Last‐glacial alluvial fan sedimentation in the Lower Derwent Valley, Tasmania

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Abstract
Alluvial fans of the last glacial age in the lower Derwent Valley in southeastern Tasmania were built by debris flows and stream flows. The deposits were derived from periglacial and nivational slope mantles at the highest altitudes in the fan catchments, and from regolith of uncertain affinities at the lowest elevations. The apical and middle parts of the fans commonly consist of coarse‐grained debris flow and water‐laid deposits, while the distal deposits are predominantly water‐laid sheetflood silts and clays. Channels are important in the proximal parts of the fans and are mostly filled with water‐laid sediments, less commonly by debris flow deposits. A first approximation to the mean velocity of the last stage of debris flow in a channel is between 4 m/sec and 6 m/sec. A group of ‘water‐laid’ sediments shows very poor separation of particle sizes, and two explanations have been offered for these sediments. Firstly, the large quantities of unsorted debris available from the catchments produced moderately well‐sorted slurries, or, secondly, openwork stream gravels were infiltrated by later water flows charged with fines. The streams crossing the fans also produced water flows with relatively low suspended sediment concentrations resulting in reasonably good sorting. The debris flows and ‘water‐laid’ muddy sediments cannot be distinguished using CM plots. The debris flows display considerable variation in thickness and grain‐size characteristics, which is attributable to properties inherent in debris flow behaviour.