Andrew Terhorst, John Rogers

A 12 km2 area off Simon's Town in NW False Bay, South Africa, was surveyed using side-scan sonar and a single-beam echosounder. The survey revealed six distinct patterns of acoustic reflectivity or acoustic facies.

The first facies show Cape Peninsula Granite outcrops, matching onshore patterns, with lineaments reflecting the principal WNW-ESE joint direction. The second facies indicate stationary, long-crested, trochoidal wave ripples, likely formed by currents from southeasterly gales. The third facies show an uneven grey tone representing calcareous gravelly sand derived from marine organisms in the shallower western areas. The fourth facies appear as 'cloud-like' and 'tongue-like' light patches, indicating windows of underlying rippled quartzose sand. The continuous light tone of the fifth facies represents a blanket of fine, rippled, quartzose sand in the deeper eastern regions. The sixth facies consists of medium-grey patches within Facies 5, possibly representing coarse sediment, pending further confirmation.

Analysis of sediment samples shows that the calcareous and quartzose sediments mix according to the Folk & Ward (1957) sediment-mixing model. Quartzose sands probably originate from Late Pleistocene regressive dunes reworked during the Holocene transgression. Modern calcareous sediments originate from carbonate-secreting organisms either attached to granite outcrops or unattached on the seafloor surface. The sub-tidal environment is predominantly calm, with occasional high-energy conditions due to southeasterly gales influencing sediment movement.