Facies analysis and tectonic significance of lacustrine fan-deltaic successions in the Pliocene–Pleistocene Mugello Basin, Central Italy

Abstract

The Mugello Basin is an intermontane asymmetric basin, trending WNW–ESE and filled with Pliocene–Pleistocene alluvial and lacustrine deposits. The study focuses on the sedimentary succession deposited at the basin's northern margin and uses facies analysis to reconstruct the margin's depositional and deformation history. The controversial concepts of “fan delta” and “hyperconcentrated flow,” adopted in this study, are firstly discussed. The early Pleistocene succession consists of two higher order, coarsening-upward successions (time span ∼10 5 years), separated by an angular unconformity and referred to as the lower fan delta (LFD) and upper fan delta (UFD) systems. They show an alternation of alluvial fan, fan delta front, lacustrine, and floodplain facies associations. The alluvial facies indicate sediment dispersal by flashy, hyperconcentrated flows in the form of sheetfloods and moderately channelized flows. Deposits of the former predominate in the LFD system and deposits of the latter in the UFD system, and this change is attributed to the greater volumes of water yielded by enlarged fan catchments. The fan deltas are of Gilbert and shoal water types, indicating variable water depths at the lake margin. Each of the successions consists of fining- and coarsening-upward cyclothems of medium (tens of meters) and small scale (meters). These different-scale cycles are interpreted as the sedimentary response to pulses of a compressional deformation of the basin margin at variable frequencies, related to the contemporary Northern Apennine active thrusts (ca. 50 km away from the basin). The high-frequency flood events are attributed to episodes of heavy rainfall. The episodes of compression downwarped the basin margin alluvium, made the central lake shift periodically toward the margin, and created progressive unconformities in the sedimentary succession. The immediate effects of a compressional pulse included lake transgression and accentuation of the structural hinge of the basin margin, causing a decline of sediment supply from the catchments. As the hinge relief was subsequently reduced by denudation, the alluvial fans prograded and fan deltas were formed in normal conditions of graben subsidence. The time lag determined the amount of fine-grained sediment accumulated at the lake margin, and thus the water depth and fan delta type.