Paper No. 7
Presentation Time: 9:45 AM


HEREFORD, Richard, AMOROSO, Lee, BEARD, Sue and HOUSE, P. Kyle, U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff, AZ 86001,

Depositional evidence of precursors to the Colorado River system east of the Kaibab Plateau before regional canyon cutting at 6–5 Ma is scarce. Two such deposits are the ~16–6 Ma Bidahochi Formation and the alluvium of the Crooked Ridge paleoriver of Lucchitta et al. (2011). New geologic mapping shows that alluvium pre-dating canyon cutting crops out at two localities along a 60-km SW-descending slope from White Mesa to The Gap, 66-km NE of Grand Canyon.

White Mesa, with 1 km of inverted relief relative to the Colorado River, is a 19-km long remnant paleovalley eroded into Cretaceous (K) and Jurassic (J) sandstone and shale that contains 30–40 m of alluvium and a sanidine-bearing tuff undergoing age determination. Five paleochannels containing gravel lenses with abundant K sandstone clasts converge on the paleovalley—three from the NE, and one each from the N and S—increasing its preserved width to 5 km. A narrow, sinuous 26-km long ridge of J sandstone capped by < 1 m of sandy lag gravel connects the White Mesa and Crooked Ridge outcrops. Extending SW 11 km beyond the sinuous ridge, the Crooked Ridge paleochannel is 0.08–1.5 km wide and contains 30–33 m of alluvium.

Alluvium at both outcrops is lithologically similar, predominantly well-stratified gray clay interbedded with poorly sorted fine sand having distinctive pale-grayish shades of green, red, and yellow. Gravel beds are uncommon; clasts are > 50 percent K sandstone and 25–40 percent quartzite and chert. Deposition was likely in a relatively low energy fluvial system with abundant fine-grained overbank deposits.

Lucchitta et al. (2011), reviving interest in the relation of lag gravel on southern White Mesa to the origin of Grand Canyon, describe volcanic and quartzite clasts derived from the San Juan Mountains 320 km NE. However, the quartzite clasts show evidence of reshaping. Converging channels and the dearth of primary far-travelled detritus suggest that the source terrain was K strata N, NE, and S of White Mesa that was overlain to the NE by Paleogene deposits originating from and near the San Juan Mountain area. The White Mesa alluvium may correlate temporally with all or part of the Bidahochi Formation, based on inclusion of 20 Ma detrital zircon (Price et al., 2012), the mid-Miocene termination of 1.6 km of denudation, and the timing of widespread late Neogene canyon cutting.

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