Published online by Cambridge University Press: 09 July 2018
The turbiditic facies of the Aquitanian-Oligocene of the Algeciras and Bolonia Units consist of lutitic-marly beds alternating with varied sandstones. 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 primary porosity (quartzose sandstones) and led to the development of allophane and kaolinite/dickite in later diagenetic stages. In rocks with high contents of biotitic matrix, chlorites and iron oxides were produced, together with the development of secondary porosity. In feldspathic sandstones, the early diagenetic processes produced chlorite, illite, illite-smectite mixed-layers and smaller amounts of kaolinite. In calcareous sandstones, the presence of an early cement prevented, to a certain extent, the dissolution of unstable fragments and restricted the development of clay minerals. The movement of fluids was not important during this early diagenetic stage in controlling the variety or abundance of mineral cements; the controlling factors were the different chemical conditions developed in the pore-solution of the various sandstone types. In spite of similar sandstone burial conditions, the different sandstone lithologies continued to determine the diagenetic processes, 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, Fe-dolomites and Fe-chlorites. A later alkaline diagenetic phase was responsible for the development of calcite, feldspar and illite. The circulation of fluids in these stages remained limited, although there was probably some movement from the lutitic-level fluids to the overlying sandstones.