2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 18
Presentation Time: 1:30 PM-5:30 PM


MOZLEY, Peter S.1, CAMPBELL, Andrew R.2 and LYNN, Richard D.1, (1)Dept. of Earth and Environmental Science, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801, (2)Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, mozley@nmt.edu

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).