The mudstone composition as reflected in the sedimentary evolution of a turbidite basin. The example of the Agnone Flysch (Molise, Italy)

Chemical and mineralogical signatures of mudstones from the Agnone Flysch turbidite successions (late Tortonian to early Messinian) deposited in the Lagonegro-Molise foredeep basin reveal new insight for the sedimentary evolution of the southern Apennines during late Miocene. Geochemical and mineralogical signatures of the Verrino and Poggio Villanelle members of the Agnone Flysch mark a complex sedimentation history testifying to a multi-source area probably due to structural changes and depositional basin physiography. The studied mudstones show chemical variations in term of Ca vs. Si and Al. Mineralogical analyses confirm a decrease of carbonate minerals and an increase of silicate phases passing from Verrino Member to Poggio Villanelle Member samples. Compositional data of the mudstone samples of both members mainly record a derivation from felsic rocks due to the Calabrian terranes unroofing. Poggio Villanelle Member samples further show an enrichment in Cr, Ni, Fe e Mg in accordance with higher chlorite content suggesting a mafic supply probably related to an ophiolitic source successively exposed. Thus, those trends based on chemical and mineralogical data suggest a change in relative abundance of a detritus characterized by different compositions through time and space. Whole rock geochemistry of the studied mudstones indicate that they are from the first-cycle sediments. Palaeoweathering processes in source areas were moderate as shown by CIA (Chemical Index of Alteration) and CIA’ (modified Chemical Index of Alteration) values of the mudstones of both members (Verrino and Poggio Villanelle members). The trends observed in the paleoweathering diagrams suggest source areas characterized by moderate weathering in non-steady-state conditions which have changed toward the upper portion of the sedimentary succession. Those changes are mainly related to the geodynamic conditions of the source areas where active tectonism allows erosion of all zones within weathering profiles developed on source rocks. The paleocurrent data and the compositional analysis of the mudstones and sandstones indicate a provenance (supply) from the western, north-western, and south-western sectors of the basin where carbonate platforms and magmatic, metamorphic and sedimentary rocks cropped out.