Chapter 12 Some Aspects of Diagenesis in Contourites

This chapter discusses some aspects of diagenesis in contourites. It distinguishes among (1) occurrences of rhythmic variations that can be linked to primary sedimentary processes, such as turbidite, contourite, and hemipelagite deposition, (2) diagenetic processes associated with fine-grained contourites, and (3) diagenetic processes associated with coarse-grained contourites. The mechanisms of element redistribution along a redox gradient may only be active under certain conditions. These conditions are frequently found together in fine-grained contourites, indicating (1) that sufficient organic carbon is available to act as a reducing agent driving a redox reaction; (2) that sufficient time is available to allow relocation to take place, and (3) that the redox-sensitive elements – especially Mn(II) and Fe(II) – are present in a form which allows their transport to major redox boundaries. Diagenesis of fine-grained contourites involves carbonate, manganese, and iron-oxidation processes. The deposition of sandy and silty contourites is controlled by winnowing processes. Laminations in the silty contourites of the Weddel Sea represent variability in the current, producing preferential winnowing of silty coarsening-up, and fining-up sequences. These contourites are formed at slow accumulation rates. One of the most significant consequences of the grain-winnowing concentrations concerns glaucony. The green- grain concentration compared to the bulk sand fraction suggests that glaucony abundance is not constant through time.