An Oligocene subaqueous PDC deposit: inferences on its depositional mechanisms

Abstract:

We studied and discussed the depositional mechanisms of a PDC deposit recognized in the Oligocene turbite system of the Val d'Aveto Formation (Northern Italian Apennines). The deposit is characterized by a sandy-sized base (4 m-thick), overlain by a massive conglomerate (46 m-thick), whose gravel-sized detritus is overall represented by substrate-derived clasts (~80%), rather than volcaniclastic ones. Optical, diffractometric (XRD) and minero-chemical (SEM-EDS) analyses on matrix samples reveal a welded volcanic felty texture, rich in plagioclase and horneblende, with accidental metamorphic rock fragments and loose crystals that tend to upward increase along the deposit. Lithological association and roundness of gravel-sized detritus, together with the accidental lithics and loose crystals in the matrix fraction, suggest that this PDC ran channelized in a continental (mountain?) drainage before entering the basin. Channelization in this type of setting 1) enhanced the PDC erosion capacity during the motion and 2) partially cooled it. Flow reached the water at temperatures which triggered the formation of a trapped gas carapace, able to inhibit the water-flow mixing (Sparks et al. (1980-Journal of Volcanology and Geothermal Research). This preserved the current from a shocking water mix and favored its welding. Thus, we suggest that this model can be generally applied when PDC temperatures trigger a stable heat transfer between the flow and the water, which allows the generation of trapped gas carapace and preservation from phreatomagmatic explosions. In contrast, depositional mechanisms inferred by Whitman (1989-Marine Geology) and the ones proposed through the direct observation and sampling of a hot PDC at Montserrat Volcano (Trofimovs et al., 2006-Geology) are developed when fundamental role is played by phreatomagmatic explosions and violent water-flow mix.