Abstract

The petrography of fluvial sediments depends both on the characteristics of the watershed (lithology, climate, relief, and tectonics) and the transport distance. Previous studies of lithologic effects have emphasized the mineral composition of the rocks, generally not taking into account the effect of texture. We show that both grain size and petrography of basaltic components in the fluvial sediments depend on the texture of source basalts (percentages of glass, phenocrysts, and vesicles), which affects pathways of disintegration by weathering. We compare two watersheds in the Middle Atlas Mountains in Morocco that present the same physiography and outcrops of Pleistocene basalts with the same chemical and mineral composition, but different textures. The results are: (1) If glass is lacking, basalt disintegrates mainly into silts and clays by separation of crystals as weathering progresses. Such basalts do not contribute much to the mainly sand-size fluvial sediment. In fluvial sand, the modal percentages of augite, olivine, and basalt fragments vary greatly in comparison to the percentages of phenocrysts in the source basalt: coarse fluvial sands are depleted in augite and olivine and enriched in basalt fragments. Fine-medium fluvial sands are enriched in augite and olivine and depleted in basalt fragments. (2) If the basalt is glassy, the basalt-fragment proportion of all the sand-size fluvial material increases. Subaerial disintegration of basalt gives an abundant sand fraction that is rich in rock fragments, because of a millimeter-scale network of microfissures in the saprolite. In the sand-size sediment fractions, augite and olivine modal percentages in relation to those of rock fragments are close to the phenocryst percentages in the source basalt. Silts and clays from sediments derived from glassy basalt contain more labradorite and smectite, respectively, than sediments derived from nonglassy basalt. (3) Vesicles favor silty disintegration of nonglassy basalt, but they do not appear to influence significantly the disintegration of glassy basalt into sand.

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