Channel formation and migration by mass-flow processes in the Lower Carboniferous fluviatile Fell Sandstone Group, northeast England

Small, steep-sided narrow channels, locally developed within the braided fluviatile Lower Carboniferous Fell Sandstone of northeast England, are attributed to mass-flow processes initiated by channel-bank slumping. The channels have sharp undisturbed margins and are filled with moderately well-sorted medium to coarse sandstone identical to the sandstone which they cut. Most channel-fills are structureless, but a few show diffuse concentric marginal laminations. Laminations also occur towards the top of channel fills, but they are less regular and show low-amplitude trough-like undulations. Channel margin inclinations range from 13° to 55°, and channel depths vary between 1·7 m and 2·5 m. The channels, which are only seen in profile, are orientated perpendicular to the local and regional palaeocurrent trend, and cut through structureless sandstone and planar cross-stratified sandstone characterized by compound and compound-complex bar bedforms. Only one channel has both margins preserved; and of the remainder (seven) only one has its northern margin preserved. Undercutting and retrogressive slumping of the sandy channel bank is thought to have initiated a subaqueous sediment gravity flow, which moved across the main channel along a transverse scour at the base of the slip face of a large composite bar bedform. Such scours have strong asymmetric profiles with the steeper slope along the base of the active slip face (northern side) and the lower slope on the downstream (southern) side of the scour. Thus, the northern channel margin was steeper, less stable and more prone to collapse than the southern margin, and is almost never preserved. Flow reconstruction indicates that on entering the channel a hydraulic jump developed at the change in slope, and the flow became more turbulent and erosive. Scoured sand incorporated into the flow increased concentration and shearing along the boundaries. Reverse shear was also exerted by the overlying current, on top of the flow, but the central part remained relatively unaffected. The presence or absence of marginal laminations is probably related to marginal shearing and flow dilution. Deposition occurred in response to deceleration and frictional freezing of the sand flow.