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Geology & Cartography Division, Oxford Brookes University, Gipsy Lane, Oxford OX3 0BP, UK
A series of stacked fluvio-lacustrine sequences in the Westphalian of the northern part of the Southern North Sea can be correlated over areas of greater than 2500 km2. Each sequence is similar in terms of thickness (100–300 m) and the internal arrangement of lithofacies but they differ in colour and in the amount of sandstone and coal that they contain. A typical sequence consists of a sand-rich lower unit, a mud-dominated middle unit which fines upwards towards its top and a mud-dominated upper unit which generally coarsens upwards. The main reservoir-prone intervals are confined to the lower-middle parts of sand-rich units and consist of coarse-grained braided or low-sinuousity channel deposits. These fluvial sandstones are laterally extensive and are neither typical of incised valley fills (in terms of stratal geometries) nor of progradational systems tracts (in terms of facies distribution). Instead they appear to have been deposited during periods of widespread fluvial aggradation caused by transgression as a result of relative lake-level rise. Each lower sand-rich unit is overlain by a dark mudstone with a high gamma ray expression which is interpreted as an initial (lake) flooding surface marking the onset of predominantly lacustrine and mire sedimentation associated with the development of the fining-upwards mud-dominated unit. The top of each fining-upwards unit is marked by a maximum gamma ray peak interpreted as a maximum flooding surface which defines the end of lake transgression. Above the maximum flooding surface, each coarsening-upwards mud-dominated unit shows increasing deltaic influence upwards associated with a slowing in the rate of relative lake level rise. The tops of some of these units are eroded suggesting that in some case deltaic progradation was followed by a relative fall in lake level; in other cases the boundary with the overlying transgressive sand-rich unit in the next sequence is wholly conformable. Coal mires developed in coastal areas, particularly during transgression when coarse-grained sediment was trapped up-dip on the coeval alluvial plain. Relative changes in lake level had a periodicity of 1.5 Ma or less indicating that they may be related to third order eustatic cycles.
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