Submerged shoreline preservation and ravinement during rapid postglacial sea-level rise and subsequent “slowstand”

Abstract

Submerged shorelines hold much potential for examining the interplay between the rate of sea-level rise and geomorphic setting, and informing the development of models of contemporary shoreline behavior. This paper describes the sedimentary architecture of a submerged barrier shoreline complex off Durban, South Africa. The complex consists of several barriers that have survived the postglacial transgression and associated erosive ravinement processes. The main shoreline sequence (–60 m) dates to 11,690 ± 90 calibrated (cal) yr B.P. and rests on a Pleistocene lagoonal deposit (35,395 ± 592 cal yr B.P.). The entire barrier shoreline complex is truncated by a strongly diachronous wave ravinement surface. The ravinement surface seaward of the main –60 m shoreline is steep, but the gradient declines across and landward of the subordinate landward shoreline complexes. The steep ravinement surface is attributed to fast sea-level rise (possibly associated with meltwater pulse 1B), while the gentle ravinement surface is associated with a subsequent slowing of the rate of sea-level rise (to 0.15 mm yr –1 ). Preservation of the main barrier and back-barrier deposits through overstepping is associated namely with rapid transgression and increased back-barrier accommodation. The smaller barriers landward of the main barrier were preserved through overstepping related to gentle antecedent gradients, despite more intense ravinement during a very slow rise in sea level (slowstand). This process was assisted by a sediment deficit. The resulting post-transgressive drape is also influenced by antecedent topography created by the barriers themselves; damming along the landward sides of overstepped barriers thickens the drape and creates a temporal disconnect between the migration of the shoreface and more landward elements of the littoral system. In examining the rates of shoreline migration associated with the overstepped barrier system, these are far greater than those calculated for the predicted rates of shoreline change on similarly steep coastal profiles. Future rates of shoreline migration appear to be insufficient to cause overstepping, and rollover or erosional responses are more likely.