Fan-delta sedimentation in the Silurian Coralliferous Formation of southwest Wales: implications for the structure of the southern margin of the Welsh Basin

The uppermost Llandovery to lower Wenlock Coralliferous 'Group' in SW Pembrokeshire is here redefined as a single formation with two members. The Coralliferous Formation is approximately 150 m thick and comprises a basal unit of granule- to pebble-grade rudite beds, the Renney Slip Member, overlain by interbedded mudstones and fine sandstones of the Deadman's Bay Member. The Renney Slip Member lies unconformably above the Skomer Volcanic Group and includes 12.4 m of coarse grained, granule- and pebble-rich rudites, with beds up to 0.94 m thick. Three lithofacies are recognized within this unit: coarse, granule-rich rudite beds are interpreted as a variety of mass flow deposits, some of which have been reworked in a marine environment; thick sandstones with planar and ripple lamination are shoreface to offshore transition zone deposits; silty mudstones interbedded with very fine grained sandstones represent marine offshore deposits, formed largely below mean storm wave base. These facies associations, and abundant bioturbation, indicate an environment with a strong marine influence, and a proximal source of coarse grained sediment. The Renney Slip Member is reinterpreted in the context of a fan delta depositional model. At least seven cycles of deposition are recognized, each showing an upwards-fining pattern, representing deposition from fan delta, shoreface-transition zone to open marine environments. These patterns of deposition are attributed to localized tectonic movements causing variations in relative sea level. At the time the Renney Slip Member was deposited, the southern Welsh Basin margin is interpreted as a fault-block extensional margin, with the landmass of Pretannia to the south. Though fan-delta deposition took place southwards against the uplifted footwall of the Wenall Fault, the basin margin lay to the south of the Ritec Fault.