Exotic clasts of Upper Cretaceous Southalpine units (N Italy) point to uplift and erosion of uppermost Austroalpine (Eastalpine and Transdanubian) and E-Southalpine sources

Abstract

: This study aims at identifying the source area(s) of two populations of exotic pebbles/cobbles present as drift-wood dropstones in the upper Turonian to Coniacian pelagic layers of the Scaglia Rossa, as well as gravity flow deposits in the Santonian Sirone Conglomerate of the Lombardian Flysch. The clast assemblage indicates that the main source(s) were the structurally highest Austroalpine units of the Eo-Alpine orogenic edifice of the Eastern Alps and the Transdanubian Range; the erosion of such units produced Gosau-type gravels that, after local elaboration in high-energy settings, were resedimented into the Southalpine basins. Despite the affinities shown by the clast composition with the lithofacies of the Lombardian stratigraphic succession, origin from a central-Southalpine early orogenic retrobelt edifice can be reasonably excluded, since the onset of uplift and denudation of the Southern Alps occurred in the Late Oligocene. In Santonian times, a complementary source fed the Sirone Conglomerate and likely contributed the Kainach Gosau basin with clasts of Southalpine rocks. This source is inferred to have coincided with an emerging belt located in the eastern Southalpine domain, produced by transpression along the proto-Periadriatic Lineament. Ophiolitic detritus may have reached the Central Alpine and Southalpine areas during the Early Cretaceous; in contrast, the exhumation and uplift of the Koralpe–Wölz high-pressure belt and of its uppermost Austroalpine nappe cover at 89–84 Ma, likely separated the Southalpine retrobelt basins from ‘northern’ sources.