Oxygen isotopic compositions sandstones

There seems to be no obvious systematic difference in the amount or type of calcite cementation in sandstones that have been buried to depths of 4-5 km and temperatures of more than 150°C versus otherwise unconsolidated sandstones that have not been deeper than 1.5 km and that have not been subjected to temperatures of more than around 70°C (Plate 9 Table 1). This suggests that most calcite cementation is probably complete at shallow to moderate burial depths and at relatively low temperatures, although examples of later calcite precipitation that postdates initial quartz cementation have been documented (e.g. Saigal Bjor-lykke, 1987 Walderhaug, 1990 Taylor Soule, 1993). The oxygen isotopic compositions of calcite... 

Fig. 19. Covariation of carbon and oxygen isotopic composition of carbonate cements from the Hibernia Field. Average of Avalon Sandstone from Hutcheon et al. (1985).

Fig. 25.4. Oxygen and carbon stable isotopic compositions of calcite ( ) and dolomite ( ) cements from Lyons sandstone (Levandowski et al., 1973), and isotopic trends (bold arrows) predicted for dolomite cements produced by the mixing reaction shown in Figure 25.3, assuming differing CO2 fugacities (25, 50, and 100) for the Fountain brine. Fine arrows, for comparison, show isotopic trends predicted in calculations which assume (improperly) that fluid and minerals maintain isotopic equilibrium over the course of the simulation. Figure after Lee and Bethke (1996).

Fig. 5. Oxygen and carbon isotopic composition of early-formed dolomite cements in sandstones of the central San Joaquin basin.

Fig. 15. Cross-plot of carbon and oxygen isotope values for the Tirrawarra Sandstone siderites. The bulk-rock isotope signatures are the cumulative product of the varying proportions of the different siderite cement generations (SI, S2, S3). The pure end-member compositions of the different siderite cement generations can be estimated from samples which are dominated by a single cement generation (circled areas). Only the end-member isotope compositions of the different cement generations should be taken into consideration when evaluating multigeneration siderite cements.

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