian reservoir units (e.g. coal seam correlations in the Langsettian and Duckmantian of the Murdoch, Caister and Chiswick fields; and correlation of intra-reservoir units within the Murdoch–Caister reservoir sandstone unit). Case studies have been presented for the Murdoch field in McLean & Murray (1996) and McLean and Davies (1999), and for the Caister field in Ritchie & Pratsides (1993).
 
The correlation of lithostratigraphical units between well sections is the fundamental palynostratigraphic methodology in hydrocarbon exploration. Claims that palynology provides only low-resolution biostratigraphy in the offshore Carboniferous (e.g. to stage level; Leeder et al. 1990b) are now seen as untenable (see [[:File:YGS_CHR_02_CARB_FIG_01.jpg|Figure 1]]). Correlation of miospore biozones now equals or exceeds the level of resolution achieved by most other Carboniferous fossil groups, with the notable exception of the ammonoids. Further, the framework provided by the biozones allows more detailed intra-biozonal correlations to be made. These may be based upon relatively short-lived palaeoclimatic or facies signals within the palynological data, or upon data from other disciplines such as geochemistry or wireline log analysis (Leeder et al. 1990b, Besly et al. 1993, Davies & McLean 1996, Pearce et al. 2005). Detailed analysis of closely sampled sections may allow the recognition of palynologically distinctive mud-stone or coal units. Correlation of these has provided the basis for detailed subdivision and correlation of Westphalian reservoir units (e.g. coal seam correlations in the Langsettian and Duckmantian of the Murdoch, Caister and Chiswick fields; and correlation of intra-reservoir units within the Murdoch–Caister reservoir sandstone unit). Case studies have been presented for the Murdoch field in McLean & Murray (1996) and McLean and Davies (1999), and for the Caister field in Ritchie & Pratsides (1993).
  
The development of a higher-resolution miospore biozonation has also allowed more detailed lithostratigraphical interpretations. In particular, the recognition of biozone boundaries associated with some of the principal Westphalian marine bands provides a basis for refined correlations. Note that the Aegiranum, Maltby, Amaliae and Listeri marine bands are not associated with biozone boundaries in the scheme of Clayton et al. (1977), and the same is true for the Cambriense, Aegiranum, Maltby and Amaliae marine bands in the scheme of Maynard et al (1997; [[:File:YGS_CHR_02_CARB_FIG_01.jpg|Figure 1]]). In the current study, emphasis has been placed upon the relationship of palynostratigraphy to the distribution of marine bands, and biozone boundaries identified accordingly. This means that the present scheme has a relatively high-resolution correlation potential. Further, the stratal subdivisions that it provides are likely to be genetically related in a sequence stratigraphical or palaeoclimatological sense.
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The development of a higher-resolution miospore biozonation has also allowed more detailed lithostratigraphical interpretations. In particular, the recognition of biozone boundaries associated with some of the principal Westphalian marine bands provides a basis for refined correlations. Note that the Aegiranum, Maltby, Amaliae and Listeri marine bands are not associated with biozone boundaries in the scheme of Clayton et al. (1977), and the same is true for the Cambriense, Aegiranum, Maltby and Amaliae marine bands in the scheme of Maynard et al (1997; [[:File:YGS_CHR_02_CARB_FIG_01.jpg|Figure 1]]). In the current study, emphasis has been placed upon the relationship of palynostratigraphy to the distribution of marine bands, and biozone boundaries identified accordingly. This means that the present scheme has a relatively high-resolution correlation potential. Further, the stratal subdivisions that it provides are likely to be genetically related in a sequence strati-graphical or palaeoclimatological sense.
  
 
Application of the scheme in the area on the northern margin of the Southern North Sea Basin has allowed the recognition of a major intra-Westphalian unconformity. Here, sequences towards the centre of the basin preserve a more-or-less complete record of Langsettian, Duckmantian and early to mid-Bolsovian coal-bearing strata overlain by redbeds of late Bolsovian age. To the northeast, the strata immediately beneath the redbeds become older, although palynological evidence from the redbeds indicate that these remain of mid-Bolsovian age. Such age interpretations are difficult to reconcile with the lithostratigraphy of Cameron (1993b), but, in the lithostratigraphy of Besly (2002), the base of the Lower Ketch Formation unconformably overlies the Cleaver and Westoe formations (Figure 5). In well 44/21-3 (Pearce et al. 2005), the Lower Ketch Formation rests on early Bolsovian coal-bearing rocks of the Upper Cleaver Member (''sensu ''Besly 2002). Use of the new biozonation in wells slightly farther north indicates that strata above the horizon of the Aegiranum Marine Band are not preserved, and that the redbeds lie upon late Duckmantian strata. In wells still farther northeast the Ketch Formation shows progressive onlap of the early Duckmantian, Langsettian and Namurian. In such cases the base of the Ketch Formation is evident in the absence of several biozones across the unconformity (Pearce et al. 2005).
 
Application of the scheme in the area on the northern margin of the Southern North Sea Basin has allowed the recognition of a major intra-Westphalian unconformity. Here, sequences towards the centre of the basin preserve a more-or-less complete record of Langsettian, Duckmantian and early to mid-Bolsovian coal-bearing strata overlain by redbeds of late Bolsovian age. To the northeast, the strata immediately beneath the redbeds become older, although palynological evidence from the redbeds indicate that these remain of mid-Bolsovian age. Such age interpretations are difficult to reconcile with the lithostratigraphy of Cameron (1993b), but, in the lithostratigraphy of Besly (2002), the base of the Lower Ketch Formation unconformably overlies the Cleaver and Westoe formations (Figure 5). In well 44/21-3 (Pearce et al. 2005), the Lower Ketch Formation rests on early Bolsovian coal-bearing rocks of the Upper Cleaver Member (''sensu ''Besly 2002). Use of the new biozonation in wells slightly farther north indicates that strata above the horizon of the Aegiranum Marine Band are not preserved, and that the redbeds lie upon late Duckmantian strata. In wells still farther northeast the Ketch Formation shows progressive onlap of the early Duckmantian, Langsettian and Namurian. In such cases the base of the Ketch Formation is evident in the absence of several biozones across the unconformity (Pearce et al. 2005).

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