Abstract The overwhelming documentation of coarse-grained alluvial fans dominated by mass flow contrasts with the scarce accounts of finer grained, traction-dominated alluvial fans. To fill this gap, we present sedimentological and architectural data from a set of sand-rich, streamflow-dominated Pleistocene fans flanking the eastern Upper Valdarno Basin, Italy. The routing of sand-rich sediment resulted from the fast, intense weathering of the feldspar-rich, carbonate-deprived sandstone bedrock underlying the fan catchments. Although capable of entraining large boulders, high water discharge sustained tractional reworking along the proximal facies belts, hindering mass flow. The medial facies belts have a channelized, braided planform and are dominated by processes hardly distinguishable from those characterizing fluvial environments. Along the distal facies belts, extensive overbank tracts are composed of terminal splays and crevasse lobes, the spatial arrangement of which controlled the evolution of through-going tributary channels connected to the axial basin drainage. This study focused on the sedimentary processes, stratal architecture and morphodynamics of the alluvial fans and considered the effect of bedrock inheritance on their development. The results underline how granulometry and mass flow dominance are not distinctive of alluvial fan sedimentation per se and indicate how the critical detection of piedmont, radial palaeomorphology is crucial in the identification of ancient alluvial fans.

Abstract Alluvial fans and fluvial fans are the most common depositional landforms along the margins of continental sedimentary basins. Their occurrence is determined by the area, relief and hydrology of the catchment, which ultimately control the relative ratios of sediment v. flow discharge and runoff regimes. Fundamental morphological and process distinctions exist between these two kinds of deposystem, which are seen as essentially different facies associations and internal architectures for the corresponding deposits. Alluvial fans commonly develop over short radial distances (hundreds of metres to a few kilometres) and are constructed by ephemeral, flash flow events that lead to poor organization of the sedimentary facies and overall architecture of the corresponding deposits. By contrast, fluvial (mega)fans are fed by proper river systems, which aggrade much larger volumes of clastic sediment over distances of up to a few hundred kilometres. Distinct channel belt and overbank domains are developed with a marked heterogeneity in the distribution of sedimentary facies, represented by hierarchically well-organized fluvial deposits. The general properties of alluvial and fluvial fans reflect the different potentials for the corresponding successions to host economically attractive oil and gas resources and the different approaches required in exploration and prediction.

Abstract The relative importance of tectonics, climate, base level and source lithology as the primary controls on the evolution of alluvial fans is highly debated. This study examines the role of upstream catchment characteristics on the evolution of alluvial megafans by examining three Quaternary fans (the Kalahrud, Zefreh and Mughar fans) along the flanks of the Kohrud Mountain Range in central Iran. These fans formed in a tectonically active basin under arid to semi-arid climatic conditions. The key differences between the evolutionary trends of these fans are that their catchments are underlain by different bedrock types and they have different catchment shapes and outlet characteristics. The catchment of the Kalahrud fan is in a sedimentary terrain with limited sediment supply, whereas the bedrock lithologies of the Zefreh and Mughar fans are fractured and weathered igneous rocks. However, the evolution of the Mughar fan is also controlled by the tilting of the catchment towards a wide apex and lateral shifting in the catchment outlet/fan feeder channel position. These variables resulted in relatively large-scale incision in the Kalahrud and Mughar fans that is absent in the aggradational trend of the Zefreh fan. Upstream lithological and structural controls are the dominant drivers behind the development and evolution of alluvial megafans.