Neoproterozoic Chuar Group (̃800-742 Ma), Grand Canyon: a record of cyclic marine deposition during global cooling and supercontinent rifting

Abstract Chuar Group sediments were deposited in a marine cratonic basin synchronously with marine deposition in other basins of western North America and Australia. The top of the Chuar Group is 742 Ma; thus, this succession offers a potentially important record of global climatic and tectonic changes during the mid-Neoproterozoic — a critical time leading to possible global glaciation during supercontinent rifting. Chuar Group cycles and sequences may record glacioeustatic fluctuations during the climatic transition into the Sturtian Ice Age (∼750–700 Ma), as well as extension related to the dispersal of Rodinia. The mid-Neoproterozoic Chuar Group (1600 m thick) is predominantly composed of mudrock with subordinate meter-scale beds of dolomite and sandstone. Facies analysis leads to the interpretation of a wave- and tidal-influenced marine depositional system. Diagnostic marine features include: (1) marine fossils and high local pyrite content in the mudrock facies; (2) mudcracked mud-draped symmetric ripples and reverse flow indicators in the 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). Nearly all cycles have mudrock bases indicating subtidal water depths. Dolomite-capped cycles shallow to peritidal environments (peritidal cycles), some indicating subaerial exposure of varying degrees (exposure cycles). Sandstone-capped cycles shallow to peritidal environments with no evidence of prolonged exposure (peritidal cycles). Non-cyclic intervals represent the deepest water depths (∼10s of meters). Peritidal cycles and non-cyclic intervals dominate the lower and middle Chuar Group. In the upper Chuar Group, the percentage of exposure cycles increases, as does the thickness of individual cycles and non-cyclic intervals, and the degree of subaerial exposure in the cycle caps. Correlation of cycles across 10 km shows that many cycles are laterally continuous and some vary in thickness. Some thickness changes are coincident with local and regional extensional structures. These cycle trends reflect a resolvable mixture of tectonic and climatic controls. Cycles are interpreted to be high frequency and glacioeustatically-controlled based on comparison of cycle character and thickness to Phanerozoic examples, and lateral continuity of many cycles, respectively. Cycles in the lower and middle Chuar Group show characteristics suggestive of low-amplitude sea-level changes similar to meter-scale cycles from global greenhouse climates in the Phanerozoic. Upper Chuar Group cycles show characteristics suggestive of moderate-amplitude sea-level changes similar to Phanerozoic greenhouse–icehouse transition climates, indicating an increase in continental ice volume. Variability in cycle thickness and lateral pinching of some cycle caps is controlled by variable tectonic subsidence, evidenced by thickening trends coincident with active rift-related structures, and also by variability in sediment supply. 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 (150–775 m thick) are defined based on dolomite-poor to dolomite-rich stratigraphic intervals. Long-term changes in cycle cap-lithology may be driven by long-term changes in climate, similar to short-term glacioeustatic controls on meter-scale cycle cap lithology. Large-scale thickness variability in these sequences is due to local differential tectonic subsidence.