DIAGENETIC DEVELOPMENT OF CLAY AND RELATED MINERALS IN DEEP-WATER SANDSTONES (S SPAIN) - EVIDENCE OF LITHOLOGICAL CONTROL

The turbiditic facies of the Aquitanian-Oligocene of the Algeciras and Bolonia Units consist of lutitic-marly beds alternating with varied s andstones. Early diagenetic processes and the development of secondary porosity in sandstones were controlled largely by sedimentary facies. Mineral dissolution processes were important in rocks with high prima ry porosity (quartzose sandstones) and led to the development of allop hane and kaolinite/dickite in later diagenetic stages. In rocks with h igh contents of biotitic matrix, chlorites and iron oxides were produc ed, together with the development of secondary porosity. In feldspathi c sandstones, the early diagenetic processes produced chlorite, illite , illite-smectite mixed-layers and smaller amounts of kaolinite. In ca lcareous sandstones, the presence of an early cement prevented, to a c ertain extent, the dissolution of unstable fragments and restricted th e development of clay minerals. The movement of fluids was not importa nt during this early diagenetic stage in controlling the variety or ab undance of mineral cements; the controlling factors were the different chemical conditions developed in the pore-solution of the various san dstone types. In spite of similar sandstone burial conditions, the dif ferent sandstone lithologies continued to determine the diagenetic pro cesses, especially the degrees of compaction and dissolution. Certain burial processes are common to all levels: the development of reducing conditions is indicated by the development of Fe and Mn carbonates, F e-dolomites and Fe-chlorites. A later alkaline diagenetic phase was re sponsible for the development of calcite, feldspar and illite. The cir culation of fluids in these stages remained limited, although there wa s probably some movement from the lutitic-level fluids to the overlyin g sandstones.