Outcrop and seismic expression of stratigraphic patterns driven by accommodation and sediment supply turnarounds: Implications on the meaning and variability of unconformities in syn-orogenic basins

Abstract

Tectonically-complex settings present accommodation and sediment supply changes with patterns and rates for which the current sequence stratigraphy paradigms are not designed. In the Tertiary Piedmont Basin (TPB) and Peri-Adriatic Basin (PAB), outcrop and seismic examples demonstrate that the observed stratal and stacking patterns cannot be entirely explained using conventional sequence-stratigraphic models. Therefore, it is of paramount importance to use a model-independent more comprehensive approach encompassing advanced sequence-stratigraphic concepts combined with process changes, while being able to consider the morphostructural complexity that characterizes these margins and their changes induced by basin reshaping.Abrupt relative sea level falls generated by uplift or basin inversion may exceed several hundreds of meters, resulting in wedge-margin progressive unconformities characterized by subaerial and subaqueous erosional truncation. A progressive increase in sediment supply occurs, expressed by increasing volume and size of mass-transport complexes overlain by forced-regressive deltas, as the maximum sediment supply is delayed until after the main uplift. Different accommodation/sediment supply ratios may also occur at the same time along different margins of the same basin, generating a diachronism in the T-R or R-T cycles, adding further complexity to the variability produced by autogenesis.On clastic shelf margins characterized by an increasing rate of relative sea level rise, such as in case of increasing rollback velocities and related flexural tilting, or following an orogenic collapse, sediment supply may not keep pace with increasing accommodation so that initially retrogradation and basinward condensation occur, marked by omission surfaces. However, when the rate of subsidence increases, the succession is punctuated by multiple subaqueous erosional unconformities marking phases of basinward tilting leading to the oversteepening of basin margins and abrupt deepening. The downwarping usually produces large-scale subaqueous erosional surfaces passing laterally into paraconformities, so hinged-margin drowning unconformities affecting clastic shelves occur, associated with regional stratigraphic gaps.The re-establishment of the slope equilibrium profile implies high volume of sediments eroded from drowned deltas and shelves, feeding turbidites deposited at the toe of above-grade slopes. These turbidites can be therefore considered as high accommodation-high sediment supply systems. This suggests that turbidites are delivered basinward not only due to bypass at sequence boundaries or during the highstand progradation of supply-driven deltas, but also due to abrupt accommodation creation on hinged-shelf margin wedges.The great variability of tectonically-driven unconformities generated under either decreasing or increasing accommodation suggests that the features described in the TPB and the PAB are probably not uncommon, controlled by linked dynamic turnarounds of accommodation, sediment supply and stratigraphy taking place throughout the development of basin reorganizations.