Depositional Processes In Saldanha Bay And Langebaan Lagoon

ABSTRACT

This study deals with the physical aspects of sedimentation in

Saldanha Bay and Langebaan Lagoon. On the basis of detailed textural investigations

the depositional history of the study area has been established.

The sediments in the bay and in the lagoon consist of a fine terrigenous

quartz population and a coarser skeletal carbonate population, which have been

mixed in various proportions. In order to gain size parameters that are more

closely related to the hydraulic nature of depositional processes observed in

the marine environment, grain size analyses were performed with an automatically

recording settling tube system. The instrument was developed in the

course of this study. Construction costs were kept extremely low without,

·however, impairing the scientific requirements of the instrument~

Over 500 sediment samples were recovered on a closely- spaced grid;

in each case, a size analysis was performed on the total sample and on the

insoluble, terrigenous fraction. By subtracting the terrigenous size distribution

from that of the total sediment, the relevant size parameters of the

bioclastic component were calculated. In this manner 1500 individual size :

distributions were avaible for interpretation.

In Saldanha Bay, a number of distinct energy zones can be ?istinguished

which are clearly related to the wave-refraction pattern. A centrally-exposed

zone is lined on both sides by semi-exposed zones, which in turn are followed

by a sheltered zone in the northen bay and a lagoon/bay transitional zone in

the south. The transitional zone is characterized by the interaction of

ocean-waves and tidal ebb-currents leaving the lagoonal system via the outflow

channels that are situated in the southern, wave-sheltered part of Saldanha Bay.

Physiographically the bay consists of a narrow, inshore sand prism

that merges with thicker sand bodies that have accumulated in the northen,

sheltered parts of the bay and in the bay/lagoon transitional zone, where a

tidal delta is well defined. Both semi-exposed zones are covered by a thin

veneer of sediment, usually under 0,5m thick. In the centrally-eA-posed parts

of the bay an extensive abrasion platform.is observed. It stretches between

the inshore sand prism, which terminates approximately along the llm depth

contour, and the onset of the South Channel sand sheet at about -15m •