Editing Carboniferous miospore biostratigraphy of the North Sea

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To date there are only two miospore biozonation schemes in the public domain specific to the North Sea Carboniferous. The scheme for the Westphalian by McLean (1995a) is an early draft of that presented here for the same interval. The biozones described for the whole Carboniferous by Maynard et al. (1997) are vaguely defined and of limited stratigraphical resolution ([[:File:YGS_CHR_02_CARB_FIG_01.jpg|Figure 1]]).
 
To date there are only two miospore biozonation schemes in the public domain specific to the North Sea Carboniferous. The scheme for the Westphalian by McLean (1995a) is an early draft of that presented here for the same interval. The biozones described for the whole Carboniferous by Maynard et al. (1997) are vaguely defined and of limited stratigraphical resolution ([[:File:YGS_CHR_02_CARB_FIG_01.jpg|Figure 1]]).
  
Accumulation of palynostratigraphical data over the past 30 years and more has allowed the development of a miospore biozonation scheme specific to the North Sea. This is based upon the synthesis of data from more than 13000 productive palynological samples from more than 300 Carboniferous well sections. The biozonation includes data from both coal seams and mudstone/siltstone lithologies. The fact that different lithologies and facies yield different palynological assemblages (both of miospore taxa and total organic debris) is well known (Neves 1958) and provides the basis for high-resolution palynofacies studies (e.g. for the Westphalian of the Caister field by Ritchie & Pratsides 1993, and for the late Westphalian onshore Netherlands by Van de Laar & Van der Zwan 1996). It is also recognized that the stratigraphical ranges of most, if not all, miospore taxa seen in coal seams are shorter than their ranges in clastic strata. Consequently, the best palynological dataset includes assemblages from all lithological types. The biozonation is homotaxial (''sensu ''Scott 1965) in approach (i.e. it is based upon the relative order of appearances and disappearances of taxa), but remains comparable to the framework provided by the onshore zonations. The onshore biozones were consciously defined to reflect the evolution of the Carboniferous microflora. As such, their bases are principally defined by the first appearances of certain morphologically distinctive and stratigraphically restricted taxa. However, because of the scarcity of cored sections in the North Sea, the offshore biozones are defined, where possible, on the last stratigraphical occurrences (range tops or first downhole appearances) of zonal taxa. This method makes the biozones applicable to ditch cuttings material. Selection of zonal taxa is based upon the consistent occurrence of the range top or base of each taxon in several well sections. Consequently, several relatively common taxa, which, given their representation in onshore biozones would be expected to be stratigraphically significant, have proved not to be (at least within the limitations of the database). For example, the intra-Namurian range top of the highly distinctive form ''Ibrahimispores magnificus ''occurs at inconsistent levels with respect to certain other taxa taken as standards, and so is excluded as a significant biostratigraphical event. The range base of ''Cadiospora magna ''(traditionally considered to be a Westphalian D marker taxon, e.g. Smith & Butterworth 1967, Maynard et al. 1997) occurs irregularly in the mid- and early Bolsovian of the North Sea UK sector. Other taxa (e.g. ''Fragilipollenites radiatus ''and ''Pteroretis primum''), although morphologically distinctive, occur so rarely as to preclude the recognition of consistent range limits. On the other hand, some taxa, particularly in the late Namurian (e.g. ''Ahrensisporites beeleyensis, Alatisporites nudus ''and ''Triquitrites nodosus'') are rare but have consistent stratigraphical limits, and so are considered significant.
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Accumulation of palynostratigraphical data over the past 30 years and more has allowed the development of a miospore biozonation scheme specific to the North Sea. This is based upon the synthesis of data from more than 13000 productive palynological samples from more than 300 Carboniferous well sections. The biozonation includes data from both coal seams and mudstone/siltstone lithologies. The fact that different lithologies and facies yield different palynological assemblages (both of miospore taxa and total organic debris) is well known (Neves 1958) and provides the basis for high-resolution palynofacies studies (e.g. for the Westphalian of the Caister field by Ritchie & Pratsides 1993, and for the late Westphalian onshore Netherlands by Van de Laar & Van der Zwan 1996). It is also recognized that the stratigraphical ranges of most, if not all, miospore taxa seen in coal seams are shorter than their ranges in clastic strata. Consequently, the best palynological dataset includes assemblages from all lithological types. The biozonation is homotaxial (''sensu ''Scott 1965) in approach (i.e. it is based upon the relative order of appearances and disappearances of taxa), but remains comparable to the framework provided by the onshore zonations. The onshore biozones were consciously defined to reflect the evolution of the Carboniferous microflora. As such, their bases are principally defined by the first appearances of certain morphologically distinctive and stratigraphically restricted taxa. However, because of the scarcity of cored sections in the North Sea, the offshore biozones are defined, where possible, on the last stratigraphical occurrences (range tops or first downhole appearances) of zonal taxa. This method makes the biozones applicable to ditch cuttings material. Selection of zonal taxa is based upon the consistent occurrence of the range top or base of each taxon in several well sections. Consequently, several relatively common taxa, which, given their representation in onshore biozones would be expected to be stratigraphically significant, have proved not to be (at least within the limitations of the database). For example, the intraNamurian range top of the highly distinctive form ''Ibrahimispores magnificus ''occurs at inconsistent levels with respect to certain other taxa taken as standards, and so is excluded as a significant biostratigraphical event. The range base of ''Cadiospora magna ''(traditionally considered to be a Westphalian D marker taxon, e.g. Smith & Butterworth 1967, Maynard et al. 1997) occurs irregularly in the mid- and early Bolsovian of the North Sea UK sector. Other taxa (e.g. ''Fragilipollenites radiatus ''and ''Pteroretis primum''), although morphologically distinctive, occur so rarely as to preclude the recognition of consistent range limits. On the other hand, some taxa, particularly in the late Namurian (e.g. ''Ahrensisporites beeleyensis, Alatisporites nudus ''and ''Triquitrites nodosus'') are rare but have consistent stratigraphical limits, and so are considered significant.
  
 
Definitions of the Dinantian, Namurian and Westphalian biozones are shown on [[:File:YGS_CHR_02_CARB_FIG_02.jpg|Figure 2]], [[:File:YGS_CHR_02_CARB_FIG_03.jpg|Figure 3]], [[:File:YGS_CHR_02_CARB_FIG_04.jpg|Figure 4]] respectively. Full citations for all taxa are given in the Appendix. Individual biozones are defined by the range top or base of a particular miospore taxon and represent interval or partial-range biozones. As such, they differ from the standard approach to Carboniferous palynostratigraphy, where assemblage biozones (Smith & Butterworth 1967) or concurrent range biozones (Neves et al. 1972, 1973, Clayton et al. 1977, Owens et al. 1977) have traditionally been used. It is recognized that any one defining taxon will not always be represented in a section. So, some associated taxa whose range tops or bases are known to occur ''in proximity to ''that of the defining taxon are also indicated.
 
Definitions of the Dinantian, Namurian and Westphalian biozones are shown on [[:File:YGS_CHR_02_CARB_FIG_02.jpg|Figure 2]], [[:File:YGS_CHR_02_CARB_FIG_03.jpg|Figure 3]], [[:File:YGS_CHR_02_CARB_FIG_04.jpg|Figure 4]] respectively. Full citations for all taxa are given in the Appendix. Individual biozones are defined by the range top or base of a particular miospore taxon and represent interval or partial-range biozones. As such, they differ from the standard approach to Carboniferous palynostratigraphy, where assemblage biozones (Smith & Butterworth 1967) or concurrent range biozones (Neves et al. 1972, 1973, Clayton et al. 1977, Owens et al. 1977) have traditionally been used. It is recognized that any one defining taxon will not always be represented in a section. So, some associated taxa whose range tops or bases are known to occur ''in proximity to ''that of the defining taxon are also indicated.

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