Vadose zone cements

Despite the complexities and variations in surficial environments of precipitation, vadose zone cements in arid environments have a number of distinctive characteristics. 

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). 

Both authors calculations also indicated that it is possible for solutions of reasonable compositions for natural waters to produce mixtures of freshwater and seawater that were undersaturated with respect to calcite but supersaturated with respect to dolomite. This observation is a cornerstone for some dolomitization models that are discussed later in this chapter. It is also important to note that the extent of undersaturation which results from mixing is strongly dependent on the initial Pco2 °f the dilute water when it is in equilibrium with calcite. Waters high in CO2 can cause more extensive dissolution. If these waters enter a vadose zone where CO2 can be degassed, they will become supersaturated and calcium carbonate can precipitate. This process provides an excellent mechanism for cementation near the water table. Because the water table can oscillate vertically, a considerable zone of cementation can result. 

Aeolianites that become submerged below the groundwater table may become cemented in the phreatic environment. In this zone, pore spaces between sand grains are completely filled with water and any cements derived from the interstitial waters are often isopachous in nature (Muller, 1971). Large solution volumes and longer residence times in the phreatic zone can result in coarser spar compared with the vadose zone. Increases in temperature, degree of supersaturation and NaCl content appear to result in larger cement crystal sizes (Badiozamani et al., 1977). 

Permeability in the vadose zone tends to be higher in finer sediments because flow occurs preferentially along grain surfaces rather than the centre of large pores. Finer sediments have more surfaces on which vadose flow can occur (Palmquist Johnson, 1962 Hillel, 1980 Jury et ai, 1991 Mozley Davis, 1996). If cementation is limited by the supply of Ca " and/or HCO3" to the precipitation site, vadose cements should occur preferentially in the finer sediments (Mozley Davis, 1996). 

Calcite precipitation under phreatic conditions can continue uninterrupted by an air-water interface (Morse Mackenzie, 1990). Thus, isopachous or drusy, poikilotopic and blocky spar cements are most often associated with precipitation in the phreatic zone (Jacka, 1970 Folk, 1974 Retallack, 1990 Bums Matter, 1995). Sparry cements can also form in the vadose zone as calcans or crystic nodules, but they are associated with soil zonation, highly dense micritic cements and nodules (Weider Yaalon, 1982). Because these cements are not associated with such features they are unlikely to represent calcans or crystic nodules. 

Cements near the vadose end-member are associated with typical vadose features however, these features are less apparent than in type 2 tabular units, and sparry void filling cements are sometimes more abundant than micrite. As stated previously, micrite-spar cement textures could have initially formed in the vadose zone as pendant and meniscus envelopes around grains or groups of grains (Jacka, 1974 Reeves, 1976 Warren, 1983). Upon burial, these initial vadose cements would provide sites for further calcite precipitation and the unfilled voids could subsequently be filled with sparry calcite in the phreatic zone (Jacka, 1970 Funk, 1979). The vadose contribution to cementation may have been overlooked in the past because of this overprinting. 

The main mechanism for the precipitation of calcite in the vadose zone was transpiration-induced drying, and not evaporation (transpiration does not fractionate oxygen, whereas evaporation does (Quade et al., 1989 Ceding Quade, 1993). Evaporation removes the lighter oxygen (by fractionation), making the 6 0 values in the vadose cement heavier. 

Cements from type 3 tabular (vadose) units are similar to vadose cements in luminescent characteristics. Cements from type 3 tabular (phreatic) units sometimes exhibit regular zoning in cathodoluminescence. Generally no zonation is present in calcite crystals under plain light, suggesting that crystal growth may not be multigenerational. 

Cementation in the phreatic zone occurred preferentially in zones of high primary permeability, whereas vadose cementation occurred principally in association with soil development. Pedogenic carbonates may have served as nucleation sites for later phreatic cementation, leading to complex zones of mixed pedogenic and phreatic cements. 

Carbonate cements are often among the dominant components of diagenesis and hence are of decisive importance in determining the reservoir quality of sandstone sequences. Despite this, the timing, the geochemical conditions of precipitation and dissolution, as well as the source and fate of these cements are not fully understood. In continental and near-shore sediments, cements commonly precipitate as calcretes and dolocretes in the vadose and phreatic zones, and attain a variety of mineral-ogical, textural and distribution patterns as well as elemental and isotopic compositions. These cements form lenses and layers of densely cemented... 

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