Editing Fluvial sandbody architecture, cyclicity and sequence stratigraphic setting – implications for hydrocarbon reservoirs: the Westphalian C and D of the Osnabrück–Ibbenbüren area, northwest Germany

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Olsen (1990) recognized two scales of cyclicity from Devonian meandering channel systems from east Greenland, one of the order of ''c''. 20 m thick and a higher-order one at ''c. ''100 m. He attributed these to climatic variations as a result of changes in Earth’s orbital parameters, reflecting 20000 yr Milankovitch precession cycles with modulation by ''c. ''110000 yr eccentricity cycles. Although it is possible that Milankovitch orbital forcing could account for the third-order cycles described here, the structural setting in northern Germany at this time makes it more likely that these cycles represent the product of changes in the balance between tectonically induced accommodation and climatically modulated sediment supply. Similar climatic controls on Carboniferous and Devonian non-marine successions have been described respectively by Glover & Powell (1996) and McKie & Garden (1996).
 
Olsen (1990) recognized two scales of cyclicity from Devonian meandering channel systems from east Greenland, one of the order of ''c''. 20 m thick and a higher-order one at ''c. ''100 m. He attributed these to climatic variations as a result of changes in Earth’s orbital parameters, reflecting 20000 yr Milankovitch precession cycles with modulation by ''c. ''110000 yr eccentricity cycles. Although it is possible that Milankovitch orbital forcing could account for the third-order cycles described here, the structural setting in northern Germany at this time makes it more likely that these cycles represent the product of changes in the balance between tectonically induced accommodation and climatically modulated sediment supply. Similar climatic controls on Carboniferous and Devonian non-marine successions have been described respectively by Glover & Powell (1996) and McKie & Garden (1996).
  
It is proposed that times of hinterland uplift resulted in greater orographic precipitation and hence the basal channel-belt-dominated parts of each third-order cycle reflect the rapid expansion of fluvial systems as the amount of clastic flux outstrips the accommodation space available. The change in fluvial style upwards through a cycle and the increased frequency of coals and other floodplain deposits indicates a decrease in the efficiency of the fluvial systems, a reduction in stream gradients and an elevation of the groundwater table. In a hydrographically enclosed system such as this, high rates of precipitation and any concomitant basinal subsidence would increase both the amount of sediment aggradation and raise the base level. Thus, relative base-level (effective lake-level) rise results in the progressive drowning of fluvial systems. The resulting decrease in the efficiency of the fluvial systems to transport the coarse clastic fraction is thought to be the main cause of the change from low- to relatively high-sinuosity fluvial systems and ultimately into floodplain conditions.
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It is proposed that times of hinterland uplift resulted in greater orographic precipitation and hence the basal channel-belt-dominated parts of each third-order cycle reflect the rapid expansion of fluvial systems as the amount of clastic flux outstrips the accommodation space available. The change in fluvial style upwards through a cycle and the increased frequency of coals and other floodplain deposits indicates a decrease in the efficiency of the fluvial systems, a reduction in stream gradients and an elevation of the groundwater table. In a hydrographically enclosed system such as this, high rates of precipitation and any concomitant basinal subsidence would increase both the amount of sediment aggradation and raise the base level. Thus, relative base-level (effective lake-level) rise results in the progressive drowning of fluvial systems. The resulting decrease in the efficiency of the fluvial systems to transport the coarse clastic fraction is thought to be the main cause of the change from low-to relatively high-sinuosity fluvial systems and ultimately into floodplain conditions.
  
 
This hypothesis is difficult to prove conclusively, as evidence for intra-cycle variations in climate is lacking in these successions. However, climatically controlled facies changes have been proven from the Westphalian D and younger successions of northern Germany and the UK, and primary redbed facies, including calcretes and localized alluvial fans, all indicate deposition under increasingly drier and more arid conditions, linked to the growth of a rain shadow associated with uplift of the Variscan mountains (Besly 1987, 1988). It is also known that in younger Westphalian D and Stephanian successions, the effects of increased evapotranspiration are manifested by a change to better-drained palaeosols and an absence of coals upwards through a cycle. This may indirectly support the view that climatic controls operated during earlier times.
 
This hypothesis is difficult to prove conclusively, as evidence for intra-cycle variations in climate is lacking in these successions. However, climatically controlled facies changes have been proven from the Westphalian D and younger successions of northern Germany and the UK, and primary redbed facies, including calcretes and localized alluvial fans, all indicate deposition under increasingly drier and more arid conditions, linked to the growth of a rain shadow associated with uplift of the Variscan mountains (Besly 1987, 1988). It is also known that in younger Westphalian D and Stephanian successions, the effects of increased evapotranspiration are manifested by a change to better-drained palaeosols and an absence of coals upwards through a cycle. This may indirectly support the view that climatic controls operated during earlier times.

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