I terrazzi vallivi dell 1° ordine nei bacini dei Fiumi Metauro e Foglia (Appennino Marchigiano)

The 1st-order valley terraces of the Metauro and Foglia Rivers (Marche Apennines, ItaIy)

Abstract

No specific study about the oldest valley terraces of the Umbria-Marche Apennines (T1 or «1st-order terrace») has been carried out until now, except when considered together with more recent terrace units. This paper provides new data about the areal and vertical distribution and about the lithostratigraphic sedimentologic features of the oldest valley terraces in the Marche Apennines. The recognition and correlation of the various terrace patches is based on morphostratigraphic criteria. Every 1st-order valley terrace recognised in the two main north-Marche fluvial systems (Metauro and Foglia Rivers) has been described, analysed and measured. It has been possible to point out that the terraces attributed so far to the 1st order as a matter of fact represent two different principal cycles of terracement (and consequently two principal orders of terraces, here called T1a and Tlb). The series of these two terraces, separated from each other by a slope about 20 m-high partially carved out of bedrock, is well evident in the lower reach of the Metauro River, where also the two most recent terrace orders (T2 and T3) are well developed. The recognition of two different terrace events and the reconstruction of the along valley profiles as they were at the time of T1a and T1b, allow us to point out a generalised vertical uplift of the studied area. The approximate parallelism of the T1a and Tlb long profiles shows that the uplift kept almost constant areally during the T1a-T1b time interval. The slight downstream convergence, which perturbates the approximate parallelism, can be related to a downvalley tilt component of the tectonic movements. The areal distribution of the T1 terrace alluvium in the Metauro River indicates a 14 km south-eastward migration of its ending reach in post-Tl time. The evolution stages and the generative mechanisms of the T1 are analogous to those of the 2nd- and 3rd-order terraces, a!ready described and illustrated by the writers (Nesci & Savelli, 1986, 1990). In particular both the alluvium reincision and bedrock entrenchment stages, and the braided fluvial and alluvial fan depositional stages have been recognised. The T1a and Tlb aggradation stages seem to belong to the end of two main pre-Riss anaglacial phases.