Fluvial sediment during the early Holocene in the Marchean valleys (central Italy)

Abstract

The periadriatic valleys of central Italy were affected by a general aggradation under braided river system conditions during most of the Middle and Upper Pleniglacial. Anastomosing rivers characterized by a fine sedimentation with peat layers, established during the interstadial phases (Denekamp-Arcy, Hengelo, Kesselt, Tursac and Pre-Bolling). The general Pleistocene aggradation seems to have ended in the Late Glacial, although further dating is necessary to framework the events more precisely. After a general downcutting of the Pleistocene deposits, slightly before 8,260±100 yr B.P. travertine deposition started in the mountain areas, with the damming of river valleys and the production of swampy-lacustrine deposits up to some 10 m thick. Travertine deposition continued until the 2nd millennium B.C., when it practically ceased. A section in a quarry in the middle Potenza river valley (Smorlesi Quarry) shows a sequence made up of fine sediments (alternating silty-clayey layers, alluvial soils and cross-bedded sandy layers). Within the sandy sediments, in the middle-upper part of the sequence, ceramic fragments were also found. A fireplace that was found at about 11 m from the top of the sequence, dates back to 7,210 ± 90 yr B.P. (ROME -508). Approximately 1 km downvalley, close to Fontenoce, a channel was identified at the top of the same sedimentary unit; this channnel contains two superimposed layers with Eneolithic industry, separated by overflood silts. The dating of these layers gave ages of 4,680±100 and 4,700±100 yr B.P. Sequences up to 7-8 m thick, showing the same stratigraphic and morphochronological characteristics, have been recognized in the middle Tenna River valley. At the San Gualtiero quarries, a charcoal layer from the base of an alluvial sequence generally composed of fine-grained sediments gave an age of 7,620±80 yr B.P. Moreover, Bronze age ceramic fragments and charcoal fragments 3,570±70 yr B.P. old were discovered at the base of a more than 10 m deep channel, which cuts the whole sequence and the underlying Upper Pleistocene alluvial deposits. These age values suggest a progressive deepening of the alluvial plain as the work of a single and sinous channel up to 20 m deep. From Late Glacial to 8,000 yr B.P., a slow aggradation process followed. After the 2nd millennium B.C., the deposition of fine sediments ended and a rapid deepening of thalwegs is recorded. These modifications may be attributed either to climatic changes or to the effect of Man impact after the general occupation of the slopes for agriculture and farming purposes.