Geomorphological evolution of the southern coastal zone of Kenya

Abstract

Information on chronology, terrace levels and morphological elements associated with former sea level of southern Kenya are reviewed. Additional palaeontological and lithostratigraphical evidence is presented that may be relevant to the interpretation of major palaeogeographical events that occurred particularly during the Pleistocene. This paper therefore provides a description of the active processes in the genesis and development of coastal landscape in southern Kenya. Up to eight former sea level still-stands can be recognized by comparing the heights of coastal terraces formed during the Pleistocene. These can be grouped into four main levels; namely the Ganda Terrace (over 20 m + MSL), the Kilifi Terrace (15–20 m + MSL), the Malindi Terrace (7–10 m + MSL) and the Shelly Beach Terrace (5 m + MSL). The lowest terrace is the modem reef platform, and which occupies a level of 2 m + MSL. Although it is difficult to ascertain whether this lowest marine terrace is of Pleistocene or Holocene age, it can be related to a sea level pause at its present position about 30 000 years ago. The present study demonstrates that the development of this coast is associated with eustatic, isostatic and tectonic movements. The raised coral reef formations and other marine features present between 10 and 20 m + MSL are the evidence for such processes. They have been dated at Middle Pleistocene to Early Holocene, a period which encompasses the Würmian Glaciation, when sea level regressed to 120 m—MSL lower than that of modem times. There is no evidence to support the contention that the postglaciation eustatic rise in sea level ever reached above 10 m + MSL levels over the last 20 000 years. This is because during that time the Kenyan coast had been uplifted by above 60 m as a result of neotectonic movements associated with isostatic response to crustal loading. The presence of parallel step faults on all but the Shelly Beach terrace and the absence of a true fore-reef facies in the raised coral reefs are the main evidence of tectonic uplift of the coastal zone.