Concretions Namorado Sandstone, calcite

Calcite is the most abundant cement in the Namorado Sandstone, but is patchy at many scales of occurrence. Commonly, calcite forms discrete and tightly cemented concretions embedded in weakly... 

Fig. 6. Typical lithofacies of the Namorado Sandstone. Fine- and very fine.grained sandstones arranged as turbidite cycles, each capped by shale and siltstone. Pervasively calcite-cemented beds resulted from concretion coalescence. Both individual concretions and calcite-cemented beds are discontinuous on the lateral scale as indicated by core analysis of a recently drilled horizontal well. Concretions show variable shapes and intensity of cementation, and a gradual vertical transition to partially cemented and porous intervals.

Two hypotheses have been put forward to explain the origin of calcite concretions in the Namorado Sandstone (i) growth was essentially controlled by local sources of carbonate and early bacterial processes, with the initial calcite precipitating either at... 

Further, the availability and proximity of a large source of carbonate material, namely the bioclasts and carbonate intraclasts within the Namorado Sandstone, probably provided supersaturation conditions and sites of nucleation for intense calcite cementation. Sombra et al. (1995) attributed the tabular geometry of calcite-cemented zones to the lateral coalescence of concretions in carbonate clast-rich units in the Namorado Sandstone. 

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