Editing Category:Foraminiferal zonation

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The foraminifera of the Chalk Group in the North Sea Basin were outlined by Wilkinson ''et al''. (1993, 1994).  The biomarkers were selected for borehole samples collected by open-hole methods and those chosen were well known, easily identified and based on those commonly used by industry.  Definitions were based on the first down-hole occurrences (i.e. extinctions) and a pragmatic approach was taken; a refined or detailed biostratigraphical subdivision of the Chalk Group was not attempted.  As the biomarkers offshore were based on 'tops', they do not fit exactly with the onshore biozonation outlined by Wilkinson (2000), which was based on inceptions, extinction and acmes (biomarkers or bioevents within the zones were also recognised).  Nevertheless, the biomarkers of the North Sea succession can be related to the onshore biozones, and the purpose of this report is to relate the two schemes.
  
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==Overview==
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The biomarkers identified for offshore sequences were, by necessity, defined from the top down. Each biomarker represents a single event and no attempt was made to place them into a zonal scheme.  The biomarkers of the Chalk Group in the North Sea Basin are placed, herein, within the context of the biozones recognised in southern and eastern England.
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Foraminifera from the Chalk Group have been taxonomically described for many years and since the 1970’s biostratigraphical applications have also been discussed. It is becoming more evident that foraminiferal inceptions and extinctions can be tied in with lithostratigraphical markers (eg. marl seams, flint bands) and macrofaunal events (eg. sponge beds, ''Inoceramus''-rich beds), to provide a very detailed subdivision of the Chalk Group.  Unfortunately, earlier biostratigraphical accounts often failed to record detailed stratigraphical and lithological information relating to  sample positions within the succession, thus limiting their usefulness.  
 
  
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The unpublished scheme used herein for the Chalk Group of the UK  recognises a maximum of 27 foraminiferal zones, and has been compiled from a variety of published and unpublished sources (Bailey, 1978; Swiecicki, 1980; Bailey ''et al''., 1983, 1984; Hart ''et al''., 1989; Carter & Hart, 1977, Hart, 1982; Jarvis ''et al''., 1988; Mortimore, 1986). Data for offshore areas (King ''et al''., 1989) mostly relates to uncored hydrocarbon wells, and the biostratigraphical scheme for these is based on the extinction horizon (‘Last Appearance Datum’ or ‘First Downhole Occurrence’) of key taxa.; caving in these boreholes makes the first appearance uphole, or inception, an unreliable datum.
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==Descriptions of offshore foraminiferal biomarkers in the Chalk Group==
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The foraminifera biomarkers recognised in the Upper Cretaceous Shetland Group are not included in this synthesis. Being based on First Downhole Occurrences (FDOs), the highest possible stratigraphical positions for the Biomarkers are given here. However, in some cases, similar faunas may be found lower compared to the foraminiferal zones.  Distributions are related to the species ranges of southern and eastern England.
  
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The zones, numbered 1 to 27 from oldest to youngest, are pre-fixed 'BGS' to differentiate them from previously published schemes, and where possible, they are related to the UKB zones of Hart ''et al''. (1989), macrofaunal zones and lithostratigraphical marker horizons.
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In the following account Biomarkers in the offshore UK Chalk Group are systematically described from youngest (Biomarker 1) to oldest (Biomarker 15). Alternatively, for information about a particulat Biomarker, select it from the summary table (summary correlation of offshore foraminiferal biomarkers and BGS foraminiferal zones).
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==Descriptions of foraminiferal biozones in the Chalk Group==
 
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In the following account the BGS foraminiferal zones of the Chalk Group are systematically described from oldest (BGS 1) to youngest (BGS 27). Alternatively, for information about a particular zone, select it from the summary table (summary correlation of foraminiferal and macrofaunal zones)
 
  
 
[[category:Biostratigraphy of the Chalk Group | 009]]
 
[[category:Biostratigraphy of the Chalk Group | 009]]

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