Abstract

Carbonate concretions and laterally extensive cemented layers are common features in the Prairie Canyon Member of the Mancos Shale at its type locality in western Colorado. Stable-isotope and petrographic data were obtained from these cemented zones in an attempt to determine: (1) how the cemented layers form, (2) whether or not they can be chemically correlated, and (3) if 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 stable-isotope analysis were drilled from 10 grids, with a typical grid spacing of 10 cmX10 cm. Gridded samples were obtained from two continuous cemented 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 remarkably 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 per mil PDB), which result from incorporation of 13 C-enriched carbon during methanogenesis. The edges of the cemented zones have negative delta 13 C values (down to < -9 per mil PDB), which suggests incorporation of 13 C-depleted carbon from thermocatalytic decarboxylation reactions. Oxygen isotope paleothermometry indicates that precipitation may have started at moderate temperatures ( approximately 25 degrees 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, followed by growth above and below the coalesced concretions. Such growth may have occurred in a complex manner, in which early cements precipitated throughout 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 in the Prairie Canyon Member exhibit patterns of isotopic variation different 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.

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