BRAIDED RIVER BEDFORMS AND RELATED SEDIMENTARY STRUCTURES IN THE FELL SANDSTONE GROUP (LOWER CARBONIFEROUS) OF NORTH NORTHUMBERLAND

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Abstract
SUMMARY: The Lower Carboniferous Fell Sandstone Group in northern Northumberland is composed of lenticular or sheet-like bodies of cross-stratified quartz arenites which alternate with shales and ripple-laminated siltstones. Although correlation is difficult due to the lack of fossils or marker horizons, those exposures described can be fixed stratigraphically by reference to the overlying Scremerston Coal Group. The sedimentary structures and bedforms developed in the sandstones include unimodal trough and tabular cross-stratification, channel fills with penecontemporaneous deformation structures, and dunes complete forms of which are preserved in some cases. These features are characteristic of deposition at and near the boundary between the lower and upper flow regimes, where small fluctuations in gradient, flow stage and discharge migration may radically alter the style and type of deposition. The depositional environment is interpreted as alternating between low-gradient meandering and higher-gradient braided perennial rivers, with variable discharge, in channel systems several kilometres wide. The relatively low variation in lithology, sedimentary structures and palaeoenvironments of the Fell Sandstone, compared to groups above and below, indicates relatively stable depositional conditions. Lower formations show meandering fluvio-lacustrine deposition, while coals and marine transgressive deposits above reflect the infilling of the Northumberland Basin. A comprehensive study of the whole basin is urged.