Morphology of river channels and surface runoff of the northern Campania Plain (Italy) during LGM

The Campania Plain, southern Italy, formed as a large graben or half-graben filled with thousands of meters of Quaternary sediments. Of particular interest is the recognition of an important stratigraphic marker, the Campania Grey Tuff (CGT) deposits (39 ka B.P.) during one of the last explosive eruptions of the Campi Flegrei, a complex volcanic system along the Tyrrhenian margin of the plain. The CGT blanketed the whole area and filled morphological depressions, dipping gently towards the central region of the plain, giving origin to a thick, laterally continuous, volcaniclastic unit that covered previous marine-transitional settings, whose landforms controlled the facies distribution and post-depositional settings of the CGT deposits. Since 39 ky, most of the Campania Plain became a land area. Following the eustatic regression associated with the LGM sea-level drop, a seaward shift of the shoreline and the forced regression of paralic-shallow marine depositional systems concurrently occurred, while the rivers flowing in the inner plain likely started fluvial downcutting with formation of a major incised valley. The surface underlying the regressive phase associated to the sea level drop is clearly recognizable across the plain as it is cut in the ignimbrite deposits. The reconstruction of this surface was based on geological and geophysical data, including stratigraphic well logs and high-resolution single channel reflection profiles to image the stratigraphic pattern of the continental shelf. The 3D representation has shown the morphology of the palaeovalley developed during the LGM. The valley is characterized by a complex structure likely related to channel entrenchment and aggradation. Two main downcuttings occur in correspondence of the modern Volturno River course and the Regi Lagni Channel (formerly Clanio River). The Volturno river likely flowed almost in the present location and received the waters of the Agnena Canal (formerly the Cales River, to north) before the deepest part of the incision located close to the present-day delta area. The Clanio River came from the east, conditioned by an older volcanic dome, and created a wide bend in the plain, entering the deep incision south of the mouth of the current Volturno River. The reconstructed surface actually suggests the occurrence of a complex network of incisions that supports the reconstruction of a channel entrenchment on the plain and especially towards the coastline, which at that time was located ca. 13 km offshore. Above this surface, large remobilization of volcaniclastic material took place during the re-establishment of the fluvial system, witnessed by the abundance of reworked pyroclastic ash and clasts recognized in the alluvial deposits of the medium-upper part of the valley fill. Previous radiocarbon dating indicated that the valley started to form at ca. 37 ka B.P. The onset of the LGM renewed fluvial downcutting leaving the previously formed terraces ca. 10 m above the valley floor. These deposits occur in the subsurface below the present delta plain and have their offshore equivalent in the thick succession of upper Pleistocene prograding units deposited during the Falling Stage of the sea level (FSST), bounded at the top by the erosional surface.