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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The facies described suggest that deposition occurred on a large alluvial plain where laterally extensive fluvial channel systems formed the most important component. This is in general agreement with earlier models proposed for this part of the Westphalian in northwestern Germany (David 1990, Jankowski et al. 1993). Palaeocurrent measurements indicate that the dominant flow direction was towards the west-northwest ([[:File:YGS_CHR_05_FLUV_FIG_06.jpg|Figure 6]]). Hence, the Bolsovian to Westphalian D succession is believed to record a fluvial system fed from the highlands beyond the Variscan Front to the southeast of the district. A significant southwesterly palaeocurrent component and northeasterly increase in net-togross ratio was also recognized during this study ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]], [[:File:YGS_CHR_05_FLUV_FIG_07.jpg|Figure 7]]), and may indicate the existence, at times, of an axially fed fluvial system, derived from the northeast or east. The lack of marine facies suggests a setting far removed from the open sea.
 
The facies described suggest that deposition occurred on a large alluvial plain where laterally extensive fluvial channel systems formed the most important component. This is in general agreement with earlier models proposed for this part of the Westphalian in northwestern Germany (David 1990, Jankowski et al. 1993). Palaeocurrent measurements indicate that the dominant flow direction was towards the west-northwest ([[:File:YGS_CHR_05_FLUV_FIG_06.jpg|Figure 6]]). Hence, the Bolsovian to Westphalian D succession is believed to record a fluvial system fed from the highlands beyond the Variscan Front to the southeast of the district. A significant southwesterly palaeocurrent component and northeasterly increase in net-togross ratio was also recognized during this study ([[:File:YGS_CHR_05_FLUV_FIG_03.jpg|Figure 3]], [[:File:YGS_CHR_05_FLUV_FIG_07.jpg|Figure 7]]), and may indicate the existence, at times, of an axially fed fluvial system, derived from the northeast or east. The lack of marine facies suggests a setting far removed from the open sea.
  
The identification of two different types of channel sandbody indicates two distinct fluvial styles. The channels dominated by downstream accretion were bedload-dominated rivers, characterized by large barforms that migrated predominantly down stream. Channels were probably braided in form and produced fairly wide and extensive river tracts. The presence of different scales of channels, together with the evidence for large channel barforms, suggests that channel belts formed from multi-channel depositional systems. Their high degree of lateral mobility resulted in channel belts that are much wider than those of the active channels. Minimum channel depths must be greater than the preserved thickness of individual barforms, so deep rivers in excess of 15 m are envisaged in some instances, although they probably ranged from 10 m to 20 m. These should be considered as minimum figures and are derived from the post-compactional thicknesses of the preserved sandbodies. The width of the overall braided river system is difficult to quantify but may have been a kilometre or more.
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The identification of two different types of channel sandbody indicates two distinct fluvial styles. The channels dominated by downstream accretion were bedload-dominated rivers, characterized by large barforms that migrated predominantly down stream. Channels were probably braided in form and produced fairly wide and extensive river tracts. The presence of different scales of channels, together with the evidence for large channel barforms, suggests that channel belts formed from multi-channel depositional systems. Their high degree of lateral mobility resulted in channel belts that are much wider than those of the active channels. Minimum channel depths must be greater than the preserved thickness of individual barforms, so deep rivers in excess of 15m are envisaged in some instances, although they probably ranged from 10m to 20m. These should be considered as minimum figures and are derived from the post-compactional thicknesses of the preserved sandbodies. The width of the overall braided river system is difficult to quantify but may have been a kilometre or more.
  
The channels dominated by lateral accretion produce thinner, more heterolithic deposits. The absence of mid-channel bars suggests that the flow was generally confined within single channels. Palaeocurrent measurements indicate that flow was mainly across the barforms and that lateral accretion processes were important (cf. Smith 1987, Leeder 1999). Hence it is considered that these represent point bars and that the channel deposits were the result of highly sinuous rivers. In many instances the full channel thickness is preserved and a range of channel depths is indicated, varying from 4 m to 15 m. Again, these should be considered as minimum figures, derived from the post-compactional thicknesses of the preserved sandbodies. Active channels were narrower than the channels dominated by downstream accretion, and widths, based on preserved abandoned channel plugs and point bar length, probably ranged from 40 m to 150 m. The sandbodies produced by the process of lateral accretion are obviously larger than the original channel widths, and typically range from 100 m to 400 m across.
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The channels dominated by lateral accretion produce thinner, more heterolithic deposits. The absence of mid-channel bars suggests that the flow was generally confined within single channels. Palaeocurrent measurements indicate that flow was mainly across the barforms and that lateral accretion processes were important (cf. Smith 1987, Leeder 1999). Hence it is considered that these represent point bars and that the channel deposits were the result of highly sinuous rivers. In many instances the full channel thickness is preserved and a range of channel depths is indicated, varying from 4m to 15m. Again, these should be considered as minimum figures, derived from the post-compactional thicknesses of the preserved sandbodies. Active channels were narrower than the channels dominated by downstream accretion, and widths, based on preserved abandoned channel plugs and point bar length, probably ranged from 40m to 150m. The sandbodies produced by the process of lateral accretion are obviously larger than the original channel widths, and typically range from 100m to 400m across.
  
 
The channels dominated by lateral accretion deposited a mixture of bedload and suspended sediment load. The finer grain size of some not all) of these channels indicates that they were probably muddier than those formed by downstream accretion. Flow fluctuations are indicated by mud-filled scours on bar tops. Channel abandonment was an important process, possibly linked to meander (neck or chute) cut-off. Following abandonment, these channels formed shallow oxbow lakes that filled with a variety of low-energy deposits, mainly muds, but including peat.
 
The channels dominated by lateral accretion deposited a mixture of bedload and suspended sediment load. The finer grain size of some not all) of these channels indicates that they were probably muddier than those formed by downstream accretion. Flow fluctuations are indicated by mud-filled scours on bar tops. Channel abandonment was an important process, possibly linked to meander (neck or chute) cut-off. Following abandonment, these channels formed shallow oxbow lakes that filled with a variety of low-energy deposits, mainly muds, but including peat.

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