Since their discovery, the phyllosilicate superstructures, known for decades as mixed-layered clay minerals, have had various applications in mineralogy, petrology, oil industry, etc. The most important and handy application of the mixed-layered illite-smectite (i/s) is modeling the evolution of the interlayer structures during burial depth. To this goal, X-ray diffraction patterns of clay association is analyzed for discreet phase identification and mixed-layered order evaluation. This paper presents an analysis of a complete sedimentary deposit stretched between Miocene and Pliocene (5-8 Ma provided by two Valleys (Putna and Râmnicu Sărat) in the Focşani Basin aiming to solve a controversy concerning whether or not the sedimentary sequence was buried to the thickness it suggests (8,000 m). The sedimentary sequence in the Focsani Basin has been extensively analyzed in the last twenty years, aiming to discover the dynamics of this complex structure affected by active subduction and seismicity. The clay minerals analysis was focused on X-ray diffraction patterns of <2μm fraction of 250 samples and the R factor of the I/S ordering determined by using Środoń methods (1980 and 1984). The illite-smectite interlayer analysis showed no illitization and no ordering tendencies of the superstructures with growing thickness of the sediment which was correlated with the burial depth. The conclusion of this study does not only reveal valuable information about the evolution of the Focșani Basin but also highlights the importance of phyllosilicate superstructure analysis for understanding the complex regional dynamics.