Paper No. 3
Presentation Time: 8:35 AM


NORDT, Lee1, ATCHLEY, Stacy1, DWORKIN, S.I.2, RAMEZANI, Jahandar3 and BOWRING, S.4, (1)Department of Geology, Baylor University, PO #97354, Waco, TX 76798, (2)Department of Geology, Baylor University, PO Box 97354, Waco, TX 70798-7354, (3)Dept of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, (4)Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Building 54-1126, Cambridge, MA 02139,

Interpreting evolving late Triassic continental weathering, climates, and plant formations during initial break-up of southwest tropical Pangea has been hampered by a lack of high resolution environmental records. Based on zircon dating, Petrified Forest National Park (PEFO), Arizona contains an age model for the Chinle Formation ranging from ca. 230 to 207 Ma, providing an excellent opportunity for paleoenvironmental reconstruction from paleosols.

Using standard field and laboratory techniques, we identified, described, sampled and characterized 85 paleosols formed in alluvial mudstones in the study area. Even though there was burial compaction, post burial gley of root traces, and organic matter oxidation, chemical diagenesis was minimized because of shallow burial depths and temperatures.

Prior to 216 Ma the principal soil forming processes were pedoturbation (Bss horizons), gleization (Bg horizons), and pedoformation (Bw horizons) with calcification (Bk horizons) dominating after 216 Ma. Employing pedotransfer functions from molecular oxides, cation exchange capacity, COLE, and fine clay abundance decrease up section, whereas pH, base saturation, and bulk density increase up section. XRD patterns show a dominance of smectite in the clay fraction throughout the stratigraphic interval, with kaolinite in slightly greater abundance before 219 Ma. These geochemical trends are consistent with weathering indexes estimating mean annual rainfall between 1100 and 1300 mm before 216 Ma and between 200 to 800 mm thereafter.

Rooting patterns and paleobotanical evidence suggests that the slightly acidic soils before 216 Ma supported shrublands in unstable floodplains (Entisols) and forests in more stable floodbasins (Inceptisols and Vertisols). Intercalated with aggrading landscapes (Entisols), forests transitioned to woodlands adapted to calcareous soils (Inceptisols and Vertisols) from 216 and 209 Ma. By 209 Ma desert shrublands in alkaline soils (Aridisols) appeared, and in association with alkaline lakes. Nutrient availability and available water capacity was greatest in pre-216 Ma soils because of near neutral soil pH and low pre-burial bulk densities.