GEOMORPHOLOGY, OFFSHORE BATHYMETRY AND QUATERNARY LITHOSTRATIGRAPHY AROUND THE BOT RIVER ESTUARY

SUMMARY

The Quaternary sediments around the margin of the Bot River's closed estuary have been drilled and a succession up to 52 m thick has been analysed sedimentologically. Four main units have been identified. The basal Unit 1 is a 10 m-thick, angular, quartzose coarse gravelly mud, rich in fragments of rock and vein-quartz, and is mainly encountered in the palaeo-valley under the barrier. It is interpreted as a fluvial basal gravel deposited during the early stages of a Wurm IIa/IIb transgression. Unit 2 is 15 to 30 m thick and is consistently muddy (muddy sand to mud). It is characterized by benthic foraminifera typical of inner-shelf or estuarine environments and is interpreted as a back-barrier lagoonal sediment deposited during the latter stages of the Wurm IIa/IIb transgression. Due to compaction and its subsequent cohesiveness Unit 2 survived re-excavation during the Wurm IIb regression to −130 m, when the mouth of the palaeo-Bot River lay south of False Bay, between the Hangklip Ridge and Betty's Bay Bank. Unit 3 overlies Unit 2 in BOT-103 at Die Keel and is a 12 m-thick shelly, fine to medium sand. Unit 4 is 5 to 15 m thick and is a quartzose, medium sand under the barrier. Farther inland it can be a sandy mud and the unit can be rich in organic matter. Unit 4 was laid down during the Flandrian transgression. Unit 5 is the 15 to 30 m-thick calcareous, quartzose medium sand of the barrier environment that is found on the beach, on the dunes and also on the washover fans that are periodically fed by storm-surges breaching the foredunes. There is evidence of a post-Glacial (5,5 to 2 Ka B.P.) barrier slightly inshore of the present coast and of a Late Pleistocene (125 Ka B.P.) shoreline beside the Lamloch swamps.