Segnalazione e significato dei paleosuoli nel quadro geomorfologico della Valle Telesina (Benevento, Appennino campano)

Occurrence and significance of paleosols in the geomorphological framework of Telesina Valley (Benevento, Campanian Apennines)

Abstract

In this paper, we highlight and describe four paleosols interbedded in the Middle Pleistocene alluvial succession outcropping along the lower basin of Calore River (Telesina Valley, Campanian Apennines); we also discuss the significance of the paleosols in the Middle Pleistocene evolution of the Valley. The Telesina Valley is a morphostructural depression bordered by carbonatic massifs. The pre-Quaternary bedrock of the valley is tectonically deformed and mainly made of Upper Cretaceous - Lower Pliocene pelitic and arenaceous rocks. The sedimentary infilling of the Telesina Valley rests on these pre-Quaternary rocks. The oldest Quaternary deposits consist of carbonatic breccias (Lower Pleistocene?) on which alluvial fan deposits of Middle to Upper Pleistocene ages unconformably rest. The latter interfinger and partially overlie alluvial gravels, sands and fine deposits of the Calore River of Middle Pleistocene age; a reddish tuff (Guardia Sanframondi Ignimbrite) is interbedded in the upper part of these deposits. An Upper Pleistocene grey tuff (Campanian Ignimbrite) unconformably rests on both Quaternary deposits and pre-Quaternary bedrock. The lithological succession is completed by Holocene alluvial deposits of the Calore River consisting of silty sands and gravels. The paleosols are interbedded in the upper part of the Calore River Middle Pleistocene alluvial deposits; they are truncated by erosional discontinuities and are directly covered by Guardia Sanframondi Ignimbrite (IGS), whose ³⁹Ar/⁴⁰Ar radiometric age is 560 ± 2 ka B.P. Thus, the IGS formed during the cold MIS-14. This indicates that the underlying paleosols may have formed during the previous temperate stage (MIS-15) and the erosional discontinuities at the beginning of the MIS-14. Less developed paleosols show well expressed vertic features, while others show clear macroscopic evidence of clay illuviation: these evidence suggest a pedogenesis in a climate with strong seasonality. Literature data highlight that the climate during the MIS-15 was also characterized by abundant rainfall and relatively high temperatures. Notwithstanding these climatic conditions favourable to pedogenetic processes, the paleosols described show a weak or medium degree of pedogenetic evolution, a poor lateral continuity and are localized only in the western part of the basin¹⁴. Our hypothesis is that the biostasic conditions during the MIS-15 slowed down the alluvial aggradation without stopping it because the uplift of the bounding relieves still supplied large amounts of detritus.