LITHOSTRATIGRAPHIC AND CHEMOSTRATIGRAPHIC RELATIONSHIPS IN THE NEOPROTEROZOIC CHUAR GROUP, GRAND CANYON: IMPLICATIONS FOR CONTROLS ON C-ISOTOPE VARIABILITY OF NEOPROTEROZOIC OCEANS?
DEHLER, Carol M., Department of Geology, Utah State Univ, 4505 Old Main Hill, Logan, UT 84322-4505, chuaria@cc.usu.edu, ELRICK, Maya, Earth & Planetary Sciences, Univ of New Mexico, Northrop Hall, Albuquerque, NM 87131, CROSSEY, Laura J., Dept. of Earth & Planetary Sciences, Univ. of New Mexico, Northrop Hall, Albuquerque, NM 87131, and KARLSTROM, Karl E., Dept. of Earth and Planetary Sciences, Univ. of New Mexico, Albuquerque, NM 87131

Four lithostratigraphic sequences (150-775 m thick) within the Neoproterozoic Chuar Group are defined by dolomite-poor to dolomite-rich stratigraphic intervals. The dolomite-poor intervals (150-450 m thick) consist of meter-scale, sandstone-capped peritidal cycles with deeper-water mudrock bases, and non-cyclic intervals. The overlying dolomite-rich intervals (20-325 m thick) consist of meter-scale, dolomite-capped (and lesser sandstone-capped) peritidal and exposure cycles with deeper-water mudrock bases, and non-cyclic intervals. Meter-scale cycles are interpreted to be high-frequency and glacioeustatically-controlled by comparison of cycle character (shallowing-upward) and thickness (1-20 m thick) to Phanerozoic examples, and lateral continuity of cycles. The presence of dolomitic meter-scale cycle caps, in this otherwise siliciclastic succession, is due to rapid changes in climate related to short-term, glacioeustatic sea-level changes (in low latitudes, interglacial modes are wetter, glacial modes are drier). The lithostratigraphic sequences may reflect longer-term climate changes similar to the short-term climate changes that controlled meter-scale cycle-cap lithology, i.e., the dolomite-poor intervals represent wet climate/interglacial modes and higher sea level, and the dolomite-rich intervals represent dry climate/glacial modes and lower sea level. Variability in d13Corg and d13Ccarb from organic-rich mudrocks and dolomites in the Chuar Group (~ -2 to +12‰ PDB, relative to d13Ccarb) shows four “negative” excursions. The number and magnitude of excursions are similar to those from other mid-Neoproterozoic successions in the region and globally, and therefore, the Chuar C-curve may reflect regional and (or) global d13C trends. Chuar d13C shifts correlate with lithostratigraphic sequences: positive d13C intervals coincide with dolomite-poor intervals and negative d13C shifts coincide with dolomite-rich intervals. This relationship can be explained by changes in sedimentation rates, driven by long-term climate change and accommodated by longer-term intracratonic rifting in low latitudes, which modulated the burial of organic carbon and thus affected d13C variability. This new model may be a proxy for global d13C variability during the mid-Neoproterozoic.

Rocky Mountain - 54th Annual Meeting (May 7–9, 2002)
Session No. 11
P3: Proterozoic Paleogeography and Paleoclimate
Sharwan Smith Center: Theater
8:00 AM-12:00 PM, Wednesday, May 8, 2002
 

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