Illite/smectite diagenesis and thermal evolution of Lower Cretaceous–Paleogene successions in the Dolna Kamchiya Depression, Eastern Bulgaria

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.