Nuovi dati sulla geologia del quaternario dell'alta Valle di Zoldo (Dolomiti sudorientali)

Abstract

Deposits associated with the last glacial phase in the upper Zoldo Valley are distributed on the valley floor and on its flanks, forming a belt at an elevation of about 100 m above the valley floor. They appear to rest over a single erosional valley bottom rocky surface. On the basis of the geometric relations between deposits which are genetically linked to the main glacier and the deposits in the tributary valleys, modelling appears to have been influenced by repeated pulsations of the main glacier interfering with pulsations of glaciers present in the tributary valleys. The marked variation in erodibility of the deposits along the valley - which is attributable both to the complex structural setting of the pre-quaternary substratum and to the great heterogeneity of the stratigraphic sequence - caused a differential amount of erosion by ice masses. During the last retreat phase, most tributary glaciers outliving the main glacier directly influenced the modelling of the main valley. Several depositional units, each belonging to a distinct basin, have been recognised. They are primarily glacial deposits; fluvioglacial, fluviolacustrine and stream deposits are also recognised. Landslide extensive debris bodies, of major importance in the morphological evolution of the valley after the last local glacier retreat, have also been identified. Different modelling due primarily to a varying erodibility, has been highlighted by landslides which occurred during the most recent phases of the modelling process. The reciprocal interference between ice masses and the gravitative accumulations of surface debris caused repeated damming episodes of the main valley, followed by the formation of temporary flooded areas. This slowed down or held back transfer of detrital material, and gave a general "overflooded" appearance to the present valley bottom. The large amount of mobile material throughout the basin has largely controlled this situation. Direct age determinations indicate that local and most recent glacial episodes, responsible for these deposits, took place between Upper Pleistocene and Holocene. Pollen associations in lacustrine sediments, which are heterotopical with the oldest glacial deposit, indicate an Upper Pleistocene steppe climatic episode. Several clues (a systematic migration of the main glacier and of the postglacial watercourse towards the right side of the valley; a systematic migration of a left tributary watercourse towards the left side of the valley; the presence of deep tributary valleys on the left side only of the main valley) suggest the tilting of the north-central sector (Mt. Punta, on the valley left side) with respect to the southern sector (Col Baion, on the valley right side), during the sedimentation time of the depositional units distributed between Dont and Ponteséi.