Successioni alluvionali terrazzate nell’Appennino nord-marchigiano

Terrace alluvium successions in the Northern Marche Apennines (Italy)

Abstract

In this paper the results of geological studies about the terrace alluvium successions of the lst-, 2nd- and 3rd-order of the Northern Marche valleys are exposed. The aim is to provide a synthetic description of the terrace alluvium sequences, in particular of these which represent the main aggradation stages, and to define the stratigraphic relations between them. The main orders of terraces show cyclicity in the deposition and erosion of the major sequences; they are repeated with constant characters in each major cycle and point to particolar evolution stages inside each cycle («marker» stages and deposits). The Northern Marche terrace alluvium successions rest above complex alternations of ridges and depressions of the bedrock moulded during erosive stages preceding the aggradation. In some places, the aggradation starts with fine-grained (mostly sandy-silty) alluvium with intercalations of pebbly lenses. These deposits are very discontinuous and have been observed only at the base of the 3rd-order alluvium of the Metauro, Cesano and Conca Valleys. Some 14C dates show that they date back to more than 41 000 years B.P. in the Metauro valley and between 31 920 and moree than 44 000 years B.P. in the Conca Valley. There is no sufficient sedimentological evidence to recognise the channel patterns; we can only say that the alluvium considered is interpretable as a low-energy stream deposit. The first alluvial deposits diffused throughout every valley studied and typical of each main ter-race, are constituted by well sorted pebbly fluvial sequences with occasional sandy or silty-sandy lenses, generally more frequent downvalley. The pebbles are always markedly poligenic and reflect the lithology of the whole drainage basin; the roundness is high, but sometimes it can decrease because of the control of local factors. The outcrops almost always show large-scale grouped stratified units and assume a typical festoon-like appearance; the individual depositional bodies show thicknesses varying from a few centimetres up to 1-2 m and lateral extensions not exceeding 10 m (in transverse sections). The observed sedimentary structures allow us to refer the examined sequences to braided-river depositional environments. The aggradation of the alluvial successions prosecuted with the deposition of areally discontinuous sequences of alluvial fan. Their thick-nesses vary in relation to the extension and the lithology of the sourcebasins. In particular, calcareous lithotypes originated not very extended and relatively thick (sometimes more than 25 m) alluvial fans; on the contrary, sandy-marly-clavey lithotypes gave origin to wider and thinner (less than 4-5 m) fans. The gravelly fans show generally (with more evidence in the intermediate and distal aeras) fining-upward trends: towards the top, in fact, the grain-size decreases and the tickness and frequence of silty-sandy layers increase. Where the source basins consists of calcareous lithotypes, sometimes the top of the alluvial-fan sequences is constitued by clasts morphometrically similar to these which characterize the éboulis ordonnés, probably provided by the reworking of these slope-waste deposits. The gravelly alluvial fans, or the gravelly lenses in the «muddy» ones, show that the pebbles are markedly monogenic or provided by a very low number of formations; the pebbles are often flattened and generally they are very poorly sorted: very often elements of decimetric dimensions float into finer gravels. The pebbles are generally scarcely rounded or subrounded, even if in the distal area deposits consisting of well-rounded clasts are rather frequent. The sedimentological characteristics of the depositional sequences change from the fans-head areas to the more distal ones. In the former areas mass-flow deposits prevail, the layering is lacking or not very evident and the sequences appear mostly chaotic; in the latter zones, the mean grain-size decreases and stream-flow deposits generally prevail: in these cases, the sedimentary structures point out again braided-stream depositions environments. The alluvial-fan sequences sedimented by minor tributary streams, always rest above the braided-river ones of the trunk valleys. After their aggradation, the braided river-alluvial fan successions, have been terraced and often noticeably remoulded by gradually deepening sinuous-meandering streams: this fact is indicated both by superficial traces of ancient sinuous paleochannels and by the sedimentary structures of the deposits belonging to the terracement phase (re-incision stage). The re-incision, however, was not continuative, but has been interrupted by minor phases of aggradation and terracement. The fluvial sequences deposited during the re-incision stages are characterised by poorly-sorted gravels; in the most representative examples, the outcrops show fining-upward trends, with coarse-grained gravels (dimensions sometimes up to some decimetres) at the base and sands and fine-grained gravels at the top. In these examples, a tripartition of the sequences can be recognised as follows: a coarse-grained basal level (channel-lag); a low-angle crossbedded intermediate level (point-bar); a generally massive sandy-silty top level (flood-plain p.p.). The regularity of the said three levels is almost always disturbed by more or less frequent minor lenticular depositions bodies related to chute precesses. The deposition of the braided-river and of the alluvial-fan sequences, followed by the stratified slope-waste sequences (éboulis ordonnés), is attributable to the main glacial phases of the Middle and Upper Pleistocene and seems to belong to the intermediate-endind stages of the main anaglacials. In particular, at the beginning of the deposition of the silty-sandy alluvium dating more than 44 000 years B.P., the entrenchment of the principal streams was concluded. As regards the re-incision stage, it seems to start during the principal cataglacial phases and to continue during the first part of the interglacials.