Palaeosols in an Upper Pliocene fluvial to shallow marine succession (Valdelsa Basin, Central Italy): A sequence-stratigraphic perspective

The present study provides an example of how palaeopedology and facies analysis may be integrated for the interpretation of a cyclothemic succession in the Upper Pliocene (Piacenzian) of the Valdelsa Basin (central Italy). The stacking of facies and intervening bounding surfaces, including palaeosols, outline a hierarchy of elementary (EDS) and composite (CDS) depositional sequences within the unconformity-bounded stratigraphic units (S4-S6 synthems) which compose the succession. The focus of the study is on synthem S4 and the transition to synthem S5. Synthem S4 records the development of a distal alluvial plain dominated by floodbasin mudstone with subordinated channelized sandstone (S41), followed by the incision of a fluvial valley aggraded by the cyclical stacking of braided and low sinuosity channelized conglomerate and sandstone (S42). Synthem S5 includes lower shoreface sandstone and inner shelf mudstone related to a major transgression which affected the study area during the late Piacenzian. Evidence of soil-forming processes is well preserved within sub-unit S41 where five palaeosols (PS4a-e) are stacked within the facies architecture of EDS1c-d, both being defined as transgressive to high stand tracts. Increasing palaeosol development was observed within both EDSs, while palaeosols were generally thicker and more closely stacked within EDS1d. Palaeosols were interpreted as marking minor cycles of accommodation space creation within EDS1c, while marking a better defined high stand tract in EDS1d. Increasing upward soil development was observed within each EDS, with generally better developed soils in EDS S41d. The unconformable transition from S4 to S5 is marked by a thin veneer of slope deposits, bearing pedogenic carbonates reworked from a missing PS4f palaeosol, formed during the shaping of the erosional surface separating sub-units S41 and S42. The results of this study indicate that: 1) the sedimentary and pedogenic processes recorded in the channelized facies and palaeosols in the floodbasin facies of sub-unit S41, are coherent with a palaeoenvironmental setting dominated by seasonal climate; 2) the facies-palaeosol architecture of S4 synthem corresponds to those described in sequence-stratigraphic models applied to continental successions. The pedo-sedimentary signature of the major sea-level fluctuation recorded in the transition between S4 and S5 synthem differs from these models. In this case a well-developed and drained palaeosol, expected to record the maximum regressive surface shaped during the falling stage of sea level, is missing. This difference is related to a rapid fall and subsequent rise of relative sea level which marked the transition between S4 and S5 synthems.