Petrography and stable isotope geochemistry of Oligocene-Miocene continental carbonates in south Texas: Implications for paleoclimate and paleoenvironment near sea-level

摘要

Cenozoic sedimentary rocks in the southern Texas Gulf Coastal Plains contain abundant continental carbonates that are useful for reconstructing terrestrial paleoclimate and paleoenvironment in a region near sea-level. Our field observations and thin section characterizations of the Oligocene and Miocene continental carbonates in south Texas identified three types of pedogenic carbonates, including rhizoliths, carbonate nodules, and platy horizons, and two types of groundwater carbonates, including carbonate-cemented beds and carbonate concretions, with distinctive macromorphologic and micromorphologic features. Based on preservations of authigenic microfabrics and variations of carbon and oxygen isotopic compositions, we suggest these carbonates experienced minimal diagenesis, and their stable isotopic compositions reflect paleoclimate and paleoenvironment in south Texas. Our Oligocene and Miocene carbonate clumped isotope temperatures (T(Delta(47))) are 23-28 degrees C, slightly less than or comparable to the range of modern mean annual and mean warm season air temperature (21-27 degrees C) in the study area. These T(Delta(47)) values do not show any dependency on carbonate-type, or trends through time suggesting that groundwater carbonates were formed at shallow depths. These data could indicate that air temperature in south Texas was relatively stable since the early Oligocene. The reconstructed paleo-surface water delta O-18 values are similar to modem surface water which could indicate that meteoric water 6180 values also remained stable since the early Oligocene. Mean pedogenic carbonate delta C-13 values increased similar to -4.6%(0) during the late Miocene, most likely reflecting an expansion of C-4 grassland in south Texas. This study provides the first mid-and late Cenozoic continental records of paleoclimate and paleoecology in a low-latitude, near sea-level region. (C) 2018 Elsevier B.V. All rights reserved.