Descending Tabular Cross-Bed Sets and Bounding Surfaces from a Fluvial Channel in the Upper Carboniferous Coalfield of North-East England

The internal structure of an elongate sandbody 1.9 km across and 10 m thick is dominated by sequences of tabular cross-bed sets: trough sets are less common. Tabular sets have avalanche faces and occur as downcurrent-descending tiers within larger cosets. The sets were deposited by sandsheet bedforms and were components of larger sandwave bedforms (which now form cosets). Sandwaves descended the low-angled leeside of an ever larger slow-moving bar. Facies of cross-bedding are defined from set and coset types. Analogies with modern fluvial bars suggest that the 10 m thick sandstone was deposited as part of one large fluvial bar. Similar configurations of sets are known from the leesides of other large fluvial, tidal shelf and aeolian bedforms. A spectrum of different types of bounding surfaces separate sets and cosets. These originated when sandsheets migrated straight down, or diagonally down the lee faces of sandwaves. Such migration was probably influenced by the crestline shape of the larger sandwave bedform rather than by river flow stage.