Cyclicity of Devonian fluvial sedimentation, Cunningsburgh Peninsula, SE Shetland

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Abstract
The Cunningsburgh area, SE Shetland, contains approximately 1000 m of fluvial sediments deposited in four sub-environments. Ripple cross-laminated siltstones and sandstones represent levee-type sediments, horizontally- and solitary cross-stratified sandstones were deposited by overbank sheet floods; red mudstones with sandstone lenses represent floodbasin sediments and grouped cross-stratified sandstones are substratum sediments deposited in the channellized zone as transverse bars. point bars and megaripple channel-fills. Transition, entropy and substitutability analyses indicate that in the S of the Cunningsburgh area cyclicity was controlled by two processes: one was that of channel migration, cut-off or avulsion, producing scoured surfaces overlain by substratum cross-stratified sandstones. The other was that of overbank flooding, producing an interbedding of overbank sheet flood deposits, levee-type sediments and floodbasin fines. Further to the N, distant from the main channellized zone, overbank sheet flood deposits are more common and are interbedded with levee and floodbasin deposits. Cyclicity in this zone was, therefore, controlled essentially by overbank flooding alone but not by channel migration or shifting. The fluvial system was characterized by a central zone containing channellized high-sinuosity flow which was bordered, within the same alluvial valley, by low-lying floodplains subject to flooding by both crevassing and overtopping of levees. The main stream only rarely migrated into this outer floodplain.