Quaternary forced regression deposits in the Adriatic basin and the record of composite sea-level cycles

High-resolution seismic and sedimentological data from the Central Adriatic basin reveals a Quaternary shelf-perched wedge that is comprised of four prograding units, the top surfaces of which are truncated by regional erosional surfaces. Internal reflector geometry indicates that each unit developed during highstand to falling sea-level conditions. Falling sea level resulted in successive downward and seaward shifts of the shoreline (forced regression). Because progradation occurs as a depositional continuum from highstand to lowstand conditions, sequence boundaries are difficult to recognize and correlate on a regional scale. In fact, continued sea-level fall produces several downshift surfaces of limited lateral extent. The regional erosional surfaces that truncate and hence bound the prograding units are composite in origin. The erosion surfaces formed during times of shelf subaerial exposure and were modified by shoreface and marine erosion during each subsequent rapid sea-level rise. All of these composite erosional surfaces become conformable seaward of the youngest depositional shoreline break formed at the end of each phase of progradation. These composite sequence-bounding erosional surfaces are draped by mud that is the distal equivalent of the overlying progradational unit deposited following rapid landward shifts of the shoreline. Facies analysis and stratigraphic data in the youngest progradational unit indicate that each of the four progradational units formed in response to fourth-order (100-120 ka) cyclicity. The four progradational units stack to form an aggradational-retrogradational sequence set that records a longer-term relative sea-level rise. Such a trend can reflect regional subsidence and/or a longer-term component of rise in the Quaternary eustatic signal. In the Adriatic basin, the key factors that favour the preservation of the forced regression deposits within the Quaternary shelf-perched wedge are: (1) the composite nature of relative sea-level cycles where a longer-term relative sea-level rise interacts with shorter-term (fourth to fifth-order) sea-level cycles; (2) the asymmetry of the eustatic signal that, reinforced by local subsidence, yields relative rises of large magnitude and short duration; (3) the high amplitude of the higher-frequency fifth-order signal that drives relative sea-level falls of short duration. This kind of composite cyclicity also controlled the formation and preservation of forced regression deposits on other Quaternary continental margins as well as in ancient sedimentary successions where these deposits may occur in backstepping or aggradational multistorey sequence sets.