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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== 4. The relevance of the Flamborough outlier ==
 
== 4. The relevance of the Flamborough outlier ==
  
Stewart & Bailey (1996) reported a previously unrecognized package of sedimentary rocks of Late Palaeocene to Mid-Eocene age straddling blocks 42/29 and 47/4b, which they termed the Flamborough Outlier [[:File:YGS_CHR_03_POST_FIG_01.jpg|(Figure 1)]]. They suggested that these strata are a remnant of the previously more extensive sedimentary cover responsible for the regional burial effects identified from sonic velocity studies and studies based on AFTA, VR, etc. This, in turn, suggests that the cooling episode identified from earlier AFTA-based studies must have begun after deposition of these sediments, suggesting that cooling must have begun later than Middle Eocene times (~40 Ma). Stewart & Bailey (1996: 172) commented that the previous AFTA-based studies had been interpreted as requiring deposition of missing section during Campanian to Danian times “based on the assumption that no significant thickness of sediment accumulated on the East Midlands Shelf following the onset of uplift (Bray et al. 1992)”. This statement is not accurate. The interpretation that the now eroded sedimentary units were of Campanian to Danian age was based solely on the timing constraints derived from AFTA, showing that cooling must have begun by ~60 Ma, which requires that the additional burial required to produce the observed heating must have been deposited prior to this time. We emphasize again that the results of reassessing the regional AFTA dataset, some of which are reported here, confirm the Palaeocene timing for the onset of cooling.
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Stewart & Bailey (1996) reported a previously unrecognized package of sedimentary rocks of Late Palaeocene to Mid-Eocene age straddling blocks 42/29 and 47/4b, which they termed the Flamborough Outlier [[:File:YGS_CHR_03_POST_FIG_01.jpg|(Figure 1)]]. They suggested that these strata are a remnant of the previously more extensive sedimentary cover responsible for the regional burial effects identified from sonic velocity studies and studies based on AFTA, VR, etc. This, in turn, suggests that the cooling episode identified from earlier AFTA-based studies must have begun after deposition of these sediments, suggesting that cooling must have begun later than Middle Eocene times (~40Ma). Stewart & Bailey (1996: 172) commented that the previous AFTA-based studies had been interpreted as requiring deposition of missing section during Campanian to Danian times “based on the assumption that no significant thickness of sediment accumulated on the East Midlands Shelf following the onset of uplift (Bray et al. 1992)”. This statement is not accurate. The interpretation that the now eroded sedimentary units were of Campanian to Danian age was based solely on the timing constraints derived from AFTA, showing that cooling must have begun by ~60Ma, which requires that the additional burial required to produce the observed heating must have been deposited prior to this time. We emphasize again that the results of reassessing the regional AFTA dataset, some of which are reported here, confirm the Palaeocene timing for the onset of cooling.
  
The results presented here show quite clearly that the main erosional episode on the East Midlands Shelf must have occurred after deposition of the youngest preserved Chalk and prior to deposition of the Palaeogene strata recognized by Stewart & Bailey (1996). Evidence in support of this conclusion is seen in the estimates of missing section derived by Japsen (2000) from sonic velocities, which show no evidence of any reduction in the amount of missing section in the vicinity of the outlier of Palaeogene strata, as might have been expected if the sedimentary section is more complete in that region. Instead, values of Japsen’s (2000) “burial anomaly” remain at about 0.8–1.0 km across the region of the outlier, suggesting that, even where the Palaeogene strata are preserved, an additional ~1 km or so of strata have been deposited and eroded.
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The results presented here show quite clearly that the main erosional episode on the East Midlands Shelf must have occurred after deposition of the youngest preserved Chalk and prior to deposition of the Palaeogene strata recognized by Stewart & Bailey (1996). Evidence in support of this conclusion is seen in the estimates of missing section derived by Japsen (2000) from sonic velocities, which show no evidence of any reduction in the amount of missing section in the vicinity of the outlier of Palaeogene strata, as might have been expected if the sedimentary section is more complete in that region. Instead, values of Japsen’s (2000) “burial anomaly” remain at about 0.8–1.0km across the region of the outlier, suggesting that, even where the Palaeogene strata are preserved, an additional ~1km or so of strata have been deposited and eroded.
  
In fact, these observations lead to the conclusion that, at least in the vicinity of the outlier, erosional removal of the additional strata must have been complete prior to deposition of the Palaeogene strata preserved in the outlier. With the age of the oldest Palaeogene units in the outlier being of Late Palaeocene (Early Thanetian) age, the timing constraints from AFTA for the onset of cooling suggest that exhumation must have been extremely rapid, with the entire package of additional strata removed in as little as perhaps 5 Ma (taking the oldest limit on cooling from AFTA of 65 Ma and an age of ~60 Ma for Early Thanetian from Harland 1989). Erosion of sedimentary rocks of similar (Palaeogene) age to those in the outlier from adjacent regions of the shelf was probably achieved during the more recent (Miocene) episode of exhumation recognized in regional AFTA data, particularly onshore (see earlier discussion). This episode most likely correlates with the phase of Late Miocene inversion recognized in the SNS by Stewart & Bailey (1996) that they suggested represented the dominant erosional episode across the region.
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In fact, these observations lead to the conclusion that, at least in the vicinity of the outlier, erosional removal of the additional strata must have been complete prior to deposition of the Palaeogene strata preserved in the outlier. With the age of the oldest Palaeogene units in the outlier being of Late Palaeocene (Early Thanetian) age, the timing constraints from AFTA for the onset of cooling suggest that exhumation must have been extremely rapid, with the entire package of additional strata removed in as little as perhaps 5Ma (taking the oldest limit on cooling from AFTA of 65Ma and an age of ~60Ma for Early Thanetian from Harland 1989). Erosion of sedimentary rocks of similar (Palaeogene) age to those in the outlier from adjacent regions of the shelf was probably achieved during the more recent (Miocene) episode of exhumation recognized in regional AFTA data, particularly onshore (see earlier discussion). This episode most likely correlates with the phase of Late Miocene inversion recognized in the SNS by Stewart & Bailey (1996) that they suggested represented the dominant erosional episode across the region.
  
 
The foregoing discussion shows that many lines of evidence point to the conclusion that the offshore as well as the onshore EMS has undergone two major episodes of burial and subsequent exhumation during the Latest Cretaceous and Cainozoic. This is consistent with the conclusion reached by Japsen (1997), although the relative contributions of the two episodes may not vary exactly as he suggested. This aspect of the AFTA data from the southern North Sea will be discussed in detail, together with the regional dataset, elsewhere.
 
The foregoing discussion shows that many lines of evidence point to the conclusion that the offshore as well as the onshore EMS has undergone two major episodes of burial and subsequent exhumation during the Latest Cretaceous and Cainozoic. This is consistent with the conclusion reached by Japsen (1997), although the relative contributions of the two episodes may not vary exactly as he suggested. This aspect of the AFTA data from the southern North Sea will be discussed in detail, together with the regional dataset, elsewhere.
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Bray, R. J., P. F. Green, I. R. Duddy 1992. Thermal history reconstruction using apatite fission track analysis and vitrinite reflectance: a case study from the UK East Midlands and the southern North Sea. In ''Exploration Britain: into the next decade'', R. F. P. Hardman (ed.), 3–25. Special Publication 67, Geological Society, London.
 
Bray, R. J., P. F. Green, I. R. Duddy 1992. Thermal history reconstruction using apatite fission track analysis and vitrinite reflectance: a case study from the UK East Midlands and the southern North Sea. In ''Exploration Britain: into the next decade'', R. F. P. Hardman (ed.), 3–25. Special Publication 67, Geological Society, London.
  
Cameron, T. D. J., A. Crosby, P. S. Balson, D. H. Jeffery, G. K. Lott, J. Bulat, D. J. Harrison 1992. ''United Kingdom offshore regional report: the geology of the southern North Sea''. London: HMSO.  
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Cameron, T. D. J., A. Crosby, P. S. Balson, D. H. Jeffery, G. K. Lott, J. Bulat, D. J. Harrison 1992. ''United Kingdom offshore regional report: the geology of the southern North Sea''. London: HMSO. Cope, J. C. W., J. K. Ingham, P. F. Rawson 1992. ''Atlas of palaeogeography and lithofacies''. Memoir 13, Geological Society, London. Crowhurst. P. V., P. F. Green, P. J. J. Kamp 2002. Appraisal of (U– Th)/He apatite thermochronology as a thermal history tool for hydrocarbon exploration: an example from the Taranaki Basin, New Zealand. ''American Association of Petroleum Geologists, Bulletin ''86, 1801–819.
 
 
Cope, J. C. W., J. K. Ingham, P. F. Rawson 1992. ''Atlas of palaeogeography and lithofacies''. Memoir 13, Geological Society, London.  
 
 
 
Crowhurst. P. V., P. F. Green, P. J. J. Kamp 2002. Appraisal of (U– Th)/He apatite thermochronology as a thermal history tool for hydrocarbon exploration: an example from the Taranaki Basin, New Zealand. ''American Association of Petroleum Geologists, Bulletin ''86, 1801–819.
 
  
 
Duddy, I. R. & B. Erout 2001. AFTA®-calibrated 2-D modelling of hydrocarbon generation and migration using Temispack®: preliminary results from the Otway Basin. In ''Eastern Australian basins symposium: a refocused energy perspective for the future'', K. C. Hill & T. Bernecker (eds), 485–97. Special Publication, Petroleum Exploration Society of Australia.
 
Duddy, I. R. & B. Erout 2001. AFTA®-calibrated 2-D modelling of hydrocarbon generation and migration using Temispack®: preliminary results from the Otway Basin. In ''Eastern Australian basins symposium: a refocused energy perspective for the future'', K. C. Hill & T. Bernecker (eds), 485–97. Special Publication, Petroleum Exploration Society of Australia.
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Green, P. F. 2002. Early Tertiary palaeothermal effects in northern England: reconciling results from apatite fission track analysis with geological evidence. ''Tectonophysics ''349, 131–44.
 
Green, P. F. 2002. Early Tertiary palaeothermal effects in northern England: reconciling results from apatite fission track analysis with geological evidence. ''Tectonophysics ''349, 131–44.
  
Green, P. F., I. R. Duddy, R. J. Bray 1995a. Further discussion on Mesozoic cover over northern England: interpretation of apatite fission track data. ''Geological Society of London, Journal ''152, 416. 
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Green, P. F., I. R. Duddy, R. J. Bray 1995a. Further discussion on Mesozoic cover over northern England: interpretation of apatite fission track data. ''Geological Society of London, Journal ''152, 416. 1995b. Applications of thermal history reconstruction in inverted basins. In ''Basin inversion'', J. G. Buchanan & P. G. Buchanan (eds), 148–65. Special Publication 88, Geological Society, London.
 
 
Green, P. F., I. R. Duddy, R. J. Bray 1995b. Applications of thermal history reconstruction in inverted basins. In ''Basin inversion'', J. G. Buchanan & P. G. Buchanan (eds), 148–65. Special Publication 88, Geological Society, London.
 
  
Green, P. F., I. R. Duddy, R. J. Bray 1997. Variation in thermal history styles around the Irish Sea and adjacent areas: implications for hydrocarbon occurrence and tectonic evolution. In ''Petroleum geology of the Irish Sea and adjacent areas'', N. S. Meadows, S. Trueblood, M. Hardman, G. Cowan (eds), 73–93. Special Publication 124, Geological Society, London.  
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Green, P. F. 1997. Variation in thermal history styles around the Irish Sea and adjacent areas: implications for hydrocarbon occurrence and tectonic evolution. In ''Petroleum geology of the Irish Sea and adjacent areas'', N. S. Meadows, S. Trueblood, M. Hardman, G. Cowan (eds), 73–93. Special Publication 124, Geological Society, London. Green, P. F., J. D. Hudson, K. Thomson 2001. Recognition of tectonic events in undeformed regions: contrasting results from the Midland Platform and East Midlands Shelf, central England. ''Geological Society of London, Journal ''158, 59–73.
  
Green, P. F., J. D. Hudson, K. Thomson 2001. Recognition of tectonic events in undeformed regions: contrasting results from the Midland Platform and East Midlands Shelf, central England. ''Geological Society of London, Journal ''158, 59–73.
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Green, P. F., I. R. Duddy, K. A. Hegarty 2002. Quantifying exhumation from apatite fission-track analysis and vitrinite reflectance data: precision, accuracy and latest results from the Atlantic margin of northwest Europe. In ''Exhumation of the North Atlantic margin: timing, mechanisms and implications for petroleum exploration'', A. G. Doré, J. Cartwright, M. S. Stoker, J. P. Turner, N. White (eds), 331–54. Special Publication 196, Geological Society, London. Harland, W. B., R. L. Armstrong, A. V. Cox, L. E. Craig, A. G. Smith, D. G. Smith 1989. ''A geologic time scale 1989''. Cambridge: Cambridge University Press.
 
 
Green, P. F., I. R. Duddy, K. A. Hegarty 2002. Quantifying exhumation from apatite fission-track analysis and vitrinite reflectance data: precision, accuracy and latest results from the Atlantic margin of northwest Europe. In ''Exhumation of the North Atlantic margin: timing, mechanisms and implications for petroleum exploration'', A. G. Doré, J. Cartwright, M. S. Stoker, J. P. Turner, N. White (eds), 331–54. Special Publication 196, Geological Society, London.  
 
 
 
Harland, W. B., R. L. Armstrong, A. V. Cox, L. E. Craig, A. G. Smith, D. G. Smith 1989. ''A geologic time scale 1989''. Cambridge: Cambridge University Press.
 
  
 
Hillis, R. 1991. Chalk porosity and Tertiary uplift, Western Approaches Trough, SW UK. ''Geological Society of London, Journal ''148, 669–79.
 
Hillis, R. 1991. Chalk porosity and Tertiary uplift, Western Approaches Trough, SW UK. ''Geological Society of London, Journal ''148, 669–79.
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Stewart, S. A. & H. W. Bailey 1996. The Flamborough Outlier, UK southern North Sea. ''Geological Society of London, Journal ''153, 163–73.
 
Stewart, S. A. & H. W. Bailey 1996. The Flamborough Outlier, UK southern North Sea. ''Geological Society of London, Journal ''153, 163–73.
  
Whittaker, A., D. W. Holliday, I. E. Penn 1985. ''Geophysical logs in British stratigraphy''. Special Report 18, Geological Society, London.  
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Whittaker, A., D. W. Holliday, I. E. Penn 1985. ''Geophysical logs in British stratigraphy''. Special Report 18, Geological Society, London. Ziegler, P. A. 1990. ''Geological atlas of western and central Europe'', 2nd edn. Rotterdam: Shell Internationale Petroleum Maatshappij BV.
 
 
Ziegler, P. A. 1990. ''Geological atlas of western and central Europe'', 2nd edn. Rotterdam: Shell Internationale Petroleum Maatshappij BV.
 
 
 
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