Paleosols and associated channel-belt sand bodies from a continuously subsiding late Quaternary system (Po Basin, Italy): New insights into continental sequence stratigraphy

Previous sequence-stratigraphic work has emphasized the key role of paleosols and associated sand-dominated fluvial bodies as key features for interpreting alluvial architecture. The temporal resolution of the ancient record is, however, insufficient to fully explain the complex relationship between soil formation and the evolution of fluvial systems under changing sea-level and climate conditions. In this paper, we present a detailed record of paleosol-channel belt relationships reconstructed from the subsurface of a rapidly subsiding region (Po Plain, Italy) that spans almost all of the last glacial-interglacial cycle (similar to 120 k.y.). The studied succession preserves a systematic bipartite zonation into a thick paleosol-bearing segment close to the basin margin and a sand-dominated interval, with vertically amalgamated channel belts, in an axial position. Individual paleosols are weakly developed and represent key stratigraphic markers that can be traced basinwide into adjacent, essentially contemporaneous, unconfined channel-belt deposits. Unlike conventional models of late Quaternary alluvial-coastal plain systems, no persistent incised valley was established in the Po system during the last glacial-interglacial cycle. Continuous accommodation was the key depositional control on alluvial stratigraphy during the prolonged (similar to 90 k.y.) phase of late Pleistocene sea-level fall, which led to the deposition of a thick, dominantly aggradational alluvial succession. The development of shallowly incised, short-lived valley systems took place only at the transition to glacial stages associated with substantial sea-level drop (marine oxygen isotope stage [MIS] 3-2 transition, and possibly MIS 5-4 transition). This study shows that in rapidly subsiding settings with high rates of sedimentation, incised valley systems may be replaced by aggradationally stacked, essentially non-incised fluvial bodies. In these cases, over-bank packages bounded by immature paleosols represent the most likely alternative to the highly weathered interfluve paleosol predicted by classic sequence-stratigraphic models. Fourth-order sequence boundaries and lower-rank erosional surfaces may be easily confused at the similar to 100 k.y. scale, and transgressive surfaces, defining the onset of retrogradation, may become the most readily identifiable sequence-stratigraphic surfaces.