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45548-AC8
The Interrelationship of Sequence Stratigraphy, Paleoclimatology, and Terrestrial Ichnology in Triassic Paleosol-Bearing Alluvial Successions, Moenkopi and Chinle Formations, Southwestern United States
This study is testing the hypothesis that cyclic alluvial stratal accumulations, paleoclimatic changes, and terrestrial trace fossil occurrences are highly interrelated. Our specific goals are to: 1) reconstruct the hierarchy of stratal cyclicity and evaluate the allocyclic and autocyclic controls on deposition; 2) reconstruct the Late Triassic climate using paleopedological and ichnological approaches in order to document the history of climate change leading up to the Triassic-Jurassic extinction; 3) quantify climatic variables of pCO2 and mean annual temperature and assess possible correlations with 1 and 2 above; and 4) compare sequence-stratigraphic and paleoclimatic record estimates with those of previous studies in order to establish whether the documented changes are local, regional, or global.
Preliminary conclusions for goal 1:
Two age-equivalent Upper Triassic fluvial successions deposited on the continental interior show a three-tier hierarchy of depositional cyclicity. Meter-scale fining-upward fluvial aggradation cycles (FACs) comprise fluvial aggradational cycle sets (FACSETs) that are 4-15 m thick (avg. 8.4 m). FACSETs in turn stack into four fluvial sequences 26-48 m in thickness (avg. 41 m). Within these sequences, transgressive-systems-tract equivalents (TE) are characterized by channel sands and associated minor overbank deposits and relatively immature paleosols (i.e., high rates of deposition), whereas highstand- to falling-stage-systems-tract equivalents (HFE) are dominated by overbank muds and relatively well-developed paleosols (i.e., lower rates of deposition). These two fluvial successions, which are 200 km apart, contain age-equivalent fluvial sequences that record similar histories of deposition and pedogenesis: Sequence 1 contains only an incomplete HFE; Sequence 2 includes both the TE and HFE; Sequence 3 is an HFE; and Sequence 4 contains only a TE. Fluvial sequences likely accumulated in response to pulses of source area uplift and/or basin subsidence that induced changes in accommodation. Conversely, higher-frequency FACs and FACSETs that occur within sequences do not correlate between study areas and are likely the products of autocyclic processes, such as channel avulsion, floodplain aggradation, and channel migration. These results suggest that regionally significant tectonic episodes may be discernible in suspended-load fluvial deposits that accumulated over a broad area.
Preliminary conclusions for goal 2:
Two Upper Triassic (Late Norian through Rhaetian) stratigraphic intervals in New Mexico contain pedogenic features that reflect an arid to semiarid climate. There is little pedogenic variation throughout the strata at each location, and a typical paleosol profile is about 1 meter thick and has an AB–Bw–Bk–BC horizon succession. Bkm, Bss, Bssk, or Bssg horizons are present in some paleosols. Micromorphological features suggest dominantly well-drained sola (e.g., abundant carbonate nodules, illuviated clay) with minor periods of moist or saturated conditions (e.g., FeMn concretions, FeMn coatings and hypocoatings, sepic-plasmic fabrics). Trace fossils are abundant in these strata and are dominated by adhesive meniscate burrows and root traces. Crayfish burrows are present at one of the two localities, which suggest that there was a high water table in localized areas. No petrified wood is present in this interval, though it has been documented in many older portions of the Chinle. Depth-to-carbonate functions estimate that mean annual precipitation was between 200 and 450 +/-95 mm. Relative to location 1, location 2 has higher paleo-precipitation estimates and stronger and more abundant sepic-plasmic fabrics in thin sections. The presence of a gleyed paleosol, crayfish burrows, and slickensides at location 2 also suggests conditions wetter than at location 1. By comparing these paleosols to modern soils, this study demonstrates that the Late Triassic Western Interior during the Late Norian to Rhaetian was arid to semiarid and supported a desert shrub environment that had localized and periodic moist or saturated soil conditions.
Preliminary conclusions for goals 3 and 4:
This portion of the study applies quantitative isotopic climate proxies in order to construct two age-equivalent, relatively continuous pCO2 and temperature records that span the eight million years preceding the T-J extinction event. d13C data reveal relatively low Late Norian pCO2 levels (<500 - 1,500 ppmV), increased Rhaetian levels (>1,000 ppmV), and at least two periods of extreme pCO2 levels (>3,000 ppmV) preceding the T-J boundary. d18O data from the same time interval suggest that mean annual temperatures (MAT) increased by >10oC in association with the peak increases of pCO2 levels. These estimates are consistent with a recent GCM, which modeled the effects of increased pCO2 levels on Pangea (~200Ma) and predict severe climatic consequences including a global MAT increase of 6oC (>10oC in some regions). Based on ecosystem models for future climate-warming scenarios, this type of climatic change would likely cause a severe biological crisis. Although this data precedes the T-J boundary, many studies suggest that the mass-extinction event took place over a more prolonged period beginning in the Late Triassic. Thus, it is likely that climate was a causal mechanism for the T-J extinction.
