Sequence response to syndepositional regional uplift: insights from high-resolution sequence stratigraphy of late Early Pleistocene strata, Periadriatic Basin, central Italy

This paper deals with the depositional sequences that occur within the uppermost part of the Plio–Pleistocene Periadriatic basin fill in the southern Marche region, central Italy. The succession is an Early Pleistocene, easterly dipping clastic wedge showing an overall shallowing trend from slope clays to shallow-marine and non-marine deposits comprising two major sequences, namely Q mb and Q mc. Analysis has provided new insights into: (i) the nature of sedimentary facies and facies associations occurring within the upper part of Q mb and Q mc; (ii) the gradual contact within Q mb between regressive littoral deposits (RLD) and underlying deep-marine blue clays; (iii) the composite origin of the Emilian surface, which is a widespread erosional unconformity separating Q mb from Q mc; (iv) the cyclothemic pattern of Q mc, composed of downstepping, small-scale depositional sequences; (v) the role played by synsedimentary uplift on the stacking pattern of small-scale sequences and their internal architecture. Up to three small-scale depositional sequences have been recognised within Q mc (Q mc1, Q mc2 and Q mc3). They are up to 50 m thick and defined by previously unrecorded, lower-rank, regionally extensive surfaces. Facies organization indicates that, as a rule, in ascending stratigraphic order each small-scale sequence may includes a distinctive basal unconformity surface that can be traced from incised valleys to associated interfluves, an incised-valley fill, a transgressive surface of marine erosion, a transgressive systems tract, a highstand systems tract, a regressive surface of marine erosion and an attached falling stage systems tract. In proximal positions, the small-scale sequences are dominated by transgressive and highstand systems tracts with incised-valley fills and falling stage systems tracts absent or volumetrically much less significant. In relatively basinward locations, where the regressive surfaces of marine erosion converge with the lower sequence boundary, the falling stage systems tract may represent the entire depositional sequence. High-frequency sequences display a distinctive stacking pattern and form a tectonically induced forced regressive sequence set underlain by a composite, tectonically enhanced regressive surface of marine erosion formed by the lateral connection of lower-rank sequence boundaries.