Resedimented limestones in fault-controlled basins (Zorzino Limestone, Southern Alps, Norian, Italy): Facies types and depositional model

Abstract

The Norian succession of the Southern Alps of Italy stores thick successions of resedimented basinal carbonates (Zorzino Limestone) that were deposited in fault-controlled, anoxic, intraplatform basins developed within a vast inner platform domain (Dolomia Principale). To characterise the basinal facies, to define their distribution within these basins and to identify depositional processes, four stratigraphic logs covering about 2000 m of total stratigraphic thickness from proximal to distal basinal settings were described bed by bed. Eight different lithofacies organised in 11 bed types were identified according to sedimentary structures, texture, and composition: each bed (about 4500 beds in total) was measured individually at cm scale and classified in one of the 11 bed types.Distribution of bed types in terms of events (i.e. number of beds) and cumulative thickness (i.e. volume of sediments produced by the different depositional processes) in the four selected logs change from the proximal to the distal parts of the basin, reflecting the existence of diverse dominant depositional processes at different places within the intraplatform basins. Bed by bed analyses of thick stratigraphic logs provide important hints about the distribution of different types of depositional processes, from proximal to distal basin. Frequent slump folds and mass flow deposits, with clasts mostly deriving from the slope facies itself, indicate instability of the slope. Sediment composition and facies types document a major role played by the reworking of primary deposits formed along the slope: basinal facies mostly consist of resedimented slope deposits, produced by sliding of sediments, rich in lime mud, accumulated along the slope and low-angle proximal basin, whose instability (likely enhanced by seismic activity related to the Norian extensional tectonics) produced most of the recognised facies types. The two-step depositional processes as well as the facies associations and bed types clearly indicate a different behaviour (in terms of prevailing depositional processes and facies associations) of the studied resedimented limestone with respect to siliciclastic deposits.The distribution of facies and the detailed analyses of bed types, recording the distribution of depositional processes at basinal scale, provide a tool for the interpretation of basin morphology, from the study of stratigraphic logs, that can be applied both to outcrop and to subsurface successions.