A New Evaluation of Fining Upward Sequences in a Mud-Rock Dominated Succession of the Lower Old Red Sandstone of South Wales, UK

Summary

As much as 80% of the 4.5-km-thick succession of alluvial sediments of late Silurian to early Devonian age in South Wales is composed of mudstone, generally regarded as overbank suspension deposits. Studies of selected formations reveal a variety of lithofacies associations in these mud-dominated intervals, which suggest diverse mechanisms for mud emplacement. For example, there are distinct fining upward units, 2.5–5 m thick, that represent ephemeral channel-zone deposits in which extensive reworking of palaeo-Vertisols took place as clay pellet bedload material. The overall depositional system has many similarities with that described from the Channel Country of central Australia, where anastomosing fluvial systems are associated with distal mud sheetflood deposits. The exact channel planforms for the Old Red Sandstone systems, however, cannot be readily determined, although where larger channel sandbodies exist they have a sheet-like occurrence with a high width/depth ratio, and possibly may be the result of large-scale, low-frequency sheet-flood events. The development of palaeosols with vertic features, calcretes, alternations of desiccation and burrowing in the channel deposits suggests marked seasonality with a flashy regime. The Channel Country represents a fairly stable low-relief cratonic interior, whereas the Old Red Sandstone in South Wales was deposited on a coastal-plain in a subsiding basin setting. There are, however, similarities in terms of depositional products despite the disparity in morpho-tectonic setting, which may reflect extremely low gradients and climatic influences such as frequency and magnitude of flood events.