Organic Matter Accumulation in Sediments

In the fossil record, dark-colored organic-matter-rich layers (black shales, sapropels, petrolenm sonrce rocks in general) witness periods of time when conditions for organic matter accnmnlation in sediments apparently were particnlarly favorable. As the other extreme, massive seqnences of white- or red-colored sedimentary rocks are devoid of organic matter. Althongh these rocks may contain abundant calcareons or siliceons plankton fossils, the organic matter of the organisms apparently was destroyed before it conld be bnried in the sediments. 

---

At the boundary between uplands and wetlands there is, in some circumstances, an interaction between organic matter accumulation in sediments and the development of wetland conditions. Some level of organic matter accumulation is required to drive anaerobic metabolism. But also, because, in general, well-decomposed organic matter improves the water holding capacity of mineral soils, particularly in medium to coarse texmred sediments, and particularly if the clay mineralogy is dominated by low activity kaolinitic clays, there is a feedback between organic matter accumulation and the extent and duration of water saturation. 

Fig. 4.2 Schematic models for organic matter accumulation in sediments. A) Stagnant basin or Black Sea model (after Demaison and Moore 1980 and Stein 1991) B) Productivity model (after Rullkotter et al. 1983). OM = Organic matter.

---