Low-Mg calcite cement

A second generation of low-Mg calcite cement was precipitated in primary and secondary pores in both lenses and host rocks. This cement is moder-... 

Hendry, J.P., Trewin, N.H. Fallick, T.E. (1996) Low-Mg calcite marine cement in Cretaceous turbidites origin, spatial distribution and relationship to seawater. Sedimentology, 43, 877-900. 

Carbonate rock fragments could also be a source of calcite cement if they are available in the source area and if they survive transport. In the Palaeogene sandstones of the North Sea we have found intervals containing up to 55 vol% chalk clasts, for instance, and carbonate rock fragments have also been reported from sandstones in other basins (e.g. Richmann et al., 1980 Dickinson, 1988 Taylor, 1990). However, carbonate rock fragments are probably more stable than biogenic carbonate because of their low-Mg-calcite composition and coarser crystal size. 

Fig. 10. Plot of the Mg/Ca and Fe/Ca ratios of sparry calcite cement, (replaced) carbonate grains and fringe cement of lenses and host rocks. All carbonate is low-Mg calcite. Despite the rather large range in chemical compositions, there is no significant difference in chemical composition between the various carbonate components.

Fig. 11. Plot of oxygen and carbon stable isotopic compositions of bulk carbonate samples. Note the small range in isotopic compositions of calcite in lenses despite variable ratios between early and late calcite cement. This points to similar physical and chemical conditions during replacement of early diagenetic CaC03 cement by low-Mg calcite and late calcite cement precipitation.

Generally the calcite cements are characterized by relatively low average mol% of Fe (1.0%), Mg (0.6%) and Mn (0.2%) (Table 2). Cements in the turbiditic arenites have somewhat more Fe than those in the shelf hybrid arenites (av. 1.4% and 0.6%, respectively) (Table 2). Bioclasts such as echinoderms, originally composed of high-Mg calcite, show low Mg contents, which confirms their pseudomorphic replacement. Calcite which replaced feldspar grains is occasionally Sr rich (up to 2.1% SrC03) (Fig. 6F Table 2). 

Trace amounts of Fe, Mg and Mn are present in virtually all calcite cements that have been measured in the central basin. Samples are characterized by relatively high Fe and Mg, but relatively low Mn contents. Compositions have a characteristic (Fe + Mn)/Mg ratio of about 3/1 (Fig. 9). Late cements tend to be more enriched in Ca (Boles Ramseyer, 1987), which may be due to late precipitation of calcite during the dissolution of Ca-plagioclase at burial temperatures greater than about 70-80 °C. Fe and Mn contributions to the pore water from this source would presumably be negligible. Marine pore waters at both North and South Coles Levees have up to 1800 mg/1 Ca (see analysis in Fisher Boles, 1990), due to Ca enrichment from feldspar dissolution (these values are considerably higher than originally reported by Boles Ramseyer (1987)). 

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