The Evolution of Grand Valley in the Upper Colorado River Basin: A Diluvial Hypothesis for the Geneses of Alpine Terraces
Abstract
Rivers are thought to form fluvial terraces. The geneses of terraces depend on climatic changes because of the variations of riverbed slopes and sediment fluxes. Particularly, generating erosional-strath terraces requires beveling a surface, depositing a laterally extant sedimentary layer, and abandoning, accompanied by vertical incisions. Amounting, much evidence now suggests that large flows of water and sediments are necessary for strath terrace geneses. Thus, one can ask: is the solely fluvial explanation sufficient for the geneses of terraces? I invoke catastrophic diluvial processes as an alternative explanation for the geneses of alpine terraces, drawing the evidence from those in the Grand Valley area of the upper Colorado River Basin.
For decades, researchers suggested that the terraces of the upper Colorado River are of fluvial origin. Unfortunately, their conclusions lacked age control and the link to climate changes, and analyses of the matrices of the terraces. This study offers such data and a new interpretation on the terraces. Five Colorado River terrace/floodplain matrices were examined, using electrical resistivity tomography (ERT) method, the bedrock Mancos Shale at depths (< 25 Ω-m) and the overlying terrace clasts were identified (>100–1000’s Ω-m). The terraces consist of aggregates of river gravels, cobbles, and even boulders, indicating that the formative discharges exceeded those of seasonal floods or river flows. New luminescence dates for the fill/depositional terraces in the tributaries concur that episodic jökulhlaups from Grand Mesa occurred between 85 and 58 ka, post-dating Bull Lake (MIS 6) and pre-dating Pinedale glaciations (MIS 2).
Arguably, contemporaneous to the fill terraces, the strath terraces formed under the repeated cycles of 1) rapid beveling of the substrate, 2) lateral deposition of gravels and boulders, and 3) rapid vertical incision and abandonment, resulting in the Holocene-modern floodplains. Hence, the suggestion that the terraces in the area formed by episodic glacial floods instead of fluvial processes.
My suggestion for terrace geneses through glacial outburst flooding is supported by similar data from other jökulhlaup-prone places in addition to the Rocky Mountains. Conceivably, with more investigations of alpine fluvial terraces elsewhere, a wide recognition of diluvial, rapid terrace formation will result.
Subject
geomorphologystrath terraces
fill terraces
Grand Valley
Grand Mesa
electrical resistivity tomography
optically stimulated luminescence
Citation
Jeon, Kyungho (2020). The Evolution of Grand Valley in the Upper Colorado River Basin: A Diluvial Hypothesis for the Geneses of Alpine Terraces. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /193060.