Reevaluation of the Crooked Ridge River—Early Pleistocene (ca. 2 Ma) age and origin of the White Mesa alluvium, northeastern Arizona

Abstract

Essential features of the previously named and described Miocene Crooked Ridge River in northeastern Arizona (USA) are reexamined using new geologic and geochronologic data. Previously it was proposed that Cenozoic alluvium at Crooked Ridge and southern White Mesa was pre-early Miocene, the product of a large, vigorous late Paleogene river draining the 35-23 Ma San Juan Mountains volcanic field of southwestern Colorado. The paleoriver probably breeched the Kaibab uplift and was considered important in the early evolution of the Colorado River and Grand Canyon. In this paper, we reexamine the character and age of these Cenozoic deposits. The alluvial record originally used to propose the hypothetical paleoriver is best exposed on White Mesa, providing the informal name White Mesa alluvium. The alluvium is 20-50 m thick and is in the bedrock-bound White Mesa paleovalley system, which comprises 5 tributary paleochannels. Gravel composition, detrital zircon data, and paleochannel orientation indicate that sediment originated mainly from local Cretaceous bedrock north, northeast, and south of White Mesa. Sedimentologic and fossil evidence imply alluviation in a low-energy suspended sediment fluvial system with abundant fine-grained overbank deposits, indicating a local channel system rather than a vigorous braided river with distant headwaters. The alluvium contains exotic gravel clasts of Proterozoic basement and rare Oligocene volcanic clasts as well as Oligocene-Miocene detrital sanidine related to multiple caldera eruptions of the San Juan Mountains and elsewhere. These exotic clasts and sanidine likely came from ancient rivers draining the San Juan Mountains. However, in this paper we show that the White Mesa alluvium is early Pleistocene (ca. 2 Ma) rather than pre-early Miocene. Combined 40 Ar/ 39 Ar dating of an interbedded tuff and detrital sanidine ages show that the basal White Mesa alluvium was deposited at 1.993 ± 0.002 Ma, consistent with a detrital sanidine maximum depositional age of 2.02 ± 0.02 Ma. Geomorphic relations show that the White Mesa alluvium is older than inset gravels that are interbedded with 1.2-0.8 Ma Bishop-Glass Mountain tuff. The new ca. 2 Ma age for the White Mesa alluvium refutes the hypothesis of a large regional Miocene(?) Crooked Ridge paleoriver that predated carving of the Grand Canyon. Instead, White Mesa paleodrainage was the northernmost extension of the ancestral Little Colorado River drainage basin. This finding is important for understanding Colorado River evolution because it provides a datum for quantifying rapid post-2 Ma regional denudation of the Grand Canyon region.

Crooked Ridge River (Crr) Redux

“…The detrital zircon data presented by Hereford et al (2016) are not clear. Apparently, the younger grains include 18 with a weighted mean average age of ~33 Ma; the 12 youngest grains in this range have a weighted mean average ~23 Ma, and the two youngest grains have imprecise ages of ~15 and ~19 Ma.…”

“…Not "rare", as stated by Hereford et al (2016) Hereford et al (2016) proposed that CRR is "…no older than ca. 25 Ma…" on the basis of detrital zircon ages from the CRR alluvium, and ca.…”

“…According to Hereford et al, 2016, the lithologic characteristics of the Blue Point alluvium "resemble" those of the alluvium on White Mesa, on which grounds the Blue Point alluvium is declared to be part of the White Mesa alluvium. Therefore, the age of the tuff interbedded with the Blue Point alluvium is taken to be a primary evidence for the age of the White Mesa (and CRR) alluvium, that is, ~2 Ma.…”

“…Later, Hereford et al (2016) contended that CRR was of only local significance because the exotic clasts were derived from recycling of pre-existing piedmont gravels; that its valley was not large; that alluvial deposits on White Mesa are the significant stratigraphic unit, so the name Alluvium of Crooked Ridge should be dropped in favor of Alluvium of White Mesa; and that the presently-exposed alluvium, and thus (according to them) the river as a whole, has an age of ~2 Ma.…”

“…Lucchitta et al (2013) concluded that ancestral Crooked Ridge river carried clasts of all lithologies exposed along its course from the San Juan Mountains to the Kaibito Plateau Figs. 1, 2 and 4) Hereford et al (2016) cited Cather et al (2008) for the basis of their idea that the exotic pebbles at Crooked Ridge were recycled from ancient gravel deposits on a regional piedmont described and depicted by Cather et al, 2008, Figure 14, p. 28.…”