The newly recognized Nant Brianne turbidite system was a focus of laterally supplied coarse-grade sediment deposition that, along with the Caban–Ystrad Meurig system, punctuated late Hirnantian to early Telychian, mudstone-dominated slope apron deposition along the SE margin of the southern Welsh Basin. Geological mapping coupled with detailed biostratigraphy enable the depositional influence of sea-floor topography, an active Llyn Brianne Fault and eustatic sea-level changes to be tracked. The latter may represent ‘far field’ effects relating to the retreat and advance of contemporary Gondwanan ice sheets. Slope apron mudstone facies reveal a strong response to high order eustatic events; however, the response of contemporary, easterly sourced, coarse-grade turbidite systems was more complex, with some periods of increased sand and gravel input coinciding with times of rising global sea level. These anomalous relationships are explained by invoking a series of smaller-scale movements in marine base level. It was the interaction of these lower order events with the main eustatic cycles that appears to have been the primary control on sand and gravel input to the Welsh Basin. An early Telychian expansion of the Nant Brianne turbidite system records a marked increase in tectonically generated sediment at a time of palaeo-plate collision between Baltica/Avalonia and Laurentia before intra- and peri-basinal faulting led to its abandonment as a supply path to the basin centre.

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