Spatial distribution of carbon and oxygen isotopes in laterally extensive carbonate-cemented layers: Implications for mode of growth and subsurface identification

Carbonate concretions and laterally extensive cemented layers are common fe atures in the Prairie Canyon Member of the Mancos Shale at its type localit y in western Colorado. Stable-isotope and petrographic data were obtained f rom these cemented zones in an attempt to determine: (1) how the cemented l ayers form, (2) whether or not they can be chemically correlated, and (3) i f it is possible to differentiate them from concretions in the subsurface. The cement in both the layers and concretions consists of ferroan dolomite and lesser amounts of stoichiometric dolomite and ankerite. Samples for sta ble-isotope analysis were drilled from 10 grids, with a typical grid spacin g of 10 cm x 10 cm, Gridded samples were obtained from two continuous cemen ted layers at several locations along the layers (2.3 m to 1.5 km apart on the same layer), and from isolated and stratabound concretions, Taken together, all the cements (regardless of spatial position) show a rem arkably linear correlation (R-2 = 0.98) between delta(13)C and delta(18)O, Systematic center-to-edge decreases in both delta(13)C and delta(18)O occur in each of the cemented zones and are persistent over lateral intervals of at least 1.5 km, The cores of the cemented zones have positive delta(13)C values (up to > 10 parts per thousand PDB), which result from incorporation of C-13-enriched carbon during methanogenesis. The edges of the cemented z ones have negative delta(13)C values (down to < -9 parts per thousand PDB), which suggests incorporation of C-13-depleted carbon from thermocatalytic decarboxylation reactions. Oxygen isotope paleothermometry indicates that p recipitation may have started at moderate temperatures (similar to 25 degre es C) and ended at temperatures as high as 92 degrees C, The spatial distribution of the isotopic data in the cemented layers can be explained as resulting from the coalescence of stratabound concretions, fo llowed by growth above and below the coalesced concretions. Such growth may have occurred in a complex: manner, in which early cements precipitated th roughout the thickness of the layer but were more abundant in the core zone . Given the lateral persistence of the pattern of isotopic variation in the cemented layers, geochemical correlation of the layers may be possible in the subsurface. In addition, because laterally persistent cemented layers i n the Prairie Canyon Member exhibit patterns of isotopic variation differen t from concretions (layers have a more asymmetrical vertical distribution), it should be possible to distinguish between the two styles of cementation from subsurface core data alone.