Calcite rims

Calcite cementation in bioclastic hybrid and lithic arenites of the Bismantova-Termina succession is pervasive along layers and concretionary horizons. Cementation in the hybrid shelf arenites was mostly precompactional and began with marine calcite rims, syntaxial overgrowths on echinoderms, K-feldspar and dolomite overgrowths, chloritic clay rims, framboidal pyrite and heulandite, followed by... 

Secondary porosity contributes more than half (6.7%) to the total porosity (average.of 11%, excluding tight-sandstone zones) of the Catalina Sandstone. Of the available drill cores from wells 0-35 and K-18, 45.7 and 17.5%, respectively, are completely cemented by early ferroan calcite. The difference between the two wells may be due to the fact that the sandstone beds are thicker in well K-18. Where the early ferroan calcite is absent, mechanical compaction and quartz overgrowths have reduced the primary porosity further. In K-18, the framework grains are largely coated by a micritic-calcite rim, approximately 25 pm thick, which prevented silica cementation. Dissolution of the early ferroan calcite cement contributed most of the... 

TABLE 10. Parameters of the RIM, RIMl, RIM2 and SM potentials (first, seeond, third and fourth line) used for calcite (RIM and SM) and aragonite (RIM, RIMl and RIM2). 

Rim thickness appears to be correlated with object size (Metzler et al., 1992). However, rims around grains of calcite, tochilinite and other alteration products may be artifacts of alteration rather than preaccretionary features. Hua et al. 

In pelite 466457 a large garnet comprises an inner zone that is relatively free of inclusions sinrounded by a later generation of garnet with a crescent shape making an incomplete outer rim. This outer rim is strongly fractured and is partially altered to biotite. The innermost part of the inner zone contains abundant small quartz and calcite inclusions (Fig. 6b). 

Even though this sediment represents marine offshore deep-water turbidites, meteoric flow could have reached the reservoir just after turbidite deposition, during a sea-Ievel lowstand (Fig. 13), when both the platform and part of the slope were partially exposed. The relatively low burial rate during the early history of the Namorado Sandstone favours this hypothesis. Meteoric waters resulted in the formation of blocky calcite that replaces bioclasts or occurs as the inner rims of mouldic pores (see Carvalho et al., 1995). 

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