Stratigrafia e sedimentologia di una sequenza fluvioglaciale wurmiana del centro di Udine

Abstract

Late Pleistocene glaciofluvial sequences were deposited in outwash plain environments at the southern margins of Tagliamento Glacier in the Udine area (High Friulian Plain, northeastern Italy). The deposits are formed essentially by gravel, boulder gravel and sandy gravel of coalescing fans, locally interbedded with glacigenic sequences including diamicton units. The 6-m thick sequence of Udine described here consists mainly of clast-supported gravels overlying an older conglomerate bedrock. The glaciofluvial lithofacies association occurs as a fining-upward sequence of distinct gravel units. The following units have been recognised (from bottom to top): - massive or crudely bedded clast-supported boulder gravel (Gm); - stratified cobbly and pebbly gravel with trough crossbeds (Gt); - horizontal stratified and imbricated pebbly gravel (Gh); - gravel, sandy gravel and granule beds with planar crossbedding (Gp). A fersiallitic brown topsoil developed on the upper part of the outwash gravel sequence infilling many cone-shaped weathering pockets. The lower unconformity between glaciofluvial sediments and matrix-supported conglomerate indicates a major erosional break of unknown age. The four principal gravel lithofacies (Gm, Gt, Gh, Gp) are interpreted to be the sedimentary product of a braided proglacial system which evolved rapidly from proximal ice-contact fans into distal finer outwash plains. The lithofacies (Gm) represents sedimentation of high-density debris flows. The scour and fill feature of the lithofacies (Gt) shows that this unit was deposited as a filling of minor fluvial channels during flood events. (Gh) is interpreted to be a multiple lag deposit connected with stable marginal floodplains. Finally, the plane crossbedding of (Gp) indicates longitudinal bar deposits growing from older bar remnants. The glaciofluvial sequence of Udine is attributed to a Late Pleistocene glacial stage (Late Würm of glacial alpine chronostratigraphy) which occurred around 20 000 years b.P.