Illite-smectite mixed layers in sicilide shales and piggy-back deposits of the Gorgoglione Formation (Southern Apeninnes): geological inferences

A characteristic mechanism of foredeep, migration and duplex accretion in the Southern Apennines is responsible for the emplacement of clay nappes as roof units (e.g., Sicilide Unit) which, in turn, are covered by siliciclastic sediments (e.g., Gorgoglione Formation) accumulated in piggy-back basins. In this context the clay mineralogy was used as a tool to study the emplacement of thrust sheets using a detailed powder X-ray diffraction studies of illite-smectite mixed layers (I-S) clay minerals. The data show that the Gorgoglione Formation has a low thermal history given the presence of the appraisable amount of random I-S with low illite content (R0; %I=30-50); the Sicilide Succession studied experienced a strong tectonic load as indicated by the high illite content and ordered I-S (R1 and R3; %I=60-95). This load is higher than that observed nowadays, suggesting that deepening and exhumation occurred in a short-time range (during Lower Miocene), before the deposition of the Gorgoglione Formation in the piggy-back basin. The Torrente Cerreto Unit is considered as an olistostrome, because it has variable illite content and I-S order (%I=30-75; R0 and R1).