The upper Messinian post-evaporitic succession of Serredi Quarry (southern Tuscany, Italy): new stratigraphic data and geochronology of an ash layer.

An ash layer, about 1 cm thick, has been found during a reinvestigation of the upper Messinian-Zanclean succession exposed at Serredi Quarry (Fine Basin, Northern Apennines), one of the best known Neogene Tuscan sections. The Serredi Quarry succession (about 175 m thick) has been subdivided into three intervals: the lower (18 m thick) and middle (132 m thick) ones are separated by an unconformity and are both referable to the Messinian post-evaporitic phase, whereas the upper interval (25 m thick) represents marl deposition following the Pliocene transgression. The lower interval consists of relatively deep-lacustrine, anoxic, thinly laminated mudstones with subordinate sands, whereas the middle one includes lacustrine mudstones (occasionally with pedogenic features) with sands, diagenetic gypsum-rich mudstones, limestones, gravels and selenite gypsum draped by gypsarenites. These lithofacies are typically arranged into marginal-lake, cyclical stacking patterns. The ash layer is located in the lower interval. It is faintly laminated and composed of almost pure fine-grained glass shards and rare pumices. These characteristics suggest a primary deposition from a pyroclastic fallout. Chemical analyses were carried out on well preserved pumices and glass shards were used for fission-tracks dating, which provided an age of 5.35 + 0.32 Ma. Nevertheless, its age can be further constrained by the fact that i) the ash layer is about 145 m below the Miocene-Pliocene boundary, and ii) the intervening deposits mainly consist of mudstones with occasional pedogenic features, which are likely to comprise sedimentary hiatuses. Accordingly, the ash layer probably emplaced in the older part of the determined age interval. On the whole, the geochronology, geochemistry and the stratigraphic framework of the first ash layer found in the post-evaporitic deposits of Tuscany could represent valuable tools for regional stratigraphic studies on the Lago Mare phase and long-distance correlation.