Ane P. Wiersma, P.J.F. Frantsen, P.N.W. Verhoef

The southern coastal zone of Kenya, is formed by a combination of

isostatic and differential tectonic movements and eustatic sea-level

oscillations. This has resulted in a complex setting with interacting

depositional environments and associated morphology. For a large-scale

construction project, sand of sufficient quality is required. Due to

lack of available resources, the potential for nearby sand occurrences

needed to be explored. Because of the complexity and size of this area

a geophysical survey was especially designed to explore near surface

sand occurrences. The survey consisted of a combination of different

acquisition techniques and included a combined high-resolution swath

bathymetry / side-scan sonar system and a chirp sub-bottom profiler.

Over a total of 780 kilometer track length data was acquired with

these systems across the 25 kilometer long and 4 km wide area of

interest.The bathymetric data provide insight in surface processes and

different types of sedimentary environments, such as fan systems and

incised channels. The side-scan-sonar data give an indication of the

surface sediment types, locations of obstructions and rocky outcrops.

From the chirp sub-bottom profiles the thickness and bedding

structures of the buried sediments could be derived and were used to

plan core locations. 79 vibrocores were executed, the sediment logs

were used during the interpretation of the geophysical data. The

combination of different datasets resulted in a synthesis, describing

the evolution of the southern coastal zone of Kenya, improving our

understanding of the sedimentary dynamics en associated depositional

environments. The results show high-resolution information on the near

surface sand resources of the region and also include additional

features of interest of the sedimentary system, such as outcrops that

should be avoided.