Geological inheritance and its role in the geomorphological and sedimentological evolution of bedrock-hosted incised valleys, lake St Lucia, South Africa

Abstract

The morphology of Lake St Lucia, Africa's largest estuary, has been strongly influenced by antecedent topography throughout its Quaternary evolution. The 5 × 20 km southern basin is bounded to landward by a 50 m-high Early/Mid-Pleistocene dune ridge and to seaward by a 100 m-high Late Pleistocene/Holocene dune barrier. Nine seismic units (A-I) are identified in the enclosed basin, along with two regionally developed sequence boundaries (SB1 and SB2) and two tidal ravinement surfaces (tRS1 and tRS2). The lowermost sequence boundary (SB1) is related to regional late-Pliocene hinterland uplift. SB2 is related to the last glacial maximum (LGM) when sea levels fell ∼130 m below present. Lowstand fluvial sediments occupy the base of both SB1 and SB2 incisions. Thick central basin deposits dominate the valley fills, more so in the Pliocene age valleys, the occurrence and thickness of which is attributed to sheltering by a seaward barrier-dune system as well as to the low gradient settings. Prograding tidal bedforms occupy the minor incisions formed within the tidal ravinement surfaces, marking the onset of lagoonal conditions in the area. The system is capped by lagoonal deposits, deposited above tidal flats. A series of prograding spits mark the early stages of lagoonal segmentation in the Holocene. The preservation of only one incision and fill sequence prior to the development of an LGM-aged valley succession is attributed to geological inheritance whereby older valleys were repeatedly re-incised by younger valleys throughout the Pleistocene, causing partial preservation of Pleistocene fills up to the most recent cycle. The older fill remnant relates to a larger magnitude base level fall prior to the Quaternary, which survived erosion during subsequent sea-level falls. Adjacent topography in the form of rocky cliffs and the seaward fronting barrier limit the accommodation space in which infilling material can be hosted outside the major valley network. The competent basement rock into which these valleys have incised protected lowstand fluvial sediments from subsequent tidal ravinement.