Carbonate cements phreatic

Figure 7.42. Comparison between (A) an idealized plot of variation in 8180 and 813C for carbonates subjected to vadose and phreatic meteoric diagenesis (after Lohmann, 1988) with (B) the meteoric alteration trend observed for the Key Largo Limestone, Florida, U.S.A. (after Martin et al., 1986). The critical trend in isotopic composition is termed the meteoric calcite line. This trend may be modified at the water recharge surface where evaporation is an important process, caliche is formed and the diagenetic phases are depleted in 13C derived from soil-gas CO2. Another modification can occur distally to the recharge area where precipitating carbonate cements may have isotopic ratios nearly equivalent to dissolving phases.

Badiozamani K Mackenzie F.T. and Thorstenson D.C. (1977) Experimental carbonate cementation Salinity, temperature, and vadose-phreatic effects. J. Sediment. Petrol. 47, 529-542. 

Beckner J. R. and Mozley P. S. (1998) Origin and spatial distribution of early vadose and phreatic calcite cements in the Zia Formation, Albuquerque Basin, New Mexico, USA. In Carbonate Cementation in Sandstones. Distribution Patterns and Geochemical Evolution (ed. S. Morad). International Association of Sedimentologists, Oxford, vol. 26, pp. 27-52. 

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.

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... 

In this paper the terms dolocrete and calcrete are used to indicate sediments extensively cemented by displacive dolomite and calcite under conditions ranging from the soil horizon to shallow phreatic. Conversely, we use the expression carbonate cement in sandstones where is no evidence of displacive, near-surface precipitation. Kaolin is used as a... 

A few experiments have been successfully performed at low temperatures to simulate carbonate diagenetic processes for example, cements have been precipitated on skeletal carbonate sands in experimental reaction chambers designed to mimic vadose and phreatic meteoric cementation (Thorstenson et al., 1972 Badiozamani et al., 1977). These cements are remarkably similar in composition and morphology to those found in rocks cemented in the meteoric... 

The dominant types of cementation in the Zia Formation are pedogenic and phreatic. By definition, the spatial distribution of pedogenic carbonate is a function of the spatial distribution of palaeo-sols, which is a function of facies architecture and the length of time a particular surface was exposed. Most pedogenic carbonate in the Zia Formation is poorly developed, discontinuous and associated with finer-grained sediments in overbank fines (OF), crevasse splay (CS), and interdune (ID) facies associations (see Table 1 Fig. 6). Unlike discontin-... 

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. 

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