STABLE ISOTOPIC COMPOSITION OF UPPER-CRETACEOUS CARBONATES FROM BIG BEND NATIONAL PARK, TEXAS

Stable isotopic ratios of ancient soil carbonates are widely used to infer the C isotopic composition of soil-respired CO₂ and O isotopic composition of meteoric waters. Stable isotopic ratios were determined using conventional acid reaction and mass spectrometric methods. In an attempt to decipher the origin of terrestrial deposits in Big Bend National Park, we determined the C and O isotopic compositions of sedimentary carbonates. Isotopic ratios were obtained with VG SIRA-12 mass spectrometer and are reported relative to V-PDB using the ‰ delta notation. The carbonates occur in floodplain and lacustrine deposits of the Upper Cretaceous Javelina Fm. Four groups representing different origins are recognized. Pedogenic (Group 1): Microcrystalline calcite nodules (d¹⁸O -2.3 to -7.4, d¹³C -5.1 to -13.1). Lacustrine (Group 2): Carbonates containing algal oogonia and gastropod steinkerns (d¹⁸O -3.6 to -10.9, d¹³C of -7.0 to -12.4). Early Diagenetic (Group 3): Carbonate nodules, void-fillings, and coatings with radial-fibrous texture (d¹⁸O -5.3 to -8.9, d¹³C -8.4 to -16.3). Late Diagenetic (Group 4): Sparry calcite cements, void- and fracture-fillings (d¹⁸O -5.2 to -13.2, d¹³C -0.7 to -10.9). Carbonates of groups 1, 2, and 3 are found reworked as clasts in Javelina fluvial channel deposits, and so all precipitated early in its depositional history. Isotopic values for groups 1, 2, and 3 overlap significantly, with groups 2 and 3 being slightly lower in d¹⁸O than 1. Isotopic values for group 4 carbonates overlap slightly with these, but also have lower d¹⁸O values and significantly higher d¹³C values. This suggests that the primary isotopic compositions of groups 1, 2, and 3 may not have been dramatically altered during diagenesis. However, the similarity in isotopic values among pedogenic, lacustrine, and early diagenetic carbonates, and the slightly lower values in lacustrine carbonates compared to pedogenic ones, suggests that simple interpretation of pedogenic carbonates in terms of meteoric water composition may not be appropriate. The slightly elevated isotopic values of pedogenic carbonates may suggest that they precipitated from soil waters concentrated by evaporation compared to surface and ground waters. This study suggests the use of caution in relating O isotope values of ancient pedogenic carbonate to meteoric waters without prior knowledge of hydrology of the local groundwater.