Facies associations of rain-generated versus crater lake-withdrawal lahar deposits from Quaternary volcanoes, central Italy

Two syneruption lahar deposit successions from Quaternary Italian volcanoes are presented, displaying different facies associations interpreted to reflect different water sources. The lahar deposits associated with the White Trachytic Tuff Cupa (WTTC) ignimbrites from the Quaternary Roccamonfina volcano, located 150 km to the southeast of Rome, have been interpreted in terms of rain-generated lahars. The WTTC ignimbrites are made of more than 1 km3 of loose pumice and lava lithic debris emplaced along the hyperbolic slope of the volcano at ca. 300 ka during an interglacial period characterised by mild and wet climate. The lahar deposits are organised in a coarsening-upward, aggradational, and back-stepping succession of medium- to thick-bedded, progressively juvenile-poorer, non-cohesive debris flow to fluvial deposits. Box-shaped channels cut the WTTC ignimbrites along the steep upper slopes. Channels are filled with lava lithic-rich fluvial to hyperconcentrated-flow sand and conglomerate, which are interpreted as lag deposits related to processes of bulking due to the removal of light pumice and ash debris from the upper slope. Along the lower slopes of the volcano and in the surrounding ring plains where the average slope inclination decreases to few degrees, lahars emplaced an aggradational succession of bedded, ash-rich, hyperconcentrated-flow deposits entirely derived from WTTC components. The succession coarsens upward with increasing presence of lava-rich conglomerate lenses, fluvial in origin, interpreted to record the progressive restoration through time of the drainage network. The succession is cut by incised gullies filled with polygenetic fluvial deposits which indicate the restoration of intererruption condition. By contrast, the ca. 23-ka, small-volume, Peperino Albano phreatomagmatic eruption from Colli Albani volcano, located 30 km to the southeast of Rome, emplaced a valley-ponded, block and ash ignimbrite, which, along the western slope of the volcano, grades laterally into a single, far-reaching, thick lahar deposit. The lahar deposit coarsens upward from coarse-ash, hyperconcentrated-flow deposit into a lithic-block-rich, debris-flow deposit. This lahar deposit has been interpreted to be directly derived from a pyroclastic flow and particularly related to the entrance of the pyroclastic flow into a pre-existing maar crater lake along the pyroclastic-flow path. The basal sand-size, hyperconcentrated-flow deposit is interpreted to represent early deposition from the fast frontal flood wave, whereas the coarse lithic-rich debris-flow deposit at the top may represent the rear of the lahar. The separation of the two facies can be related to processes of ‘hydraulic sieving’ operated by the lake water, which couples with ash particles, leaving behind the coarser fraction.