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

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[[File:YGS_CHR_03_POST_FIG_03.jpg|thumbnail|Figure 3 AFTA data from well 47/25-1.  
 
[[File:YGS_CHR_03_POST_FIG_03.jpg|thumbnail|Figure 3 AFTA data from well 47/25-1.  
 
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[[File:YGS_CHR_03_POST_FIG_04.jpg|thumbnail|Figure 4 Palaeotemperatures determined from AFTA and VR data in well 47/25-1 .]]
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[[File:YGS_CHR_03_POST_FIG_04.jpg|thumbnail|Figure 4 Palaeotemperatures determined from AFTA and VR data in well 47/25-1 define a linear depth profile (solid line), subparallel to the present-day temperature profile (dashed line) derived from corrected BHT values in this well, and offset to higher temperatures by ~30–40°C. This suggests that heating was attributable primarily to deeper burial, with little or no difference in heatflow compared to the present day.]]
  
[[File:YGS_CHR_03_POST_FIG_05.jpg|thumbnail|Figure 5 Fitting a linear palaeotemperature profile to the constraints derived from AFTA and VR data in SNS well 47/25-1 [[:File:YGS_CHR_03_POST_FIG_04.jpg|(Figure 4)]] .]]
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[[File:YGS_CHR_03_POST_FIG_05.jpg|thumbnail|Figure 5 Fitting a linear palaeotemperature profile to the constraints derived from AFTA and VR data in SNS well 47/25-1 [[:File:YGS_CHR_03_POST_FIG_04.jpg|(Figure 4)]] and extrapolation to an assumed palaeosurface temperature of 20°C allows estimation of the range of palaeogeothermal gradients and removed section that are consistent with the data within 95% confidence limits, as shown by the shaded zone. The methods employed in constructing this Figure, and the assumptions embodied in the analysis, have been described, for example by Bray et al. (1992), Crowhurst et al. (2002) and Green et al. (2002). Also shown is the range of values of removed section defined by sonic velocity data from Triassic and Upper Cretaceous units in this and adjacent wells, from Japsen (2000). For a palaeogeothermal gradient close to the present-day value of 35.4°Ckm<sup>–1</sup>, allowed values of removed section derived from the AFTA and VR data are highly consistent with those derived from the sonic velocity data, and data from three independent systems give consistent indications of 600–1000m of missing post-Chalk section]]
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[[File:YGS_CHR_03_POST_FIG_06.jpg|thumbnail|Figure 6 Schematic illustration of the reconstructed burial and exhumation history for well 47/25-1, derived from AFTA and VR data. This history is based on compacted sedimentary rock thicknesses, for simplicity, and highlights only the major features, viz: no detectable Variscan effects; maximum burial depths in the Palaeocene, with 600–1000 m of additional Late Cretaceous section removed during Cainozoic exhumation; rapid exhumation in the Early Cainozoic, followed by further deposition of Palaeogene units and a Late Miocene episode of exhumation (based on arguments discussed in the text related to the Flamborough outlier). Note that the most recent episode is not revealed by AFTA data in this well, and should be regarded as speculative. Delineation of this aspect of the history forms a focus of continuing work in the region.]]
  
[[File:YGS_CHR_03_POST_FIG_06.jpg|thumbnail|Figure 6 Schematic illustration of the reconstructed burial and exhumation history for well 47/25-1, derived from AFTA and VR data.]]
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[[File:YGS_CHR_03_POST_FIG_07.jpg|thumbnail|Figure 7 Schematic illustrations of reconstructed burial and exhumation histories and thermal histories for well 47/25-1, onshore well Rufford-1, and Edale Shales from outcrop in the southern Pennines, based on results presented here and in Green et al. (2001). The exact form of the Mesozoic history, particularly for the southern Pennines, is highly speculative, and only the points of peak palaeotemperature and burial in the Early Tertiary are well constrained, but the data discussed here suggest that the broad nature of the history is probably reliable. The earlier palaeothermal event revealed by AFTA in this area is shown as a discrete Triassic hydrothermal event, although this remains speculative. Palaeosurface temperatures have been held constant at 20°C prior to Late Cretaceous times in the thermal reconstructions, as the data are not sensitive to this part of the history. AFTA data from the Rufford-1 well provide strong evidence for two distinct episodes of cooling during the Cainozoic (Green et al. 2001), although this part of the history for the other two situations remains somewhat speculative at present.]]
 
 
[[File:YGS_CHR_03_POST_FIG_07.jpg|thumbnail|Figure 7 Schematic illustrations of reconstructed burial and exhumation histories and thermal histories for well 47/25-1, onshore well Rufford-1, and Edale Shales from outcrop in the southern Pennines.]]
 
  
 
[[File:YGS_CHR_03_POST_TAB_01.jpg|thumbnail|Table 1 Sample details and apatite fission-track age data.]]
 
[[File:YGS_CHR_03_POST_TAB_01.jpg|thumbnail|Table 1 Sample details and apatite fission-track age data.]]
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== Summary ==
 
== Summary ==
  
Previous AFTA studies in the UK southern North Sea and adjacent onshore areas have generated considerable discussion, particularly concerning the timing and magnitude of additional burial and subsequent uplift and erosion. New AFTA and VR results confirm and extend the conclusions of these earlier studies. In the southern Pennines, Carboniferous strata cooled from maximum palaeotemperatures about 100°C or more in Late Palaeozoic times, and from a peak palaeotemperature of ~80°C in Early Palaeogene times. For reasonable palaeogeothermal gradients, this palaeotemperature suggests burial by 1–2 km of Late Palaeozoic and Mesozoic rocks prior to Cainozoic exhumation. New AFTA and VR data from offshore well 47/25-1 also show that rocks of Carboniferous to Upper Cretaceous age were buried more deeply prior to exhumation that began between 90 and 40 Ma. Data from neighbouring wells refine this timing to 65–55 Ma. Combining the AFTA and VR data from the 47/25-1 well with sonic velocity-based constraints on palaeoburial suggests that an additional 800±200 m of section were deposited in 30 million years or less, prior to the onset of exhumation in Palaeocene times. A distinct Neogene phase of exhumation is not resolved from these data, although regional evidence suggests a significant proportion of the total missing section may have been removed during Neogene times. “Palaeoburial” of Carboniferous source rocks and their subsequent exhumation, recorded in AFTA, VR and sonic velocity data from the southern North Sea, have played an important role in defining and shaping the occurrences of hydrocarbons across the region.
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Previous AFTA studies in the UK southern North Sea and adjacent onshore areas have generated considerable discussion, particularly concerning the timing and magnitude of additional burial and subsequent uplift and erosion. New AFTA and VR results confirm and extend the conclusions of these earlier studies. In the southern Pennines, Carboniferous strata cooled from maximum palaeotemperatures about 100°C or more in Late Palaeozoic times, and from a peak palaeotemperature of ~80°C in Early Palaeogene times. For reasonable palaeogeothermal gradients, this palaeotemperature suggests burial by 1–2 km of Late Palaeozoic and Mesozoic rocks prior to Cainozoic exhumation. New AFTA and VR data from offshore well 47/25-1 also show that rocks of Carboniferous to Upper Cretaceous age were buried more deeply prior to exhumation that began between 90 and 40 Ma. Data from neighbouring wells refine this timing to 65–55Ma. Combining the AFTA and VR data from the 47/25-1 well with sonic velocity-based constraints on palaeoburial suggests that an additional 800±200 m of section were deposited in 30 million years or less, prior to the onset of exhumation in Palaeocene times. A distinct Neogene phase of exhumation is not resolved from these data, although regional evidence suggests a significant proportion of the total missing section may have been removed during Neogene times. “Palaeoburial” of Carboniferous source rocks and their subsequent exhumation, recorded in AFTA, VR and sonic velocity data from the southern North Sea, have played an important role in defining and shaping the occurrences of hydrocarbons across the region.
  
 
== Introduction ==
 
== Introduction ==
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== 1. Previous AFTA studies ==
 
== 1. Previous AFTA studies ==
  
Application of AFTA to samples from outcrops and hydrocarbon exploration wells on the East Midlands Shelf (EMS), and to samples from the UK southern North Sea (SNS) wells, has shown that the sedimentary section in this region has experienced major Cainozoic cooling (Green 1989, Bray et al. 1992, Green et al. 2001). Results in sedimentary rocks of Carboniferous to Triassic age from outcrops on the onshore EMS reveal cooling from palaeotemperatures of 70–90°C beginning some time between 65 and 55Ma (Palaeocene). Results from subsurface samples confirm this episode and also provide improved definition of the cooling history, revealing an additional subsequent cooling episode from lower peak palaeotemperatures, which began some time between 25 Ma and 5 Ma (Miocene). Vitrinite reflectance data from Carboniferous units in EMS wells are highly consistent with the Palaeocene palaeotemperatures defined by AFTA (Bray et al. 1992, Green et al. 2001), and it is clear that, in these wells, Carboniferous units cooled from their maximum postdepositional palaeotemperatures in Palaeocene times, which effectively dates the termination of active hydrocarbon generation from Carboniferous source rocks in the region.
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Application of AFTA to samples from outcrops and hydrocarbon exploration wells on the East Midlands Shelf (EMS), and to samples from the UK southern North Sea (SNS) wells, has shown that the sedimentary section in this region has experienced major Cainozoic cooling (Green 1989, Bray et al. 1992, Green et al. 2001). Results in sedimentary rocks of Carboniferous to Triassic age from outcrops on the onshore EMS reveal cooling from palaeotemperatures of 70–90°C beginning some time between 65 and 55Ma (Palaeocene). Results from subsurface samples confirm this episode and also provide improved definition of the cooling history, revealing an additional subsequent cooling episode from lower peak palaeotemperatures, which began some time between 25Ma and 5Ma (Miocene). Vitrinite reflectance data from Carboniferous units in EMS wells are highly consistent with the Palaeocene palaeotemperatures defined by AFTA (Bray et al. 1992, Green et al. 2001), and it is clear that, in these wells, Carboniferous units cooled from their maximum postdepositional palaeotemperatures in Palaeocene times, which effectively dates the termination of active hydrocarbon generation from Carboniferous source rocks in the region.
  
Attempts to understand the mechanisms responsible for the elevated Palaeocene palaeotemperatures and subsequent Cainozoic cooling, and also the exact timing at which cooling began, have been the subject of some discussion. Green (1989) reported AFTA data from five EMS wells. He suggested that Palaeocene palaeogeothermal gradients were indistinguishable from present-day values, and that 1–2 km of section have been removed by Cainozoic uplift and erosion. Bray et al. (1992) came to a similar conclusion, on the basis of a more rigorous statistical analysis of palaeotemperatures derived from AFTA and VR data from these wells. Bray et al. (1992) also reported that similar effects had been detected in wells from the offshore (SNS) portion of the EMS.
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Attempts to understand the mechanisms responsible for the elevated Palaeocene palaeotemperatures and subsequent Cainozoic cooling, and also the exact timing at which cooling began, have been the subject of some discussion. Green (1989) reported AFTA data from five EMS wells. He suggested that Palaeocene palaeogeothermal gradients were indistinguishable from present-day values, and that 1–2km of section have been removed by Cainozoic uplift and erosion. Bray et al. (1992) came to a similar conclusion, on the basis of a more rigorous statistical analysis of palaeotemperatures derived from AFTA and VR data from these wells. Bray et al. (1992) also reported that similar effects had been detected in wells from the offshore (SNS) portion of the EMS.
  
 
Although results of sonic velocity studies of wells in the region (Hillis 1991, 1993) supported the estimates of Cainozoic exhumation derived from AFTA and VR data, Holliday (1993) and Smith et al. (1994) considered these amounts to be unrealistically large, on the basis of regional geological trends. These concerns were echoed more recently by Holliday (1999). Specific comments included doubts about the validity of extrapolating linear palaeogeothermal gradients to estimate removed section, questions concerning the most appropriate values of palaeosurface temperature, and, on the basis of criticisms by McCulloch (1994) that were shown to be erroneous by Green et al. (1995a), the precise timing at which cooling began.
 
Although results of sonic velocity studies of wells in the region (Hillis 1991, 1993) supported the estimates of Cainozoic exhumation derived from AFTA and VR data, Holliday (1993) and Smith et al. (1994) considered these amounts to be unrealistically large, on the basis of regional geological trends. These concerns were echoed more recently by Holliday (1999). Specific comments included doubts about the validity of extrapolating linear palaeogeothermal gradients to estimate removed section, questions concerning the most appropriate values of palaeosurface temperature, and, on the basis of criticisms by McCulloch (1994) that were shown to be erroneous by Green et al. (1995a), the precise timing at which cooling began.
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Despite these concerns, subsequent work has supported the conclusions of these early AFTA studies. The general validity of the approach employed in the EMS wells has been confirmed by application to controlled situations in various parts of the world, where geological evidence provides independent constraints on both the amount of removed section and the timing of cooling. In such situations, estimates from AFTA are highly consistent with the independent geological constraints (e.g. Green et al. 1995b, Crowhurst et al. 2002), suggesting that the approach can be used with confidence in less well controlled settings.
 
Despite these concerns, subsequent work has supported the conclusions of these early AFTA studies. The general validity of the approach employed in the EMS wells has been confirmed by application to controlled situations in various parts of the world, where geological evidence provides independent constraints on both the amount of removed section and the timing of cooling. In such situations, estimates from AFTA are highly consistent with the independent geological constraints (e.g. Green et al. 1995b, Crowhurst et al. 2002), suggesting that the approach can be used with confidence in less well controlled settings.
  
More specifically, reassessment of AFTA data from the Rufford-1 well (Green et al. 2001), located on the onshore EMS, has confirmed both the Palaeocene timing for the onset of cooling and the requirement for about 1450 m of post-Triassic cover removed during Cainozoic exhumation, much of which may have been removed during the Neogene. This most recent interpretation employs a palaeosurface temperature of 20°C, as suggested by Holliday (1993), coupled with a Palaeocene palaeogeothermal gradient about 30 per cent higher than the present-day value. Both these factors serve to reduce the amount of additional section required to explain the observed Palaeocene palaeotemperatures from those originally estimated by Green (1989) and Bray et al. (1992), although the amounts are still higher than suggested simply from regional geological tends, which would suggest a maximum of about 800–900 m (Green et al. 2001). Reasons for this discrepancy are the subject of continuing investigations in the region.
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More specifically, reassessment of AFTA data from the Rufford-1 well (Green et al. 2001), located on the onshore EMS, has confirmed both the Palaeocene timing for the onset of cooling and the requirement for about 1450m of post-Triassic cover removed during Cainozoic exhumation, much of which may have been removed during the Neogene. This most recent interpretation employs a palaeosurface temperature of 20°C, as suggested by Holliday (1993), coupled with a Palaeocene palaeogeothermal gradient about 30 per cent higher than the present-day value. Both these factors serve to reduce the amount of additional section required to explain the observed Palaeocene palaeotemperatures from those originally estimated by Green (1989) and Bray et al. (1992), although the amounts are still higher than suggested simply from regional geological tends, which would suggest a maximum of about 800–900m (Green et al. 2001). Reasons for this discrepancy are the subject of continuing investigations in the region.
  
 
The identification of significant Neogene exhumation in the results from the Rufford-1 well (Green et al. 2001) is consistent with the suggestion by Japsen (1997) that much of the Cainozoic exhumation in and around the UK southern North Sea may have taken place during the Neogene, although the suggestion by Japsen (1997) that Palaeocene exhumation was restricted principally to onshore areas is shown to be incorrect by the results presented here.
 
The identification of significant Neogene exhumation in the results from the Rufford-1 well (Green et al. 2001) is consistent with the suggestion by Japsen (1997) that much of the Cainozoic exhumation in and around the UK southern North Sea may have taken place during the Neogene, although the suggestion by Japsen (1997) that Palaeocene exhumation was restricted principally to onshore areas is shown to be incorrect by the results presented here.
  
Recent AFTA results from the Lake District of northwest England (Green 2002) have also confirmed previous results from that region (which were also the subject of some discussion), and have finally provided a geologically plausible explanation of Palaeocene palaeotemperatures in that region as being attributable to a combination of elevated basal heat flow and moderate amounts (generally 700–1550 m) of additional Late Palaeozoic to Mesozoic burial (with Cainozoic cooling caused by subsequent exhumation and reduction in heatflow). In this context, the results from the EMS and SNS, discussed above, form part of a highly consistent regional picture.
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Recent AFTA results from the Lake District of northwest England (Green 2002) have also confirmed previous results from that region (which were also the subject of some discussion), and have finally provided a geologically plausible explanation of Palaeocene palaeotemperatures in that region as being attributable to a combination of elevated basal heat flow and moderate amounts (generally 700–1550m) of additional Late Palaeozoic to Mesozoic burial (with Cainozoic cooling caused by subsequent exhumation and reduction in heatflow). In this context, the results from the EMS and SNS, discussed above, form part of a highly consistent regional picture.
  
 
With results from a variety of sources pointing to a consistent regional framework, the present study was undertaken in order to eliminate some of the remaining areas of uncertainty regarding the magnitude and mechanisms of Cainozoic palaeothermal effects across the region shown in [[:File:YGS_CHR_03_POST_FIG_01.jpg|Figure 1]].
 
With results from a variety of sources pointing to a consistent regional framework, the present study was undertaken in order to eliminate some of the remaining areas of uncertainty regarding the magnitude and mechanisms of Cainozoic palaeothermal effects across the region shown in [[:File:YGS_CHR_03_POST_FIG_01.jpg|Figure 1]].
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To emphasize the importance of Early Tertiary effects in the southern Pennines, new AFTA and VR results are presented [[:File:YGS_CHR_03_POST_TAB_01.jpg|(Table 1)]], [[:File:YGS_CHR_03_POST_TAB_02.jpg|(Table 2)]] from the Namurian Mam Tor Sandstone and Edale Shales, collected from the vicinity of Mam Tor, near Castleton on the northern flank of the Derbyshire Dome [[:File:YGS_CHR_03_POST_FIG_01.jpg|(Figure 1)]]. The principles involved in application of AFTA and VR, and the extraction of thermal history solutions from these data, have been outlined elsewhere (e.g. Green et al. 2001, 2002, Crowhurst et al. 2002) and are not repeated here. AFTA data from two samples of Mam Tor Sandstone are illustrated in [[:File:YGS_CHR_03_POST_FIG_02.jpg|Figure 2]], together with the resulting thermal-history solutions. Note that AFTA does not constrain the entire thermal history of the host rock. Rather, the data are dominated by the major palaeothermal events that have affected the sample, and extraction of thermal-history solutions from the data are designed with this in mind (Green et al. 2002).
 
To emphasize the importance of Early Tertiary effects in the southern Pennines, new AFTA and VR results are presented [[:File:YGS_CHR_03_POST_TAB_01.jpg|(Table 1)]], [[:File:YGS_CHR_03_POST_TAB_02.jpg|(Table 2)]] from the Namurian Mam Tor Sandstone and Edale Shales, collected from the vicinity of Mam Tor, near Castleton on the northern flank of the Derbyshire Dome [[:File:YGS_CHR_03_POST_FIG_01.jpg|(Figure 1)]]. The principles involved in application of AFTA and VR, and the extraction of thermal history solutions from these data, have been outlined elsewhere (e.g. Green et al. 2001, 2002, Crowhurst et al. 2002) and are not repeated here. AFTA data from two samples of Mam Tor Sandstone are illustrated in [[:File:YGS_CHR_03_POST_FIG_02.jpg|Figure 2]], together with the resulting thermal-history solutions. Note that AFTA does not constrain the entire thermal history of the host rock. Rather, the data are dominated by the major palaeothermal events that have affected the sample, and extraction of thermal-history solutions from the data are designed with this in mind (Green et al. 2002).
  
In both samples, the AFTA data clearly require at least two major episodes of heating and cooling [[:File:YGS_CHR_03_POST_FIG_02.jpg|(Figure 2)]]. In each sample, the earlier event is required to explain the fission-track age data, with apatite grains over a range of Cl contents (up to 0.7 wt per cent Cl in sample RD41-47) giving ages consistently younger than the value expected if the sample has not been significantly heated since deposition (horizontal bars in the age versus Cl plots in [[:File:YGS_CHR_03_POST_FIG_02.jpg|Figure 2]]). Both the pooled fission-track age of 211±10 Ma in sample RD41-47 and the central age of 158±14 Ma in sample RD41-46, are much younger than the depositional age of the host rock, again showing that the samples must have been much hotter at some time in the past. Evidence for the more recent event in each sample comes from the track length data. Comparison of the measured length distributions with those expected if the samples have remained at near-surface temperatures since deposition [[:File:YGS_CHR_03_POST_FIG_02.jpg|(Figure 2)]] shows that a large proportion of the tracks in each sample are shorter than expected on this basis, although a smaller proportion of tracks do have lengths closer to the expected range, suggesting that the samples have indeed spent some time at temperatures close to surface values.
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In both samples, the AFTA data clearly require at least two major episodes of heating and cooling [[:File:YGS_CHR_03_POST_FIG_02.jpg|(Figure 2)]]. In each sample, the earlier event is required to explain the fission-track age data, with apatite grains over a range of Cl contents (up to 0.7 wt per cent Cl in sample RD41-47) giving ages consistently younger than the value expected if the sample has not been significantly heated since deposition (horizontal bars in the age versus Cl plots in [[:File:YGS_CHR_03_POST_FIG_02.jpg|Figure 2]]). Both the pooled fission-track age of 211±10Ma in sample RD41-47 and the central age of 158±14Ma in sample RD41-46, are much younger than the depositional age of the host rock, again showing that the samples must have been much hotter at some time in the past. Evidence for the more recent event in each sample comes from the track length data. Comparison of the measured length distributions with those expected if the samples have remained at near-surface temperatures since deposition [[:File:YGS_CHR_03_POST_FIG_02.jpg|(Figure 2)]] shows that a large proportion of the tracks in each sample are shorter than expected on this basis, although a smaller proportion of tracks do have lengths closer to the expected range, suggesting that the samples have indeed spent some time at temperatures close to surface values.
  
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.
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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 290Ma and 220Ma, whereas sample RD41-47 reached a maximum palaeotemperature in excess of 110°C and began to cool between 240Ma and 180Ma. Results from sample RD41-46 suggest a subsequent peak palaeotemperature of 80 to 90°C from which cooling began some time between 80 and 40Ma, 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 30Ma.
  
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 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).
+
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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The relationship between fission-track ages of individual apatite grains and chlorine content are also shown in [[:File:YGS_CHR_03_POST_FIG_03.jpg|Figure 3]] for the two deepest samples. In these plots, the horizontal black lines show the trends predicted from the default thermal history scenario. In sample GC290-5, apatites containing between 0.0 and 0.3 weight per cent Cl give ages significantly less than the predicted values, whereas apatites with higher-Cl contents give older ages, closer to the expected values. This reflects the greater sensitivity of the grains with lower-Cl contents, which are more easily reset than those with higher-Cl contents. In sample GC290-6, all single grain ages are less than predicted, and the single grain containing almost 0.7 weight per cent Cl has undergone less age reduction than the lower-Cl grains. These observations again emphasize that these samples have been hotter at some time in the past.
 
The relationship between fission-track ages of individual apatite grains and chlorine content are also shown in [[:File:YGS_CHR_03_POST_FIG_03.jpg|Figure 3]] for the two deepest samples. In these plots, the horizontal black lines show the trends predicted from the default thermal history scenario. In sample GC290-5, apatites containing between 0.0 and 0.3 weight per cent Cl give ages significantly less than the predicted values, whereas apatites with higher-Cl contents give older ages, closer to the expected values. This reflects the greater sensitivity of the grains with lower-Cl contents, which are more easily reset than those with higher-Cl contents. In sample GC290-6, all single grain ages are less than predicted, and the single grain containing almost 0.7 weight per cent Cl has undergone less age reduction than the lower-Cl grains. These observations again emphasize that these samples have been hotter at some time in the past.
  
The single-grain age data in the lower-Cl grains in these two samples are highly consistent at about 50 Ma (allowing for scatter attributable to appropriate analytical uncertainties). Trends of single-grain age versus Cl content such as these, with consistent ages over the range of lowest Cl contents, clearly show that the fission-track ages of the lower-Cl grains in each sample have been totally reset and, therefore, the measured fission-track ages reflect the time at which the samples began to cool to palaeotemperatures low enough for tracks to be retained. (Note that the measured ages are not equal to the time of cooling, because of the effects of annealing of tracks formed during the period following the onset of cooling, and the age data must be considered in tandem with the track-length data in order to define the actual time at which cooling began.)
+
The single-grain age data in the lower-Cl grains in these two samples are highly consistent at about 50Ma (allowing for scatter attributable to appropriate analytical uncertainties). Trends of single-grain age versus Cl content such as these, with consistent ages over the range of lowest Cl contents, clearly show that the fission-track ages of the lower-Cl grains in each sample have been totally reset and, therefore, the measured fission-track ages reflect the time at which the samples began to cool to palaeotemperatures low enough for tracks to be retained. (Note that the measured ages are not equal to the time of cooling, because of the effects of annealing of tracks formed during the period following the onset of cooling, and the age data must be considered in tandem with the track-length data in order to define the actual time at which cooling began.)
  
Thermal-history solutions derived from the AFTA data in samples from the well are summarized in [[:File:YGS_CHR_03_POST_TAB_02.jpg|Table 2]], which also summarizes estimates of maximum palaeotemperature derived from VR data from the Jurassic section in this well. If we assume that results from this well represent the effects of a synchronous cooling episode, estimates of the onset of cooling from AFTA in the five samples suggest that cooling from maximum palaeotemperatures began some time between 90 Ma and 40 Ma. The detail of the track-length data in these samples also suggests a possible later cooling episode from a lower palaeothermal peak. This most likely represents the Neogene cooling identified from AFTA onshore (Green et al. 2001) and also suggested by Japsen (1997). However, the detail of the Cainozoic cooling history is beyond the scope of this contribution and is not pursued here.
+
Thermal-history solutions derived from the AFTA data in samples from the well are summarized in [[:File:YGS_CHR_03_POST_TAB_02.jpg|Table 2]], which also summarizes estimates of maximum palaeotemperature derived from VR data from the Jurassic section in this well. If we assume that results from this well represent the effects of a synchronous cooling episode, estimates of the onset of cooling from AFTA in the five samples suggest that cooling from maximum palaeotemperatures began some time between 90Ma and 40Ma. The detail of the track-length data in these samples also suggests a possible later cooling episode from a lower palaeothermal peak. This most likely represents the Neogene cooling identified from AFTA onshore (Green et al. 2001) and also suggested by Japsen (1997). However, the detail of the Cainozoic cooling history is beyond the scope of this contribution and is not pursued here.
  
Cooling beginning between 90&nbsp;Ma and 40&nbsp;Ma is consistent with the Palaeocene cooling identified in AFTA data from onshore wells and outcrop data (reviewed earlier), and the simplest interpretation of these data is that results from the 47/25-1 well also represent the effects of a regional cooling episode that began in the interval 65–55&nbsp;Ma. Although it is true that, at the limits of the data, cooling in the 47/25-1 well may have begun at any time between 90&nbsp;Ma and 40&nbsp;Ma, synthesis of unpublished results from other offshore wells in the vicinity of this well also provide a tighter timing constraint to the interval 65–55&nbsp;Ma for the onset of cooling. Therefore, it seems beyond reasonable doubt that the preserved sedimentary units in the offshore EMS, as well as the onshore, have undergone major cooling through Cainozoic times, beginning at about 60&nbsp;Ma.
+
Cooling beginning between 90Ma and 40 Ma is consistent with the Palaeocene cooling identified in AFTA data from onshore wells and outcrop data (reviewed earlier), and the simplest interpretation of these data is that results from the 47/25-1 well also represent the effects of a regional cooling episode that began in the interval 65–55Ma. Although it is true that, at the limits of the data, cooling in the 47/25-1 well may have begun at any time between 90Ma and 40Ma, synthesis of unpublished results from other offshore wells in the vicinity of this well also provide a tighter timing constraint to the interval 65–55Ma for the onset of cooling. Therefore, it seems beyond reasonable doubt that the preserved sedimentary units in the offshore EMS, as well as the onshore, have undergone major cooling through Cainozoic times, beginning at about 60Ma.
  
 
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&nbsp;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]]).
+
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&nbsp;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&nbsp;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&nbsp;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&nbsp;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).
+
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.
Line 97: Line 96:
 
== 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&nbsp;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&nbsp;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.
+
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&nbsp;km across the region of the outlier, suggesting that, even where the Palaeogene strata are preserved, an additional ~1&nbsp;km or so of strata have been deposited and eroded.
+
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&nbsp;Ma (taking the oldest limit on cooling from AFTA of 65&nbsp;Ma and an age of ~60&nbsp;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.
+
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.
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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., 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., 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., 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., 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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