DELTAIC SEDIMENTATION UNITS IN THE UPPER CARBONIFEROUS OF NORTHERN ENGLAND

In the Kinderscout Grit (Namurian, R₁c) of the southern half of the Central Pennine Basin, England, there occur tabular, isolated, cross-bedded sets of coarse sandstone, between 4 and 40 m thick. The sets are up to 1 km wide and the foresets, in plan, are convex in the direction of dip. The sets interfere laterally to form extensive sheets. Foresets dip at up to 27° and are sometimes themselves internally cross-bedded in trough-shaped sets, here termed “intrasets”. The large sets are thought to be deltaic sedimentation units rather than sandwaves. In the delta top environment in which these sets were deposited, a high rate of bed-load sediment supply and a sudden deepening were required to initiate the sets. Medium-scale cross-bedding, thought to have been laid down in migrating, fluviatile channels overlies the large-scale sets and represents the topset member of the sedimentation units. From the spatial arrangement of the large- and medium-scale cross-bedding, it is possible to distinguish those areas where deposition took place during the deepening, from those where it took place essentially after the deepening. The causes of the deepening, which must have been on at least a basinal scale, may have been eustatic or tectonic.