Slope processes in weathered volcaniclastic deposits within the city of Naples: The Camaldoli Hill case

Abstract

We describe the slope processes acting on Camaldoli Hill, the main volcanic feature of the Neapolitan area whose geological evolution and setting have been reconstructed. The backbone of the hill includes the remnants of two partially superposed tuff cones, lying between the Campanian Ignimbrite (CI) and the Neapolitan Yellow Tuff (NYT). This sequence is mantled by pyroclastic, anthropogenic and epiclastic deposits, with abrupt thickness and facies variations. The structural setting of the hill mainly results from several phases of reactivation of the CI caldera faults which were active until about 9.5 ka. Deformation younger than 15 ka is evidenced by landslide deposits, caused by slope instability from volcano-tectonism, and by a high-angle erosional unconformity, formed in response to a base-level lowering. A stratigraphic analysis of the reworked deposits at the foot of the slopes allowed us to define both depositional mechanisms and sedimentation rates. The results of combined volcanological, geomorphological and engineering-geological studies permitted us to constrain and quantify past geological processes and hypothesis about the future evolution of the hill. Present-day slope processes on Camaldoli Hill are largely controlled by the presence of weathered and reworked deposits, whose nature and thickness have been analysed and mapped in detail. Four main kinds of slope processes have been recognized: falls and toppling failures from NYT; small-scale slides in the weathered and pedogenized loose cover; mixed events, represented by slides evolving to hyperconcentrated flows, mud flows and debris flows; and areal and linear erosion. Consequently, a high number of mass movements not previously documented have been mapped. At the same time, an insight into the sedimentation rate due to the overall slope processes, covering a time-span of about 5 ka, was given. Some final considerations regarding landslide hazard are presented in the context of the most suitable remedial works.