Middle Pleistocene to Holocene glacio-eustatic sequences of the Po Basin (Italy): Towards improved geological mapping of the Quaternary period

Unconformity-Bounded Stratigraphic Units (UBSUs) can provide an invaluable approach to the geological mapping of Quaternary successions by delineating large-scale stratigraphic patterns that reflect changes in accommodation and sedimentation mostly dictated by glacio-eustatic variations. Through the example of the Po Basin, we document that a mosaic of 115 stratigraphic units, which reflect local nomenclature from 45 official geological maps, can be grouped within a markedly simpler and unambiguous framework of few regionally mappable UBSUs using the concepts of cyclostratigraphy. Synthems that developed in response to 100-kyr, eccentricity-driven glacio-eustatic fluctuations, with characteristic transgressive-regressive facies patterns and a diagnostic pollen signature, represent the building block of the Middle Pleistocene-Holocene subsurface stratigraphy and are here suggested to serve as a primary guide to geological mapping well beyond the regional scale. Synthem boundaries correspond to transgressive surfaces and correlative fluvial abandonment surfaces: they are marked by remarkable landward facies shifts that developed at the onset of interglacial periods in response to abrupt sea-level rise. Synthems may include internal unconformities of local to regional extent that reflect transition to glacial/cold periods and that are correlated to laterally extensive fluvial channel-belt sand/ gravel bodies. Synthem boundaries can be tracked with good continuity through the depocenters; in contrast, they are correlated only tentatively into uplifted fluvial terraces and glacial/fluvioglacial successions buried or cropping out discontinuously at basin margins or across buried thrust fronts. Prominent key horizons demarcate the onset of the present (MIS1), last (MIS5e), and penultimate (MIS7) interglacials. Such surfaces offer the highest potential for global correlation.