Valley pond and ignimbrite veneer deposits in the small-volume phreatomagmatic ‘Peperino Albano’ basic ignimbrite, Lago Albano maar, Colli Albani volcano, Italy: influence of topography

The ca. 23-ka, small-volume, basic phreatomagmatic Peperino Albano ignimbrite, from the polygenetic Albano maar (Colli Albani volcano, central Italy) shows valley pond facies as well as veneer deposits along the maar rim and along topographic ridges. Valley pond facies is characterised mainly by massive structure and chaotic texture and can be up to 30 m thick. Veneer deposit facies is characterised by parallel to low-angle cross-stratified bedforms alternating fines-depleted lapilli-sized layers, and massive, matrix-supported beds. Occurrence of uncharred wood remains and accretionary lapilli suggests temperature of emplacement comprised between 246° and 100°C. We have interpreted the lateral facies variations in terms of temporal and spatial variations of the sediment supply from the transport system to the depositional system of the pyroclastic flow. Ignimbrite veneer facies at the maar rim may reflect pulsatory eruption dynamics, whereas valley pond facies may reflect the bulking of the pyroclastic flow inside the valleys and consequent high sedimentation rates. Ignimbrite veneer facies at topographic ridges has been interpreted to reflect detachment processes of more concentrated undercurrents draining within valleys from the more dilute upper part of the pyroclastic flow that climbs the ridges. The present interpretation suggests that processes of pyroclastic flow transformation downcurrent and induced by topography are not necessarily peculiar of hot, high-mobility pyroclastic density currents. The more likely source of water interacting with magma is interpreted to be groundwater contained within the karstic aquifer located at approximately 1 km below the ground level. This is inferred by both the large amount of limestone xenoliths present in the Peperino Albano and the absence of vesicular juvenile clasts, the latter implying that magma–water interaction occurred before gas exolution processes were significant.