Sedimentology, architecture and depositional setting of the fluvial Spireslack Sandstone of the Midland Valley, Scotland: insights from the Spireslack surface coal mine

The Spireslack surface coal mine exposes a section in the Carboniferous Lawmuir Formation (Brigantian) into the Upper Limestone Formation (Arnsbergian). This paper describes the stratigraphy exposed at Spireslack and, in so doing, names for the first time the Spireslack Sandstone, a distinctive erosively based, sandstone-dominated unit in the Upper Limestone Formation. The Spireslack Sandstone consists of two fluvial sandstone channel sets and an upper, possibly fluvio-estuarine, succession. From an analysis of their internal architectural elements, the channel sets are interpreted as a low-sinuosity, sand-dominated, mixed-load fluvial system in which avulsion and variations in sediment load played a significant part. The lower channel set appears to be confined to erosional palaeovalleys of limited lateral extent and significant relief. The upper channel set is much more laterally extensive and shows evidence of a generally lower sediment load with a greater degree of lateral accretion and flooding. Consequently, the Spireslack Sandstone may represent a system responding to base level changes of higher magnitude and longer duration than the glacio-eustatic scale commonly attributed to Carboniferous fluvio-deltaic cycles. The Spireslack Sandstone may represent an important correlative marker in the Carboniferous of the Midland Valley and may provide an alternative analogue for some Carboniferous fluvial sandstone stratigraphic traps.