Fluvial sequence stratigraphy and paleoclimate of the Upper Triassic (Norian-Rhaetian) Chinle Strata, northern New Mexico.
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Cleveland, David M.
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Two age-equivalent Upper Triassic (Norian and Rhaetian) fluvial successions in New Mexico were evaluated from a sequence-stratigraphic and paleopedologic perspective. Meter-scale fining-upward fluvial aggradation cycles (FACs) comprise fluvial aggradational cycle sets (FACSETs) 4-15 m thick. FACSETs in turn stack into four fluvial sequences 26-48 m thick. Fluvial sequences correlate between the two locations and likely accumulated in response to pulses of source area uplift and/or basin subsidence. 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. Each location contains abundant paleosols of varying maturity with relatively consistent features throughout the strata. A typical paleosol profile has features similar to modern Aridisols, is about 1 meter thick, and has an AB–Bw–Bk–BC horizon succession. Depth-to-carbonate functions estimate that mean annual precipitation was between 200 and 450 +/-95 mm. Root traces in the paleosols are typically shorter than a meter and less than a centimeter wide. By comparing these paleosols to climate-sensitive characteristics of 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. Quantitative isotopic climate proxies are applied to the isotopic composition of pedogenic carbonates in order to construct two age-equivalent, relatively continuous pCO2 and temperature records that span the eight million years preceding the Triassic-Jurassic (T-J) boundary. The [delta]13C data reveal relatively low Late Norian pCO2 levels (<500 to 1,000 ppmV), increased Rhaetian levels (>1,500 ppmV), and at least two periods of extreme pCO2 levels (~3,000 ppmV) preceding the T-J boundary. [delta]18O data from the same time interval suggest that mean annual temperatures (MAT) increased by 7 - 9 [degrees] C in association with the peak increases of pCO2 levels. The T-J boundary is associated with widespread marine and terrestrial extinctions, and it is possible that climate was a significant driving mechanism of the Late Triassic biotic crisis.