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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.

A coupled three-dimensional physical-biological model was developed in order to simulate the ecological functioning and potential impacts of land-derived inputs in the southwest lagoon of New Caledonia. This model considered pelagic biogeochemical cycling of organic matter, taking into account advection and diffusion processes driven mainly by local wind fields and freshwater discharges. Modeled phytoplankton dynamics were strongly correlated with both freshwater nutrient inputs and wind-driven hydrodynamic processes, the latter resulting in a large input of oceanic water from the southeast part of the lagoon under trade wind conditions. In situ data obtained during the summer (January 1998) under trade wind conditions supported predicted concentration gradients along several coast to reef transects and provided a validation of the coupled physical-biogeochemical model. An additional sensitivity analysis showed that the alteration of the biogeochemical parameters did not strongly affect the results of the model. Freshwater inputs of nutrients were simulated using a realistic scenario corresponding to the summer rainy season of 1997-1998 in New Caledonia. Despite occasional flooding events from the main rivers considered in these simulations, no significant meso-scale phytoplankton bloom was identified. Hydrodynamically driven dispersion and rapid uptake of nutrients by phytoplankton were sufficient to spatially constrain the impact of river inputs and maintain oligotrophic conditions. The fine spatial grid of our three-dimensional model demonstrated that eutrophication in the southwest lagoon of New Caledonia is confined to the most restricted coastal embayments, while most of the lagoon experiences sustained oligotrophic conditions

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