Geology and stratigraphy of the southern sector of the Neogene Albegna Basin (Grosseto, Tuscany, Italy)

The Neogene Basin of the Albegna River is located to east of the Middle Tuscany Ridge and its southern extension ends close to the M. Argentario. Moreover, it is disposed along a significant structural antiapenninic lineament. The oldest Neogene sediments cropping out in t he Albegna Basin (Lignitiferous Unit - T Unit) are related to the late Tortonian; they have been interpreted to be deposited into a shallow lacustrine environment. During the early Messinian, the lacustrine facies evolves to brackish condition (Acquabona-Spicchiaiola Unit - M1). This event can be correlated with a similar one known in the Southern Tuscany basins located to the west of the Middle Tuscany Ridge. The third Miocene unit (Lago-Mare Unit - M3), late Messinian in age, unconformably rests on T and M1 units. It was deposited in a fluvio-lacustrine system. Early Pli ocene deposits (Pliocene I Unit - P1) rest on the Miocene with a basal transgressive facies and are moinly represented by blue clays of outer marine shelf environment. The Middle Pliocene (Pliocene II Unit - P2), is mainly composed of inner shelf terrigenous sediments unconformably overlying different biozones of the Early Pliocene. Such sediments mark a second marine event and are followed by continental-lacustrine deposits (Pliocene III Unit - P3), closing the Neogene sedimentation of the Albegna Basin. The Pleistocene deposits are widely spread; the most ancient (Chiani-Tevere-Montescudaio Unit - Q1) deposited in a fluvio-lacustrine system during the Early Pleistocene. The subsequent sediments (Pleistocene II Unit - Q2) consist of marsh lithofacies and tuffite layers derived from the Latera volcanic apparatus. The age of 320-240 Ky of the tuffite layers constrains Q2 to the Middle Pleistocene. Marine Pleistocene deposits (Pleistocene III Unit - Q3) are present in the westernmost part of the basin; they can be referred to the Late Pleistocene (Tyrrhenian) both for their altitude (not more than 15 m) and close distance to the coast. The ancient and recent alluvial deposits, with an estimated age younger than 240 ky, are separated into four orders of terraces.