Abstract
Abstract The Great Unconformity is an iconic geologic feature that coincides with an enigmatic period of Earth's history that spans the assembly and breakup of the supercontinent Rodinia and the Snowball Earth glaciations. We use zircon (U-Th)/He thermochronology (ZHe) to explore the erosion history below the Great Unconformity at its classic Grand Canyon locality in Arizona, United States. ZHe dates are as old as 809 ± 25 Ma with data patterns that differ across both long (∼100 km) and short (tens of kilometers) spatial wavelengths. The spatially variable thermal histories implied by these data are best explained by Proterozoic syn-depositional normal faulting that induced differences in exhumation and burial across the region. The data, geologic relationships, and thermal history models suggest Neoproterozoic rock exhumation and the presence of a basement paleo high at the present-day Lower Granite Gorge synchronous with Grand Canyon Supergroup deposition at the present-day Upper Granite Gorge. The paleo high created a topographic barrier that may have limited deposition to restricted marine or nonmarine conditions. This paleotopographic evolution reflects protracted, multiphase tectonic activity during Rodinia assembly and breakup that induced multiple events that formed unconformities over hundreds of millions of years, all with claim to the title of a “Great Unconformity.”
Highlights
The Great Unconformity is exposed along the length of the Grand Canyon in northwestern Arizona, United States (Fig. 1) and separates the Cambrian Tonto Group from the underlying Paleoproterozoic basement or MesoproterozoicNeoproterozoic Grand Canyon Supergroup
Evidence of erosion during all of these periods is preserved in the Grand Canyon Supergroup of the Upper Granite Gorge (UGG; Fig. 1C); in unconformities within the Unkar Group (>800 Ma), disconformities between the Cardenas Basalt, Nankoweap Formation and the Chuar Group, and the unconformity separating the Chuar Group and Sixtymile Formation/Tapeats
We suggest that variable Neo- tion histories across major faults
Summary
The Great Unconformity is exposed along the length of the Grand Canyon in northwestern Arizona, United States (Fig. 1) and separates the Cambrian Tonto Group from the underlying Paleoproterozoic basement or MesoproterozoicNeoproterozoic Grand Canyon Supergroup It represents as much as 1.2 b.y. of missing time (Timmons and Karlstrom, 2012). The Lower Granite Gorge (LGG) does not preserve the Grand Canyon Supergroup, which makes it unclear whether the LGG and UGG share a common Neoproterozoic history Together, these geologic relationships suggest a multiphase and possibly spatially variable history of Great Unconformity development. We acquired ZHe data for four samples each from the LGG and UGG (Tables S1 and S2 in the Supplemental Material) Seven of these samples are Precambrian granitoid basement collected near river level, and one is the 729 ± 0.9 Ma Walcott Member Tuff near the top of the Chuar Group (Fig. 1D). intrasample variation in its eU zonation pattern (Fig. S2E), which may help explain its substantial ZHe date-eU scatter
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