Stratigraphic and Geomorphological analysis of the Late Quaternary Campochiaro alluvial fan and implications for the Bojano basin (Molise)

Abstract

As a part of a study aimed at constraining the geological evolution of the Bojano Basin (Molise, Southern Apen-nines), the area of Campochiaro alluvial fan has been studied in detail. New geomorphic, stratigraphic, pedologic and radiometric data complete previous geological knowledge about this area (Russo & Terribile, 1995). The Bojano Basin is an intermontane depression located between the Matese massif and Sannio Hills. Extensional tectonic activity since Middle Pleistocene developed in this area a tectonic graben (Coltorti & Cremaschi, 1981; Naso et al, 1998). The main faults of this graben, trending WNW-ESE, are located at the base of the slopes of Matese Mountain and Sannio Hills. Historical and instrumental seismicity suggest that extensional tectonic is still active (Esposito et al., 1987; Federici et al. 1992). The Campochiaro alluvial fan is located in the south-eastern side of the Bojano Basin: Fan deposition is now inactive and the fan deposits are entrenched by the La Valle Creek and Quirino River. Boreholes data indicates that fan deposits are thicker than 90 m. Three alluvial fan units have been distinguished based on geomorphological data: The height of the alluvial fan surfaces above the current talweg is respectively more than 30 m (I order), between 15 m and 20 m (II order), and less than 10 m (111 order). The master fault is active as suggested by large fault scarps located at the base of Colli della Defenza slope. Moreover, it cuts pliocenic paleosurfaces with more than 600 m of vertical displacement. Detailed mountain topographic profiles along the fault scarps suggest a Holocene recent vertical slip-rate between 0.7 and 1 mm/yr. Another fault, synthetic with the master fault and located valleyward, cuts the two older units of Campochiaro alluvial fan, The estimated recent vertical slip-rate is between 0.3 and 0.8 mm/yr. Detailed stratigraphic analysis in the third unit allowed us to constrain the recent evolution of the Campochiaro alluvial fan: About 20 m of conglomerates have been divided in three different stratigraphic units, separated by paleosoils and sandy levels. Radiometric data on paleosoils and bone fragments dates the deposition of the younger alluvial fan between the Last Maximum Glacial stage and the Late Glacial period in a cold and arid climatic context. Thus, the deposition rate of the alluvial fan in this period was between 0.7 and 0.8 mm/yr. Soil profiles data suggests similar time of soil development on the three alluvial fan surfaces. Thus, the older alluvial fan units must also be Upper Pleistocene in age. The current entrenchment of the alluvial fan started between the end of the Late Glacial period and the beginning of Holocene. This data allows us to constrain in the same period the end of lacustrine deposition in the Bojano Basin and the arrival of tephra material (very likely related to the Neapolitan Yellow Tuff eruption from the Phlegrean Fields).