A mixed bioclastic/siliciclastic flood-tidal delta in a micro-tidal setting: depositional architectures and hierarchical internal organization (Pliocene, southern Apennine, Italy)

Tide-dominated depositional systems are very common in macro tidal and meso tidal settings. They are less developed in micro tidal marine areas where sediments are dispersed mostly under the effects of waves and currents. In some specific coastal settings, the influence of the dominant hydrodynamics can be reduced or attenuated by the presence of promontories forming engulfed sectors or by the occurrence of submarine passageways or straits. In these conditions, as well as in micro tidal settings, the tidal influence can be amplified, producing a significant signature in the sedimentary record. A number of tide-influenced deposits can be recognized in the Neogene of the Southern Apennine, Italy, although the Mediterranean area is characterized by minor tidal ranges. Spectacular exposure of middle to upper Pliocene deposits cropping out around Tricarico allows the analysis of the architecture and internal complexities of a mixed bioclastic-siliciclastic succession deposited in a thrust-top basin. Undulations forming along the hinge of an anticline favored the onset of seaway conditions, which produced hydraulic amplification of marine currents flowing towards the chain and subjected to tidal influences. The mixed deposits of Tricarico exhibit prominent large-scale, unidirectional cross stratification and a suite of additional dune- and ripple-bedded structures of various dimensions. Cross stratification can be subdivided into four hierarchical levels based on their increasing degree of internal complexity of different ranks (from first-order to fourth-order sets). Processes invoked for the formation of such a complex suite of larger- and smaller-scale sedimentary structures are related to cyclical events, such as high-frequency sea-level oscillations and tidal cycles of various durations. First-order sets are interpreted as produced by migrating subaqueous dunes along a SSW-NNE-trending seaway at water depths below the wave base. These sets exhibit bed-thickness vertical patterns which have been related to the influence of highfrequency base-level oscillations that occurred during dune accumulation, producing alternating stages of accelerating and decelerating currents. Second-order cross stratification has been interpreted to have formed by dunes with varying sinuosity, superposition, and flow conditions, under the effect of varying current strengths but constant sediment production. Formset successions were produced by large compound dunes and are considered as the expression of low-energy and decaying dune fields that developed during times of decreasing sediment transport. Cross lamination of third-order and fourth-order sets shows series of bundles and couplets of coarser and finer laminae which, at a different scale, recorded repeated cycles of tidal ranges of different amplitudes. These considerations allow us to propose an original depositional model represented by a flood-tidal delta, which questions the absence of macro tidal sedimentation within the purportedly micro tidal oceanographic setting of the Mediterranean during the Pliocene.