Weathering and Relative Durability of Detrital Minerals in Equatorial Climate: Sand Petrology and Geochemistry in the East African Rift

This article investigates how, where, and to what extent the mineralogical and chemical composition of sand-sized sediments is modified by extreme weathering in modern equatorial settings, with the ultimate goal of learning to read climate from the sedimentary record. To single out the weathering effect, we studied the compositional trends of fluvial sands along the western branch of the East African Rift between 5°S and 5°N. The relative durability of different detrital components, as well as potential hydraulic-sorting and grain-size effects, were assessed by comparing samples with similar provenances in different climatic and environmental conditions or of different size classes within the same sample. Sands of equatorial central Africa at the headwaters of the Congo and Nile basins display the full spectrum of petrologic suites characterizing rift-shoulder and volcanic rift provenances. Unlike in arid Arabia, quartzose sands are not restricted to areas where detritus is recycled from prerift sedimentary covers. In a hot humid climate, weathering can effectively obliterate the fingerprint of parent rock lithology and produce a nearly pure quartz residue even where midcrustal basement rocks are being actively uplifted and widely unroofed. In such settings garnet is destroyed faster than hornblende, and zircon faster than quartz. Weathering control on detrital modes is minor only in the rain shadow of the highest mountains or volcanoes, where amphibole-dominated quartzofelicdspathic metamorphiclastic sands (Rwenzori Province) or clinopyroxene-dominated feldspatholithic volcaniclastic sands (Virunga Province) are generated. Our detailed study of the Kagera basin emphasizes the importance of weathering in soils at the source rather than of progressive maturation in temporary storage sites during stepwise transport and shows that the transformation of diverse parent rocks into a quartzose "white sand" may be completed in one sedimentary cycle in hydromorphic soils of subequatorial lowlands. Micas and heavy minerals, which are less effectively diluted by recycling than main framework components, offer the best key to identify the original source-rock imprint. The different behavior of chemical indexes such as the CIA (a truer indicator of weathering) and the WIP (markedly affected by quartz dilution) helps us to distinguish strongly weathered first-cycle versus polycyclic quartz sands.