Facies analysis, cyclostratigraphy and carbon-isotope stratigraphy of the neoproterozoic Chuar Group, Grand Canyon, Arizona

Facies analysis of Chuar Group strata indicates a wave- and tidal-influenced marine depositional system. Diagnostic marine features include: (1) marine fossils and high local pyrite content (mudrock and dolomite facies); (2) mudcracked mud-draped symmetric ripples and reverse flow indicators (sandstone facies); (3) facies associations between all facies; and (4) no unequivocal terrestrial deposits. Chuar facies stack into ∼320 dolomite- and sandstone-capped meter-scale cycles (1–20 m thick) and non-cyclic intervals of uniform mudrock facies (20–150 m thick). Correlation of cycles across 10 kms shows that many cycles are laterally continuous and some vary in thickness. Some thickness changes are coincident with local and regional extensional structures.; Cycles are interpreted to be high-frequency and glacioeustatically-controlled by comparison of cycle character and thickness to Phanerozoic examples and lateral continuity of cycles, respectively, and indicate an overall increase in continental ice volume. The presence of dolomite caps, in this otherwise siliciclastic succession, is interpreted to be due largely to rapid changes in climate related to short-term glacioeustatically-controlled sea-level changes. Four crude lithostratigraphic sequences (150 to 775 m thick) are defined based on dolomite-poor to dolomite-rich stratigraphic intervals, and may have been driven by longer-term climate changes similar to those that controlled changes in meter-scale cycle cap lithology.; C-isotope variability in δ13C_org and δ 13C_carb from organic-rich mudrocks and dolomites in the Chuar Group reflects a mixture of global and local controls. H/C ratios of >0.49 indicate that Chuar mudrocks have not been thermally altered and intrabasinal correlation of δ13C_org values indicates that most Chuar mudrocks record near-primary values. Although some Chuar dolomites reveal early diagenetic alteration, their δ13C_dol values typically fall near those of coeval “least-altered” dolomites or organic-rich mudrocks (relative to dolomite values).; The number and magnitude of excursions in the Chuar Group are similar to those from mid-Neoproterozoic regional and global C-curves. Dolomite-poor/dolomite-rich lithostratigraphic sequences, interpreted as wet-to-dry long-term climate modes linked to continental glaciations, are in phase with many of the large-magnitude Chuar C-isotope shifts. These relationships suggest that changes in sedimentation rates, driven by long-term climate change, could have controlled large-scale C-isotope variability in the Chuar Group, and possibly, globally.