Diagenesis of Paleogene sandstones in the Padesh strike-slip basin, Southwestern Bulgaria

Abstract

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.