Editing Post-Carboniferous burial and exhumation histories of Carboniferous rocks of the southern North Sea and adjacent onshore UK

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Estimates of maximum palaeotemperature derived from AFTA and VR data in the 47/25-1 well are plotted against depth (from KB) in [[:File:YGS_CHR_03_POST_FIG_04.jpg|Figure 4]]. The values derived from VR show some scatter, but overall are consistent with the palaeotemperatures indicated by the AFTA data. One value appears to be much lower than the majority, which we interpret as representing suppression of the reflectance level in this sample, similar to the Edale Shales from outcrop, discussed earlier. Omitting this lower VR value, the combined palaeotemperature constraints from AFTA and VR define a linear palaeotemperature profile, subparallel to the present-day temperature profile (also shown in [[:File:YGS_CHR_03_POST_FIG_04.jpg|Figure 4]]) but offset to higher values by a difference of about 40°C.
 
Estimates of maximum palaeotemperature derived from AFTA and VR data in the 47/25-1 well are plotted against depth (from KB) in [[:File:YGS_CHR_03_POST_FIG_04.jpg|Figure 4]]. The values derived from VR show some scatter, but overall are consistent with the palaeotemperatures indicated by the AFTA data. One value appears to be much lower than the majority, which we interpret as representing suppression of the reflectance level in this sample, similar to the Edale Shales from outcrop, discussed earlier. Omitting this lower VR value, the combined palaeotemperature constraints from AFTA and VR define a linear palaeotemperature profile, subparallel to the present-day temperature profile (also shown in [[:File:YGS_CHR_03_POST_FIG_04.jpg|Figure 4]]) but offset to higher values by a difference of about 40°C.
  
These features of the palaeotemperature profile suggest that heating was predominantly caused by deeper burial. [[:File:YGS_CHR_03_POST_FIG_05.jpg|Figure 5]] shows the results of quantitative analysis of the palaeotemperature constraints derived from AFTA and VR data (Bray et al. 1992) to define the range of values of palaeogeothermal gradient and missing section that are consistent with these data. Assuming a palaeogeothermal gradient similar to the present-day value of 34.5°Ckm–1, and a palaeosurface temperature of 20°C as advocated by Holliday (1999), results from this well require 500–1100 m of removed section (from the upper and lower limits of the shaded region in [[:File:YGS_CHR_03_POST_FIG_05.jpg|Figure 5]]).
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These features of the palaeotemperature profile suggest that heating was predominantly caused by deeper burial. [[:File:YGS_CHR_03_POST_FIG_05.jpg|Figure 5]] shows the results of quantitative analysis of the palaeotemperature constraints derived from AFTA and VR data (Bray et al. 1992) to define the range of values of palaeogeothermal gradient and missing section that are consistent with these data. Assuming a palaeogeothermal gradient similar to the present-day value of 34.5°Ckm–1, and a palaeosurface temperature of 20°C as advocated by Holliday (1999), results from this well require 500–1100m of removed section (from the upper and lower limits of the shaded region in [[:File:YGS_CHR_03_POST_FIG_05.jpg|Figure 5]]).
  
As also illustrated in [[:File:YGS_CHR_03_POST_FIG_05.jpg|Figure 5]], this amount of missing section is highly consistent with estimates in the region of 600–1000 m derived from sonic velocity data from both Late Cretaceous and Triassic strata in this and neighbouring wells by Japsen (2000). Thus, evidence from AFTA, VR and sonic velocity data are consistent with a scenario involving 800±200 m of additional section. This section must have been deposited subsequent to deposition of the youngest preserved Chalk in the 47/25-1 well (of Coniacian age, based on Cameron et al. 1992; fig. 82) and prior to the onset of cooling, which synthesis of AFTA from all wells suggests must have been prior to 55 Ma. Thus, all data point to a history involving a considerable thickness of sediment being deposited within an interval of not much more than 30 Ma and subsequently eroded, possibly in two stages based on tentative evidence from AFTA in the 47/25-1 well (above) and more conclusive evidence from AFTA data onshore (Green et al. 2001).
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As also illustrated in [[:File:YGS_CHR_03_POST_FIG_05.jpg|Figure 5]], this amount of missing section is highly consistent with estimates in the region of 600–1000m derived from sonic velocity data from both Late Cretaceous and Triassic strata in this and neighbouring wells by Japsen (2000). Thus, evidence from AFTA, VR and sonic velocity data are consistent with a scenario involving 800±200m of additional section. This section must have been deposited subsequent to deposition of the youngest preserved Chalk in the 47/25-1 well (of Coniacian age, based on Cameron et al. 1992; fig. 82) and prior to the onset of cooling, which synthesis of AFTA from all wells suggests must have been prior to 55Ma. Thus, all data point to a history involving a considerable thickness of sediment being deposited within an interval of not much more than 30Ma and subsequently eroded, possibly in two stages based on tentative evidence from AFTA in the 47/25-1 well (above) and more conclusive evidence from AFTA data onshore (Green et al. 2001).
  
 
Note that palaeogeothermal gradients slightly higher than the present-day value would be allowed by the palaeotemperature constraints from this well (up to ~52°Ckm–1), but the comparison with results based on analysis of sonic velocity data presented by Japsen (2000) suggest that a situation involving a palaeogeothermal gradient similar to the present-day value is more likely for this well.
 
Note that palaeogeothermal gradients slightly higher than the present-day value would be allowed by the palaeotemperature constraints from this well (up to ~52°Ckm–1), but the comparison with results based on analysis of sonic velocity data presented by Japsen (2000) suggest that a situation involving a palaeogeothermal gradient similar to the present-day value is more likely for this well.

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