Submerged barrier shoreline sequences on a high energy, steep and narrow shelf

摘要

Late Pleistocene-Holocene shoreline sequences and associated shelf stratigraphy are described from a high gradient, high wave-energy shelf. Although shoreline preservation by overstepping is considered unlikely in such an environment as a result of intense ravinement during shoreline translation, the development and preservation of two distinct shorelines at - 100 m and - 60 m are evident and attributed to century to millennial scale periods of stillstand. During these periods shoreline equilibrium forms developed, characterised by early diagenesis of beachrock and aeolianite. Between these features the shelf is dominated by erosional surfaces reflecting ravinement during periods of slowly rising sea-level. Where shorelines are not preserved, areas of scarping in the ravinement surface, at similar depths to the adjoining shorelines, represent rocky headlands separating the sandy coastal compartments where the depositional shorelines formed. Shoreline preservation by overstepping was facilitated by rapid relative sea-level rise associated with meltwater pulses 1A and IB, although the features preserved are likely to represent only the cemented cores of the coastal dunes and linear beachrock outcrops formed. The steep antecedent shelf gradient fostered the erosion of significant portions of the shorelines by ravinement, with loose sediment re-organised into a thick post-ravinement drape. The removal of shorelines during ravinement was countered by the rapid rate of sea-level rise and early sub-tropical diagenesis. It is clear that due to the combined effects of subtropical diagenesis, long periods of stillstand and drowning by meltwater pulses, the governing controls on shoreline preservation can be over-ridden by combinations of local (climatic) and global (eustatic) factors.