Paleogeographic reconstruction of Segrino Lake area: Southern Alps, Northern Italy

The origin of the Southern sub-Alpine lakes was intensively discussed in the past century. Morphological observations, combined with seismic reflection acquisitions provided the fluvio-glacial origin of them. However, the formation and the post-Messinian evolution of the smaller lakes between the two branches of the Como Lake, Southern Alps, Italy, was only marginally investigated so far. This area is regionally named Triangolo Lariano, and several authors hypothesised the post-Messinian evolution of Segrino Lake area connecting its formation with the initial Messinian incision followed by the morainic block during the Last Glacial Maximum (LGM). The proposed study re-evaluates the origin of Segrino Lake as well as the lower Triangolo Lariano area, using morphological observation, orthophoto interpretation, detailed Terrain analysis on high-resolution DEM and finally, the interpretation and correlation of borehole stratigraphy. The results highlight a complex morphological evolution of the area up to the pre-Messinian times. In fact, considering the morphology and the geological characteristics of the bedrock as well as the morphometry of the area, fluvial and glacial phases were observed. Deep incised valleys linked with the Messinian Sea level change, and a complex drainage system are clearly detectable on the field and from detailed Terrain analysis. Finally, to reconstruct the paleogeography and order the chronology of the geological events that have occurred in the area, a set of borehole data were interpreted. These analyses allowed to observe an alternation of strata characterized by heterogeneous and interstratified deposits, that reflected a sequence of lacustrine, fluvial, glacial, lacustrine, and fluvial deposits. In fact, the careful evaluation of the stratigraphy highlights the presence of a pre-Messinian lacustrine phase in the area North of Segrino Lake. In addition, fluvial deposits, suspended valleys and paleo-meanders suggest a strong erosive phase dating back to the Messinian age. During this period, the Lambro River deeply incised into the bedrock forming the actual Segrino Valley. Subsequently, the glaciation phase remodelled the area, depositing erratic boulders and morainic material that caused changes in the drainage settings. In particular, the morainic barrier South of Segrino Lake is responsible for the formation of a new lake in the Segrino-Canzo area as well as in the lower part of the study area were the Pusiano and Alserio Lakes are located nowadays. In the following period the deglaciation and the new hydrological asset of the area led to a shrinking of Segrino-Canzo Lake, and finally a drainage inversion of Segrino Lake, with the outflow directed towards North, and the formation of an alluvial fan which isolated the actual Segrino Lake. Finally, the hypothesis already formulated in the past by some other authors, regarding the presence of a lake that he would fill the study area after the LGM, is therefore supported. New evidence due to the available borehole stratigraphy allowed us to recognize a new and more complex and highly heterogeneous evolution of the study area from Messinian time onwards.