STABLE ISOTOPE COMPOSITION OF OLIGOCENE-PLIOCENE PEDOGENIC AND GROUNDWATER CARBONATES IN THE TEXAS COASTAL PLAIN: IMPLICATIONS FOR DIAGENESIS AND PALEOCLIMATE

The Oligocene-Pliocene sedimentary rocks in south Texas record past climate and environment in a low altitude region. The Catahoula, Oakville, Fleming, and Goliad formations contain abundant pedogenic and groundwater carbonates which may be useful in reconstructing paleoclimate and paleoenvironment if primary isotopic compositions are preserved. We integrate field observations, studies of carbonate texture on thin sections, and stable isotope composition of micritic and sparry carbonate in order to evaluate diagenesis. We then use the δ¹⁸O and δ¹³C values of micritic pedogenic and groundwater carbonates to reconstruct the paleoclimate and paleoenvironment in south Texas during the last ~40 Myr. The pedogenic carbonate nodules and laminar horizons are micritic, but contain abundant vugs and veins filled with calcite spar. The groundwater carbonate cement is composed of microsparry calcite. Preliminary results show that the average bulk carbonate δ¹⁸O value is -4.5 ± 0.3 ‰, and stable through time. The value is similar to published δ¹⁸O values of Quaternary pedogenic carbonates in south Texas, and differs from the δ¹⁸O value of calcite spar by -1.1 ‰. The lack of isotopic variation between spar and bulk carbonate, and the constant isotope values through time imply that prevalent late diagenesis may have influenced oxygen isotope composition, or the late Cenozoic global cooling signal was buffered by other regional climate factors. The average bulk carbonate δ¹³C values increased from -8.6 ± 0.4 ‰ during the Oligocene to -4.9 ± 0.4 ‰ during the late Miocene. The δ¹³C values of calcite spars within the Miocene and Pliocene samples are 3.0-3.3 ‰ lower than the values of bulk carbonate, but the δ¹³C values of spar within the Oligocene samples are similar to the values of bulk carbonate. The data suggest calcite spar formation during late diagenesis, but the micritic carbonate recorded original paleoenvironment information. In which case, the observed increase in δ¹³C values may reflect global expansion of C₄ plants during the late Miocene. Future work will focus on high-resolution sampling of micritic carbonate for stable isotope composition analysis in order to fully evaluate diagenesis, and collecting stable isotopic compositions of micritic carbonate to build a paleoclimate record in the Texas coastal plain.