Il bacino lacustre villafranchiano di Steggio (Prealpi Venete, Treviso): primi dati stratigrafici ed evidenze neotettoniche

Abstract

A new lacustrine sequence of Viliafranchian age has been discovered inside a small intravallive trough (Valcavasia Valley) extending between the Brenta and Piave river valleys in a piedmont area at the foot of Mt. Grappa. The deposits of the palaeo-lake, assigned to a new lithostratigraphic unit named "Steggio clay beds", are essentially formed of grey clay and a highly organic black clay. There are also less frequent interbedded layers of gravel, deposited by fluviatile-lacustrine processes. These lenticular gravel beds testify the activity of interdistributary subaqueous channels related to an influent alluvial fan located at the western border of the palaeo-lake. The lacustrine deposits have been ascribed to middle Viliafranchian on the basis of the fossil mammals, particularly Eucladoceros senezensis, found in the lower stratigraphic sequence. Palaeomagnetic analyses of three stratigraphic units have shown normal polarity, probably corresponding to one of the two Reunion's Subepochs occurring at the proximity of the Pliocene - early Pleistocene limit. A system of five subparallel faults has been recognized in the upper stratigraphic levels of the lacustrine sedimentary complex. These faults are NE-SW oriented and exhibit a prevailing left-hand strike slip character. Direct measurements on fault planes gave dip slips varying from 7 to 17 cm and strike slips of about 55+60 cm. Because of the Viliafranchian age of the lacustrine deposits, this fault system is a significant geological evidence of the neotectonic activity characterizing the prealpine area.