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 MPl4a) and is about 2000 m thick; the upper one is Middle Pliocene in age (biozones MPl4b p.p. - MPl5a) 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. Stage 1 (Early Pliocene - lower part of biozone MPl4a). An immature north-trending drainage network develops on the northern slope of the Forcuso Mt.- Mattina Mt. ridge. A regionally extended ruditic basal interval takes form during this stage. It lies unconformably on various tectonic units, and is made up of reddish coarsegrained deposits, showing a north-ward facies migration from proximal alluvial cone to near-shore facies. Some turbiditic deposits are present but their relationships with the gravel deposits are not well defined. Stage 2 (Early Pliocene - upper part of the biozone MPL4a). A progressive but generalized drowning of the alluvial plain sectors produce a landward migration of the shoreline with a widening of the marine and coastal facies. Stage 3 (Early Pliocene - upper part of the biozone MPL4a). Some sectors of the basin are affected by compressive syn-sedimentary tectonics, locally marked by the development of syntectonic intraformational unconformities (ANADON et alii, 1986) in the sedi- mentary record. The non-deformed sectors show regressive trend successions with a gradual transition from sandy-silty neritic-epibathyal lithofacies to marginal sandy coastal lithofacies. Some very discontinuous lagoonal facies have been observed in the littoral sequences. Stage 4 (Early Pliocene - uppermost part of the biozone MPL4a - starting of the Middle Pliocene?) - A north-east-ward progressive fillingup of the depozones with coarse-grained coastal and alluvial deposits produces the sea-ward shoreline migration and the restoring of generalized subaerial conditions. A more developed hydrographic network allows the rapid transport and storage of very large volumes of coarse-grained clastic sediments, which were eroded from the internal sectors of the chain. Stage 5 (Early-Middle Pliocene limit and lower part of the biozone Mpl4b) - A generalized hiatus represent the lower part of the biozone Mpl4b. The stratigraphic gap is due to emersion of the marginal palaeosector of the foredeep, which is probably linked to compressive tectonic activity. Stage 6 (Middle Pliocene - upper part of the biozone Mpl4b) - A new progressive widening of the marine basin is documented by coastal ruditic and biocalciclastic facies. The outcropping sequences record a rapid transition from ruditic-arenitic coastal facies to pelitic neritic facies. Stage 7 (Middle Pliocene - upper part of the biozone Mpl4b and biozone Mpl5a) - During the stage, regressive sequences develop. They are characterized by pelitic epibathyal to circalittoral facies, which gradually are replaced by marginal foreshore sandy-silty facies. During the Late Pliocene a regional tectonic phase breaks off the environmental conditions which allowed the marine and coastal sedimentation in the area.
Evoluzione tettono-sedimentaria e paleogeografica dei depocentri «wedge-top» nell'ambito del «foreland basin system» pliocenico dell'Appennino meridionale (settore Irpino-Dauno)
Ciarcia S.;
2003-01-01
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 MPl4a) and is about 2000 m thick; the upper one is Middle Pliocene in age (biozones MPl4b p.p. - MPl5a) 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. Stage 1 (Early Pliocene - lower part of biozone MPl4a). An immature north-trending drainage network develops on the northern slope of the Forcuso Mt.- Mattina Mt. ridge. A regionally extended ruditic basal interval takes form during this stage. It lies unconformably on various tectonic units, and is made up of reddish coarsegrained deposits, showing a north-ward facies migration from proximal alluvial cone to near-shore facies. Some turbiditic deposits are present but their relationships with the gravel deposits are not well defined. Stage 2 (Early Pliocene - upper part of the biozone MPL4a). A progressive but generalized drowning of the alluvial plain sectors produce a landward migration of the shoreline with a widening of the marine and coastal facies. Stage 3 (Early Pliocene - upper part of the biozone MPL4a). Some sectors of the basin are affected by compressive syn-sedimentary tectonics, locally marked by the development of syntectonic intraformational unconformities (ANADON et alii, 1986) in the sedi- mentary record. The non-deformed sectors show regressive trend successions with a gradual transition from sandy-silty neritic-epibathyal lithofacies to marginal sandy coastal lithofacies. Some very discontinuous lagoonal facies have been observed in the littoral sequences. Stage 4 (Early Pliocene - uppermost part of the biozone MPL4a - starting of the Middle Pliocene?) - A north-east-ward progressive fillingup of the depozones with coarse-grained coastal and alluvial deposits produces the sea-ward shoreline migration and the restoring of generalized subaerial conditions. A more developed hydrographic network allows the rapid transport and storage of very large volumes of coarse-grained clastic sediments, which were eroded from the internal sectors of the chain. Stage 5 (Early-Middle Pliocene limit and lower part of the biozone Mpl4b) - A generalized hiatus represent the lower part of the biozone Mpl4b. The stratigraphic gap is due to emersion of the marginal palaeosector of the foredeep, which is probably linked to compressive tectonic activity. Stage 6 (Middle Pliocene - upper part of the biozone Mpl4b) - A new progressive widening of the marine basin is documented by coastal ruditic and biocalciclastic facies. The outcropping sequences record a rapid transition from ruditic-arenitic coastal facies to pelitic neritic facies. Stage 7 (Middle Pliocene - upper part of the biozone Mpl4b and biozone Mpl5a) - During the stage, regressive sequences develop. They are characterized by pelitic epibathyal to circalittoral facies, which gradually are replaced by marginal foreshore sandy-silty facies. During the Late Pliocene a regional tectonic phase breaks off the environmental conditions which allowed the marine and coastal sedimentation in the area.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
