Discovery of lacustrine varves belonging to the Preboreal Chronozone in the Ligurian-Piedmontese Apennines (Val Vobbia-Genova).

Abstract

The discovery of a clayey deposit of 2.90m thickness, covered by ordinary Holocene flood alluvium, was discovered along a 600-metre tract of the bed of the Vobbia River, in the Ligurian-Piedmontese Apennines. This outcrop, sporadically exposed on the right and left banks of the rives bed, at its confluence with the Rio Rabbiosa, by the flood of 23 October, 1999, contains an 80 cm thick varve interval, overlain in a perfectly horizontal position, by a 16-metre long conifer trunk, 50 cm in diameter towards the caulis and 25 cm towards the apex. The morphological study of the left orographic slope of relevant this tract of the Val Vobbia, at the site of the Zan Bridge, shows traces of a slip plane and slump that, because of the not yet eroded amount of material involved, must have blocked the principal river course when its mass was intact The subsequent formation of a dam impoundment permitted the deposition of clayey material. In particular, the textural analysis of the varve of sediment sequence has highlighted that the summer layer of sandy-silty material was laid down by currents with excess load and the winter one by the deposition of uniformly suspended, fines material. The authors conclude that during the entire period of the formation of the varves the climatic conditions caused the complete freezing of the lake surface during winter. The radiocarbon analysis of the ligneous material (Sample 2) recovered from a summer varve 25 cm under the conifer trunk gave a calibrated age (intercept) Beta-145740-RSd-Cal 9,250 B.C. (Cal 11,200 B.R), while the conifer trunk above (Sample 1) provided a calibrated age (intercept) Beta-143343-Rsa Cal 9,240 B.C. (Cal 11,190 B.R) It is therefore evident that the clayey deposits, the conifer trunk and the formation of the dam impoundment are contemporaneous and can be dated to the Preboreal period.