Different stacking patterns along an active fold-and-thrust belt—Acerenza Bay, Southern Apennines (Italy)

Traditional sequence stratigraphic models provide limited understanding of the internal complexity and variability of mixed siliciclastic-carbonate strata accumulated in tectonically active settings. Coeval Lower Pleistocene (Gelasian) shallow-marine, mixed siliciclastic-carbonate depositional wedges accumulated within an active piggyback basin along the southern Italy fold-and-thrust belt are characterized by similar internal architecture of sequences but different stacking patterns. In particular, four coastal wedges (up to 30 m thick each), just a few kilometers (~2 km) apart from each other, show aggradational versus progradational stacking patterns related to their location within a deforming piggyback basin. In all the studied sections, mixed siliciclastic-carbonate strata form isolated sedimentary wedges organized into three vertically stacked transgressive-regressive sequences bounded by sharp flooding surfaces. Aggradational versus progradational internal architecture results from (1) local syndepositional compressive and/or extensional tectonics controlling differential uplift and subsidence, and (2) sediment supply characterized by a combination of intrabasinal and extrabasinal siliciclastics and carbonates. Aggradation occurs in areas showing a balance between both accommodation and sediment supply, and siliciclastic and carbonate fractions. Progradation is typical of supply-dominated areas located close to the active anticline, and dominated by the carbonate fraction. The present work documents the local variability of stratal stacking patterns and sediment supply (siliciclastic-carbonate ratio). We highlight the limitations of using sequence architectures and systems tracts for base-level change and basin reconstructions in tectonically active settings. It is important not only to correctly interpret the stacking pattern, but also to increase our understanding of the type of sediment (siliciclastic vs. carbonate) and sedimentation rate, sedimentation loci, and subsurface predictions.