Southern Alpine lakes — Hypothesis of an erosional origin related to the Messinian entrenchment

Abstract

All the major lakes lying south of the Alps (Maggiore, Lugano, Como, Iseo and Garda) are cryptodepressions, unlike the northern Alpine lakes. Their glacial origin is discussed and refuted on the basis of geological, structural and geomorphic arguments. A major erosional phase is recorded south of the Alps after the deposition of the Gonfolite Formation (a synorogenic to postorogenic sedimentary wedge representing a deep-sea fan, ranging in age from Late Oligocene to Middle Miocene) and prior to the Pliocene transgression. The deposits of the Gonfolite have been deeply incised near Como: a tentative reconstruction of the inferred Messinian paleocanyon is proposed. The erosional surface can be followed under the Po Plain by means of seismic reflection profiles and is controlled by hundreds of deep exploratory wells. According to the available surface and subsurface data, the erosional phase can be dated at the Late Miocene (Messinian): it is related to the dramatic drop in sea level resulting from the isolation of the Mediterranean basin(s) from the world ocean at the close of the Miocene. The depressions presently occupied by the southern Alpine lakes are interpreted as canyons deeply incised along either former paleovalleys (as is the case for Lake Como) or along tectonically controlled embayments (as is the case for Lake Garda) during the evaporitic drawdown. The depressions created by the entrenchment have been eventually occupied and partly reexcavated by glaciers during the ice ages. Frontal moraines dammed the southernmost edge of the valleys, which are presently occupied by the lakes.