Elsevier

Chemical Geology

Volume 81, Issues 1–2, 25 January 1990, Pages 157-162
Chemical Geology

Research paper
13C and 18O compositions of carbonates from a cyclic carbonate-evaporite rock sequence: Evidences for meteoric water input

https://doi.org/10.1016/0009-2541(90)90045-9Get rights and content

Abstract

Twenty-five carbonate rock samples from a core section composed of seven thick rhythmic carbonate-evaporite sequences (Lower McKnight Formation, Early Cretaceous, south Texas, U.S.A.), were analyzed for carbon and oxygen isotopic compositions. Results show the carbonate rocks to have δ13C = −0.38 to + 1.68‰ and δ18O = −4.60 to −2.48‰ vs. PDB, consistent with the reported world-wide values for carbonate rocks of Cretaceous age. Despite this low ∼2‰ overall variation, δ18O shows more variability than δ13C throughout the core section studied. Furthermore, δ13C and δ18O are covariant, becoming progressively lighter toward the overlying evaporites within each carbonate-evaporite sequence. The tendency toward lighter isotopic compositions together with the covariance between δ13C and δ18O in these carbonates is thus not reconcilable with a simple model of carbonate deposition in a marginal evaporitic basin upon continuous evaporation of seawater. Instead, the data imply contributions from isotopically light meteoric waters.

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    This research was supported by a Junior Faculty Research Fellowship of the University of Oklahoma.

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