MIS 5e at San Giovanni di Sinis (Sardinia, Italy): Stratigraphy, U/Th dating and 'eustatic' inferences

摘要

The most representative Late Pleistocene ("Tyrrhenian") outcrop in Sardinia (San Giovanni di Sinis) was re-visited based on detailed sedimentological and stratigraphical analysis supported by U-series dating of fossil corals. The stratigraphy shows shoreface-backshore sandstones overlying an erosional surface cut on vertebrate-bearing layers. Facies analysis and sequence-stratigraphic approaches suggest "eustatic" changes during the overall depositional interval of the marine sequence. Disconformities distinguish two coastal units suggesting a lateral shifts of the depositional environments driven by millennial-scale frequency sea level fluctuations, never exceeding a maximum height +1 to +3 m and by a subsequent rise to +5 ÷ +5.5 m asl. These deposits have historically been assigned to MIS 5, on the basis of ⅰ) their palaeontological content (warm "Senegalese" fauna), ⅱ) lithostratigraphic interpretations and ⅲ) amino-acid, geochro-nological data and generally attributed to the high sea-level episode of MIS 5e sub-stage, through sequential interpretation. U-series measurements in colonies of Cladocora caespitosa from the main outcrop of San Giovanni di Sinis and other minor deposits from the eastern side of the isthmus and from Capo San Marco, yielded ~(230)Th-ages confirming an assignment of the embedding sediments to MIS 5e. Similar measurements in a pelecypod valve (Cerastoderma sp.) and a vermetid colony at San Marco yielded much younger apparent ages, tentatively attributed to late "diagenetic" U-uptake processes. Both the first minor pulses and the most important "eustatic" pulse recorded at San Giovanni di Sinis cannot be assigned unequivocally to one of the MIS 5e sea-level oscillations proposed in the recent literature, due to dating inaccuracies and uncertainties, but it seems probable that it occurred during an early part of the interval.