PARSING GRAND CANYON’S GREAT UNCONFORMITY-- COMPOSITE EROSION SURFACE FROM AT LEAST THREE EPISODES BETWEEN 1,350 AND 508 MA

John Wesley Powell recognized two major unconformities, one below the Unkar Group and the other below the Cambrian Tonto Group. He interpreted these as surfaces that record erosion of ancient mountains to flat surfaces and establishment of new base levels before the next sediments were deposited. We constrain the timing and magnitude of erosion on these surfaces, plus a third erosion surface, that together make up a composite Great Unconformity. The latter includes: 1) a sub-Unkar Group Great Nonconformity (400-500 Ma missing) that records 25 km of basement exhumation between 1.35 Ga and deposition of the Bass Formation at 1.25 Ga; 2) a sub-Chuar Group Angular Unconformity (~325 Ma missing) that records tilting of the sub-Unkar nonconformity, formation of fault blocks of Shinumo Sandstone, and about 2 km of erosion in upthrown blocks between 1.10 Ga and 775 Ma; and 3) a Chuar-Cambrian Unconformity that is locally a disconformity and locally an angular unconformity with >200 Ma missing and 2 km of erosion to remove the Chuar Group in most places before the Cambrian. Detrital zircons in both Unkar and Chuar groups show unroofing of underlying successions to basement before the next was deposited. Clarence Dutton was the first to use the term Great Unconformity. We use the term Great Unconformity (1.3 Ga to 200 Ma missing) where flat-lying Paleozoic rocks overly tilted Precambrian rocks representing different amounts of basement exhumation. New dating of the Sixtymile Formation, highest unit beneath the Great Unconformity, reveals it to be a Cambrian unit that records 530-508 Ma uplift, tilting, and erosion. There is no evidence for Snowball Earth- related (717-630 Ma) glacially-driven deep basement exhumation in Grand Canyon. Instead, basement regolith below Cambrian strata is considered dominantly subaerial; it is thin (< 3 m thick), characterized by breccias, and exhibits generally extremely low (1-2 m) relief (although moderate local relief < 80 m occurs against Shinumo Sandstone monadnocks). The Grand Canyon region also lacks evidence that a long-lived exposed regolith gave rise to the first biomineralized fossils of Cambrian time. Instead, early regolith was generally removed with each basin succession and timing of the last erosion interval 530-508 Ma and the main Sauk II transgression post-dated the Cambrian explosion.