Analisi di facies dei sedimenti del “bacino di Frattaguida” (Perugia-Terni). Nota preliminare

Facies analysis of the “Frattaguida basin” sediments – (Perugia-Terni). Preliminary report

Abstract

The «Frattaguida basin» is localized between the Nestore and the Chiani revers valley in the Umbria region. The basin is a trough prevailingly incised into Oligo-Miocene turbiditic sediments and then infilled by gravelly-sandy materials. This preliminary report analyses the sediments in the central and northern part of the «Frattaguida basin». The study has recognizcd three different lithofacies: – Lithofacies 1, at the base, is separated by an unconformity from the upper part of the Oligo-Miocene sandy turbiditic sediments (Macigno Formation). This facies, on an average not more than 10 m thick, is constituted by an heterogeneous conglomerate, clay supported. These sediments have been prevailingly deposited by debris flows. – Lithofacies 2, which is about 60 in thick, overlies the previous or directly the Macigno Formation. Interbedded gravels and sands are the main sediments of this facies; each couple is a finin upward sequence. We suppose that lithofacies 2 was deposited by braided stream flowing from the North to the South. – Lithofacies 3, at the top of the Frattaguida sedimentary sequence, is generally composed of sands with occasional, penny rounded carbonate gravels. Paleosoils have been locally recognised. The sediments of lithofacies 3 were probably deposited in a fluvio-lacustrine environment. The present fluvial flow is towards the NE whereas in the past the flow was towards the SW. The flow reversal is related to faulting.