INFLUENCE OF DEPOSITIONAL ENVIRONMENT ON COVARIATION OF OXYGEN AND CARBON ISOTOPES IN MARINE DOLOMITE CEMENTS

We examined the δ¹³C and δ¹⁸O values of dolomite cements in the Book Cliffs of Utah, and compared them to a compilation of isotopic values for marine dolomites. Cements in fluvial, upper and lower shoreface sandstones of the Desert and Grassy Members of the Cretaceous Blackhawk Formation have a wide range of isotopic values (δ¹⁸O -15 to -5 ‰ PDB, δ¹³C -4 to 2 ‰ PDB) and show distinct linear isotopic trends, which vary systematically with depositional environment. Dolomites in lower and upper shoreface sandstones show strong positive correlations of δ¹³C and δ¹⁸O (r = 0.97 and 0.76, respectively); those in fluvial sandstones also have a linear isotopic trend, but rather than showing systematic covariation, are characterized by relatively invariant δ¹³C values (~3 ‰ variation) and δ¹⁸O values that vary by ~10 ‰. Our compilation of data from the literature demonstrates that the strong positive correlation observed it the shoreface sandstones is the most common trend seen in dolomite cements from marine clastic systems. Furthermore, most dolomites exhibiting a strong positive correlation between δ¹³C and δ¹⁸O plot along approximately the same trend line, with slopes that range narrowly between ~1.5 and 2.5. The exceptions to this are the shoreface dolomites, which, with one exception, plot along trend lines with lower slopes (~0.4 and 0.7) and are displaced slightly towards more ¹⁸O-depleted values. Thus, it appears that the depositional environment of marine dolomite cements systematically influences the resultant isotopic values, such that more near-shore dolomites have more ¹⁸O-depleted values and trend lines with lower slopes than more offshore dolomites. The fluvial dolomites from the Book Cliffs are further displaced towards ¹⁸O-depleted values and have a trend line with an even lower slope (-0.1), and thus can be considered an extension of this pattern. The strong linear covariation in the marine dolomites is most likely due to precipitation over a range of burial depths, in which the δ¹⁸O values become ¹⁸O depleted with increasing precipitation temperature and the δ¹³C values become ¹³C depleted due to thermal decarboxylation reactions (i.e., both variables are directly linked by increasing T).