Abstract

Among the southern Apennines Pliocene foreland basin system successions, the wedge-top depozone sequences are well exposed in the basins of Benevento, Ariano Irpino, Daunia, Ofanto valley and Potenza. The basins are characterized by syn-depositional compressive stress and by a polyphase evolution, as already recognized for the Ofanto piggy-back basin (Hippolyte et alii, 1994). The Pliocene wedge-top deposits are traditionally referred to the Ariano Unit (Ippolito et alii, 1973) and are formed by marine to alluvial mainly clastic successions up to about 3000 m thick. They lie with a regional unconformity on several tectonic units of the orogenic wedge. This paper provides outcrop and oil-well data on the Ariano Unit deposits, cropping out in the Irpinia-Daunia region near the front of the chain in the northern segment of the southern Apennine arc. Detailed analysis of lithology and sedimentary facies have been carried out, supported by integrated biostratigraphical data, and stratigraphic and sedimentological correlation. The study contributes to defining the tectono-stratigraphic and palaeogeographic evolution of the orogenic margin of the southern Apennines Pliocene foredeep basin. Two major depositional sequences have been recognised, having about four hundred thousand years of duration. The lower one has an Early Pliocene age (biozone MP14a) and is about 2000 m thick; the upper one is Middle Pliocene in age (biozones MP14b p.p.--MP15a) and is 700 m thick; both are characterised by mainly transgressive basal facies and very thick regressive top facies. Facies associations related to alluvial, lagoonal, coastal marine and shelf marine sedimentation environments have been identified and interpreted. The gravel alluvial facies associations are represented both by channelled-flow gravel deposits and by mass-flow deposits; the coarse grained sediments, in the regressive trend can be identified, generally, as weathered in a gravel braided river environment (Miall, 1996). The lagoonal environment is represented by pelitic sediment rich of brackish water ostracofauna. The coastal marine environments are constituted by sandy-conglomeratic facies associations and sandy facies associations, which show shoreface and foreshore typical sedimentary structures and localised traces of biological activity and remains of littoral environment shells. Also near-shore gravel facies associations, related to fluvial systems, are recognisable in Baronia. The shelf marine facies associations are represented by bioturbated sandy or silty marly clays with sedimentation below the normal wave base; more distal sandy-pelitic turbiditic facies are present, outcropping along the Ufita river talweg. The shelfal-coastal marine deposits suggest a very high sedimentation rate, which is over 3.5 mm/yr. The sedimentary, palaeoenvironmental and palaeogeographic evolution has been strongly influenced in the study area by the coeval tectonic evolution of the chain. In compressional orogenic systems, such as the Southern Apennines, tectonic activity can largely overprint the effects of eustatic sea-level fluctuations, so that significant changes in the relative sea level can be mostly produced by flexural subsidence plus thrust activity. It can be traced through various evolutionary stages. In the lower Early Pliocene, the area was probably still emerged; it was affected by Adriatic marine transgressions only from the higher part of the Early Pliocene.

You do not currently have access to this article.