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 palaeobathymetric 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.

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Using the first high-resolution geophysical data set collected from the uThukela Banks Marine Protected Area (MPA), we reveal a plethora of hitherto unknown or poorly resolved seabed features. In tandem with several remotely operated vehicle dives, we improve on the previous National Biodiversity Assessment map for the area and reveal a more complex picture of the seabed geology and geomorphology on which the MPA is predicated. The upper slope (-120 m and deeper) is dominated by small canyons, gullies and rills that occasionally extend to the shelf edge and form a series of slumps. Suspected cold-water corals were imaged on the interfluves of the Thukela Canyon. The mid to outer shelf (-60 to -100 m) is mostly rocky, and is composed of Pliocene-age siltstones for the most part. Aeolianite shorelines are found at depths of 60 m and 100 m, in which palaeo-lagoons and parabolic aeolian dune systems are also preserved. These features provide habitat for mesophotic corals and demersal fishes. Overlying and abutting hard rock substrates are unconsolidated sandy sediments that are mobilised by the inshore movement of the Agulhas Current. An inshore mud belt characterised by pockmarks associated with free gas expulsion is mapped for the first time. A well-developed palaeo-drainage pattern is also revealed, posing exciting new opportunities for the study of benthic communities associated with palaeo-estuaries and lagoons now exposed at the seabed. Several new habitats, both inside and out of the MPA boundaries, should form the basis for future research within the MPA, in addition to informing expansions of the MPA.