STRATIGRAPHIC REVISION, PROVENANCE, AND NEW AGE CONSTRAINTS OF THE NANKOWEAP FORMATION AND CHUAR GROUP, GRAND CANYON SUPERGROUP, GRAND CANYON ARIZONA

The Nankoweap Formation (~100 m thick) sits unconformably above the 1.2-1.1 Ga Unkar Group (2.1 km thick) and is overlain by the ca. 770-742 Ma Chuar Group (1.6 km thick) and overlying Sixtymile Fm (~60 m thick) of the Grand Canyon Supergroup, Arizona. It has long been considered an unconformity-bound, tectonostratigraphically separate unit between the Unkar and Chuar groups, with a depositional age of about 900 Ma inferred from paleomagnetic data, or <942 Ma based on a prior detrital zircon analysis .

New detrital zircon U-Pb analyses on two Nankoweap samples show small but significant age populations (total n=6) of ~800-770 Ma grains, placing the Nankoweap Formation in the same tectonostratigraphic unit as the Chuar Group, and further constraining the age of the base of the Chuar Group to <800 Ma. A stratigraphic revision is suggested here to include the Nankoweap Fm. as the basal unit in the Chuar Group, expanding the Chuar Group in thickness and lessening the duration represented by the group. Previously reported intraformational faults in the Nankoweap Formation therefore record syn-extension related to movement on the Neoproterozoic-age Butte fault and nascent rifting of Rodinia, consistent with structural features observed in the Chuar Group. The duration of the slight angular unconformity at the base of Nankoweap Formation is now constrained to be about 300 Ma (1100-800 Ma).

Other detrital zircon populations from the Nankoweap Formation include dominant age peaks at 1070, 1100, 1170, 1340, and 1450 Ga, with small populations in the 1600-2000 Ga range and a few Archean grains. The middle Chuar Group sandstone samples show an alternation/mixing between a provenance signature similar to that seen below in the Nankoweap Fm and a signature with a dominating 1750 Ma age peak, with a few 800-770 Ma grains. Sixtymile Fm. sandstone samples have two dominant age peaks, one at 1700-1750 Ga and one at 1450 Ga. The overall provenance change from a mixed Laurentian signature to a basement-dominated signature likely reflects an unroofing sequence whereby Unkar sediments were reworked into the Chuar basin, concomitant with and followed by dominant basement reworking.