The study area is within the Tornillo Basin in Big Bend National Park in west Texas. This study examines 43 paleosols spanning the late Cretaceous and early Tertiary with the specific goal of characterizing mineral assemblage and abundance in B-horizons of paleosols associated with two previously identified periods of elevated temperature and atmospheric CO2 (green house events). Mineral assemblage and abundance was determined using X-ray diffraction (XRD).
The minerals that were identified include quartz, smectite, calcite, feldspar, and kaolinite. Calcite is absent (exclusive of pedogenic nodules) in the oldest paleosols from the Aguja Fm., has relatively high abundance in the younger Javelina Fm., and is present in smaller amounts in the youngest Black Peaks Fm. Finely disseminated calcite in these paleosols is apparently detrital and its presence is largely controlled by parent material provenance.
Smectite abundance is highest in paleosols that formed during the Middle Maastrichtian green house event. Additionally, these paleosols are distinctive because of their lack of feldspar. This time period of elevated temperature and atmospheric CO2 is therefore clearly identifiable by the mineral assemblage of its paleosols which are the result of enhanced chemical weathering. In contrast, paleosols of the Late Maastrichtian green house event do not show evidence of increased smectite production. This is most likely due to the abundance of detrital calcite in these paleosols that would have inhibited the hydrolysis of feldspar.