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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.
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This content will become publicly available on September 5, 2026
Ages of boulder armored benches document varying Fremont River tributary incision rates, Teasdale-Torrey lowlands, Utah, USA
Although rates of fluvial incision across the Colorado Plateau are known reasonably well, rate variability through time and its controlling processes are still poorly understood. We used boulder armored benches from the Teasdale-Torrey lowlands reach of the Fremont River in the northwestern Colorado Plateau (Utah, USA) as temporal markers to determine regional incision rates and explore controls on rate variability. Bench gravels are sourced from Tertiary volcanic rocks capping nearby Boulder and Thousand Lakes Mountains. The sedimentology of bench deposits suggests that most form from mass movement with later fluvial reworking. Volcanic boulders are tougher than the local sedimentary bedrock, which promotes boulder armoring and topographic inversion. Thirty-seven boulder cosmogenic 3He exposure ages from 11 different benches range from >600 ka to ca. 100 ka. Soil carbonate stages from two benches are in good agreement with surface exposure ages. Averaged Fremont River and tributary incision rates determined from bench exposure ages are 32% faster for tributaries off of Thousand Lakes Mountain (0.41 m/k.y.) than tributaries off of Boulder Mountain (0.28 m/k.y.). This difference in incision rate may be due to Laramideage structures limiting incision for the tributaries that drain Boulder Mountain and extensive Pleistocene ice caps on Boulder Mountain creating a wider and thicker boulder armor slowing incision.
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- Award ID(s):
- 2223354
- PAR ID:
- 10644079
- Publisher / Repository:
- Geosphere
- Date Published:
- Journal Name:
- Geosphere
- Volume:
- 21
- Issue:
- 5
- ISSN:
- 1553-040X
- Page Range / eLocation ID:
- 941 to 961
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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