An official website of the United States government
Abstract Landscape evolution is driven by factors like tectonics and climate, and unraveling such factors can reveal the history recorded in landscape morphology. The northern U.S. Cordillera features many potential drivers, such as the Yellowstone plume, the extrusion of a large igneous province, and the drainage of large lakes. Among this complex geologic history, the drivers of transient incision in the Clearwater and Salmon watersheds of central Idaho are not well understood. To constrain the pattern of regional incision, we analyze the morphologies of 80 individual tributaries underlain by single lithologies. From north to south across our study area, knickpoint elevations increase from about 800 to 2,200 m, and incision depths increase from about 300 to 1,200 m. We use both numerical and analytical models to demonstrate that such a gradient could represent spatial variations in rock uplift. These findings suggest that transience is driven by a spatially variable increase in rock uplift that has disrupted a low‐relief paleolandscape, and the high steepness values of main drainages suggest that high rock‐uplift rates are still maintained to the present. Changes in rock uplift may be related to interactions between the Yellowstone plume and the lithosphere, although base level fall from the drainage of the Lake Idaho down the proto‐Snake River may be superimposed over these patterns in rock uplift. We show that careful, quantitative analyses of river profiles in geologically complex regions can differentiate between the influences of rock uplift and far‐field base level changes.
more »
« less
Late Miocene or older canyon incision in the northern U.S. Cordillera shown by erosion rates, incision models, and basalt flow ages
Deep canyons along the Salmon, Snake, and Clearwater rivers in central Idaho, USA suggest long-lasting transient incision, but the timing and drivers of this incision are not well understood. The perturbation of the Yellowstone hotspot, eruption of flood basalts, and drainage of Lake Idaho all occurred within or near to this region, but the relationship among these events and incision is unclear. Here, we utilized in situ 10Be cosmogenic radionuclide concentrations for 46 samples (17 new) of fluvial sediment across the region to quantify erosion rates, calibrate stream power models, and estimate incision timing. We estimate that transient incision along the Salmon River began prior to ca. 10 Ma. However, canyon age decreases to ca. 5 Ma or earlier farther to the north. For a group of tributaries underlain by basalt, we use the age of the basalt to estimate that local transient incision began between ca. 11.5 and 5 Ma. Based on these timing constraints, the canyons along the Salmon and Clearwater rivers predate the drainage of Lake Idaho. We argue that canyon incision was triggered by events related to the Yellowstone hotspot (e.g., basalt lava damming, subsidence of the Columbia Basin, reactivation of faults, and/or lower crustal flow). Furthermore, our models suggest basalt may be more erodible than the other rock types we study. We show that lithology has a significant influence on fluvial erosion and assumptions regarding river incision model parameters significantly influence results. Finally, this study highlights how geodynamic processes can exert a significant influence on landscape evolution.
more »
« less
- Award ID(s):
- 2103713
- PAR ID:
- 10579289
- Publisher / Repository:
- The Geological Society of America
- Date Published:
- Journal Name:
- GSA Bulletin
- ISSN:
- 0016-7606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
40Ar/39Ar detrital sanidine (DS) dating of river terraces provides new insights into the evolution and bedrock incision history of the San Juan River, a major tributary of the Colorado River, USA, at the million-year time scale. We dated terrace flights from the San Juan−Colorado River confluence to the San Juan Rocky Mountains. We report >5700 40Ar/ 39Ar dates on single DS grains from axial river facies within several meters above the straths of 30 individual terraces; these yielded ∼2.5% young (<2 Ma) grains that constrain maximum depositional ages (MDAs) and minimum incision rates. The most common young grains were from known caldera eruptions: 0.63 Ma grains derived from the Yellowstone Lava Creek B eruption, and 1.23 Ma and 1.62 Ma grains derived from two Jemez Mountains eruptions in New Mexico. Agreement of a DS-derived MDA age with a refined cosmogenic burial age from Bluff, Utah, indicates that the DS MDA closely approximates the true depositional age in some cases. In a given reach, terraces with ca. 0.6 Ma grains are commonly about half as high above the river as those with ca. 1.2 Ma grains, suggesting that the formation of the terrace flights likely tracks near-steady bedrock incision over the past 1.2 Ma. Longitudinal profile analysis of the San Juan River system shows variation in area-normalized along-stream gradients: a steeper (ksn = 150) reach near the confluence with the Colorado River, a shallower gradient (ksn = 70) in the central Colorado Plateau, and steeper (ksn = 150) channels in the upper Animas River basin. These reaches all show steady bedrock incision, but rates vary by >100 m/Ma, with 247 m/Ma at the San Juan−Colorado River confluence, 120−164 m/Ma across the core of the Colorado Plateau, and 263 m/Ma in the upper Animas River area of the San Juan Mountains. The combined dataset suggests that the San Juan River system is actively adjusting to base-level fall at the Colorado River confluence and to the uplift of the San Juan Mountains headwaters relative to the core of the Colorado Plateau. These fluvial adjustments are attributed to ongoing mantle-driven differential epeirogenic uplift that is shaping the San Juan River system as well as rivers and landscapes elsewhere in the western United States.more » « less
-
This study assesses the impact of fold-thrust belt driven deformation on the topographic evolution, bedrock exhumation and basin formation in the southeastern Peruvian Andes. We do this through a flexural and thermokinematically modelled balanced cross-section. In addition, published thermochronology samples from low-elevation (river canyons) and high-elevation (interfluves) and Cenozoic sedimentary basin datasets along the balanced cross-section were used to evaluate the age, location, and geometry of fault-driven uplift, as well as potential relationships to the timing of ∼2 km of canyon incision. The integrated structural, thermochronologic, and basin data were used to test the sensitivity of model results to various shortening rates and durations, a range of thermophysical parameters, and different magnitudes and timing of canyon incision. Results indicate that young apatite (U-Th)/He (AHe) canyon samples from ∼2 km in elevation or lower are consistent with river incision occurring between ∼8–2 Ma and are independent of the timing of ramp-driven uplift and accompanying erosion. In contrast, replicating the young AHe canyon samples located at >2.7 km elevation requires ongoing ramp-driven uplift. Replicating older interfluve cooling ages concurrent with young canyon ages necessitates slow shortening rates (0.25–0.6 mm/y) from ∼10 Ma to Present, potentially reflecting a decrease in upper plate compression during slab steepening. The best-fit model that reproduces basin ages and depositional contacts requires a background shortening rate of 3–4 mm/y with a marked decrease in rates to ≤0.5 mm/y at ∼10 Ma. Canyon incision occurred during this period of slow shortening, potentially enhanced by Pliocene climate change.more » « less
-
Abstract Bedrock erosion and canyon formation during extreme floods have dramatically altered landscapes on Earth and Mars. Grand Coulee was carved by outburst floods from Pleistocene glacial Lake Missoula and is the largest canyon in the Channeled Scabland, a megaflood‐scoured landscape in the northwestern USA. Quantifying paleo‐discharge is required to understand how landscapes evolve in response to extreme events, but there are few constraints on the magnitude of the floods that incised Grand Coulee; hence, we used hydraulic modeling and geologic evidence to quantify paleo‐flood discharges during different phases of canyon incision. When upper Grand Coulee was incising by headward waterfall retreat, the paleo‐discharge was 2.6 × 106 m3s−1, which produced shear stresses great enough to cause the waterfall to retreat via toppling of basalt columns. The largest possible flood through upper Grand Coulee, a Missoula flood which raised glacial Lake Columbia to a stage of 750 m, produced a modeled discharge of 7.6 × 106 m3s−1. The discharges associated with waterfall retreat and drainage of glacial Lake Columbia are >80% and ∼50% lower, respectively, than the 14–17 × 106 m3s−1discharge predicted by assuming the present‐day topography was inundated to the elevation of high‐water marks. Due to bedrock incision, high‐water marks may overestimate paleo‐flow depth in canyons carved by floods, hence bedrock erosion should be considered when estimating paleo‐discharge in flood‐carved canyons. Our results indicate that outburst floods with discharges and flow depths much lower than those required to inundate high‐water marks are capable of carving deep canyons.more » « less
-
Abstract Pleistocene outburst floods from the drainage of glacial Lake Missoula carved bedrock canyons into the Columbia Plateau in eastern Washington, USA, forming the Channeled Scabland. However, rates of bedrock incision by outburst floods are largely unconstrained, which hinders the ability to link flood hydrology with landscape evolution in the Channeled Scabland and other flood-carved landscapes. We used long profiles of hanging tributaries to reconstruct the pre-flood topography of the two largest Channeled Scabland canyons, upper Grand Coulee and Moses Coulee, and a smaller flood-eroded channel, Wilson Creek. The topographic reconstruction indicates floods eroded 67.8 km3, 14.5 km3, and 1.6 km3 of rock from upper Grand Coulee, Moses Coulee, and Wilson Creek, respectively, which corresponds to an average incision depth of 169 m, 56 m, and 10 m in each flood route. We simulated flood discharge over the reconstructed, pre-flood topography and found that high-water evidence was emplaced in each of these channels by flow discharges of 3.1 × 106 m3 s−1, 0.65–0.9 × 106 m3 s−1, and 0.65–0.9 × 106 m3 s−1, respectively. These discharges are a fraction of those predicted under the assumption that post-flood topography was filled to high-water marks for Grand and Moses Coulees. However, both methods yield similar results for Wilson Creek, where there was less erosion. Sediment transport rates based on these discharges imply that the largest canyons could have formed in only about six or fewer floods, based on the time required to transport the eroded rock from each canyon, with associated rates of knickpoint propagation on the order of several km per day. Overall, our results indicate that a small number of outburst floods, with discharges much lower than commonly assumed, can cause extensive erosion and canyon formation in fractured bedrock.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/b36524.1
