Statistical analysis of gravel deposits: a powerfull tool for reconstructing changes in landscape evolution

rnformat1on about the geomorph1c-sedimentary landscape evolution in response to tecton1cs an d/or relat1ve sea-level changes. The Lower Pleistocene deposits croppmg aut 1n the Lower Cec1na Valley are a useful means far reconstructing past changes m the supply area because they represent an ancient coastal zone. When marine sediments are reworked and conglomeratic layers are d1scont1nuous the clast litholog1cal analys1s becomes an invaluable tool to determine stratigraphic correlations between outcrops. Grave l deposits are part1cular powerfully far th1s arm because they reflect the lithologies cropping out m the anc1ent dramage area s. Three gravel layers have bee n recognized in the study area, belong1ng to three d1fferent Allostratigraphic Units (AUs) and clearly d1splaced by tectonics. Two statistica l analyses of gravel clasts, mtegrated by facres and paleontolagical data, from the Lower Pleistocene deposits in the Lower Cecina Valley {Tuscany, Italy) have been combined to unravel changes in the palaeo-drainage system. Data from sixteen outcrops were collected and 6400 clasts described. Although facies analysis, mrcrofmacro-palaeontology and petrographic characteristics of the gravel depos1ts have h1ghllghted the presence of three Aus, clast lithology difference has resulted to be the mai n discriminator in their identificati an. Cluster and principal component analyses of the 6400 clasts have permitted to i m prove the stratrgraphy of the Lower Plerstocene depos1ts and to constrain the re-routing ofthe_lower palaeo-Cecina River from a supposedly SE-NW to the present E-W drrect1on. The dramage systems in zone characterized by the presence of half-grabens with intrabasin transfer zones tends to intersect the main tectonic lineaments in arder to cut the footwall and pass to the hanging-wall basins. By contrast,_ when transfer zones are not present the drainage system tends to arrange 1tself parai lei to the depression of the half-graben and remains in the hanging-wall, like the major river of the Tuscany. The absence of deltaic depos1ts related to the dra1nage system of the paleo-Cecina River in the outsk1rt of the study area confrrm that during the beginning of the Early Pleistocene the river flowed northward. The diversion of the paleo-Cecina River took piace dunng the Early Pleistocene when a strike-slip fault started to create the Lower Cecina Valley