Deep-burial diagenesis of the siliciclastic Ordovician Natal Group, South Africa

The siliciclastic Ordovician Natal Group, distributed along the east coast of South Africa, is a predominantly fluvial sequence deposited by braided streams. Although the strata are not metamorphosed, they have been strongly affected by deep burial diagenesis that left many burial diagenetic textures and structures. Well-preserved pressure solution pits and overcompaction fractures on pebbles and cobbles are found in the conglomeratic units near the base of the succession, whereas convex–concave and sutured grain-contacts, fractured arenaceous grains, deformed detrital mica, concertina-shaped muscovite, and deep-burial albite and calcite cements are ubiquitous in the arenaceous units higher in the sequence. In addition, pressure-shadows and stylolites, illitization of clay minerals, and recrystallization of quartz cement are also common. These diagenetic textures and structures demonstrate that the Natal Group had been in a deep-burial environment. Thickness data of the overlying strata and fluid-inclusion temperatures show that the burial depths for the Natal Group probably ranged between 4800 and 7200 m. This conclusion is also supported by the high crystallinity indices of illite taken from the succession.