Problemi di cartografia geologica relativa ai depositi quaternari del F° 471 “Irsina”. Il conglomerato di Irsina: mito o realtà?

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Collocazione:
Il Quaternario Italian Journal of Quaternary Sciences, 17(2/1), 2004, 391-404
Autore/i:
Luisa SABATO, Marcello TROPEANO & Piero PIERI
Abstract:

Il F° 471 “Irsina” (Basilicata) ricade lungo il confine catena-avanfossa, per cui risulta costituito da due porzioni rappresentative di areemolto differenti sia dal punto di vista morfo-strutturale, che stratigrafico. In particolare, nella parte orientale del foglio affiorano depositiquasi esclusivamente quaternari e ricadenti nell’area della Fossa bradanica. La parte superiore ed esposta del riempimento di talebacino di avanfossa è rappresentata da una successione di sedimenti di tipo regressivo, caratterizzata in basso, e per quasi tutto ilsuo spessore, da emipelagiti siltoso-argillose (Formazione delle Argille subappennine), e nella parte alta da depositi grossolani chegiacciono in contatto transizionale e/o erosivo sulle emipelagiti.Il quadro stratigrafico-deposizionale che deriva dai nuovi rilevamenti effettuati nell’area orientale del Foglio Irsina si differenzia daquanto proposto nella vecchia Carta Geologica d’Italia e nella letteratura prodotta fino agli inizi degli anni ’90: all'interpretazione cheproponeva la presenza di una successione di sedimenti costituenti un sistema aggradazionale, nei quali non era precisata la separazionefra sedimenti costieri e continentali, e nei quali la suddivisione formazionale era basata esclusivamente su criteri litologici, si proponeora quella che riconosce un insieme di sedimenti, di età e quote decrescenti da O a E, derivanti da sistemi progradazionali ditransizione formatisi alternativamente durante cicli di variazione relativa del livello del mare, cui si sovrappongono, in contatto didiscontinuità, depositi continentali.In particolare, alla formazione nota col nome di Conglomerato di Irsina, che era descritta e cartografata sia come un unico depositoaggradazionale in contatto stratigrafico sulla sottostante formazione indicata col nome Sabbie di Monte Marano, sia come un depositosincrono dalle aree più interne (600 m s.l.m.) a quelle più esterne (400 m s.l.m.) rispetto alla catena, venivano attribuiti litosomi conglomeraticiriconosciuti ora come corpi posti a diversa altezza stratigrafica, di diversa origine ambientale ed, in alcuni casi, in nettadiscordanza fra loro. Il nuovo rilevamento geologico, associato all’analisi di facies, ha infatti permesso di individuare nell’area del F°471 “Irsina”, una successione sabbioso-conglomeratica (indicata col nome formazionale di Sabbie e Conglomerati di Monte S. Marco)al cui interno si riconoscono diversi corpi conglomeratici intercalati che possono essere attribuiti a quattro membri posti a quote edaltezze stratigrafiche differenti, e di età progressivamente più giovane da O a E.

The area of the F° 471 “Irsina”, on scale of 1: 50.000 (CARG), is located in Basilicata region (southern Italy), along the chain-foredeepborder of the South-Apennines orogenic system. So, the eastern part of the map regards the Bradanic Trough (foredeep) domain,while the western one regards deformed units of the chain domain. In particular, the eastern part is represented almost by Quaternary deposits, more or less in horizontal arrangment, which correspond to the upper part of the in-fill succession of the foredeep basin. Theoutcropping succession of the basin is represented by regressive deposits characterized below by hemipelagites of the Argille subappennineFormation, and above by coarse-grained deposits. These latter lie on the hemipelagites of the Argille subappennine Fm. in,alternatively, conformable or erosional contact. The new geological survey of the F° 471 Irsina and the facies analysis carried out onthe outcropping deposits, allow us to present a more detailed stratigraphy of the Quaternary Bradanic Trough deposits, in comparisonwith that one showed by the old geological survey on scale 1:100.000.The upper Pliocene-lower Pleistocene silty-clays of the Argille subappennine Fm. are stratigraphically overlain by the Sabbie eConglomerati di Monte S. Marco Formation. This formation is made up of lower Pleistocene up to 60 m thick sands, sandstones andconglomerates interpreted as offshore-transition to shoreface systems prograding onto- and distally passing to- hemipelagites of theArgille subappennine Fm. Several conglomeratic bodies are interbedded in the Sabbie e Conglomerati di Monte S. Marco Fm., andlocally some of them are in erosion on the Argille subappennine Fm. These bodies have a sigmoidal shape, are clinostratified and showthicknesess variable from some meters to over 60 m. The deposits of these bodies are referred to transitional environments, mostly characterizedby deltaic facies; they crop out at different heights above present-day sea level, and their age becomes younger from W to E, from the higher to the lower. According to their stratigraphic position, some of these bodies represent different enter points along thesame paleoshoreline; they may be considered coeval and grouped in four members here called: Conglomerato di Giumentarizzo,Conglomerato di Madonna della Pietà di Irsina, Conglomerato di Monte S. Angelo, Conglomerato di Sant’Antonio Abate. Erosively overlying either the Sabbie e Conglomerati di Monte S. Marco Fm. or the conglomeratic members, a sandy-conglomeraticdeposit is observed. This latter, named Sabbie e Conglomerati di Fosso Macello Formation, is up to 10 m thick and is related to alluvialenvironments.The new stratigraphic setting results quite different from that proposed by the previous Italian Geological Cartography on scale of1:100.000, in which the Bradanic Trough infill succession was characterized by aggradation of shallow marine and continental deposits.In particular, the Conglomerato di Irsina Formation was described and mapped as a single aggradational deposit overlying theSabbie di Monte Marano Formation, and having the same age from the inner areas (600 m above s.l.) to the outer ones (400 m aboves.l.). In the Irsina area, where the Conglomerato di Irsina Fm. was established, the new geological survey demonstrates that this depositis really composed by three conglomeratic lithosomes having different age and different environmental and stratigraphic meaning: afirst lithosome is composed of foreshore conglomerates that locally represent the upper part of a regressive sandy succession; thesecond lithosome is composed of clinobedded deltaic conglomerates erosionally overlying the previous sandy-conglomeratic successionand, distally, the clay deposits; a third lithosome is represented by a continental sandy-conglomeratic deposit erosionally overlyingthe previous ones. In the old Italian Geological Cartography on scale of 1:100.000 these three lithosomes were represented as onetabular body, aggrading and partially etheropic with the underlying Sabbie di Monte Marano Fm. So, the deposits that we now distinguishinto three lithosomes (upper part of the Sabbie e Conglomerati di Monte S. Marco Formation, Conglomerato di Madonna della Pietà di Irsina Member and Sabbie e Conglomerati di Fosso Macello Formation), in the old survey, by mistake, were included only inthe Conglomerato di Irsina Formation.Also in other localities we observe the occurrence of different conglomeratic lithosomes not distinguished in the old survey. In short, itdoes not exsist an only conglomeratic and coeval formation on the top of the regressive bradanic succession, so the Conglomerato diIrsina Formation, as established in the sixties and known in the literature, has to be amended.Furthermore, the new data demonstrate that the deposits and the upper surface of the bradanic reliefs do not represent an only regressiveepisode, but a serie of regressive episodes recorded during the tectonic uplift of the bradanic basin. In fact, the stratigraphic organizationshow a progradational trend rather than an aggradational one. On this subject, the different heights which the regressive depositsoutcrop to (from about 600 m a.s.l., up to 400 m a.s.l. in the surveyed area) are not the result of a differential uplifting of the basinafter the marine sedimentation, but the recording of the regional uplift occurring during the sedimentation.

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