High-frequency cycles within Pleistocene forced-regressive conglomerate wedges (Bradanic area, southern Italy) filling collapse scars

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ABSTRACT

Middle Pleistocene sedimentation in the Bradanic area (foreland basin of the southern Apennines) took place in a setting in which tectonic tilting created anomalous shelf gradients and where slope failures repeatedly occurred at the front of quickly prograding sedimentary wedges. A coarse-grained wave-dominated coastal/deltaic complex consisting of two downstepping, forced-regressive wedges prograded during the falling stage of a middle Pleistocene glacio-eustatic fluctuation (P. lacunosa Zone). Conglomerate clinoforms show a dramatic increase in relief and steepness as a result of the infill of collapse scars.

Compaction of prodelta mudstones, due to the load of the scar-filling conglomerates, resulted in a slight deformation of the fills into broad synformal structures, with gentle anticlinal closures at their distal ends. Accommodation space was created, resulting in splitting of the prograding bodies into high-frequency cycles which are thought to reflect the interaction of localized compactional subsidence with autocyclic shifting of river mouths and/or climatically induced variations in sediment supply. Available chronologic constraints suggest average durations of small-scale cycles on time-scales shorter than those of Milankovitch orbital frequencies.

This case history highlights the importance of some factors which are often neglected in sequence stratigraphic models, e.g. physiography of the sea floor and localized sediment loading. On one hand, the bulk of falling-stage wedges can be preserved as they prograde into intraplatform depressions represented by collapse scars; on the other, significant accommodation space is created by compactional subsidence, leading to the splitting of a simple progradational unit into multiple, small-scale cycles.

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