Cicli continentali tardoquatemari lungoi tratti vallivi mediani delle Marche settentrionali

Late Quaternary continental cycles in the intermediate valley reaches of the Northern-Marchean area

Abstract

The results of geological and geomorphological studies on the continental deposits of the intermediate reaches of the Northern-Marchean valleys are exposed. In particular, the stratigraphic relation between alluvial and detritic sequences and among these and the main erosional phases are examined. Three major continental cycles related to the main valley terraces (TI, T2, T3) have been recognized. Each cycle can be subdivided into an erosive hemicycle and into a depositional one. The erosive hemicycle starts with the re-incision of the deposits of the preceding cycle (stage R) and terminates with the entrenchment of the valley-stream into the bedrock (stage E), which creates on the valley floors complex alternations of ridges and depressions. During the stage R, thin sandy-pebbly alluvial sequences (Fs) are deposited by sinuous-meandering streams. The depositional hemicycle consists of various stages (alluvial and slopewaste depositional sequences), each one characterized by a typical "guide-deposit". The alluvial successions start with pebbly fluvial sequences of braided streams (Fb) resting above the bedrock reliefs and depressions modelled during the preceding stage E. These deposits are followed by areally discontinuous alluvial-fan sequences (Ca). The slope-waste successions have been subdivided in stratified-slope deposits (Ds) (grèzes litées and éboulis ordonnés) and «chaotic deposits» (Dc). This latter group comprises various types of slope-waste deposits (among which the deposits known with the term of "paleofrana") produced by different mass-movement processes. The Ds sequences rest on the alluvial successions, close the depositional hemicycle and are related to wide rectifications of the landscape. The stratigraphic collocation of the Dc sequences is various and often uncertain. An important phase of Dc deposition seems however related to the Ca - Ds transition; a second phase could have taken place during the last phases of Ds deposition. The main continental cycles, pointed out by the major valley terraces (TI, T2, T3) can be interpreted as the result of the interaction between the regional tectonic uplift of the Northern-Marchean area and the main late-Quaternary climatic cycles (respectively Mindel, Riss, Würm glacial stages and relative interstages). The erosive hemicycle seems to start during late-glacial stages and to end at the beginning of the subsequent major glacial stage. In particular the erosive hemicycles can start (beginning of the stage R) independently of tectonic control and only as a result of the changed meteo-climatic conditions in landscapes "over-alluviated" and covered with wide detritic sheets. Subsequently, the control of tectonic uplift should prevail in an environment characterized by a noteworthy decrease of the detritic production (end of stage R and stage E). The instauration of extremely cold climates during the following major glaciation is related to a gradual deforestation of slopes, a considerable increase in detritic production and a concomitant decrease of streams capacity. By the gradual intervening of these conditions deposition of Fb - Ca - Ds successions is determined. In particular the Ds deposits represent the maximum glacial peaks, characterized in the Northern-Marchean area by very cold and climates. During the depositional hemicycle the effect of tectonic uplift was generally exceeded by the rate of continental and climate-controlled sedimentation.