A new perspective on sequence stratigraphy of syn-orogenic basins: insights from the Tertiary Piedmont Basin (Italy) and implications for play concepts and reservoir heterogeneity

Abstract

AbstractThe 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.