CONTRASTING SEDIMENT GRAVITY FLOW PROCESSES IN THE LATE LLANDOVERY, RHUDDNANT GRITS TURBIDITE SYSTEM, WELSH BASIN

The Rhuddnant Grits turbidite system was deposited within an elongate, fault-bounded trough in the late Llandovery (Telychian) Welsh Basin. Two groups of sandstones are identified within the system: high-matrix sandstones and laminated sandstones. The high-matrix sandstones are m edium to very thick bedded, fine to very coarse-grained muddy sandston es. The high-matrix sandstone beds are almost entirely structureless a nd have several features indicative of deposition from high density tu rbidity currents, probably undergoing late stage debris flow behaviour (e.g. grain size discontinuities, inverse grading, floating clasts). The laminated sandstones are thin to very thin bedded, fine-grained an d have a distinctive mud/silt lamination. Tractional structures and co nvolution are common in these beds. They were probably deposited by sl ow moving, dilute turbidity currents. Dissimilar palaeocurrent vectors and estimates of flow properties from the two types of sandstone supp ort the contrasting nature of the depositing flows. A coarsening and t hickening upwards trend is identified in the laminated sandstones of t he Rhuddnant Grits Formation. This trend is not reflected in the high- matrix sandstone . beds. Although the high-matrix sandstones appear in packets or groups within the laminated sandstone background, they wer e otherwise deposited in an entirely random manner throughout the expo sed system. This may suggest that the two types of sandstone are the r esult of different triggering mechanisms at source, or of contrasting flow properties developed early in the flow histories.