Siliciclastic shelf margin sedimentology and relative sea-level changes
The effects of changes in relative sea-level on clastic shelf margins are examined utilizing two case studies, the passive Quaternary to Recent of Sable Island Bank, Nova Scotia, and the active Silurian shelf of Dyfed, southwest Wales. Within the overall transgressive systems tract of the Sable Island Sand and Gravel Formation, siliciclastic material is continually reworked in shallow water by modern storm-dominated, tidally-modulated flows in an attempt to reach equilibrium and textural grade with the bank-top hydrodynamics. Over the past 11,000 years marked transgressive marine ravinement of the bank-top has cannibalised considerable volumes of sediment into the dynamic realms of eustatically produced accommodation space. Sediment transport, together with bedform and grain-size variations reflect both progressive sorting and the depth dependent fluid power relationship. Bedload sediment transport is predominantly geostrophic, and is seen to be topographically steered at shelf-edge breaks in slope, a process related to the hydraulic fence concept. Through this process, sediment by-pass into deep water is hindered, and spatially significant upward coarsening/thickening sandbodies are developing, most notably at East Bar. Taken a stage further, shoal aggradation of this sediment supply dominated coastal system has enabled the emergence of the cuspate/elongate shelf-edge Sable Island. The effects of changes in relative sea-level on various scales is described from the Coralliferous and Gray Sandstone Groups of southwest Wales. Six Vailian sequences are recognised within a thick basinward prograding clastic wedge. During lowstands of relative sea-level, shelfal incision and sediment by-pass occurred, with sediment supplying deep water fan systems immediately to the north of the study area. Valley-fills vary in depositional environment from low sinuosity fluvial channels (Sequence I), estuarine tidal channels (Sequences II to V), and mixed estuarine/high sinuosity fluvial channels of Sequence VI.