Fell Sandstone lithostratigraphy south-west of Berwick-upon-Tweed: implications for the regional development of the Fell Sandstone

Abstract

SUMMARY The Fell Sandstone Group in north-east England forms part of a fluvio-deltaic Lower Carboniferous Visean sequence that was deposited within the Northumberland Basin and the smaller Tweed Basin following their initiation by rifting and subsidence in the late Devonian/early Carboniferous. The Fell Sandstone was deposited by a large braided-river complex which drained westwards into a shallow, low-energy tideless sea. Outcrop studies have indicated that the Fell Sandstone Group is composed almost entirely of sandstone with a paucity of overbank fines (less than 20%). However, a number of boreholes drilled for a hydrogeological study in the Tweed Basin indicate that in this region the Fell Sandstone Group may contain 50 to 60% mudstone. Seven mappable sandstone units have been identified within the field area, separated by laterally persistent mudstone units. Pumping test data indicate that many of the sandstone units are in good communication, whereas others are in isolation. Thickness variations between wells indicate that many of the sand units have a lenticular geometry. Regional compilation of borehole data through the Fell Sandstone across the Northumberland Basin highlights a number of lateral facies changes which cannot be assigned simply to a relationship of distance from source. In the early Carboniferous a number of faults within the Northumberland Basin were active during the deposition of the Fell Sandstone. Local variations in facies and sandstone thickness within the Fell Sandstone Group can be explained using simple tilted fault-block models. Where a particular fault changes polarity of throw local thickness and extent of