Abstract

During the Early Pleistocene, mouth bar fan deltas formed on the northern margin of the shallow water lacustrine Valdarno Basin, which developed during the last phases of the Northern Apennines evolution. One fan delta, studied in detail here, is a coarse-grained, fluvial-dominated system and shows characteristics different from the classic Gilbert-type model. The apex is morphologically and sedimentologically similar to those reported for alluvial fans of humid climates. Very course-grained, poorly organized deposits, dumped as lobes during intense floods or laid down in braided channels, predominate. Downfan, in the transition zone, braided channels change into stable straight channels lacking evidence of lateral accretion. These consist of lenticular pebbly sand and sandy bodies included within massive mud and fine sand. In the subaqueous zone the straight distributary channels, cut into muddy lacustrine deposits, are filled with sand. Distally, such channels give way to lenticular bodies composed of sands arranged into coarsening- and thickening-upward sequences. The occurrence of drowned channels is explained by scouring during periods of low water stand or due to hyperpycnal flows. The very shallow water, the small size, and the low energy of the lake favored the formation of mouth-bar fan deltas instead of Gilbert-type foresets.

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