Concretions platy

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... 

Platy concretions Clays, clay-rich silty sand... 

Fig. 6. Stratigraphical column showing details of the relationships between grain size, lithology, sorting and degree of cementation in sediments from the Chamisa Mesa and Canada Pillares Members. Note correlation between coarser and better sorted channel (CH) associations and good cementation. Nodular, platy and rod-shaped concretions are associated with crevasse splay (CS), paleosol (P), interdune facies (ID) and overbank fine (OF) sediments. Scattered ovoid to elongate concretions dominate in the cross-stratified aeolian facies (EC). Note that the coarser portions of aeolian sandsheets (ES) are preferentially cemented.

Platy concretions are small (5-50 cm diameter), flat, irregularly shaped masses that most often occur in groups or masses with a consistent planar orientation, and are often subparallel to bedding. This type of concretion is usually associated with crevasse splay deposits (CS), overbank fines (OF), palaeosol horizons (P), and interdune (ID) deposits (Table 1 Fig. 6). The surfaces of platy concretions commonly have 1-3 cm diameter pits, tubes or grooves (Fig. lOA), although smooth surfaces are... 

Fig. 10. (A) Platy concretion from the middle of the Unnamed Member. Notice the millimetre-sized tube (T) and groove (G) structures. Divisions on the scale are in centimetres. (B) Photomicrograph of alveolar textures (A) from a platy concretion in the Unnamed Member. (C) Rod-shaped concretions from an aeolian sandstone in the Chamisa Mesa Member. Note that several of the rods branch and thin downwards. Divisions on the scale are in centimetres. (D) Radial spar (microcodium) microtexture.

Other outcrops are characterized by millimetresized calcite-filled fractures that are in places irregular, unoriented and fenestral, and sometimes resemble small folds (Fig. 11C). Original sedimentary structures are generally not preserved. These units may also be associated with tubular, rod and platy concretions. These outcrops exhibit alveolar and... 

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... 

Platy concretions have been described by several workers as resulting from initial disruption of relict bedding similar rod concretions have been described as rhizocretions (Kappa, 1980b Esteban Kappa, 1983 Retallack, 1988, 1990). 

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.

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