GEOCHEMICAL AND MINERALOGICAL CRITERIA FOR THE IDENTIFICATION OF ASH LAYERS IN THE STRATIGRAPHIC FRAMEWORK OF A FOREDEEP - THE EARLY MIOCENE MT CERVAROLA SANDSTONES, NORTHERN ITALY

Dacitic-rhyodacitic and rhyolitic calc-alkaline volcaniclastic sedimen ts forming lithified ash layers have been found within the Mt. Cervaro la Sandstones in two turbidite systems (Torre Amorotti and Scoltenna) of Upper Aquitanian-Lower Burdigalian age, Mount Cervarola sandstones have undergone intense diagenetic alteration leading to the complete r ecrystallization of glass shards in the lithified ash layers. The volc anic origin of some sediments has been determined by means of petrogra phical observations, whereas mineralogy and geochemistry were used to recognize other lithified ash layers and to discriminate between volca niclastic and non-volcaniclastic normal sediments. The most significan t parameters are the occurrence of interstratified illite-smectite and of almost stoichiometric diagenetic calcite in lithified ash layers, whereas in normal sediments illite, and more abundant carbonates are o bserved. Distribution of first-row transition metals (V, Cr, Ni) was o riginally different between the two sediment types, but significantly diagenetically depleted in lithified ash layers compared with both non -volcaniclastic sediments and unaltered northern ash layers. V, Cr and Ni were particularly efficient when used in the geochemical discrimin ation between volcaniclastic and normal sediments, coupled with ''immo bile'' elements (Zr and Ti). The possibility of rapidly distinguishing volcaniclastic sediments, including also those that underwent deep di agenesis and burial, has been tested using a wider data set including sediments of different tectonic environments and with variable diagene tic histories. Lithified ash layers occur in five distinct stratigraph ic positions within the Mt. Cervarola Sandstones and may be traced ove r a distance of 65 km. These ash layers represent a new regional const raint for the correlation and the regional modelling of this complex f oredeep turbidite system, Hemipelagic facies and thin-bedded turbidite environments are demonstrated to favour the preservation of volcanicl astic sediments.