Abstract

The Suguta Valley occupies the present axis of the Gregory Rift south of Lake Turkana. Since the Pliocene, faulting has been focused along a 35 km wide belt between the Ng’iro basement uplift and the Loriu Plateau, producing an eastward thickening asymmetric basin. The oldest exposed volcanic rocks are basalts, tuffs and ashes dated 4.2–3.8 Ma. These were followed by a shift to trachytic volcanism, starting at about 3.8 Ma and continuing at least to 2.7–2.6 Ma. Feeder dykes to the trachytes strike NW–SE, suggesting a NE–SW least horizontal stress direction in the Early Pliocene. The trachytes were followed by a return to basaltic and alkali basaltic volcanism, which continues to the present. The structural evolution of the Suguta rift was similar to parts of the central and southern Gregory Rift, with early, broad basin development followed by constriction of deformation to an axial zone of grid faulting. This later structuring rotated flows and strata by as much as 35° in the past 3.8 Ma, or 9° per Ma. Rotational rates of this magnitude are extremely high in comparison to other continental rift settings. Formation of the Suguta basin also resulted in renewed uplift of the Ng’iro footwall and large-scale monoclinal flexuring of volcanic flows in the vicinity of the basin border faults. The intense basement structuring and accompanying footwall uplift led to the formation of large-scale land slips that carried large volumes of rock into the basin axis. Patterns of fluvial and deltaic sedimentation similarly reflect direct control by rift structures.