Concretions tabular

OF Overbank fines Massive, crudely bedded silts and muds (Fm) finely laminated to rippled silts and muds (FI) laminated silt, sand, and clay (Fsc) silts and clays w/rhizocretions (Fr) Tabular to thin and lobate 0.1-3 m thick 0.5 m to 0.2 km in lateral extent Muds and silts Poorly cemented, isolated nodules, platy, and rod concretions... 

P Palaeosol horizons Palaeosols on sand (Ps) palaeosols on silts and clays (Psc) silt and clays w/rhizocretions (Fr) massive sand w/rhizocretions (Smr) Tabular to discontinuous and patchy 0.1-1 m thick 0.1 to > 1 km lateral extent Muds, silts, very fine to medium silty and clayey sand/sandstones Nodular, platy and rod concretions, and type 2 and type 3 (vadose) tabular units... 

EC Cross-stratified aeolian dune bodies Trough cross-bedded sand (Ste) planar laminated sand (Spe) low angle cross-bedded sand (Sle) horizontally laminated sand (She) ripple cross-laminated sand (Sre) massive sand (Sme) Tabular, lenticular and wedge-shaped 1-3 m thick > 1 km lateral extent Fine to lower coarse, moderately to well sorted sand/sandstone Scattered ovoid to elongate concretions and small type 1 and type 3 phreatic tabular units... 

Massive to brecciated and mottled type-2 tabular unit with abundant branching rod Concretions. 

Scattered nodules and rod concretions in a mudstone "matrix, interbedded with thin type-3 (vadose and phreatic) tabular lenses. 

The isotopic composition of the various calcite types does not vary greatly. Carbon isotope values (5 C) range from -3.0 to -5.5%o pdb, whereas oxygen isotope values (5 0) range from -7.3 to -13.6%o PDB (Fig. 14). 5 C values for nodular, platy, rod-shaped concretions and type 2 tabular units are generally heavier than those of other types... 

Vadose (nodular, platy, and rod concretions) o Mixed type-3 (vadose)tabular 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). 

Fig. 2. Sketch showing the diverse patterns of calcite cement in sandstones of this study (from McBride el al., 1995). (A) Sandstone beds inteibedded with mudrock are completely cemented by calcite (1), cemented in two rows of spheroidal concretions (Borello Formation only, 2), cemented only in the lower one-third (3), or are cemented in concretions located chiefly at the base of the beds (4). (B) Stacked sandstone beds are cemented in tabular concretions parallel with bedding (2) or parallel with faults...

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. 

Tabular alumina grains constitute the filler of many refractories, whether they are fashioned as bricks or not fashioned castable refractory concretes whose binder is cement, itself with high alnmina content [BOC 99]. 

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