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

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 backbarrier
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.