PROBLEMI DI CARTOGRAFIA GEOLOGICA RELATIVA AI DEPOSITI QUATERNARI DEL F° 471 “IRSINA”. IL CONGLOMERATO DI IRSINA: MITO O REALTÀ?

Abstract

Sabato L., Tropeano M. & Pieri P., Geological mapping problems regarding quaternary deposits of the F° 471 Irsina. The
Conglomerato di Irsina: myth or reality? (IT ISSN 0394-3356, 2004).
The area of the F° 471 “Irsina”, on scale of 1: 50.000 (CARG), is located in Basilicata region (southern Italy), along the chain-foredeep
border 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. The
outcropping succession of the basin is represented by regressive deposits characterized below by hemipelagites of the Argille subappennine Formation, 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 on
the outcropping deposits, allow us to present a more detailed stratigraphy of the Quaternary Bradanic Trough deposits, in comparison
with 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 e
Conglomerati di Monte S. Marco Formation. This formation is made up of lower Pleistocene up to 60 m thick sands, sandstones and
conglomerates interpreted as offshore-transition to shoreface systems prograding onto- and distally passing to- hemipelagites of the
Argille subappennine Fm. Several conglomeratic bodies are interbedded in the Sabbie e Conglomerati di Monte S. Marco Fm., and
locally some of them are in erosion on the Argille subappennine Fm. These bodies have a sigmoidal shape, are clinostratified and show
thicknesess variable from some meters to over 60 m. The deposits of these bodies are referred to transitional environments, mostly characterized by 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 the same 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-conglomeratic
deposit is observed. This latter, named Sabbie e Conglomerati di Fosso Macello Formation, is up to 10 m thick and is related to alluvial
environments.
The new stratigraphic setting results quite different from that proposed by the previous Italian Geological Cartography on scale of
1:100.000, 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 the
Sabbie di Monte Marano Formation, and having the same age from the inner areas (600 m above s.l.) to the outer ones (400 m above
s.l.). In the Irsina area, where the Conglomerato di Irsina Fm. was established, the new geological survey demonstrates that this deposit is really composed by three conglomeratic lithosomes having different age and different environmental and stratigraphic meaning: a first lithosome is composed of foreshore conglomerates that locally represent the upper part of a regressive sandy succession; the second lithosome is composed of clinobedded deltaic conglomerates erosionally overlying the previous sandy-conglomeratic succession and, distally, the clay deposits; a third lithosome is represented by a continental sandy-conglomeratic deposit erosionally overlying the previous ones. In the old Italian Geological Cartography on scale of 1:100.000 these three lithosomes were represented as one tabular body, aggrading and partially etheropic with the underlying Sabbie di Monte Marano Fm. So, the deposits that we now distinguish into 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 in the Conglomerato di Irsina Formation.
Also in other localities we observe the occurrence of different conglomeratic lithosomes not distinguished in the old survey. In short, it
does not exsist an only conglomeratic and coeval formation on the top of the regressive bradanic succession, so the Conglomerato di
Irsina 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 regressive episode, but a serie of regressive episodes recorded during the tectonic uplift of the bradanic basin. In fact, the stratigraphic organization show a progradational trend rather than an aggradational one. On this subject, the different heights which the regressive deposits outcrop 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 basin after the marine sedimentation, but the recording of the regional uplift occurring during the sedimentation.