Abstract The metamorphic basement units of the Upper Ouémé watershed in Benin have been investigated to identify the structural controls on aquifer properties, groundwater flow and water balance at large scale. Spatial analysis of borehole and hydrogeophysical data suggests that large-scale weathering profiles, aquifer transmissivity and storage properties are better correlated to a palaeo-weathering surface. Multi-model analysis, combined with assessment of nine transient numerical groundwater models against observations, suggests the best conceptualizations are those where hydraulic conductivity and specific yield are distributed within a weathered zone determined through interpolation of weathered zone thickness. When compared to previous studies, the general groundwater balance of simulated models suggests the groundwater system contributes, on average, 49.8 m 3 s −1 to the river flow (mostly during the rainy season). The same volumetric flow would be lost to groundwater evapo-transpiration and deep/lateral drainage of the catchment. Borehole abstraction (about 7.5 m 3 s −1 ) represents only 6% of the average groundwater recharge and 1% of the average rainfall. This suggests that despite relatively low borehole productivity, the basement aquifer system still has an important unused potential for rural to mid-scale water supply and that, at present, the main external drivers for groundwater resource sustainability are changes in climate and land use.

Abstract We reanalyse the global distribution of gullies in order to provide a set of observational constraints that models of gully formation must explain. We validate our results derived from the global data with four detailed case studies. We show that the availability of steep slopes is an essential factor to consider when assessing the spatial distribution and abundance of gullies. When the availability of steep slopes is taken into account, it reveals, with a few exceptions, that gullies are found almost uniformly across the whole 30°–90° latitude band. Our analysis also reveals that massive ice deposits are anti-correlated with gullies, and that the undulations in the equatorwards limits of the gully distribution could be explained by longitudinal variations in maximum surface temperatures (controlled by variations in surface properties, including thermal inertia and albedo). We find a sharp transition in both hemispheres between pole-facing gullies, which extend from 30° to 40°, to a more mixed, but dominantly equator-facing orientation of gullies polewards of 40°. We have no definitive explanation for this transition but, based on previous studies, we suggest it could be linked to the availability of near-surface ice deposits.

Abstract The Kosi River in India is well known in the fluvial fan literature because of its well-documented avulsive dynamics and because of the relationships between changes in the course of the river and megafan aggradational processes. The radial configuration of the Kosi drainage network was instrumental in the recognition of large, low-gradient, fluvial-dominated counterparts of alluvial fans, commonly defined as megafans, and the system forms a well-constrained example contributing to the recent development of the concept of distributive fluvial systems. A major flood inundated the Kosi Megafan in August 2008. The available data on the temporal evolution of the flood inundation patterns illustrate how the exceptional discharge event travelled across the megafan surface using the pre-existing distributary channel network, how the anthropogenic infrastructure affected the flood path and propagation, and the type of geomorphic changes that were induced by this catastrophic hydrological event. In spite of the large discharge involved in the flood, the fan drainage network appears not to have been significantly modified by the event, probably because the flood wave followed a pre-existing channel network for avulsion and down-fan propagation. Most of the evident aggradation took place on the proximal and medial domains along a well-defined, radially oriented sector of the fan. The observed pattern of flood propagation and associated sedimentation provide important clues to understanding the processes operating during exceptional discharge events, which are an integral part of the long-term, avulsion-dominated evolution of such systems.

Abstract The development of more predictive models for the distribution patterns of large ore deposits and districts is critical for future discovery success in mineral exploration. Some studies have suggested that the distribution of orogenic Au and porphyry Cu deposits appears ordered with a periodic spacing in some mineral provinces, but it remains unclear if spatial periodicity is a common feature of diverse mineral systems. We present evidence for spatial periodicity of large mineral deposit clusters along 20 major structural corridors from nine world-class mineral provinces with five ore deposit types (orogenic Au, porphyry Cu, sediment-hosted Cu and Zn-Pb-Ag, and diamondiferous kimberlites). For orogenic Au deposit clusters, spatial periodicity commonly occurs around 30 to 40 km (range 19–50 km) in the Eastern Goldfields (Australia), Abitibi (Canada), and Sierra Foothills (United States) provinces. Periodicity of moderate- to giant-sized sediment-hosted Cu deposit clusters occurs around 27 km in the Central African Copperbelt (Zambia, Democratic Republic of Congo). Large porphyry Cu deposit clusters show periodicity around 65 to 122 km in the American Cordillera (United States, Mexico, Chile). Large shale-hosted Zn-Pb-Ag deposit clusters have a periodicity around 116 km in the Carpentaria province (Australia). Finally, kimberlite clusters have a spatial periodicity around 121 to 237 km in southern and central Africa. We also observed a dual periodicity along some structural corridors, with smaller deposits located at half the spacing of giant deposits. Whereas the mineral provinces studied were selected on the basis that they seemed to show spatial periodicity, many other provinces worldwide do not appear to display spatial periodicity of ore deposits. We link our results to the phenomenon of self-organization, which explains emergence of large-scale spatial (and temporal) order in complex systems as an effective mechanism to dissipate large energy gradients. As the best examples of spatial periodicity of ore deposits identified to date are associated with some of the world’s best endowed mineral provinces, it is possible that overall province endowment is linked to both the degree of self-organization and the magnitude of regional energy gradients. Further research is required to identify relevant underlying geologic causes for spatial periodicity. Nevertheless, we provide two case studies suggesting that the intersection of preexisting basement fault sets with at least semiregular spacing may be a common control on spatial periodicity of mineral deposits. Where spatial periodicity of mineral deposits is observed to occur, it can improve the predictive capacity of exploration models and ore discovery rates.