AN APPROACH FOR INVESTIGATING THE DEPOSITIONAL MECHANISM OF FINE-GRAINED SURGE DEPOSITS - THE EXAMPLE OF THE DRY SURGE DEPOSITS AT LA-FOSSA-DI-VULCANO

Fine-grained, thinly laminated surge deposits have been studied in order to understand the physical properties of pyro-clastic deposits with a large tractional component and to investigate on type and number of laminae deposited from a single surge cloud. The examples are taken from the dry surge deposits of the Fossa cone (Vulcano island), where there is a good knowledge of the stratigraphy, good exposures of the deposits, and an absence of secondary structures. Analysis of texture and structure allows all the bed facies to be grouped into 4 principal types. Type 1 is made up of a loose, coarse and medium ash in dark grey, gently undulating laminae, sometimes with cross laminae, that alternate with pale grey laminae. Type 2 consists of grey ash in beds characterized by limited lateral continuity, normal grading and a flat, non-erosive base. Type 3 is made up of submillimeter, ochreous laminae, plane-parallel or gently undulating. Type 4 consists of massive beds of fine ash, ochre in colour, characterized by notably lateral continuity and constant thickness. Determination of the nature, morphology, and size of the grains, shows that a close relationship exists between the relative abundance and shape of components and the textural and grain-size characteristics of the bed facies. The results suggest that a single surge event could deposit either a set of plane-parallel to transverse laminae (as Type 1 and Type 3), or a pair of laminated beds (as Type 2 and 4), depending on the flow velocity, the effective grain size and density of the solid phase, and the particle concentration in the eruptive cloud. The almost complete lack of lithics suggests that magma-water interaction occurred in sub-surface conditions across a fragmented magma surface. The efficiency of the interaction was primarily controlled by the ascent velocity of the magma in the conduit and its physical properties.