The record of the Messinian salinity crisis in the Tertiary Piedmont Basin (NW Italy): The Alba section revisited

摘要

The Alba succession (Tertiary Piedmont Basin, NW Italy) preserves the northernmost record of the Messinian salinity crisis (MSC) and was deposited on the southern margin of a wide wedge-top basin, related to the involvement of the Piedmont Basin in the Apennine compressional tectonics. Pre-MSC sediments consist of a cyclic succession of marine euxinic shales and calcareous marls, deposited under the influence of precession-modulated climate changes, and document the progressive restriction of the basin prior to the onset of the MSC. They are followed by the Primary Lower Gypsum unit (PLG), deposited during the first MSC stage (from 5.96 to 5.60. Ma). These sediments show a clear precession-related cyclic stacking pattern and record the lateral transition from a shallow water marginal setting in the SW to a deeper one in the NE. In marginal settings, six PLG cycles are recognised, truncated by an erosional unconformity placed at the base of the post-evaporitic sediments. The lowermost five cycles are composed of massive and banded selenite beds separated by thin shale intervals. A sharp change, evidenced by the appearance of the branching selenite facies, is recorded by the 6th gypsum bed that represents a distinctive marker bed, here called Sturani key-bed, that can be mapped throughout the study area. Basinward, the lower PLG cycles are transitional to decimetre-thick carbonate-rich layers interbedded to euxinic shales, that are overlain by the Sturani key-bed. Above the marker bed, other seven PLG cycles are present. The gypsum beds form thinner bodies compared to the Sturani key-bed and are characterised by a greater amount of fine-grained terrigenous fraction, suggesting an increase of continental runoff related, in turn, to humid climate conditions at the end of the first MSC stage. PLG cycles are followed by slumped mudstones and clastic gypsum beds that correspond to the resedimented and chaotic facies (Resedimented Lower Gypsum), deposited in the Mediterranean basins during the second MSC stage (from 5.60 to 5.55. Ma). They are in turn overlain by continental and brackish water facies with Lago Mare fossil assemblages, recording the final stage of the MSC.The Messinian succession of Alba provides the opportunity to reconstruct the lateral facies transition between marginal and distal settings and to shed new light on the deep water MSC sedimentary record. Moreover, the appearance of the branching selenite facies from the 6th PLG cycle upward provides a tool for properly placing the Piedmont record in the MSC chronostratigraphic framework, allowing us to investigate the influence of climate gradients on the sedimentary response to the Mediterranean salinity crisis.