LITHOSTRATIGRAPHIC AND CHEMOSTRATIGRAPHIC RELATIONSHIPS IN THE NEOPROTEROZOIC CHUAR GROUP, GRAND CANYON: IMPLICATIONS FOR CONTROLS ON C-ISOTOPE VARIABILITY OF NEOPROTEROZOIC OCEANS?

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 d¹³C_org and d¹³C_carb from organic-rich mudrocks and dolomites in the Chuar Group (~ -2 to +12‰ PDB, relative to d¹³C_carb) 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.