Abstract

Red-bed strata of the Nankoweap Formation unconformably overlie the ∼l,100-m.y.-old Cardenas Lavas of the Unkar Group in the eastern Grand Canyon. An unconformity and an apparent disconformity are present. At most places the upper member of the Nankoweap overlies the Cardenas, and locally an angular discordance can be recognized that reflects the truncation of 60 m of Cardenas. This unconformity also underlies a newly recognized ferruginous sandstone of probable local extent that underlies the upper member and that herein is called the ferruginous member of the Nankoweap. Truncation of the Cardenas beneath the ferruginous and upper members locally may have been as much as 300 m. Basal conglomeratic sandstone of the upper member locally overlies the ferruginous member with apparent disconformity, reflecting a probable hiatus in deposition.

Stratigraphic and structural relationships indicate that a ferruginous weathered zone was developed on an erosionally truncated section of the Cardenas before, and perhaps during, the time of deposition of the ferruginous member. Erosion of the ferruginous weathered zone provided material for the ferruginous member of the Nankoweap. The ferruginous weathered zone locally was faulted against unweathered Cardenas before deposition of the upper member of the Nankoweap.

The Nankoweap Formation is disconformably overlain by marine strata of the Chuar Group. Three distinct units separated by unconformities (the Unkar Group, Nankoweap Formation, and Chuar Group) thus are recognized in the Grand Canyon Series of Walcott. Following current Stratigraphic practice, the Grand Canyon Series is herein redesignated the Grand Canyon Supergroup.

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