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Laura J. Crossey

Publications and source records attributed to Laura J. Crossey.

3 recordsLinked to original sources
USGS2018

Carving Grand Canyon’s inner gorge: A test of steady incision versus rapid knickzone migration

A recent study posits that much of the 240-m-deep inner gorge of Grand Canyon was carved between 500 and 400 ka via passage of a migrating knickzone with incision rates of ~1600 m/Ma during that time period; this was based on dating of a ca. 500 ka travertine deposit perched on the rim of the inner gorge, near Hermit Rapid, and a ca. 400 ka travertine drape that extends to within 60 m of river level nearby. However, a new U/Th age of 517 ± 13 ka on the same travertine drape challenges this model of a migrating knickzone and punctuated incision. The presence of ca. 500 ka travertine just 95 m above river level requires that most of the inner gorge was carved before that time. The resulting maximum bedrock incision rate of 230 m/Ma is consistent with independent results from sites up and downstream and with models for semi-steady Quaternary bedrock incision and dispels problems with the transient incision model. Downstream from the Hermit Rapid area, dikes present on both sides of the canyon have been used to support the migrating knickzone model. We report a new 40 Ar/ 39 Ar age of 517 ± 16 ka on one of these dikes, but argue that they don’t necessarily gauge incision. Field observations suggest that the discontinuous travertine deposits, near Hermit Rapid, were deposited by springs that emanated from the Redwall-Muav aquifer, mantled the Tonto Platform, and locally built downwards into the inner gorge and tributary canyons. The range of U/Th ages from ca. 10–600 ka suggests these were long-lived spring systems. The travertine cements predominantly angular to subrounded locally derived clasts consistent with deposition on hillslopes and by tributaries. Well-rounded gravels are exceedingly rare but have been used to suggest that the Colorado River was at the rim of the inner gorge at ca. 500 ka. No exotic Colorado River clasts, derived from the area outside of Grand Canyon, were observed by us. In-place gravel from the main stem or tributaries (e.g., from paleo–Hermit Creek) within the travertine deposits can be reconciled with existing data, if: (1) travertine was deposited at ca. 2 Ma, which is approximately when the steady incision model suggests the inner gorge began to incise; (2) a 500 ka lava dam in the Lava Falls Rapid area, 140 km downstream, backed water and sediment up to the rim of the inner gorge in the Hermit area; or (3) regional climate-driven aggradation took place at 500 ka.

Grand Canyon↗
USGS2016

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

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.

Arizona↗