THE HASLINGDEN FLAGS (NAMURIAN G 1 ) OF SOUTH-EAST LANCASHIRE: BAR FINGER SANDS IN THE PENNINE BASIN

Abstract

Summary The sedimentary sequence between the Gastrioceras cancellatum Marine Band and the Rough Rock in south-east Lancashire comprises a wide range of lithologies ranging from mudstone through siltstone to ripple cross-laminated and cross-bedded medium-grained sandstones. The sequence consists broadly of two coarsening upwards sequences, each dominantly of mudstone and siltstone in its lower part and each capped by a sandstone member, the Lower and Upper Haslingden Flags respectively. Isopachytes on these sandstones show them to form elongate lobes thinning eastwards from a maximum exposed development around Blackburn. Palaeocurrent directions are dominantly towards the east. Both sandstones are cross-bedded in their central upstream regions and, in the Upper Haslingden Flags, a complex internal geometry involves large scale, low-angle cross-bedding up to 27 metres thick. The overall geometry and lithofacies assemblage is compared with that of a modern bar-finger sand and, in the Upper Haslingden Flags, the internal structure of the actual distributary mouth bar is shown. These deltas differ in type from the usual Millstone Grit delta in their finer grain size, their body geometry and in their palaeocurrent pattern. These points of difference raise the possibility of a western source area for at least some of the Millstone Grit and also imply that a different hydrological regime may have operated there.