Editing Dinantian and Namurian depositional systems in the southern North Sea

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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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