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The diagenesis of mixed-layer illite/smectite (I/S) minerals in Lower Cretaceous–Paleogene successions from the Dolna Kamchiya Depression was studied, using X-ray diffraction analysis of the clay fraction (<2 μm) from core samples. The proportion of illite in I/S and degree of ordering increase with depth irrespective of the geological age, indicating that highly expandable I/S compositions were progressively illitized during the burial evolution. Lowest smectite values are recorded in the Lower Cretaceous deposits, whereas in the Paleogene sections are documented great regional variations in the I/S mineralogy, caused by differential basin subsidence. The transition from randomly interstratified (R0) to R1-ordered I/S occurs between depths of 2200 m and 2400 m, and crosscuts the major stratigraphic boundaries. The variable patterns of I/S depth profiles resulted from the combined influence of temperature, burial history, sedimentation and subsidence rates, and basin geodynamic types on diagenesis. The rapid increase in illite content in the I/S clays across the main unconformities reflects the great thickness of eroded sediments during uplift and denudation events and/or elevated heat flow. The reconstructed palaeogeothermal gradient for the Eocene after corrections for decompaction and erosion would correspond to the values for foreland basin settings reported in the literature. Application of the I/S geothermometer to the Lower Cretaceous–Paleogene rocks demonstrates a general trend of increased organic maturity toward southeast in relatively isochronous deposits. The new results appear to be the first record for the effect of a multiphase basin evolution on the smectite illitization in sedimentary successions from the Dolna Kamchiya Depression.

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Sandstones are a major rock type in the 2500 m thick Palaeogene sedimentary fill of the Padesh strike-slip basin. The main diagenetic alterations include mechanical compaction and carbonate cementation while dissolution, transformation, recrystallization, replacement and chemical compaction were of lesser significance during burial. These postdepositional processes and their products are discussed in terms of micropetrographic characteristics, spatial distributional patterns, mechanisms of formation, controlling factors, and temporal sequence. The diagenetic changes are related to eodiagenesis, mesodiagenesis, and telodiagenesis but most processes occurred during the mesodiagenetic stage. The sandstone diagenesis was controlled by the depositional facies, climate conditions, primary mineralogy and fabric, distribution and composition of the associated shales, chemistry of the pore fluids, sedimentation and subsidence rates, burial depth, and basin thermal regime. The most peculiar feature is recorded in basal sandstone strata of the Palaeogene succession which display evidence for minor selective dissolution and subsequent kaolinite precipitation in the produced secondary pores as a result of thermal maturation of organic matter in the adjacent black shales. Several lines of evidence indicate a hyperthermal character of the Padesh Basin thus resembling other “hot-type” sedimentary basins. The present study contributes to deciphering the diagenetic pathway of siliciclastic deposits in strike-slip basins characterized by elevated heat flow and can be applied for the evaluation of their hydrocarbon system potential. It also supports previous findings that thermal maturation of kerogene in carbonate-free, organic-rich shales may influence the mesodiagenetic alteration of adjacent sandstones in response to increased temperature.