Organogenic dolomite

To understand the process of formation of these organogenic dolomites, it is necessary to look at the chemistry of the pore waters in which they form. Figure... 

Figure 8.25. Some chemical and diagenetic properties of organic-rich marine sediments as a function of depth based on DSDP interstitial water profiles. A. Schematic gradients of SO42-, total alkalinity, Ca2+ and Mg2+ in pore waters, and zones of sulfate reduction, methanogenesis and fermentation. Magnesium diffuses into the sediment and organogenic dolomite forms at depth. B. Logarithm of calculated saturation states of interstitial waters with respect to dolomite. Dolomite saturation=0. All these pore waters are oversaturated with respect to dolomite. (After Compton, 1988.)...

The profile of Mg2+ in Figure 8.25 indicates downward diffusion of this constituent into the sediments. Mass balance calculations show that sufficient Mg2+ can diffuse into the sediments to account for the mass of organogenic dolomite formed in DSDP sediments (Baker and Bums, 1985 Compton and Siever, 1986). In areas of slow sedimentation rates, the diffusive flux of Mg2+ is high, and the pore waters have long residence times. Dolomites form under these conditions in the zone of sulfate reduction, are depleted in 13c, and have low trace element contents. With more rapid sedimentation rates, shallowly-buried sediments have shorter residence times, and dolomites with depleted 13C formed in the sulfate-reduction zone pass quickly into the underlying zone of methanogenesis. In this zone the DIC is enriched in 13C because of the overall reaction... 

Compton J.S. (1988) Degree of supersaturation and precipitation of organogenic dolomite. Geology 16, 318-321. 

Slaughter, M. Hill, R.J. (1991) The influence of organic matter in organogenic dolomitization. J. sediment. Petrol., 61, 296-303. 

Dolomite cement components were all sourced from outside the sand body, most probably from local or basinal mudrocks. Stable isotope data indicate a mixed organogenic-marine carbonate source, and precipitation at relatively low temperatures (s70°C, if pore fluids were sourced from clay mineral dehydration reactions during deep burial of Carboniferous mudrocks in the Rathlin basin 55°C if they were locally sourced). Thermobaric mass transfer was enhanced by tectonic pulsing and dolomite precipitation was driven by CO2 degassing. 

The finely banded argillaceous sediments are illitic or kaolinitic with a constant content of about 10% mixed-layer minerals and <5% chlorite. Carbonates are generally represented by micro- to macro-crystalline dolomites and organogenic limestones. The thickness of those shallow-marine Upper Carboniferous sediments is 25-90 m. 

The Lower Permian deposits are represented by intercalation of gypsified dolomites and gypsums with spare interlayers of sand rock and siltstones (thickness from 80 to 115 m). Above it, with the erosion, Middle Permian sedimentary rocks are deposited. Carbonate rock here is represented by marls, lime dolostones and dolomitic limestones, mostly organogenous, but sometimes silicified. Carbonate members are alternate with terrigenous members (sand rocks, siltstones, clay) and make, as a whole, till 75% from the volume of Middle Permian (Shevelev 2012). 

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