The raised coral reef complex of the Kenyan coast: Tridacna gigas U-series dates and geological implications

The Kenyan coast is characterized by a raised fossil reef complex cut by a series of morphological terraces. Shallow subtidal coralgal facies containing Tridacna gigas shells are found at different heights along the coast. Alpha-spectrometric U-series methods were applied to 18 T. gigas samples from different locations along this reef complex with an elevation range from 0 to 15 m above present sea level to obtain chronological information. Apparent U–Th ages, based on the assumption of closed system behaviour, correspond to early marine isotope stage (MIS) 5 (n = 17) and MIS 7 (n = 1). However, initial 234U/238U activity ratios exhibit a wide range, in many cases much greater than present seawater, which is likely to be associated with diagenetic alteration and migration of U-series isotopes. For this reason, we attempted a form of open system isochron dating after separating the Tridacna samples into three different groups on the basis of current elevation and distribution along the coast. An ‘isochron’ age of 120 ± 4 ka (1 sigma) was obtained for the higher elevation group, placed in the terraced central coastal area; another age, statistically undistinguishable from the previous one, of 118 ± 7 ka (1 sigma) was obtained for the group confined along the northern coast. These two groups correspond to a transgressive–regressive cycle connected to the maximum sea level highstand during the MIS 5e. A third ‘isochron’ age of 100 ± 4 ka (1 sigma) was obtained for the group confined along the southern coast, encompassing part of the isotopic substages MIS 5c and d. Based on these data, and on the ecology of T. gigas whose optimal present-day depth range can be considered of 3–10 m below sea level, a maximum tectonic coastal uplift rate of between, respectively, 0.12 and 0.18 mm a-1 was calculated for the period since formation of these shells, and then the paleobathymetry of Tridacna facies has been inferred.