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

Abstract

Dacitic-rhyodacitic and rhyolitic talc-alkaline volcaniclastic sediments forming lithified ash layers have been found within the Mt. Cervarola 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 recrystallization of glass shards in the lithified ash layers. The volcanic origin of some sediments has been determined by means of petrographical observations, whereas mineralogy and geochemistry were used to recognize other lithified ash layers and to discriminate between volcaniclastic and non-volcaniclastic normal sediments. The most significant 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 observed. Distribution of first-row transition metals (V, Cr, Ni) was originally 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 discrimination between volcaniclastic and normal sediments, coupled with “immobile” elements (Zr and TO. The possibility of rapidly distinguishing volcaniclastic sediments, including also those that underwent deep diagenesis and burial, has been tested using a wider data set including sediments of different tectonic environments and with variable diagenetic histories. Lithified ash layers occur in five distinct stratigraphic positions within the Mt. Cervarola Sandstones and may be traced over a distance of 65 k km. These ash layers represent a new regional constraint for the correlation and the regional modelling of this complex foredeep turbidite system. Hemipelagic facies and thin-bedded turbidite environments are demonstrated to favour the preservation of volcaniclastic sediments.