Construction and evolution of submerged deltaic bodies on the high energy SE African coastline: The interplay between relative sea level and antecedent controls

Abstract

This paper investigates the interplay between allocyclic controls and antecedent topography in the evolution of submerged coastal landforms, including a back-stepped delta. Using high-resolution tools, we examine the wave-dominated Thukela shelf, and define the major seismic units. Key features identified comprise incised valleys scoured into bedrock, that have overspilled to form lagoons at depths of 50 m. These are in turn overlain by two prograding and backstepped sandy delta systems at 40 m and 32 m depth respectively. The deltas interfinger with muddy prodelta deposits and are truncated by the Holocene ravinement, overlain by the contemporary prodelta of the Thukela River system. A bedrock high separates two physically separate strato-morphological zones; landward a sediment stripped, steep and shallow nearshore zone, and seaward a gentle zone downdip where the deltaic accumulations are sited. Delta development was favoured during sea-level stillstands at −40 m and −32 m respectively. The step-back of the deltas corresponds to sharp increases in the rate of sea-level rise associated with meltwater pulses. The overall gentle palaeo-bathymetric gradient has moderated erosion associated with rising sea level, preserving a sandy back-stepping delta and a draping mud clinoform. Submerged delta positioning relates to underlying incised valleys, suggesting a synchronous transgressive evolution of the drainage and the delta. Incised valley network positioning is further governed by Late Pliocene aged growth-faults in the basement rocks. The geological framework has acted as a recurring primary control to the geomorphic evolution of the area, partitioning accommodation for sediment accumulation and moderating the efficiency of ravinement.