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

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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.
 
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.
  
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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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 80Ma and 40Ma. 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 (1.2–2km, 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.
  
As discussed earlier, some previous studies have favoured an interpretation of the southern Pennines as a long-term high since end-Carboniferous times, with the region receiving little or no sedimentary cover during the Late Palaeozoic and Mesozoic times. However, the results presented here suggest instead a history more similar to that recently advocated for the Lake District Block (Green 2002), involving a former cover of up to 1 km or more of Late Palaeozoic and Mesozoic sediments, subsequently removed during Cainozoic exhumation. Such an interpretation is supported by sonic velocity data from wells onshore that clearly show a trend in estimates of “post-Cretaceous uplift” (more strictly exhumation) increasing from east to west and reaching values about 1.5 km immediately to the east of the southern Pennines (Whittaker et al. 1985).
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As discussed earlier, some previous studies have favoured an interpretation of the southern Pennines as a long-term high since end-Carboniferous times, with the region receiving little or no sedimentary cover during the Late Palaeozoic and Mesozoic times. However, the results presented here suggest instead a history more similar to that recently advocated for the Lake District Block (Green 2002), involving a former cover of up to 1km or more of Late Palaeozoic and Mesozoic sediments, subsequently removed during Cainozoic exhumation. Such an interpretation is supported by sonic velocity data from wells onshore that clearly show a trend in estimates of “post-Cretaceous uplift” (more strictly exhumation) increasing from east to west and reaching values about 1.5km immediately to the east of the southern Pennines (Whittaker et al. 1985).
  
 
This trend of course also implies prior burial by corresponding thicknesses of cover rocks, which, combined with the evidence from AFTA presented here, suggests the former presence of a continuous cover of Late Palaeozoic and Mesozoic sediments over the entire region. This implies, in turn, that all of the present-day upland regions of northern England were probably completely submerged by the Chalk, in sharp contrast to conventional depictions of the Late Cretaceous palaeogeography of the region (Fraser & Gawthorpe 1990, Fraser et al. 1990, Ziegler 1990, Cope et al. 1992).
 
This trend of course also implies prior burial by corresponding thicknesses of cover rocks, which, combined with the evidence from AFTA presented here, suggests the former presence of a continuous cover of Late Palaeozoic and Mesozoic sediments over the entire region. This implies, in turn, that all of the present-day upland regions of northern England were probably completely submerged by the Chalk, in sharp contrast to conventional depictions of the Late Cretaceous palaeogeography of the region (Fraser & Gawthorpe 1990, Fraser et al. 1990, Ziegler 1990, Cope et al. 1992).

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