The sedimentary history of a debris‐flow dominated, Devonian alluvial fan–a study of textural inversion

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Abstract
Karlskaret fan, with a radius of less than 11/2 km and dominated by debris‐flow conglomerates, is one of numerous alluvial fans built out from the fault margins of Hornelen Basin (Devonian, Norway). The fan body is more than 170 m thick proximally, consists of four main coarsening‐upwards segments and thins distally by a rising of its base and by a vigorous interfingering with very fine‐grained sediments originating from an adjacent, impinging floodbasin system.Within the entire fan body, and within individual lobes, is a proximal‐distal (and vertical) facies change from sheet‐like, polymodal debris‐flow conglomerates through matrix‐rich conglomerates that are commonly distorted by loading, slumping and faulting, to remarkably sheet‐like, matrix‐rich granule sandstone of subaqueous debris‐flow origin.Because the alluvial fan prograded into an actively aggrading floodbasin the primary fanglomerates, themselves having been subject to some sorting on the fan surface, incorporated large quantitites of very fine sediments. This inclusion of fines, effectively a textural inversion on the lower fan reaches, frequently led to remobilization and resedimentation of material beyond the fan toe. Anomalous maximum particle size/bed thickness relationships and a variety of graded textures within these resedimented beds suggest deposition in lacustrine areas of the adjacent floodbasin.

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