Biogenic sedimentary rocks

Type Carbon content (/ wt.% C) Bulk density (/kg.m ) Gross caloric value (/MJ.kg- ) Description 

Crude oil 83-87 810-985 38.5MJ.dm Petroleum is a brownish green to black liquid 

Natural gas - 38.4MJ.m Mainly methane with other alkanes and minute amount of hydrogen sulfide 

Lignite 70-75 640-860 23.9-25.5 Dark-brown with ligneous debris 

Bituminous coal 80-95 670-910 24.4-32.6 Friable grayish brown solid 

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Sedimentary rocks are classified depending on how they were formed. They are divided into four groups (a) clastic sedimentary rocks , (b) biochemical (or biogenic) sedimentary rocks, (c) chemical sedimentary rocks and (d) other sedimentary rocks formed by impact, volcanism and other processes. 

Biochemical or biogenic sedimentary rocks are formed from calcareous skeletons of organisms. The main constituent mineral is calcite (CaCO ). Limestone is the main rock type in this category. 

Thick sedimentary pile from middle Miocene to late Pliocene is exposed in the Oga Peninsula, northern Honshu, Japan (Fig. 1.153). Age of the sedimentary rocks has been determined by microfossil data. Thus, the sedimentary rocks in the Oga Peninsula where type localities of Miocene sedimentary rocks in northern Japan are well exposed have been studied to elucidate the paleoenvironmental change of the Japan Sea (Watanabe et al., 1994a,b). Kimura (1998) obtained geochemical features of these rocks (isotopic and chemical compositions) and found that regional tectonics (uplift of Himalayan and Tibetan region) affect paleo-oceanic environment (oxidation-reduction condition, biogenic productivity). However, in their studies, no detailed discussions on the causes for the intensity and periodicity of hydrothermal activity, and temporal relationship between hydrothermal activity, volcanism and tectonics in the Japan Sea area were discussed. They considered only the time range from ca. 14 Ma to ca. 5 Ma. 

The biogenic soft parts that become buried in the sediments are also transformed into sedimentary rocks, predominantly shale. Geologic uplift fallowed by chemical weathering leads to the oxidation of the organic carbon, i.e.. 

CH4 and CO, derived mostly from biogenic sources on land, volcanic and diagenetic processes, and a small flux from weathering by atmospheric O2 of fossil organic matter in sedimentary rocks exposed at Earth s surface. 

As has already been mentioned, oxidation of methane led to the formation of free carbon which apparently was involved in biochemical cycles. Therefore the oldest sedimentary rocks could have contained both juvenile carbon, produced in chemical oxidation of methane, and biogenic carbon, formed in the decomposition of organisms. 

In this chapter we first outlined a conceptual model for the relationship between primary geological processes involved in the formation of fine-grained, siliciclastic-biogenic sediments and sedimentary rocks and a selected set of... 

Elsewhere in the Barberton Mountain Land is a wide array of mid-Archean volcanic and sedimentary rocks, ranging up to >3.5 Ga old. Some material is clearly biogenic (Westall et al., 2001), with highly fractionated carbon isotopes —27%o), but may be of non-Archean age. Thus the case for mid-Archean biotic material in Barberton remains open. 

For the past 1.4x10 years or so, U weathered from igneous, metamorphic, or sedimentary rocks generally has been dispersed in the form of the dissolved ion, or one of its complexes. The reaccumulation of U generally has been facilitated by reduction reactions which result indirectly from biogenic processes. The breakup of pre-existing rocks provided the raw materials for the building of new rocks and those built at, or very near the surface of the earth are sedimentary rocks. There are a number of such rocks where U has become enriched. 

Middle Proterozoic rocks of 1800 to 900m.y. in age are characterized by thick marine biogenic and chemical carbonate strata, evaporites, fine-grained clastic sedimentary rocks, and extensive, thick subaerial tholeiitic basalts, and quartz arenite and conglomerate. Ophiolitic and calc-alkaline volcanic rocks are absent in sequences of this age. ... 

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