A mineralogical study on the clay-rich sediments of the Lagonegro and Liguride Units of the Southern Apennines fold and thrust belt has been carried out. The area sampled is located in the Lucanian sector of the Southern Apennines between Monte Sirino and Monte Alpi. Thirty-five samples have been collected from the Calcaricon selce, Scisti silicei, Galestri and Crete nere formations and analysed by X-ray diffraction (XRD) method. Data confirm that the Lagonegro and Liguride Units belong to different paleogeographic domains. Their principal differences consist of: Feldspars distribution: Na-plagioclase/K-feldspar ratio is greater in the Liguride Unit than in the Lagonegro Unit. Clay minerals distribution in the fine grained fraction (<2 mu m): Lagonegro Unit is characterized by kaolinite (mean value 32%), I/S (mean value 12%) and I/V and/or I/Chl (mean value 26%) mixed layers, chlorite (mean value 15%), mica (mean value 14%) and small amounts of smectite (<4%). Liguride Unit contains chlorite (mean value 35%), I/S mixed layers (mean value 33%), mica (mean value 20%), and small amounts of kaolinite (mean value 5%). Content of illitic layers in the I/S, I/V and/or I/Chl mixed layers: Lagonegro Unit is characterized by an illitic content of 60-80% in the I/S mixed layers and 80% in the I/V and/or I/Chl mixed layers. Liguride Unit shows an illitic content ranging between 70 and 80% in the I/S mixed layers and between 80 and 90% in the I/V and/or I/Chl mixed layers. Mica crystallinity index: Liguride Unit shows values >0.42 degrees Delta 2theta , whereas the Lagonegro one has values correlatable with diagenetic (<2 mu m) and anchizone (2-16 mu m) conditions. Furthermore, I/S mixed layers have been used to calibrate a thermal-structural model in order to define the load suffered by the Lagonegro Unit. The best calibrated reconstruction provides a tectonic load of about 4,500 m caused by the emplacement of more internal thrust sheets in Miocene times, now totally eroded. The calculated exhumation rates are of 0.14 mm/year (between 14 Ma and 3.8 Ma), 0.74 mm/year (between 3.8 and 0.7 My) and 1.43 mm/year (<0.7 Ma).