Relevance of the Chiancone volcaniclastic deposit in the recent history of Etna Volcano (Italy)

Abstract

The Chiancone (CH) deposit is a volcaniclastic fan on the eastern flank of Mt. Etna. It crops out from the exit of the depression of the Valle del Bove (350 m a.s.l.) to the sea in an area which covers about 40 km 2, with a maximum thickness of about 30 m and a slope of 3–4 ° towards the east. The total volume of the deposit is unknown because the base does not crop out, but some geophysical data suggest a maximum thickness of about 300 m, leading to an estimated maximum volume of about 12 km 3 which is comparable to the volume of the Valle del Bove (VDB). Since this horseshoe-shaped valley is considered to be the CH's source region, a study of this deposit may allow an understanding of the mechanisms responsible for the origin of the VDB. On the basis of stratigraphic and sedimentary studies we divided the CH sequence into two main parts. The basal portion (Lithofacies 1) is visible only along the coast and has a width of about 6.5 km, a thickness of 1–4 m, and appears as an indurated horizontal deposit with a flat, locally eroded upper surface. We consider Lithofacies 1 as a huge mud flow which may be associated with an important eruptive event. The upper part (lithofacies 2–5) is mainly composed of fluvial beds (63% of measured thicknesses) that we attribute to events reworking the previously deposited layers. The less abundant (23%) very coarse-grained lenses were caused by fluvial floods. Thin and discontinuous pyroclastic layers are accidentally (3%) interbedded into the sequence. The upper portion of the CH sequence is up to 30 m thick and formed after 7590 ± 130 yr B.P. as established by our new radiocarbon dating. When the chemical data obtained on lava blocks sampled from lithofacies 1–4 are compared with products of older eruptive centres it may be argued that the major direction of transport occurred from west to east and involved erosion of those old eruptive centres which were located in the central-northern part of the VDB. Our data suggest the occurrence of an important eruptive event at least 7590 yr B.P. which may be associated with the deposition of the huge basal mud flow. This event was followed by fluvial reworking and deposition at a rate of at least 4 mm/yr. Extending this deposition rate to the whole thickness of the CH deposit (300 m) would imply a maximum age of 80,000–70,000 yr B.P. However, the deposition rate of the hidden part of the CH deposit was almost certainly much greater than 4 mm/yr implying an age for the onset of the CH sequence significantly less than 80,000–70,000 yr B.P. This very strongly suggests that the Trifoglietto volcano (which is older than 80,000 yr) was not involved in the opening of the VDB and the formation of the CH deposit. It also suggests that the first event of the opening of the VDB was much older than the 5000 yr proposed by some authors. It is possible that the CH was deposited during, or slightly later than, the life-span of the Ellittico volcano (40,000–15,000 yr B.P.).