Classification of Carbonate Sediments

Geologists have classified carbonates according to (1) origin (e.g., skeletal or non-skeletal, or even more precisely, foraminiferal, coralline, algal, diatomaceous, etc.), (2) mineral content (e.g., aragonitic, calcitic, etc.), and (3) depositional environments (e.g., pelagic. 

Summary diagram of JOIDES sediment classification employed after Leg 38 of the DSDP. (After Van Andel, Tj. H., Sediment Nomenclature and Sediment Classification during Phases I-III of the Deep Sea Drilling Project, In Deep Sea Drilling Project Initial Reports Methodology, Heath, G.R., ed., U.S. Government Printing Office, 

Name Shape subparticle location High-energy Low-energy 

Corals S5. 20 cm No discrete subparticles 0-35 Breinching, massive and encrusting coreils Foliose and encrusting corals Porous (porcellaneous) Cohesionless (sandy gravel) Mg-calcite aragonite 

Precipitate 0.1-0.6 mm Highly variable No discrete subpcirticles 35-200 Oohtes Pelletoids No inherent porosity (hyaline) Cohesionless (sand) Mg-calcite 

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Table 5.4. Classification of carbonate sediments based on texture and nature of allochems according to Folk (1962).

Both classifications are based on the depositional texture of the limestone Folk s emphasis is on the similarity of carbonate sedimentation to that of sandstones, resulting in a scheme employing as variables the lime mud spar cement ratio and allochem composition. Allochems may be intraclasts, ooids, fossils or... 

The adjective carbonate is generally being used as a generic description of all sediments containing calcium or magnesium carbonate. According to the classification of carbonate rocks as proposed by Clark and Walker (1977) and Meigh (1987) ... 

In order to communicate further ideas on sediment diagenesis, it is necessary to consider briefly the sedimentaiy materials that make up carbonate sediments and rocks and classifications of these materials. For detailed discussion the reader is referred to Pettijohn (1957), Folk (1959), Dunham (1962), Ham (1962), and Blatt et al. (1980). 

The grain size of the limestone formed the basis of early classifications of limestone lithic type the same approach was used for dolostones but with different modifying terms. Since the 1950 s combined genetic and descriptive limestone classifications have developed, based principally on the recognition that a carbonate sediment contains three major components (1) a framework of discrete grains or... 

Figure 5.4. Classification of mixed siliciclastic-carbonate sediments proposed by Mount (1985). It is an unfolded tetrahedron in which the apices are siliciclastic sand, carbonate allochems, siliciclastic mud, and carbonate micrite. For comparison with Folk s (1962) classification, the classification of pure fossiliferous limestone is also plotted.

Imbrie J, Purdy EG (1962) Classification of modern Bahamian carbonate sediments. In Ham WE (ed) Amer Assoc Petrol Geol, Memoir 1. Tulsa, OK, pp 253-272... 

Fookes, P.G., and Higginbottom, I.E. 1975. The classification and description of nearshore carbonate sediments for engineering purposes. Giotechnique, 25 406-411. 

Each of these solid phases can be described in terms of their mineralogy. This classification scheme is based on crystal structure and chemical composition. The most common minerals found in marine sediments are listed in Table 13.2. Most are silicates in which Si and O form a repeating tetrahedral base unit. Other minerals common to marine sediments are carbonates, sulfates, and oxyhydroxides. Less common are the hydrogenous minerals as they form only in restricted settings. These include the evap-orite minerals (halides, borates, and sulfates), hydrothermal minerals (sulfides, oxides, and native elements, such as gold), and phosphorites. 

Fig. 5.3. Organic isotope and geochemical data, Triassic Province, a Carbon isotope composition of oil and bitumenoid fractions for Paleozoic sediments, Triassic Province b Carbon isotope composition of oil from Saharian fields c Geochemical log for Takhoukht geological section based on shale pyrolysis data d Classification diagram HI-OI for Silurian and Devonian shale sediments, Triassic Province e Distribution of residual oil potentials vs. organic carbon content in Silurian and Devonian oil source rocks, Triassic Province...

Coal has also been considered to be a metamorphic rock, which is the result of heat and pressure on organic sediments such as peat. However, the discussion in favor of coal as a sedimentary rock because most sedimentary rocks undergo some heat and pressure and the association of coal with typical sedimentary rocks and the mode of formation of coal usually keep low-grade coal in the sedimentary classification system. Anthracite, on the other hand, undergoes more heat and pressure and is associated with low-grade metamorphic rocks such as slate and quartzite. Subducted coal may become graphite in igneous rocks or even the carbonate-rich rocks (carbonatites). 

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