Concretions tabular cemented units

Phreatic (ovoid and elongate concretions, and type-1 tabular units) Mixed type-3 (phreatic) tabular units A Fault cements... 

Fig. 15. Plot of 5 C versus 8 0, with individual points identified by cementation types. Vadose types include nodule, platy and rod concretions, as well as type 2 tabular units. Phreatic types include ovoid to elongate concretions as well as type 1 tabular units. In general vadose cements have heavier carbon values and lighter oxygen values than phreatic cements. Phreatic and type 3 (phreatic) units that plot with oxygen values greater than -10 are from the upper part of the Unnamed Member.

Ovoid and elongate concretions and type 1 tabular units appear to have formed principally in the phreatic zone, because they have poikilotopic and blocky spar cements, are associated with coarser, better sorted units, show preservation of original sedimentary structures, and are not associated with rhizocretions. In the Zia, preferential cementation of coarser, better sorted layers operates on the scale of both thin section and outcrop, something also noticed by Lynch (1996). Elongate concretions have been noted by other workers and attributed to groundwater flow in the phreatic zone (McBride et al., 1994, 1995 Mozley Davis, 1996). Orientations of these elongate concretions tend to be uniform within a single outcrop, often on the scale of several kilometres, as would be unexpected in vadose-zone cementation (Mozley Davis, 1996). 

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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