New field, petrographical, geochemical (major and trace element analyses) and geochronological (U/Pb zircon ages) data were obtained in the course of a mapping project financed by the European Union (Sysmin Funds) and by the Gabonese Republic. They enable precise constraints to be placed on the timing and characteristics of the Neoproterozoic evolution of SW-Gabon. The studied terrains lie at the northwestern end of the West Congo Belt. They are attributed to the West Congolian Group; the upper part of the Neoproterozoic series in this Panafrican belt. The magmatism in SW-Gabon is much less developed than in W-Congo where extensive accumulations of Neoproterozoic basic and felsic lavas exist. Until now, the only reported magmatic complex of possible Neoproterozoic age in SW-Gabon was a gabbroic intrusion from the Mayumba region, close to the Atlantic shore line. New geological mapping at the 1/200,000 scale reveals possible extensions of this basic plutonism at the base of the West Congolian Group where it occurs as dispersed doleritic bodies and isolated ultrabasic rocks. As a whole, magmatism indeed appears to constitute a subordinated part of the Neoproterozoic terrains. A precise U/Pb zircon age of 904 ± 6 Ma obtained on the Mayumba Gabbro is similar to that of extensive basaltic lavas forming the lower-intermediate part of the neoproterozoic series in Congo (Mayumbian Group). Likewise, a 867 ± 4 Ma age has been obtained on discrete occurrences of biotite granite mapped around the Mayumba Gabbro. The Mayumba Gabbro displays a tholeiitic and strongly depleted signature consistent with emplacement in an extensional zone (rift) away from any convergent margin. Likewise, the biotite granite shows a typical within-plate signature of alkaline and rather "enriched" type. Following tholeiitic to alkaline magmatism, extension in the West Congo Belt of SW-Gabon appears to have occurred all along the Neoproterozoic resulting in the deposition of a thick series of alternating detrital and carbonated rocks. Possible volcanic-derived materials in this series are limited and restricted to its lower part, which is consistent with Gabon has having been located aside the major zone of extension within the West Congolian Belt. An U/Pb zircon age of 713 ± 49 Ma obtained on an isolated occurrence of tuffaceous rock from the upper part of the lower detrital group (Louila unit) places a first constraint on the timing of the filling of the basin in Gabon. This age is consistent with attribution of a subsequent tillitic unit (Niari Tillite) to the Marinoan (ca. 635 Ma) glaciation. Following this event, the mainly carbonated Schisto-calcaire Sub-group and subsequent detrital Schisto-gréseux Sub-group (equivalent to the Mpioka Sub-group of Congo) emplaced between 635 and ca. 600 Ma. Thus sedimentation in the West-Congolian basin of SW-Gabon appears to have occurred over a period of about 300 Ma, and ended in response to compressional movements induced by the West Congolian orogenesis.