Page 53 - Sedimentology and Stratigraphy

40 Biogenic, Chemical and Volcanogenic Sediments

concentrations of organic material are of considerable
economic importance: coal, oil and gas are all prod-
ucts of the diagenetic alteration of organic material
deposited and preserved in sedimentary rocks, and the
processes of formation of these naturally occurring
hydrocarbons are considered further in 18.7.

3.6.1 Modern organic-rich deposits

Most of the dead remains of land plants decompose at
the surface or within the soil as a result of oxidation,
microbial or animal activity. Long-term preservation
Fig. 3.11 Thinly bedded banded iron formation (BIF) com- of dead vegetation is favoured by the wet, anaerobic
posed of alternating layers of iron-rich and silica-rich rock. conditions of mires, bogs and swamps and thick accu-
mulations of peat may form. Peats are forming at the
of lines of evidence that the atmosphere contained little present day in a wide range of climatic zones from
or no oxygen through much of Precambrian times. subarctic boggy regions to mangrove swamps in the
tropics (McCabe 1984; Hazeldine 1989) and contain
a range of plant types, from mosses in cool upland
3.5.4 Ferromanganese deposits areas to trees in lowland fens and swamps. Thick peat
deposits are most commonly associated with river
Nodules or layers of ferromanganese oxyhydroxide floodplains (9.3), the upper parts of deltas (12.3.1)
form authigenically on the sea floor: they are black to and with coastal plains (13.2.2). Pure peat will form
dark brown in colour and range from a few millimetres only in areas that receive little clastic input. Regular
to many centimetres across as nodules or as extensive flooding from rivers or the sea will introduce mud into
laminated crusts on hard substrates. Although these the peat-forming environment and the resulting
manganese nodules form at any depth, they form deposit will be a carbonaceous mudrock.
very slowly and are only found concentrated in deep The accumulation of organic material in subaque-
oceans (16.5.4) where the rate of deposition of any ous environments is just as important as land depos-
other sediment is even slower (Calvert 2003). its. Sapropel is the remains of planktonic algae,
spores and very fine detritus from larger plants that
accumulates underwater in anaerobic conditions:
3.6 CARBONACEOUS (ORGANIC) these deposits may form a sapropelic coal (18.7.1).
DEPOSITS Anaerobic conditions are also required to accumulate
the organic material that ultimately forms liquid and
Sediments and sedimentary rocks with a high propor- gaseous hydrocarbons: these deposits are composed of
tion of organic matter are termed carbonaceous the remains of zooplankton (microscopic animals),
because they are rich in carbon (cf. calcareous – phytoplankton (floating microscopic algae) and bac-
3.1). A deposit is considered to be carbonaceous if it teria. The formation of oil and gas from deposits of this
contains a proportion of organic material that is type is considered in section 18.7.3.
significantly higher than average (>2% for mudrock,
>0.2% for limestone, >0.05% for sandstone). Organic
matter normally decomposes on the death of the plant 3.6.2 Coal
or animal and is only preserved under conditions of
limited oxygen availability, anaerobic conditions. If over two-thirds of a rock is solid organic matter it
Environments where this may happen are water- may be called a coal. Most economic coals have less
logged swamps and bogs (18.7.1), stratified lakes than 10% non-organic, non-combustible material
(10.2.1) and marine waters with restricted circula- that is often referred to as ash. Coal can be readily
tion such as lagoons (13.3.2). Strata containing high recognised because it is black and has a low density.