AN OVERCONSOLIDATED PALEOSOL LAYER UNDERLYING THE VENICE LAGOON: THE CARANTO

The sedimentological, paleo-ecological, pedological, micromorphological, mineralogical, geochemical and palinological data obtained from a detailed study on sediment samples from 20 cores taken inside the central basin of the Venice lagoon and from the mainland area, has allowed a proper understanding of its late Quaternary evolutionary history. This study focuses on the stratigraphic importance of an altered compact calcareous mud layer, a few cm to a few m thick, locally known as caranto.

Many previous studies have indicated that the late Quaternary alluvial sediments underlying the Venice lagoon are dated from the last glacial maximum to the last phases of the Pleistocene and are overlain by an Holocene palustrine continental layer followed by a lagoonal sedimentation caused by the last marine ingression in the still present Venetian basin.

The indurated caranto layer, object of our study and laying on the top of the alluvial sequence refers to a stratigraphic gap from 18150-1750 cal BC to 3620-3360 cal BC. Radiocarbon dating of organic content of the caranto indicates for the first time that a continental late-glacial and holocenic deposition occurred from 12163-11894 cal BC and 4687-4541 cal BC. These deposits represent local sedimentation processes and indicate that alluvial activity was still present on the paleoplain.

Geochemical and mineralogical investigations of the sediments from the caranto, in general, gave no clear indications for an unambiguous characterization of this sedimentary formation.

Paleopedological, physical-chemical and micromorphological investigations have characterized the caranto level as a set of pedogenetic calcic and gley B and C horizons of a palaeosol developed on the antique alluvial plain. Pedo-features are indication of Fe-Mn and carbonate solution and re-deposition.

The same investigations performed on the palustrine level that tops the continental sequence, just preceding the last marine ingression, have indicated that this level is a paleosol, consists of A and B cambic horizons and belongs to a different pedogenetic phase.

The sedimentary sequence terminates with unaltered lagoonal sediments, related to the last marine ingression, dated 3620-3360 cal BC and is characterized by mud flats and salt marsh facies.