Abstract The Tertiary Piedmont Basin was reshaped as the Apennines progressively overrode the Alpine retroforeland. Nine basin-wide unconformities were followed by abrupt accommodation turnarounds or high-accommodation/high sediment supply intervals. Unusual stratigraphic discontinuities require a different perspective to reconcile allostratigraphy and sequence stratigraphy, highlighting a wider range of tectonically driven discontinuities (subaerial/subaqueous/regressive/transgressive), diachronism of accommodation and sediment–fairway turnarounds, unusual routing to deepwater, and relationships between fourth- and fifth-order sequences. This approach allows innovative play concepts and prediction of reservoir heterogeneity. Steep gradients and high sediment fluxes favour the development of coarse-grained deltas dominated by hyperpycnal processes and large-scale instability/sediment failures, often expressed by retrogressive slump scars, owing to over-steepening generated by drowning unconformities related to orogenic collapse or basin inversion. During increasing-accommodation intervals, subaqueous erosion leaves remnants of drowned deltas forming stratigraphic traps sealed by prodelta-slope muds. The sediments removed feed turbidites, generated during transgressive intervals and deposited at the toe of oversteepened slopes. These may eventually back-fill the slump scar system and show a fining-upward stacking-pattern owing to the re-establishment of the equilibrium profile. Another play concept relies on the combination of onlapping non-marine to marginal marine deposits against fault-bounded basement highs, sealed by marine muds deposited during the subsequent drowning.

ABSTRACT The Permian Upper Rotliegend Group in the UKCS Quads 48-49 was deposited in a mixed aeolian–fluvial–playa– lacustrine environment which displays different orders of internal cyclicity. A low-frequency backstepping–forestepping depositional sequence, which encompasses the whole Rotliegend succession in the study area, was probably influenced by a long-term tectonic control. This depositional sequence can be subdivided into five cycle sets, 30–70 m thick, designated Unit U1 to Unit U5. These units are defined by the recognition of marked shifts in the evolution of depositional systems. The cycle sets are in turn subdivided into 16 elementary cycles (15–20 m thick), bounded by regionally widespread surfaces picked at the point of lower aridity. These high-frequency cycles show drying-upward and drying–wetting-upward trends. The Base Permian Unconformity shows a consistent topographic relief, and it represents a major sequence boundary. The lower part of the sedimentary succession, dominated by the deposits of semipermanent braided streams and catastrophic floods, was deposited in relatively confined sub-basins controlled by extensional WNW–ESE-trending faults. The fluvial system merged basinward into playa and lacustrine facies associations (U1). Vertically, the succession records a climatic change from more humid conditions (U1) to a first aridity peak (U2) marked by erg expansion and a change in fluvial style, with ephemeral streams between the erg and the inherited structural highs. The aridity peak was combined with a smoother palaeotopography. The middle to upper (U3–U5) part of the succession was deposited during a phase of tectonic quiescence in which the initial pronounced palaeotopography was almost completely leveled. Following a dramatic climatic shift toward more humid conditions, an erosional surface cut deeply into the underlying erg complex. This wetting phase was responsible for sudden deactivation of erg expansion between U2 and U3, abrupt reorganisation of depositional environments, and overall backstepping of facies belts (U3). As a consequence of the maximum Silverpit Lake expansion, the depositional setting was characterised by strong lateral uniformity in the study area. Relatively confined fluvial systems fed thin and isolated sandstone lobes interbedded with lacustrine mudstones, which alternated with anhydrite-rich mudstones. These latter deposits, testifying to dry maxima, were correlative toward the margin of the basin with aeolian sandstones, highlighting the contraction of the playa lake (U4). Following this prolonged stage, the sedimentary environment was characterised by deposition of gravity-flow-dominated delta-front lobes in the study area. This depositional change suggests active progradation under relatively humid conditions which characterised the uppermost interval of the Upper Rotliegend Group (U5). The proposed hierarchy of the sedimentary succession, located at the interaction between fluvio–aeolian and playa– lacustrine depositional environments, provides a tool for the understanding of their mutual relationships in the UK sector (Rotliegend feather edge) as well as in the Dutch and German regions.