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

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Details of the thermal-history solutions for each sample are listed in [[:File:YGS_CHR_03_POST_TAB_02.jpg|Table 2]] and illustrated in [[:File:YGS_CHR_03_POST_FIG_02.jpg|Figure 2]]. Sample RD41-46 reached a maximum palaeotemperature of 100–110°C, from which cooling began some time between 290 Ma and 220 Ma, whereas sample RD41-47 reached a maximum palaeotemperature in excess of 110°C and began to cool between 240 Ma and 180 Ma. Results from sample RD41-46 suggest a subsequent peak palaeotemperature of 80 to 90°C from which cooling began some time between 80 and 40 Ma, and for sample RD41-47 the data suggest a peak palaeotemperature of 75 to 85°C from which cooling began some time between 90 and 30 Ma.
 
Details of the thermal-history solutions for each sample are listed in [[:File:YGS_CHR_03_POST_TAB_02.jpg|Table 2]] and illustrated in [[:File:YGS_CHR_03_POST_FIG_02.jpg|Figure 2]]. Sample RD41-46 reached a maximum palaeotemperature of 100–110°C, from which cooling began some time between 290 Ma and 220 Ma, whereas sample RD41-47 reached a maximum palaeotemperature in excess of 110°C and began to cool between 240 Ma and 180 Ma. Results from sample RD41-46 suggest a subsequent peak palaeotemperature of 80 to 90°C from which cooling began some time between 80 and 40 Ma, and for sample RD41-47 the data suggest a peak palaeotemperature of 75 to 85°C from which cooling began some time between 90 and 30 Ma.
  
As these two samples were taken from outcrops separated by a distance of only some tens of metres, they can be combined to suggest that cooling in the two events began in the intervals 240–220 Ma and 80–40 Ma, with respective peak palaeotemperatures of 100–105°C and 80–85°C. Mean VR values measured in four samples of Edale Shales immediately underlying the Mam Tor Sandstones are between 0.53 and 0.57 per cent, equivalent to a maximum palaeotemperature of 88–94°C [[:File:YGS_CHR_03_POST_TAB_02.jpg|(Table 2)]]. These values are lower than the corresponding estimates from AFTA, which is thought to result from the suppression of reflectance levels in these samples. Such effects are common in carbonaceous shales rich in hydrogen or sulphur (see discussion and references in Green et al. 2002), and have been previously identified in the Bowland Shales of Namurian age in the Irish Sea region, by comparison of VR data with AFTA data in adjacent sandstones (Green et al. 1997).
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As these two samples were taken from outcrops separated by a distance of only some tens of metres, they can be combined to suggest that cooling in the two events began in the intervals 240– 220Ma and 80–40Ma, with respective peak palaeotemperatures of 100–105°C and 80–85°C. Mean VR values measured in four samples of Edale Shales immediately underlying the Mam Tor Sandstones are between 0.53 and 0.57 per cent, equivalent to a maximum palaeotemperature of 88–94°C [[:File:YGS_CHR_03_POST_TAB_02.jpg|(Table 2)]]. These values are lower than the corresponding estimates from AFTA, which is thought to result from the suppression of reflectance levels in these samples. Such effects are common in carbonaceous shales rich in hydrogen or sulphur (see discussion and references in Green et al. 2002), and have been previously identified in the Bowland Shales of Namurian age in the Irish Sea region, by comparison of VR data with AFTA data in adjacent sandstones (Green et al. 1997).
  
The estimated timing for the onset of cooling from maximum palaeotemperatures in the Mam Tor Sandstones, at 240–220 Ma (Early to Mid-Triassic), is significantly later than the end-Carboniferous (~300 Ma) timing generally believed to apply to the southern Pennines (e.g. Plant et al. 1988, Ewbank et al. 1995, Hollis 1998). This may simply reflect protracted cooling following Variscan tectonism. In this regard, it may be significant that AFTA data from the Apley Barn borehole in the Oxfordshire coalfield (Green et al. 2001) also showed cooling from palaeotemperatures in excess of 110°C some time between 270 Ma and 245 Ma, distinctly later than Variscan (end-Carboniferous) events, which could be taken as evidence in support of protracted post-Variscan cooling. However, some aspects of regional geology suggest that the Carboniferous rocks of the southern Pennines were close to the surface in Triassic times (P. Gutteridge, personal communication 2002), which would suggest that the cooling seen in the AFTA data must be attributable to processes other than burial. An alternative explanation may be hydrothermal effects during Late Triassic to Jurassic times, for which a considerable body of evidence has been provided from K/Ar dating of clays associated with mineral deposits in the southern Pennines and northern England (Ineson & Mitchell 1972, Mitchell & Ineson 1988). In this case, palaeotemperatures associated with this event would have obliterated any Variscan effects in the AFTA data.
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The estimated timing for the onset of cooling from maximum palaeotemperatures in the Mam Tor Sandstones, at 240–220Ma (Early to Mid-Triassic), is significantly later than the end-Carboniferous (~300Ma) timing generally believed to apply to the southern Pennines (e.g. Plant et al. 1988, Ewbank et al. 1995, Hollis 1998). This may simply reflect protracted cooling following Variscan tectonism. In this regard, it may be significant that AFTA data from the Apley Barn borehole in the Oxfordshire coalfield (Green et al. 2001) also showed cooling from palaeotemperatures in excess of 110°C some time between 270Ma and 245Ma, distinctly later than Variscan (end-Carboniferous) events, which could be taken as evidence in support of protracted post-Variscan cooling. However, some aspects of regional geology suggest that the Carboniferous rocks of the southern Pennines were close to the surface in Triassic times (P. Gutteridge, personal communication 2002), which would suggest that the cooling seen in the AFTA data must be attributable to processes other than burial. An alternative explanation may be hydrothermal effects during Late Triassic to Jurassic times, for which a considerable body of evidence has been provided from K/Ar dating of clays associated with mineral deposits in the southern Pennines and northern England (Ineson & Mitchell 1972, Mitchell & Ineson 1988). In this case, palaeotemperatures associated with this event would have obliterated any Variscan effects in the AFTA data.
  
 
Evidence from AFTA for the more recent cooling event is more straightforward, with combined results from both samples consistent with cooling from 80°C to 85°C some time between 80 Ma and 40 Ma. This timing is consistent with the Palaeocene cooling event recognized from AFTA over a wider area of central and northern England (reviewed earlier), and the range of palaeotemperatures is similar in magnitude to values derived from AFTA data in other samples from the eastern flank of the southern Pennines by Green (1989) and Green et al. (2001). For likely values of palaeogeothermal gradient (say 30–50°Ckm–1), this palaeotemperature range suggests appreciable burial (1.2–2 km, assuming a Palaeocene palaeotemperature of 20°C) prior to Cainozoic exhumation, which is consistent with previously published results from wells and outcrop locations to the south.
 
Evidence from AFTA for the more recent cooling event is more straightforward, with combined results from both samples consistent with cooling from 80°C to 85°C some time between 80 Ma and 40 Ma. This timing is consistent with the Palaeocene cooling event recognized from AFTA over a wider area of central and northern England (reviewed earlier), and the range of palaeotemperatures is similar in magnitude to values derived from AFTA data in other samples from the eastern flank of the southern Pennines by Green (1989) and Green et al. (2001). For likely values of palaeogeothermal gradient (say 30–50°Ckm–1), this palaeotemperature range suggests appreciable burial (1.2–2 km, assuming a Palaeocene palaeotemperature of 20°C) prior to Cainozoic exhumation, which is consistent with previously published results from wells and outcrop locations to the south.

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