Depositional environments associated with accumulation of organic matter

4 Depositional environments associated with accumulation of organic matter 

In the earlier parts of this chapter we considered factors directly related to the production, degradation and preservation of organic matter.This section is concerned with a more detailed examination of some of the environments where organic-rich sediments accumulate. 

Lake size and morphology influence the thermally induced stratification of the water column and its stability, as there is a relationship between the depth of the seasonal thermocline and the maximum fetch of the water body the longer the fetch (i.e. the distance over which wind acts on the water), the deeper the thermocline. The potential effects of stratification on productivity and anoxicity have been discussed in Sections 3.2.2, 3.2.3, 3.2.4 and 3.3.4. 

In large lakes siliciclastic sediments (see Box 3.11) are normally concentrated around river mouths as beaches, spits and barriers fashioned by wave and current action. Deltas may also be present in relatively deep freshwater lakes and resemble those found in marine environments. Nearshore deposition is characterized by accumulations of terrestrial plant material, deposited on delta tops, which can form peats (see Section 3.4.2). Decomposer communities may actively rework the higher plant material deposited within the delta, their remains augmenting the organic material (particularly the lipids). Delta fronts can become over-steep and collapse, depositing turbidites in deeper water. While this results in conditions suitable for preservation of organic matter, dilution by large volumes of clastic material tends to occur (see Box 3.11). 

In offshore regions, clastic sediment is deposited by pelagic rain (i.e. fall-out of material from the water column) and by various forms of sediment gravity flow (e.g. slides and slumps as well as turbidites Reading 1986). Organic material can be mostly autochthonous, 

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