Natural Resources, School of
Stable isotope composition of calcareous paleosols and ground-water cements from the Ogallala Group (Neogene), western Nebraska
Date of this Version
Contributions to Geology, University of Wyoming, v. 29, no. 2. p. 97-109. 3 figs .• 2 tables, I appendix, May, 1992
A preliminary survey has been made of the stable isotope composition of secondary carbonates in the Ogallala Group of western Nebraska. The purpose of the study was to ascertain whether the isotopic signatures of calcareous paleosols in the fine grained facies of the Ogallala and groundwater cements in gravel facies might provide a basis for stratigraphic correlation and/or paleoclimatic interpretation.
The groundwater cemented channel gravels are modestly depicted in IRO (δ18O = –10‰) as compared to calcareous paleo'sols (δ18O = –8.0 to –9.0‰). Gravels of different age, however, have essentially identical isotopic signatures. The isotopic variability within paleosols is about as great as the variability between palcosols. As a result, the mean isotopic values for individual paleosols are not significantly different from one anothcr. Thus, stratigraphic correlation of paleosols based on isotopic signatures will not be possible unless intensive vertical and lateral sampling capable of distinguishing small differences among paleosols is undertaken. Nevertheless, when the mean 0180 values for individual paleosols are arranged in the stratigraphic order proposed by Lugn (1939), they increase upsection from about –9.0‰ to –8.0‰, suggesting that the pedogenic environment possibly became more arid through time. Unfortunately, this trend is not confirmed by a clear upsection increase in δ13C values.