Editing Dinantian and Namurian depositional systems in the southern North Sea

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[[File:YGS_CHR_04_DINA_FIG_10.jpg|thumbnail|Figure 10 Correlation panels showing the major deep penetrations of Namurian strata in the Southern North Sea Basin. ]]
 
[[File:YGS_CHR_04_DINA_FIG_10.jpg|thumbnail|Figure 10 Correlation panels showing the major deep penetrations of Namurian strata in the Southern North Sea Basin. ]]
 
[[File:YGS_CHR_04_DINA_FIG_11.jpg|thumbnail|Figure 11 An interpreted dipmeter log, shown alongside a gamma log of part of well 43/21-2.]]
 
[[File:YGS_CHR_04_DINA_FIG_11.jpg|thumbnail|Figure 11 An interpreted dipmeter log, shown alongside a gamma log of part of well 43/21-2.]]
[[File:YGS_CHR_04_DINA_FIG_12.jpg|thumbnail|Figure 12 A correlation panel for wells that penetrate typical cyclic Millstone Grit Namurian successions. ]]
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[[File:YGS_CHR_04_DINA_FIG_12.jpg|thumbnail|Figure 12 A correlation panel for wells that penetrate typical cyclic Millstone Grit Namurian successions. The succession is dominated by upwards-coarsening units and channel sandstones. The bases of cyclothems are commonly marked by a high-gamma mudstone that is likely to be a marine band. The major marine bands are labelled. The identification of those below top Kinderscoutian is tentative.]]
[[File:YGS_CHR_04_DINA_FIG_13.jpg|thumbnail|Figure 13 A cartoon showing the idealized relationships between marine bands (M), upwards-coarsening units, distributary channel sandstones and palaeovalleys in a typical Millstone Grit association.]]
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[[File:YGS_CHR_04_DINA_FIG_13.jpg|thumbnail|Figure 13 A cartoon showing the idealized relationships between marine bands (M), upwards-coarsening units, distributary channel sandstones and palaeovalleys in a typical Millstone Grit association, as seen in the Pennines and inferred for the Southern North Sea Basin.]]
[[File:YGS_CHR_04_DINA_FIG_14.jpg|thumbnail|Figure 14 Correlation panel of key wells in the Cleveland Basin and offshore Yorkshire.]]
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[[File:YGS_CHR_04_DINA_FIG_14.jpg|thumbnail|Figure 14 Correlation panel of key wells in the Cleveland Basin and offshore Yorkshire, showing major deepening episodes that may coincide with the development of the northern margin of the Southern North Sea Basin. Ages are only loosely constrained. All wells have gamma and sonic logs with an interpreted lithology.]]
  
 
'''By John D. Collinson'''  
 
'''By John D. Collinson'''  
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At outcrop, it is clear that the Fell Sandstone is the product of a major sandy braided river system (Turner & Munro 1987). The lateral mobility of the channels led to multi-storey channel units that tended to stack in hanging wall areas of syndepositionally active faults (Turner et al. 1993). Shallow boreholes and detailed mapping show that, in its outcrop area, the unit comprises a series of multi-storey sandbodies separated by mudstone intervals, some of which have marine microfaunas. The combination of low relief, active tectonics and episodic changes in relative sea level probably gave rise to shallow marine flooding of the alluvial tract along the Northumberland–Solway Trough. Across the Mid North Sea High, the depositional regime was probably very similar, with even greater stacking of channel sandbodies across Quadrants 42–43, possibly localized by tectonic control. Differential subsidence, possibly influenced by buried granite plutons, may have influenced this control, but the database is too scattered to demonstrate this. Dominant palaeoflow offshore is thought, on regional grounds, to have been to the south. To the west of the zone of intense channel stacking, higher proportions of finer-grained strata suggest overbank or shallow coastal plains with fewer and smaller channels at well 41/10-1 [[:File:YGS_CHR_04_DINA_FIG_03.jpg|(Figure 3)]], [[:File:YGS_CHR_04_DINA_FIG_06.jpg|(Figure 6)]]. This change takes place over a data gap of some 60 km and it quite possible that migration of the larger river channels to the west was blocked by tectonically controlled topography, as implied by Maynard & Dunay (1999), who suggested that faulting was the prime control. It seems equally possible that well 41/10-1 is drilled in a depositional shadow zone of a granite-cored block, with or without associated faulting [[:File:YGS_CHR_04_DINA_FIG_06.jpg|(Figure 6)]]. The thick finer-grained succession at 41/10-1 includes limestones and thin coal seams, and compares with the overlying Scremerston Formation; on lithostratigraphic grounds it should be included in that formation. However, thickness criteria suggest that it was deposited at a time when thick multi-storey channel sandstone units of the typical Fell Sandstone were being deposited along strike. Picking an equivalent of the top of the Fell Sandstone in the expanded Scremerston Formation in this well is somewhat arbitrary.
 
At outcrop, it is clear that the Fell Sandstone is the product of a major sandy braided river system (Turner & Munro 1987). The lateral mobility of the channels led to multi-storey channel units that tended to stack in hanging wall areas of syndepositionally active faults (Turner et al. 1993). Shallow boreholes and detailed mapping show that, in its outcrop area, the unit comprises a series of multi-storey sandbodies separated by mudstone intervals, some of which have marine microfaunas. The combination of low relief, active tectonics and episodic changes in relative sea level probably gave rise to shallow marine flooding of the alluvial tract along the Northumberland–Solway Trough. Across the Mid North Sea High, the depositional regime was probably very similar, with even greater stacking of channel sandbodies across Quadrants 42–43, possibly localized by tectonic control. Differential subsidence, possibly influenced by buried granite plutons, may have influenced this control, but the database is too scattered to demonstrate this. Dominant palaeoflow offshore is thought, on regional grounds, to have been to the south. To the west of the zone of intense channel stacking, higher proportions of finer-grained strata suggest overbank or shallow coastal plains with fewer and smaller channels at well 41/10-1 [[:File:YGS_CHR_04_DINA_FIG_03.jpg|(Figure 3)]], [[:File:YGS_CHR_04_DINA_FIG_06.jpg|(Figure 6)]]. This change takes place over a data gap of some 60 km and it quite possible that migration of the larger river channels to the west was blocked by tectonically controlled topography, as implied by Maynard & Dunay (1999), who suggested that faulting was the prime control. It seems equally possible that well 41/10-1 is drilled in a depositional shadow zone of a granite-cored block, with or without associated faulting [[:File:YGS_CHR_04_DINA_FIG_06.jpg|(Figure 6)]]. The thick finer-grained succession at 41/10-1 includes limestones and thin coal seams, and compares with the overlying Scremerston Formation; on lithostratigraphic grounds it should be included in that formation. However, thickness criteria suggest that it was deposited at a time when thick multi-storey channel sandstone units of the typical Fell Sandstone were being deposited along strike. Picking an equivalent of the top of the Fell Sandstone in the expanded Scremerston Formation in this well is somewhat arbitrary.
  
To the south, distal equivalents of the Fell Sandstone are unknown offshore. It is possible that continuing extension had started to create a deep basin with a well defined northern margin by this time. Fluvial sand could then have been bypassed to deeper water. On the other hand, more gradual subsidence may have created a gently inclined ramp across which deltas prograded. The succession in the Kirby Misperton-1 well can be interpreted as a possible marine equivalent but of unknown water depth. Such a transition, if correct, would compare with the increasing marine influence seen in the Fell Sandstone equivalents as they are traced down current to the southwest along the Northumberland–Solway Trough. Alternatively, they may represent a deeper-water facies. Either way, they need not provide an analogue to the depositional system that existed offshore.
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To the south, distal equivalents of the Fell Sandstone are unknown offshore. It is possible that continuing extension had started to create a deep basin with a well defined northern margin by this time. Fluvial sand could then have been bypassed to deeper water. On the other hand, more gradual subsidence may have created a gently inclined ramp across which deltas pro-graded. The succession in the Kirby Misperton-1 well can be interpreted as a possible marine equivalent but of unknown water depth. Such a transition, if correct, would compare with the increasing marine influence seen in the Fell Sandstone equivalents as they are traced down current to the southwest along the Northumberland–Solway Trough. Alternatively, they may represent a deeper-water facies. Either way, they need not provide an analogue to the depositional system that existed offshore.
  
 
==== 3.1.3 The Scremerston Formation ====
 
==== 3.1.3 The Scremerston Formation ====
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This formation is the lowest of four that can be characterized as of essentially deltaic facies. It is broadly late Holkerian to early Asbian in age. Its base is drawn at the top of the massive sandstones of the Fell Sandstone, a change that is typically quite abrupt [[:File:YGS_CHR_04_DINA_FIG_03.jpg|(Figure 3)]]. The unit is characterized by the presence of coal seams, by a variable incidence of major channel sandstones and by thin limestone beds. The clear cyclicity, which characterizes later Yoredale intervals, is less apparent in the Scremerston Formation. The gamma-log signature of typical Scremerston Formation is high frequency and irregular, with few pronounced upwards-coarsening units. Some thicker channel sandstones are present, the most conspicuous being those near the bottom of the Harton-1 well, which may be somewhat anomalous at a regional scale. Coal seams are usually quite thin, but can be up to several metres thick in the Dutch sector and onshore in Northumberland.
 
This formation is the lowest of four that can be characterized as of essentially deltaic facies. It is broadly late Holkerian to early Asbian in age. Its base is drawn at the top of the massive sandstones of the Fell Sandstone, a change that is typically quite abrupt [[:File:YGS_CHR_04_DINA_FIG_03.jpg|(Figure 3)]]. The unit is characterized by the presence of coal seams, by a variable incidence of major channel sandstones and by thin limestone beds. The clear cyclicity, which characterizes later Yoredale intervals, is less apparent in the Scremerston Formation. The gamma-log signature of typical Scremerston Formation is high frequency and irregular, with few pronounced upwards-coarsening units. Some thicker channel sandstones are present, the most conspicuous being those near the bottom of the Harton-1 well, which may be somewhat anomalous at a regional scale. Coal seams are usually quite thin, but can be up to several metres thick in the Dutch sector and onshore in Northumberland.
  
Where channel sandbodies occur, they typically range up to about 20 m in thickness. Their distribution shows no obvious pattern. Wells such as 41/1-1 and 43/5-1 are almost channel free, whereas wells such as 41/10-1, 42/10-2 and 43/2-1 have channel sandbodies scattered throughout the interval. The thicker channel bodies are probably multi-storey and may be incised. Some thin sandstone units are characterized by very low gamma values and may be transgressively reworked units similar to the Arnsbergian Harthope Ganister of the onshore outcrop (Percival 1992). Alternatively, they may be genuine palaeosol ganisters, developed beneath coal seams.
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Where channel sandbodies occur, they typically range up to about 20m in thickness. Their distribution shows no obvious pattern. Wells such as 41/1-1 and 43/5-1 are almost channel free, whereas wells such as 41/10-1, 42/10-2 and 43/2-1 have channel sandbodies scattered throughout the interval. The thicker channel bodies are probably multi-storey and may be incised. Some thin sandstone units are characterized by very low gamma values and may be transgressively reworked units similar to the Arnsbergian Harthope Ganister of the onshore outcrop (Percival 1992). Alternatively, they may be genuine palaeosol ganisters, developed beneath coal seams.
  
 
The Scremerston Formation records the development of a low-relief coastal plain traversed by river channels. The area was episodically transgressed to allow shallow-water marine limestones to accumulate. During regressive phases, shallow lakes and lagoons were filled by minor deltaic progradations and coal swamps formed on emergent areas, probably developing thick peats during the early stages of transgression. There is a suggestion that upwards-coarsening units are more common in the south, from which one might infer a more seaward setting, where greater water depths were created during transgressions. This accords with a broadly southerly palaeoflow.
 
The Scremerston Formation records the development of a low-relief coastal plain traversed by river channels. The area was episodically transgressed to allow shallow-water marine limestones to accumulate. During regressive phases, shallow lakes and lagoons were filled by minor deltaic progradations and coal swamps formed on emergent areas, probably developing thick peats during the early stages of transgression. There is a suggestion that upwards-coarsening units are more common in the south, from which one might infer a more seaward setting, where greater water depths were created during transgressions. This accords with a broadly southerly palaeoflow.
  
The thickness pattern of the Scremerston Formation is difficult to reconstruct in detail because of the scattered data and the generally incomplete penetration [[:File:YGS_CHR_04_DINA_FIG_07.jpg|(Figure 7)]]. Across the southern margin of the Mid North Sea High, thicknesses appear to be about 300 m, but, a little to the south, wells 41/14-1 and 41/15-1 suggest thickening to more than 500 m. This may reflect an offshore extension of the Stainmore Trough or a more local, tectonically controlled depocentre. At Seal Sands-1, the probable equivalents are about 1300 m thick. This contrasts with the near absence of equivalent strata on the Alston Block and illustrates the scale of syndepositional movement on the Butterknowle Fault. At Seal Sands-1, the section has only widely scattered and thin coals, suggesting a more offshore setting and one in which subsidence rates were too rapid to allow coal accumulation, a setting that might compare with the Upper Border Group, which is the distal equivalent of the Scremerston Formation at Bewcastle (Day 1970). In the Cleveland Basin, the nature of Scremerston Formation equivalents is somewhat speculative. In Kirby Misperton-1 well, an interval about 330 m thick of inferred thinly interbedded sandstones and siltstones, indicated by a highly serrated gamma trace, could be their equivalent but, without better biostratigraphy, this remains speculative. Their depositional setting is also uncertain. They could be a deepwater facies separated by an unseen slope from the deltaic cyclothems to the north.
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The thickness pattern of the Scremerston Formation is difficult to reconstruct in detail because of the scattered data and the generally incomplete penetration [[:File:YGS_CHR_04_DINA_FIG_07.jpg|(Figure 7)]]. Across the southern margin of the Mid North Sea High, thicknesses appear to be about 300m, but, a little to the south, wells 41/14-1 and 41/15-1 suggest thickening to more than 500m. This may reflect an offshore extension of the Stainmore Trough or a more local, tectonically controlled depocentre. At Seal Sands-1, the probable equivalents are about 1300m thick. This contrasts with the near absence of equivalent strata on the Alston Block and illustrates the scale of syndepositional movement on the Butterknowle Fault. At Seal Sands-1, the section has only widely scattered and thin coals, suggesting a more offshore setting and one in which subsidence rates were too rapid to allow coal accumulation, a setting that might compare with the Upper Border Group, which is the distal equivalent of the Scremerston Formation at Bewcastle (Day 1970). In the Cleveland Basin, the nature of Scremerston Formation equivalents is somewhat speculative. In Kirby Misperton-1 well, an interval about 330m thick of inferred thinly interbedded sandstones and siltstones, indicated by a highly serrated gamma trace, could be their equivalent but, without better biostratigraphy, this remains speculative. Their depositional setting is also uncertain. They could be a deepwater facies separated by an unseen slope from the deltaic cyclothems to the north.
  
 
==== 3.1.4 The Lower Limestone Formation ====
 
==== 3.1.4 The Lower Limestone Formation ====
  
This unit, of late Asbian age, is extensively penetrated by wells offshore. It is distinguished from the Scremerston Formation by a lower incidence of coal seams and by a much clearer pattern of upwards-coarsening cyclothems with limestones at their bases, features typical of a Yoredale facies [[:File:YGS_CHR_04_DINA_FIG_03.jpg|(Figure 3)]]. It is the earliest Carboniferous unit in which palaeovalleys, infilled by thick multistorey channel sandstones, are known to be developed [[:File:YGS_CHR_04_DINA_FIG_08.jpg|(Figure 8)]]. Large multi-storey channel units are also conspicuous at outcrop on the Northumberland coast (Gardiner 1984). The incoming of both clear cyclicity and incision may relate to an increasing eustatic control on sea level, driven by the onset of continental glaciation in the Southern Hemisphere.
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This unit, of late Asbian age, is extensively penetrated by wells offshore. It is distinguished from the Scremerston Formation by a lower incidence of coal seams and by a much clearer pattern of upwards-coarsening cyclothems with limestones at their bases, features typical of a Yoredale facies [[:File:YGS_CHR_04_DINA_FIG_03.jpg|(Figure 3)]]. It is the earliest Carboniferous unit in which palaeovalleys, infilled by thick multistorey channel sandstones, are known to be developed [[:File:YGS_CHR_04_DINA_FIG_08.jpg|(Figure 8)]].
  
One inferred palaeovalley sandstone in well 42/10-2 is the reservoir for a gas discovery and has been named the Whitby Member ([[:File:YGS_CHR_04_DINA_FIG_08.jpg|(Figure 8)]]; Maynard & Dunay 1999). Interpreted borehole image logs from that sandstone suggest cross bedding directed to the south. Maynard & Dunay (1999) suggested on the basis of similar log signatures that this sandstone could be correlated over an east–west distance of some 50 km to well 41/10-1; but, as the wells in question lie normal to the palaeoflow, it seems more likely that the log similarities are fortuitous. The southern limit of deltaic progradation is poorly constrained, as there are no certain penetrations of stratigraphical equivalents in the basin. This was a time when deepening of the Southern North Sea Basin may have started and so periods of low-stand incision could have coincided with the bypassing of sand to distal deepwater areas.
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Large multi-storey channel units are also conspicuous at outcrop on the Northumberland coast (Gardiner 1984). The incoming of both clear cyclicity and incision may relate to an increasing eustatic control on sea level, driven by the onset of continental glaciation in the Southern Hemisphere.
  
Thickness changes in the Lower Limestone Formation are quite small across most of the offshore area, with about 200 m being typical. Only at Harton-1, on the fringes of the Alston Block, and at Seal Sands-1, is the unit respectively thinner and thicker. The differences in thicknesses, and the fact that the Alston Block was flooded at this time, suggest diminishing differential subsidence. Also, the offshore area is considerably more sand rich than the equivalent intervals in the onshore Harton-1 and Seal Sands-1 wells, where limestones are much more important. The interval extends southwards in a Yoredale facies at least as far as well 41/24a-2, offshore from Scarborough. However, at Kirby Misperton-1 a relatively sand-free section some 300 m thick seems a likely correlative, although lack of robust biostratigraphy makes for uncertainty. The section at Kirby Misperton-1 is possibly in a slope or deepwater facies.
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One inferred palaeovalley sandstone in well 42/10-2 is the reservoir for a gas discovery and has been named the Whitby Member ([[:File:YGS_CHR_04_DINA_FIG_08.jpg|(Figure 8)]]; Maynard & Dunay 1999). Interpreted borehole image logs from that sandstone suggest cross bedding directed to the south. Maynard & Dunay (1999) suggested on the basis of similar log signatures that this sandstone could be correlated over an east–west distance of some 50km to well 41/10-1; but, as the wells in question lie normal to the palaeoflow, it seems more likely that the log similarities are fortuitous. The southern limit of deltaic progradation is poorly constrained, as there are no certain penetrations of stratigraphical equivalents in the basin. This was a time when deepening of the Southern North Sea Basin may have started and so periods of low-stand incision could have coincided with the bypassing of sand to distal deepwater areas.
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Thickness changes in the Lower Limestone Formation are quite small across most of the offshore area, with about 200m being typical. Only at Harton-1, on the fringes of the Alston Block, and at Seal Sands-1, is the unit respectively thinner and thicker. The differences in thicknesses, and the fact that the Alston Block was flooded at this time, suggest diminishing differential subsidence. Also, the offshore area is considerably more sand rich than the equivalent intervals in the onshore Harton-1 and Seal Sands-1 wells, where limestones are much more important. The interval extends southwards in a Yoredale facies at least as far as well 41/24a-2, offshore from Scarborough. However, at Kirby Misperton-1 a relatively sand-free section some 300m thick seems a likely correlative, although lack of robust biostratigraphy makes for uncertainty. The section at Kirby Misperton-1 is possibly in a slope or deepwater facies.
  
 
==== 3.1.5 The Middle Limestone Formation ====
 
==== 3.1.5 The Middle Limestone Formation ====
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At outcrop in Northumberland, the base of the Middle Limestone Formation is defined at the base of the Oxford Limestone, which occurs just above the base of the Brigantian. The top is defined by the base of the Great Limestone, which broadly coincides with the base of the Namurian. The cyclothems of the Middle Limestone Formation, at outcrop, tend to be less sandy than those of the Lower and Upper Limestone formations, and the progradational parts of cyclothems show greater reworking by wave energy (Reynolds 1992). Channel sandbodies are relatively uncommon and the majority of sandbodies are of delta-front or mouth-bar origin.
 
At outcrop in Northumberland, the base of the Middle Limestone Formation is defined at the base of the Oxford Limestone, which occurs just above the base of the Brigantian. The top is defined by the base of the Great Limestone, which broadly coincides with the base of the Namurian. The cyclothems of the Middle Limestone Formation, at outcrop, tend to be less sandy than those of the Lower and Upper Limestone formations, and the progradational parts of cyclothems show greater reworking by wave energy (Reynolds 1992). Channel sandbodies are relatively uncommon and the majority of sandbodies are of delta-front or mouth-bar origin.
  
Offshore, this formation is generally distinguished from its neighbours by a lower proportion of sandstones. It is characterized by abundant thin limestones, by rather thick upwards-coarsening cyclothems and by a low incidence of channels. The incidence of coal seams is somewhat variable between wells. Offshore, the interval appears to have a relatively uniform thickness, varying between 400 m and 500 m. A similar thickness occurs at Harton-1, suggesting that differential subsidence around the Alston Block continued to decline. Thickness calculations from well data are complicated by the Whin Sill, which is intruded at several levels at Harton-1 and in nearshore wells. Onshore, the expanded thickness at Seal Sands-1 suggests that the Butterknowle Fault remained active. It is clear that the expanded overall thickness at Seal Sands-1 is accompanied by an expansion in both the number and typical thickness of cyclothems. The sections at Kirby Misperton-1 and Cloughton-1 are both cyclothemic in character, although lacking the typical Yoredale facies limestones. The sections suggest that the main front of deltaic progradation advanced farther south during this interval to create shallow-water conditions in the Cleveland Basin. Offshore, the southern limit of the deltas is constrained between the deltaic facies of well 43/10-1 and the deepwater facies of Brigantian age at well 43/17-2. The lack of penetration of the unit in wells to the south of 43/10-1 is attributable to erosion at the base-Permian Unconformity and it likely that the transition zone broadly follows the mid-latitude of Quadrants 43–44.
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Offshore, this formation is generally distinguished from its neighbours by a lower proportion of sandstones. It is characterized by abundant thin limestones, by rather thick upwards-coarsening cyclothems and by a low incidence of channels. The incidence of coal seams is somewhat variable between wells.
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Offshore, the interval appears to have a relatively uniform thickness, varying between 400m and 500m. A similar thickness occurs at Harton-1, suggesting that differential subsidence around the Alston Block continued to decline. Thickness calculations from well data are complicated by the Whin Sill, which is intruded at several levels at Harton-1 and in nearshore wells. Onshore, the expanded thickness at Seal Sands-1 suggests that the Butterknowle Fault remained active. It is clear that the expanded overall thickness at Seal Sands-1 is accompanied by an expansion in both the number and typical thickness of cyclothems. The sections at Kirby Misperton-1 and Cloughton1 are both cyclothemic in character, although lacking the typical Yoredale facies limestones. The sections suggest that the main front of deltaic progradation advanced farther south during this interval to create shallow-water conditions in the Cleveland Basin. Offshore, the southern limit of the deltas is constrained between the deltaic facies of well 43/10-1 and the deepwater facies of Brigantian age at well 43/17-2. The lack of penetration of the unit in wells to the south of 43/10-1 is attributable to erosion at the base-Permian Unconformity and it likely that the transition zone broadly follows the mid-latitude of Quadrants 43–44.
  
 
==== 3.1.6 The Upper Limestone Formation ====
 
==== 3.1.6 The Upper Limestone Formation ====
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At outcrop, the base of this unit is drawn at the base of the Great Limestone, which roughly coincides with the ''Cravenoceras leion ''Marine Band of the basinal succession. Its age in the northern Pennines and Northumberland is Pendleian to early Kinderscoutian. Strata of Chokierian–Alportian age are not certainly present, suggesting a hiatus in the succession, although no mappable unconformity has yet been detected. The Upper Limestone Group includes some thick multi-storey channel sandbodies, which are most likely incised palaeovalleys (e.g. Elliott 1976, Hodge & Dunham 1991).
 
At outcrop, the base of this unit is drawn at the base of the Great Limestone, which roughly coincides with the ''Cravenoceras leion ''Marine Band of the basinal succession. Its age in the northern Pennines and Northumberland is Pendleian to early Kinderscoutian. Strata of Chokierian–Alportian age are not certainly present, suggesting a hiatus in the succession, although no mappable unconformity has yet been detected. The Upper Limestone Group includes some thick multi-storey channel sandbodies, which are most likely incised palaeovalleys (e.g. Elliott 1976, Hodge & Dunham 1991).
  
Offshore, the base of the Upper Limestone Group is drawn at a thick limestone that coincides broadly with an upward change to a more sandstone-rich succession. In most wells in the north, the limestone is sufficiently prominent for the boundary to be quite confidently identified. Farther south, the boundary changes character, along with the overall sedimentology, to a more basinal aspect. At well 41/24a-2, a condensed mudstone interval with prominent inferred marine bands is thought to equate with the boundary, whereas in intermediate wells (41/14-1, 41/15-1) inferred marine-band mudstones and thin limestones seem to coincide broadly. The base-Permian erosion truncates the Upper Limestone Group and makes for rather patchy preservation. Most recorded thicknesses are minimum values. These range up to just over 300 m on the Mid North Sea High. Farther south in the Cleveland Basin onshore, thicknesses exceed 500 m in the Cloughton-1 and Kirby Misperton-1 wells, and just offshore at well 41/24a-2. The lesser thickness of 415 m at 41/20-1 suggests that original depositional thicknesses probably decreased to the north. Farther north onshore, a reduced thickness (268 m) at Harton-1 was attributed by Ridd et al. (1970) to the early incoming of Millstone Grit facies. However, the recognition of Millstone Grit facies is considered somewhat insecure (Holliday, personal communication 2003) and the thickness could therefore be greater. At Seal Sands-1, base-Permian erosion prevents meaningful comparison of thicknesses, and it is not easy to judge how far into the Namurian the highly differential subsidence seen in the Dinantian persisted. By analogy (e.g. with the Craven Basin), it is likely that differential movements were much reduced.
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Offshore, the base of the Upper Limestone Group is drawn at a thick limestone that coincides broadly with an upward change to a more sandstone-rich succession. In most wells in the north, the limestone is sufficiently prominent for the boundary to be quite confidently identified. Farther south, the boundary changes character, along with the overall sedimentology, to a more basinal aspect. At well 41/24a-2, a condensed mudstone interval with prominent inferred marine bands is thought to equate with the boundary, whereas in intermediate wells (41/14-1, 41/15-1) inferred marine-band mudstones and thin limestones seem to coincide broadly. The base-Permian erosion truncates the Upper Limestone Group and makes for rather patchy preservation. Most recorded thicknesses are minimum values. These range up to just over 300m on the Mid North Sea High. Farther south in the Cleveland Basin onshore, thicknesses exceed 500m in the Cloughton-1 and Kirby Misperton-1 wells, and just offshore at well 41/24a-2. The lesser thickness of 415m at 41/20-1 suggests that original depositional thicknesses probably decreased to the north. Farther north onshore, a reduced thickness (268m) at Harton-1 was attributed by Ridd et al. (1970) to the early incoming of Millstone Grit facies. However, the recognition of Millstone Grit facies is considered somewhat insecure (Holliday, personal communication 2003) and the thickness could therefore be greater. At Seal Sands-1, base-Permian erosion prevents meaningful comparison of thicknesses, and it is not easy to judge how far into the Namurian the highly differential subsidence seen in the Dinantian persisted. By analogy (e.g. with the Craven Basin), it is likely that differential movements were much reduced.
  
 
In the north (e.g. 41/10-1 and in Northumberland), the Upper Limestone Group shows a broadly Yoredale character, with limestones present at the bases of many cyclothems. Farther south, towards the Cleveland Basin (e.g. wells 42/24a-2, 41/14-1, 41/15-1), thicker and apparently rather silty upwards-coarsening units are prominent, and limestones are scarce or absent. This suggests a transition southwards into deeper water, where marine bands replace limestones as records of highstands, and where progradations of fine-grained slopes were more important.
 
In the north (e.g. 41/10-1 and in Northumberland), the Upper Limestone Group shows a broadly Yoredale character, with limestones present at the bases of many cyclothems. Farther south, towards the Cleveland Basin (e.g. wells 42/24a-2, 41/14-1, 41/15-1), thicker and apparently rather silty upwards-coarsening units are prominent, and limestones are scarce or absent. This suggests a transition southwards into deeper water, where marine bands replace limestones as records of highstands, and where progradations of fine-grained slopes were more important.
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=== 3.2 The Dinantian of the Southern North Sea Basin ===
 
=== 3.2 The Dinantian of the Southern North Sea Basin ===
  
Hard data on the Dinantian of the Southern North Sea Basin are very scant. They are mainly confined to well 43/17-2, which penetrated the whole Namurian basin-fill succession and continued into Dinantian strata. Some 520 m of inferred Dinantian mudstones were penetrated, all probably falling within the Brigantian. These were referred to by Cameron (1993) as the Bowland Shale Formation and are broadly analogous with the Lower Bowland Shales of the Craven Basin (Earp et al. 1961) and with the mud-rich successions beneath the Edale Shales of Derbyshire, known from the Alport borehole (Stevenson & Gaunt 1971). These onshore examples differ from the succession in 43/17-2 in that they have interbedded turbiditic sandstones. The Lower Bowland Shales include the Pendleside Sandstone, which probably resulted from the bypassing of sand from Yoredale deltas on the Askrigg Block. The Dinantian mudstones at Alport have thin limestones, which were probably derived from the Derbyshire Massif carbonate platform to the south. Both of these onshore cases may be relevant in predicting what might occur offshore.
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Hard data on the Dinantian of the Southern North Sea Basin are very scant. They are mainly confined to well 43/17-2, which penetrated the whole Namurian basin-fill succession and continued into Dinantian strata. Some 520m of inferred Dinantian mudstones were penetrated, all probably falling within the Brigantian. These were referred to by Cameron (1993) as the Bowland Shale Formation and are broadly analogous with the Lower Bowland Shales of the Craven Basin (Earp et al. 1961) and with the mud-rich successions beneath the Edale Shales of Derbyshire, known from the Alport borehole (Stevenson & Gaunt 1971). These onshore examples differ from the succession in 43/17-2 in that they have interbedded turbiditic sandstones. The Lower Bowland Shales include the Pendleside Sandstone, which probably resulted from the bypassing of sand from Yoredale deltas on the Askrigg Block. The Dinantian mudstones at Alport have thin limestones, which were probably derived from the Derbyshire Massif carbonate platform to the south. Both of these onshore cases may be relevant in predicting what might occur offshore.
  
 
The relationship between Dinantian deltas on the Mid North Sea High and any coeval deepwater areas to the south, and the nature and history of the basin margin, remain conjectural. However, it is difficult to imagine a situation where sands did not bypass to distal deepwater areas once such a differentiated bathymetry developed. Early Namurian turbidite sandstones at well 43/17-2 (discussed in more detail in section 3.3 below) were presumably bypassed through Upper Limestone Group deltas and there may be similar Dinantian examples elsewhere in the basin. The main problem is to identify the time at which accelerated subsidence led to the development of deep water in the Southern North Sea Basin. Distal parts of the fluvial and deltaic systems, known from the early Dinantian of the Mid North Sea High, may have extended southwards in similar shallow-water facies before differential subsidence created the bathymetric contrasts inferred from available well data for the late Dinantian. These uncertainties impact on estimates of volumes of deepwater mudstones, which are potentially important source rocks in the basin. The Kirby Misperton-1 well may provide a clue to the timing of accelerated subsidence in the area. This is discussed further in Section 4.
 
The relationship between Dinantian deltas on the Mid North Sea High and any coeval deepwater areas to the south, and the nature and history of the basin margin, remain conjectural. However, it is difficult to imagine a situation where sands did not bypass to distal deepwater areas once such a differentiated bathymetry developed. Early Namurian turbidite sandstones at well 43/17-2 (discussed in more detail in section 3.3 below) were presumably bypassed through Upper Limestone Group deltas and there may be similar Dinantian examples elsewhere in the basin. The main problem is to identify the time at which accelerated subsidence led to the development of deep water in the Southern North Sea Basin. Distal parts of the fluvial and deltaic systems, known from the early Dinantian of the Mid North Sea High, may have extended southwards in similar shallow-water facies before differential subsidence created the bathymetric contrasts inferred from available well data for the late Dinantian. These uncertainties impact on estimates of volumes of deepwater mudstones, which are potentially important source rocks in the basin. The Kirby Misperton-1 well may provide a clue to the timing of accelerated subsidence in the area. This is discussed further in Section 4.
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As reviewed earlier, reconstruction of the palaeogeographic evolution of the Namurian depends on establishing a reliable chronostratigraphic framework through ammonoid-bearing marine bands. Offshore, only around extensively cored wells, and only in Kinderscoutian and younger sediments, is it possible to have a reasonably secure framework. Beyond and below these well constrained sections, stratigraphical resolution is less precise, because it relies on palynology and on the recognition of marine bands from gamma and spectral gamma logs.
 
As reviewed earlier, reconstruction of the palaeogeographic evolution of the Namurian depends on establishing a reliable chronostratigraphic framework through ammonoid-bearing marine bands. Offshore, only around extensively cored wells, and only in Kinderscoutian and younger sediments, is it possible to have a reasonably secure framework. Beyond and below these well constrained sections, stratigraphical resolution is less precise, because it relies on palynology and on the recognition of marine bands from gamma and spectral gamma logs.
  
Onshore, in the Pennines, the basin-fill history involves successive progradations of turbidite—slope—fluvial sequences that progressively filled sub-basins from north to south [[:File:YGS_CHR_04_DINA_FIG_09.jpg|(Figure 9)]] (Reading 1964, Ramsbottom 1966, 1969, Collinson 1988). The earliest of these is the Pendleian fill of the Craven Basin, after which there was an apparent slowdown in sediment supply, at least as seen at outcrop. This lasted until the Kinderscoutian, when basin-filling extended southwards to north Derbyshire. Progradations in the Marsdenian filled remaining basinal areas in west Lancashire, Derbyshire and Staffordshire. In the subsurface, the Gainsborough Trough was filled by major progradations ending in the Alportian (Steele 1988).
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Onshore, in the Pennines, the basin-fill history involves successive progradations of turbidite—slope—fluvial sequences that progressively filled sub-basins from north to south [[:File:YGS_CHR_04_DINA_FIG_09.jpg|(Figure 9)]] (Reading 1964, Ramsbottom 1966, 1969, Collinson 1988). The earliest of these is the Pendleian fill of the Craven Basin, after which there was an apparent slowdown in sediment supply, at least as seen at outcrop. This lasted until the Kinderscoutian, when basin-filling extended southwards to north Derbyshire.
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Progradations in the Marsdenian filled remaining basinal areas in west Lancashire, Derbyshire and Staffordshire. In the subsurface, the Gainsborough Trough was filled by major progradations ending in the Alportian (Steele 1988).
  
Offshore, wells that penetrate the Namurian no deeper than the Alportian show a sedimentary style that compares closely with the cyclic Millstone Grit of the Pennine outcrop (see [[:File:YGS_CHR_04_DINA_FIG_12.jpg|(Figure 12)]]). These higher stratigraphical levels will be discussed further below. The comparatively rare wells with deeper penetrations mostly give only a partial section through the basin-fill succession, so a full reconstruction is impossible [[:File:YGS_CHR_04_DINA_FIG_10.jpg|(Figure 10)]]. Only well 43/17-2, which penetrated late Dinantian basinal mudstones, demonstrates the full basin-fill sequence. This includes, in its lower part, thick intervals of mudstone with prominent high-gamma units, inferred to be basinal marine bands, and thin units of fine-grained turbidite sandstones, one of which shows prominent dish structures in core. Above the turbidites in 43/17-2 is a 400 m thick mudstone and siltstone succession that becomes conspicuously coarser in its upper part. This is interpreted as the main basin-filling slope progradation, analogous in context to the Kinderscoutian Grindslow Shales of Derbyshire and the Pendleian Pendle Shales of the Craven Basin. In well 42/25-1, where this interval was partially penetrated, dipmeter analysis shows large-scale interbedding of units with constant low-angle dips and units in which dips are highly variable in both magnitude and direction [[:File:YGS_CHR_04_DINA_FIG_11.jpg|(Figure 11)]]. Both types of unit are tens of metres thick. The dip patterns suggest interbedding of undisturbed slope siltstones and slumped and rotated masses of similar sediment. Such deformation is not commonly recognized in the Pennines, but it matches closely the Kinderscoutian Gull Island Formation in County Clare, interpreted as the product of a prograding slope and an associated apron of slumped material (Martinsen 1989, Collinson et al. 1991). The top of the main upwards-coarsening sequence is characterized in well 43/17-2 by particularly thick sandbodies of inferred channel origin [[:File:YGS_CHR_04_DINA_FIG_10.jpg|(Figure 10)]]. These compare with the major channels at the top of basin-filling sequences in the Pennines (i.e. Warley Wise, Kinderscout, Fletcher Bank and Roaches grits) and which may relate to low-stand incision in a shelf-edge position (e.g. Jones & Chisholm 1997, Hampson et al. 1999). However, the multiple cut and fill, which characterizes most of such channel complexes at outcrop, suggests that processes intrinsic to the depositional setting, perhaps related to distributary switching, may be as important as base-level controls.
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Offshore, wells that penetrate the Namurian no deeper than the Alportian show a sedimentary style that compares closely with the cyclic Millstone Grit of the Pennine outcrop (see [[:File:YGS_CHR_04_DINA_FIG_12.jpg|(Figure 12)]]). These higher stratigraphical levels will be discussed further below. The comparatively rare wells with deeper penetrations mostly give only a partial section through the basin-fill succession, so a full reconstruction is impossible [[:File:YGS_CHR_04_DINA_FIG_10.jpg|(Figure 10)]]. Only well 43/17-2, which penetrated late Dinantian basinal mudstones, demonstrates the full basin-fill sequence. This includes, in its lower part, thick intervals of mudstone with prominent high-gamma units, inferred to be basinal marine bands, and thin units of fine-grained turbidite sandstones, one of which shows prominent dish structures in core. Above the turbidites in 43/17-2 is a 400m thick mudstone and siltstone succession that becomes conspicuously coarser in its upper part. This is interpreted as the main basin-filling slope progradation, analogous in context to the Kinderscoutian Grindslow Shales of Derbyshire and the Pendleian Pendle Shales of the Craven Basin. In well 42/25-1, where this interval was partially penetrated, dipmeter analysis shows large-scale interbedding of units with constant low-angle dips and units in which dips are highly variable in both magnitude and direction [[:File:YGS_CHR_04_DINA_FIG_11.jpg|(Figure 11)]]. Both types of unit are tens of metres thick. The dip patterns suggest interbedding of undisturbed slope siltstones and slumped and rotated masses of similar sediment. Such deformation is not commonly recognized in the Pennines, but it matches closely the Kinderscoutian Gull Island Formation in County Clare, interpreted as the product of a prograding slope and an associated apron of slumped material (Martinsen 1989, Collinson et al. 1991). The top of the main upwards-coarsening sequence is characterized in well 43/17-2 by particularly thick sandbodies of inferred channel origin [[:File:YGS_CHR_04_DINA_FIG_10.jpg|(Figure 10)]]. These compare with the major channels at the top of basin-filling sequences in the Pennines (i.e. Warley Wise, Kinderscout, Fletcher Bank and Roaches grits) and which may relate to low-stand incision in a shelf-edge position (e.g. Jones & Chisholm 1997, Hampson et al. 1999). However, the multiple cut and fill, which characterizes most of such channel complexes at outcrop, suggests that processes intrinsic to the depositional setting, perhaps related to distributary switching, may be as important as base-level controls.
  
 
The age of the inferred slope succession in 43/17-2 is poorly constrained, but it is suggested to be Chokierian and Alportian with underlying basinal mudstones and turbidites extending back into the late Dinantian. If this age is correct, then the Southern North Sea Basin and the Gainsborough Trough were filled at broadly similar times. The intervening Humber Basin is likely to have filled in the same interval but that basin could have survived until later if nearby granite-cored blocks diverted river systems. In wells 43/28-1 and 48/3-3, cyclic deltaic facies are developed in rocks of inferred Chokierian or possibly Arnsbergian age, suggesting that infilling of any deepwater areas there took place in the Arnsbergian or earlier. Previously estimated Namurian ages in well 48/3-3 (Leeder et al. 1990) are now thought to be too young on the basis of comparisons with nearby wells. As these two wells lie to the south and east of wells 43/17-2 and 43/25-1, the basin-fill progradation across Quadrant 43 probably took place from east to west rather than from north to south. This inference, and the fact that the Arnsbergian to Alportian was a time of limited sand supply to the Pennine basins, suggest that the main sand-supply route was deflected to the east and possibly entered the Southern North Sea Basin from somewhere near the position of the Central Graben.
 
The age of the inferred slope succession in 43/17-2 is poorly constrained, but it is suggested to be Chokierian and Alportian with underlying basinal mudstones and turbidites extending back into the late Dinantian. If this age is correct, then the Southern North Sea Basin and the Gainsborough Trough were filled at broadly similar times. The intervening Humber Basin is likely to have filled in the same interval but that basin could have survived until later if nearby granite-cored blocks diverted river systems. In wells 43/28-1 and 48/3-3, cyclic deltaic facies are developed in rocks of inferred Chokierian or possibly Arnsbergian age, suggesting that infilling of any deepwater areas there took place in the Arnsbergian or earlier. Previously estimated Namurian ages in well 48/3-3 (Leeder et al. 1990) are now thought to be too young on the basis of comparisons with nearby wells. As these two wells lie to the south and east of wells 43/17-2 and 43/25-1, the basin-fill progradation across Quadrant 43 probably took place from east to west rather than from north to south. This inference, and the fact that the Arnsbergian to Alportian was a time of limited sand supply to the Pennine basins, suggest that the main sand-supply route was deflected to the east and possibly entered the Southern North Sea Basin from somewhere near the position of the Central Graben.
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Once the main basin-filling progradation had taken place, conditions apparently never reverted to a deep basin, and sediment supply balanced continuing thermal subsidence. Relatively shallow-water conditions prevailed across the Southern North Sea Basin throughout the remaining Namurian and into the Westphalian. This regime produced a markedly cyclic succession, with marine bands defining cyclothem boundaries, as well as providing a chronostratigraphic framework [[:File:YGS_CHR_04_DINA_FIG_12.jpg|(Figure 12)]]. It is generally accepted that Namurian cyclicity and, particularly, the occurrence of marine bands, was driven by eustatic fluctuations in sea level (e.g. Holdsworth & Collinson 1988, Martinsen et al. 1995). These are thought to have acted as controls, both on the base level for sedimentation and on the salinity of the water bodies into which deltaic progradations took place (cf. Collinson 1988). The dominant style of deposition is the upwards-coarsening unit, usually initiated at a marine band. The more sand-rich upper parts of the units include both gradationally and sharply based sandstones, the former interpreted as mouth bars, the latter as channels (see [[:File:YGS_CHR_04_DINA_FIG_13.jpg|(Figure 13)]]).
 
Once the main basin-filling progradation had taken place, conditions apparently never reverted to a deep basin, and sediment supply balanced continuing thermal subsidence. Relatively shallow-water conditions prevailed across the Southern North Sea Basin throughout the remaining Namurian and into the Westphalian. This regime produced a markedly cyclic succession, with marine bands defining cyclothem boundaries, as well as providing a chronostratigraphic framework [[:File:YGS_CHR_04_DINA_FIG_12.jpg|(Figure 12)]]. It is generally accepted that Namurian cyclicity and, particularly, the occurrence of marine bands, was driven by eustatic fluctuations in sea level (e.g. Holdsworth & Collinson 1988, Martinsen et al. 1995). These are thought to have acted as controls, both on the base level for sedimentation and on the salinity of the water bodies into which deltaic progradations took place (cf. Collinson 1988). The dominant style of deposition is the upwards-coarsening unit, usually initiated at a marine band. The more sand-rich upper parts of the units include both gradationally and sharply based sandstones, the former interpreted as mouth bars, the latter as channels (see [[:File:YGS_CHR_04_DINA_FIG_13.jpg|(Figure 13)]]).
  
Channel sandstones range in thickness from simple units (as little as 2 m thick) up to multi-storey units several tens of metres thick. The thinner simpler channel sandbodies are attributed to delta distributaries, whereas some of the thicker units are thought to be fills of incised palaeovalleys [[:File:YGS_CHR_04_DINA_FIG_12.jpg|(Figure 12)]], [[:File:YGS_CHR_04_DINA_FIG_13.jpg|(Figure 13)]]. Palaeovalleys are inferred on the basis of being out of scale with associated cyclothems, in cutting down to and even removing underlying marine bands, and in having significantly coarser-grained fills than the associated distributary channels. Both the coarser grain size and larger dimensions make the palaeovalley sandstones the main potential reservoirs. By analogy with onshore examples (e.g. Chatsworth Grit) and with some support from well data, palaeovalleys are inferred to be typically several tens of kilometres wide. The stratigraphical equivalent of the Chatsworth Grit is the main reservoir in the Trent field (O’Mara et al. 1999, O’Mara 2004), where it appears to fill the palaeovalley only partially. The later stages of the palaeovalley fill include a finer-grained quartzitic sandstone, thought to be a reworked transgressive channel unit of possible estuarine origin. This unit has maintained good permeabilities in spite of its small grain size. It is interesting to note that the apparently underfilled palaeovalley of the Trent field is matched at outcrop by the Chatsworth Grit, which has an overlying progradational interval, the Redmires Flags, confined to the area of the inferred palaeovalley and underlying the Cancellatum Marine Band. This underfilling may have resulted from a high rate of sea-level rise, which outstripped the fluvial valley-floor aggradation. The higher incidence of marine trace fossils in the Marsdenian of Quadrant 43 (Lawrence & Sutter 2002), compared with the situation at outcrop, suggests that the connection of the Southern North Sea Basin to the open ocean may have been to the east.
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Channel sandstones range in thickness from simple units (as little as 2m thick) up to multi-storey units several tens of metres thick. The thinner simpler channel sandbodies are attributed to delta distributaries, whereas some of the thicker units are thought to be fills of incised palaeovalleys [[:File:YGS_CHR_04_DINA_FIG_12.jpg|(Figure 12)]], [[:File:YGS_CHR_04_DINA_FIG_13.jpg|(Figure 13)]]. Palaeovalleys are inferred on the basis of being out of scale with associated cyclothems, in cutting down to and even removing underlying marine bands, and in having significantly coarser-grained fills than the associated distributary channels. Both the coarser grain size and larger dimensions make the palaeovalley sandstones the main potential reservoirs. By analogy with onshore examples (e.g. Chatsworth Grit) and with some support from well data, palaeovalleys are inferred to be typically several tens of kilometres wide. The stratigraphical equivalent of the Chatsworth Grit is the main reservoir in the Trent field (O’Mara et al. 1999, O’Mara 2004), where it appears to fill the palaeovalley only partially. The later stages of the palaeovalley fill include a finer-grained quartzitic sandstone, thought to be a reworked transgressive channel unit of possible estuarine origin. This unit has maintained good permeabilities in spite of its small grain size. It is interesting to note that the apparently underfilled palaeovalley of the Trent field is matched at outcrop by the Chatsworth Grit, which has an overlying progradational interval, the Redmires Flags, confined to the area of the inferred palaeovalley and underlying the Cancellatum Marine Band. This underfilling may have resulted from a high rate of sea-level rise, which outstripped the fluvial valley-floor aggradation. The higher incidence of marine trace fossils in the Marsdenian of Quadrant 43 (Lawrence & Sutter 2002), compared with the situation at outcrop, suggests that the connection of the Southern North Sea Basin to the open ocean may have been to the east.
  
 
In contrast with the Pennine outcrop, the Yeadonian of the Southern North Sea Basin is characterized by a relative lack of sandstone. The Rough Rock, which is present as a sheet-like multi-storey channel complex across most of Pennines and the subsurface of the East Midlands (Bristow 1988), is weakly represented offshore, and in some wells the Yeadonian strata lack sandstones of any significance. This may again relate to large-scale diversions of sand-supply routes.
 
In contrast with the Pennine outcrop, the Yeadonian of the Southern North Sea Basin is characterized by a relative lack of sandstone. The Rough Rock, which is present as a sheet-like multi-storey channel complex across most of Pennines and the subsurface of the East Midlands (Bristow 1988), is weakly represented offshore, and in some wells the Yeadonian strata lack sandstones of any significance. This may again relate to large-scale diversions of sand-supply routes.
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The deepest well in the basin probably penetrates only the highest Dinantian strata (Brigantian) and it is therefore impossible to demonstrate when this basin margin developed. It is possible that the early Dinantian saw the shallow-water depositional systems of the block extending southwards without significant change across what is now the basin. Redbeds of latest Devonian or earliest Dinantian age are features of wells onshore and the same sort of extensional subsidence regime probably prevailed offshore at that time. In that case, a phase of intra-Dinantian tectonic activity must have led to the inferred differentiated bathymetry.
 
The deepest well in the basin probably penetrates only the highest Dinantian strata (Brigantian) and it is therefore impossible to demonstrate when this basin margin developed. It is possible that the early Dinantian saw the shallow-water depositional systems of the block extending southwards without significant change across what is now the basin. Redbeds of latest Devonian or earliest Dinantian age are features of wells onshore and the same sort of extensional subsidence regime probably prevailed offshore at that time. In that case, a phase of intra-Dinantian tectonic activity must have led to the inferred differentiated bathymetry.
  
A more precise idea of when this movement took place may come by analogy with wells in and around the Cleveland Basin, Cloughton-1, Kirby Misperton-1 and 41/24-2 [[:File:YGS_CHR_04_DINA_FIG_14.jpg|(Figure 14)]]. In well 41/24-2, the section penetrates into inferred Lower Limestone Formation strata, which appear to have a broadly Yoredale cyclic character. This pattern of deposition persists through the Middle Limestone Formation to the base of the Namurian. In the lowest part of the Namurian, a unit some 50 m thick has very high gamma values and is inferred to be a mudstone with well developed marine bands. This interval is overlain by some 85 m of sandstones, apparently with finer-grained interbeds. The lower part of this interval shows an upwards-coarsening upwards-thickening trend. It is sharply overlain by an upwards-coarsening sequence just over 100 m thick, at the top of which are coarse cross-bedded channel sandstones whose age is thought to be late Pendleian, based on uncorroborated ages indicated on the composite log. The lower sandstones, which are not cored, are thought most likely to be turbidites, with a basin-filling slope progradation above. The succession therefore compares quite closely with the Pendleian fill of the Craven Basin. If the inferred stratigraphical breakdown is correct, the main deepening event at this well would appear to have occurred around the Dinantian/Namurian boundary.
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A more precise idea of when this movement took place may come by analogy with wells in and around the Cleveland Basin, Cloughton-1, Kirby Misperton-1 and 41/24-2 [[:File:YGS_CHR_04_DINA_FIG_14.jpg|(Figure 14)]]. In well 41/24-2, the section penetrates into inferred Lower Limestone Formation strata, which appear to have a broadly Yoredale cyclic character. This pattern of deposition persists through the Middle Limestone Formation to the base of the Namurian. In the lowest part of the Namurian, a unit some 50m thick has very high gamma values and is inferred to be a mudstone with well developed marine bands. This interval is overlain by some 85m of sandstones, apparently with finer-grained interbeds. The lower part of this interval shows an upwards-coarsening upwards-thickening trend. It is sharply overlain by an upwards-coarsening sequence just over 100m thick, at the top of which are coarse cross-bedded channel sandstones whose age is thought to be late Pendleian, based on uncorroborated ages indicated on the composite log. The lower sandstones, which are not cored, are thought most likely to be turbidites, with a basin-filling slope progradation above. The succession therefore compares quite closely with the Pendleian fill of the Craven Basin. If the inferred stratigraphical breakdown is correct, the main deepening event at this well would appear to have occurred around the Dinantian/ Namurian boundary.
  
 
At Cloughton-1, by comparison, the high-gamma mudstones at base Namurian are clearly present, although somewhat thicker. The rest of the Pendleian comprises an upwards-coarsening unit but possible turbidite sandstones within it appear more thinly bedded. Immediately below the high-gamma mudstones are thick sandstones and inferred coal seams suggestive of a delta top setting and again indicating a significant deepening event around the Dinantian/Namurian boundary. The Cloughton-1 well terminated a short distance into the Dinantian so that the earlier history cannot be deduced.
 
At Cloughton-1, by comparison, the high-gamma mudstones at base Namurian are clearly present, although somewhat thicker. The rest of the Pendleian comprises an upwards-coarsening unit but possible turbidite sandstones within it appear more thinly bedded. Immediately below the high-gamma mudstones are thick sandstones and inferred coal seams suggestive of a delta top setting and again indicating a significant deepening event around the Dinantian/Namurian boundary. The Cloughton-1 well terminated a short distance into the Dinantian so that the earlier history cannot be deduced.
  
At Kirby Misperton-1, the Namurian section compares quite closely with that at Cloughton-1 and 41/24-1. The gamma peaks in the basal mudstone are less conspicuous and the large-scale upwards-coarsening succession of the Pendleian is thicker and has thicker channel sandbodies in its upper part. In contrast, inferred turbidite sandstones in the lower part of the pro-gradation are thin in comparison. The Dinantian strata just below the inferred Dinantian/Namurian boundary are similar to those in the other wells with channel sandstones but no obvious coals. This again suggests a significant deepening event at the Dinantian/Namurian boundary. However, the Dinantian penetration extends for some 1150 m and this provides a unique view of the earlier history of the Cleveland Basin. The latest Dinantian channel sandstones themselves occur at the top of a major fine-grained upwards-coarsening succession, some 300 m thick. Beneath that are some 300 m of thinly interbedded sandstones and finer grained rocks, which in turn overlie a sandy succession within which there are small-scale upwards-coarsening units that probably record minor deltaic progradations. The occurrence of a thick fine-grained upwards-coarsening unit above older deltaics suggests a phase of deepening. There is no control on the age of these deepest Dinantian sediments, but on thickness grounds they are likely to extend back at least well into the Asbian. The earlier phase of deepening could therefore be of Asbian age, but without tighter control on the age of the section in the deeper parts of the well, it is not possible to be more precise.
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At Kirby Misperton-1, the Namurian section compares quite closely with that at Cloughton-1 and 41/24-1. The gamma peaks in the basal mudstone are less conspicuous and the large-scale upwards-coarsening succession of the Pendleian is thicker and has thicker channel sandbodies in its upper part. In contrast, inferred turbidite sandstones in the lower part of the pro-gradation are thin in comparison. The Dinantian strata just below the inferred Dinantian/Namurian boundary are similar to those in the other wells with channel sandstones but no obvious coals. This again suggests a significant deepening event at the Dinantian/Namurian boundary. However, the Dinantian penetration extends for some 1150m and this provides a unique view of the earlier history of the Cleveland Basin. The latest Dinantian channel sandstones themselves occur at the top of a major fine-grained upwards-coarsening succession, some 300m thick. Beneath that are some 300m of thinly interbedded sandstones and finer grained rocks, which in turn overlie a sandy succession within which there are small-scale upwards-coarsening units that probably record minor deltaic progradations. The occurrence of a thick fine-grained upwards-coarsening unit above older deltaics suggests a phase of deepening. There is no control on the age of these deepest Dinantian sediments, but on thickness grounds they are likely to extend back at least well into the Asbian. The earlier phase of deepening could therefore be of Asbian age, but without tighter control on the age of the section in the deeper parts of the well, it is not possible to be more precise.
  
 
It is clear from well 43/17-2 [[:File:YGS_CHR_04_DINA_FIG_10.jpg|(Figure 10)]] that major deepening had occurred offshore prior to the Brigantian and, therefore, it might be reasonable to speculate that such movements were associated with those that caused the Dinantian deepening in the Cleveland Basin, which are tentatively inferred to be Asbian in age. A phase of well constrained Asbian extension is also recognized in the Craven Basin, although deepening of the sea began earlier here (Kirby et al. 2000). If this age were correct, the deepening in the southern North Sea would be broadly contemporaneous with the deposition of the Scremerston Formation. A consequence of the deeping is that the shallow-water strata of the Scremerston and Lower and Middle Limestone formations of the Mid North Sea High passed into deeper-water facies to the south, possibly with bypassing of sand to contemporaneous turbidite settings. This would compare with the spilling of sand across the Craven faults in the Brigantian to give turbidite units such as the Pendleside Sandstone in the Craven Basin as equivalents of Yoredale deltaic strata on the Askrigg Block. Another consequence is that the pre-deepening Cementstones, Fell Sandstone and the earlier parts of the Scremerston Formation may extend as fluvial and deltaic facies beneath the Southern North Sea Basin. This contrasts with what is seen onshore, where these units are mainly restricted to the Northumberland and Stainmore troughs.
 
It is clear from well 43/17-2 [[:File:YGS_CHR_04_DINA_FIG_10.jpg|(Figure 10)]] that major deepening had occurred offshore prior to the Brigantian and, therefore, it might be reasonable to speculate that such movements were associated with those that caused the Dinantian deepening in the Cleveland Basin, which are tentatively inferred to be Asbian in age. A phase of well constrained Asbian extension is also recognized in the Craven Basin, although deepening of the sea began earlier here (Kirby et al. 2000). If this age were correct, the deepening in the southern North Sea would be broadly contemporaneous with the deposition of the Scremerston Formation. A consequence of the deeping is that the shallow-water strata of the Scremerston and Lower and Middle Limestone formations of the Mid North Sea High passed into deeper-water facies to the south, possibly with bypassing of sand to contemporaneous turbidite settings. This would compare with the spilling of sand across the Craven faults in the Brigantian to give turbidite units such as the Pendleside Sandstone in the Craven Basin as equivalents of Yoredale deltaic strata on the Askrigg Block. Another consequence is that the pre-deepening Cementstones, Fell Sandstone and the earlier parts of the Scremerston Formation may extend as fluvial and deltaic facies beneath the Southern North Sea Basin. This contrasts with what is seen onshore, where these units are mainly restricted to the Northumberland and Stainmore troughs.
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Chadwick, R. A., D. W. Holliday, S. Holloway, A. G. Hulbert 1995. ''The structure and evolution of the Northumberland–Solway Basin and adjacent areas''. Subsurface Memoir, British Geological Survey, Keyworth, Nottingham.
 
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Collinson, J. D. 1988. Controls on Namurian sedimentation in the Central Province basins of northern England. In ''Sedimentation in a synorogenic basin complex: the Upper Carboniferous of northwest Europe'', B. M. Besly & G. Kelling (eds), 85–101. Glasgow: Blackie. Collinson J. D., O. J. Martinsen, B. Bakken, A. Kloster 1991. Early fill of the western Irish Namurian Basin: a complex relationship between turbidites and deltas. ''Basin Research ''3, 223–42.
 
 
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Day J. B. W. 1970. ''Geology of the country around Bewcastle''. Memoir, Sheet 12 (England and Wales), Geological Survey of Great Britain. Donato, J. A. & J. Megson 1990. A buried granite batholith beneath the East Midland Shelf of the Southern North Sea Basin. ''Geological Society of London, Journal ''147, 133–40.
 
 
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Earp, J. R., D. Magraw, E. G. Poole, D. H. Land, A. J. Whiteman 1961. ''Geology of the country around Clitheroe and Nelson''. Memoir, , Sheet 68 (England and Wales), Geological Survey of Great Britain. Elliott, T. 1976. Sedimentary sequences from the Upper Limestone Group of Northumberland. ''Scottish Journal of Geology ''12, 115–24. Fraser, A. J. & R. L. Gawthorpe 1990. Tectono-stratigraphic development and hydrocarbon habitat of the Carboniferous in northern England. In ''Tectonic events responsible for Britain’s oil and gas reserves'', R. F. P. Hardman & J. Brooks (eds), 49–86. Special Publication 55, Geological Society, London.
 
 
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Martinsen, O. J., J. D. Collinson, B. K. Holdsworth 1995. Millstone Grit cyclicity revisited, II: sequence stratigraphy and sedimentary responses to changes of relative sea-level. In ''Sedimentary facies analysis'', A. G. Plint (ed.), 305–327. Oxford: Blackwell Science.  
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Martinsen, O. J., J. D. Collinson, B. K. Holdsworth 1995. Millstone Grit cyclicity revisited, II: sequence stratigraphy and sedimentary responses to changes of relative sea-level. In ''Sedimentary facies analysis'', A. G. Plint (ed.), 305–327. Oxford: Blackwell Science. Maynard, J. R. & R. E. Dunay 1999. Reservoirs of the Dinantian (Lower Carboniferous) play of the southern North Sea. In ''Petroleum geology of northwest Europe: proceedings of the 5th conference'', A. J. Fleet & S. A. R. Boldy (eds), 729–45. London: Geological Society.
 
 
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Reynolds, A. D. 1992. Storm, wave and tide-dominated sedimentation in the Dinantian Middle Limestone Group, Northumberland Basin. ''Yorkshire Geological Society, Proceedings ''49, 135–48. Ridd, M. F., D. B. Walker, J. M. Jones 1970. A deep borehole at Harton on the margin of the Northumberland Trough. ''Yorkshire Geological Society, Proceedings ''38, 75–103.
 
 
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