Sedimentology and hydrodynamics of mixed (siliciclastic-bioclastic) shallow-marine deposits of Acerenza (Pliocene, Southern Apennines, Italy)

Domenico Chiarella(*), Sergio G. Longhitano(*), Luisa Sabato(**) & Marcello Tropeano(**)
(*) Dipartimento di Scienze Geologiche, Università degli Studi della Basilicata - V.le dell'Ateneo Lucano, 10 - Potenza. Present address: Weatherford Petroleum Consultants AS, Folke Bernadottesvei 38B, 5147 Bergen, Norway. Corresponding Author: chiarella.d@libero.it (**) Dipartimento di Scienze della Terra e GeoAmbientali, Università degli Studi di Bari «Aldo Moro» - Via Orabona, 4 - Bari.

During the Pliocene, along the Apennines front of Southern Italy (Acerenza area), a sector of the wedge-top basin hosted shallow-marine siliciclastic-carbonate (-bioclastic) sedimentation. Sediments consist of mixed arenites and rudites forming an up to 30 m thick unit. Based on the recognition of textural features, sedimentary structures, and degree of segregation between siliciclastic and bioclastic particles, facies analysis revealed 10 facies grouped into 5 associations. They suggest the occurrence of either wave or current dominated environments, showing different degrees of heterolithic segregation between siliciclastic and bioclastic particles. The depositional system was characterised by a gentle sloping profile irregularly undulated by syndepositional gentle folds. Waves dominated the shallowest areas of the mixed system and the terrigenous fraction derived mainly from wave erosion of substrate (arenaceous) rocks. These areas were located at the top of anticlines at depths corresponding to that of an upper shoreface sector of a classic coastal profile. The bioclastic fraction derived from the fragmentation of an in situ heterozoan skeletal-carbonate factory. Almost constant waves activity prevented segregation of the siliciclastic and bioclastic fractions in the lower shoreface zones. In relatively deeper environment of the mixed system (offshore transition), persistent unidirectional currents dominated, with development of tide-influenced 2D and 3D dunes. Repeated oscillations of the water column in the sheltered coastal areas produced modulation of current velocity favouring segregation of the heterolithic fractions along the dune foresets. Finally, in the deepest sector of the system (offshore), pervasive bioturbation dominated causing unsegregation of the siliciclastic and bioclastic fractions.

Mixed siliciclastic-bioclastic segregated deposits, shallow-marine depositional processes, tidal-modulated currents, Pliocene, Southern Apennines.