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Summary The ‘Turkana Grit’ deposits of northern Kenya are a thick sequence of immature arkosic sediments of pre-Plio-Pleistocene age. Originally described by Murray-Hughes in the 1930s, these sediments have been variously considered as of either Mesozoic or Miocene age by subsequent authors. These extremely immature, tectonically controlled sediments clearly reflect intense local deformation; their age is of considerable interest, as they clearly signal the onset of significant tectonic activity in the central sector of the East African rift. A regional survey of the Turkana Grits as currently known, coupled with recently available information from the hitherto unmapped Kajong area in the SE Turkana basin, clearly indicates that these sediments represent two distinct episodes of local tectonism and deposition. On the basis of available radiometric and biostratigraphic evidence, the earlier of these episodes appears to be of Cenomanian-basal Palaeocene age, while the later episode dates from the mid-Miocene. These observations suggest that significant tectonic activity in East Africa commenced in the late Mesozoic rather than—as generally assumed—in the mid-Cenozoic. The earliest (late Mesozoic) episode of clastic deposition in the Kajong area was initially controlled by a set of major E-W trending transform faults; these deposits are primarily preserved in associated fault-wedge basins. The documentation of a major transform element in the Kajong area has important implications for the early development of the African Rift as a whole. We suggest here the possibility that the enigmatic distribution of the late Mesozoic rift sediments—which are restricted to the southern sector of the Western Rift and the northern sector of the Eastern Rift—reflects the fact that the original late Mesozoic rift consisted of a single graben. The southern and northern portions of this graben have been subsequently displaced by a major E-W trending dextral transform system, extending some 500 km from the SE Turkana basin to the present northern termination of the Western Rift on the Uganda-Sudan border.

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