Re-evaluation of exotic gravel and inverted topography at Crooked Ridge, northern Arizona: Relicts of an ancient river of regional extent

摘要

An ancient drainage, named Crooked Ridge river, is unique on the Colorado Plateau in extent, physiography, and preservation of its alluvium. This river is important for deciphering the generally obscure evolution of rivers in this region. The ancient course of the river is well preserved in inverted relief and in a large valley for a distance of several tens of kilometers on the Kaibito Plateau–White Mesa areas of northern Arizona. The prominent landform ends ～45 km downstream from White Mesa at a remarkable wind gap carved in the Echo Cliffs. The Crooked Ridge river alluvium contains clasts of all lithologies exposed upstream from the Kaibito Plateau to the San Juan Mountains in Colorado, so we agree with earlier workers that Crooked Ridge river was a regional river that originated in these mountains. The age of Crooked Ridge river cannot be determined in a satisfactory manner. The alluvium now present in the channel is the last deposit of the river before it died, but it says nothing about when it was born and lived. Previous research attempted to date this alluvium, mostly indirectly by applying a sanidine age obtained ～50 km away, and directly from six sanidine grains (but no zircon grains), and concluded that Crooked Ridge river was a small river of local significance, because the exotic clasts were interpreted to have been derived from recycling of nearby preexisting piedmont gravels; that its valley was not large; and that it only existed ca. 2 Ma. Our proposition in 2013 was that Crooked Ridge river came into being in Miocene and possibly Oligocene time, which is when the very high San Juan Mountains were formed, thus giving rise to abundant new precipitation and runoff. To address some of this ambiguity, we examined all available evidence, which led us to conclude that several of the interpretations by previous researchers are not tenable. We found no evidence for a preexisting piedmont from which the Crooked Ridge river exotic clasts could be recycled. Furthermore, the principal advocate of the piedmont discounted it in a later publication. Tributaries to Crooked Ridge river in the White Mesa area contain no exotic clasts that could have been derived from a local clast-rich piedmont; only the Crooked Ridge river channel contains exotic clasts. So, we conclude that Crooked Ridge river was the principal stream, that it was of regional significance, that it was headed in the San Juan Mountains, and that it existed long before it died, perhaps as early as Oligocene time, until it was captured by the San Juan River, maybe ca. 2 Ma. West and downstream from The Gap, no deposits or geomorphic features attributable to the Crooked Ridge river have been preserved, but we infer that the river joined the Colorado and Little Colorado paleorivers somewhere on the east side of the Kaibab Plateau, and then crossed the plateau along a paleovalley that approximated the present alignment of the eastern Grand Canyon. West of the Kaibab Plateau, the combined rivers perhaps flowed in a northwest-trending strike valley to an as-yet-unknown destination.