Establishing chronologies for alluvial-fan sequences with analysis of high-resolution topographic data: San Luis Valley, Colorado, USA

Abstract

On active alluvial fans, debris-flow deposits and frequent avulsions produce a rough topographic surface. As is the case in many initially rough landforms produced by catastrophic processes, the topography of alluvial fans is progressively smoothed, producing textural differences useful in establishing relative age criteria for fans. Here, we outline an approach for defining a quantitative, numerical chronology for the surfaces of alluvial fans from topographic analysis, although the method is generalizable to any arbitrary landform. Our chronology relies on predictions for the evolution of topography by purely diffusive modification. Specifically, by comparing the surface roughness of active and abandoned alluvial-fan surfaces measured from spectral transformations of topography, we can directly estimate a fan's "morphologic age," which is the product of the duration and efficiency of diffusive modification by surface processes. We tested the method on a suite of alluvial fans in the San Luis Valley, Colorado, USA, and evaluated the results against field observations and available geochronologic data. Estimated morphologic ages obey stratigraphic constraints and imply reasonable efficiencies of sediment transport. We highlight the fact that the oldest fan surfaces observed here, constrained to be older than 100 ka by U-series dating of pedogenic carbonates, have morphologic ages near the method's saturation point. In addition, many fans have morphologies that are not entirely consistent with a purely diffusive modification from the initial fan morphology recorded on active fan surfaces, likely as a result of postdepositional modification by sediment transport driven by wind and overland flow. However, we remain optimistic that morphologic dating can provide useful insights into the history of alluvial-fan activity, in particular, because our method provides a means for both computing a morphologic age and assessing the validity of the assumptions required for that computation from analysis of topography alone.

Clumped Isotopes Record a Glacial‐Interglacial Shift in Seasonality of Soil Carbonate Accumulation in the San Luis Valley, Southern Rocky Mountains, USA

“…Ages from seven separate clasts from the ZP site, previously reported by Johnstone et al. (2018), yielded similar ages ranging from 19.8 ± 0.1 to 26.2 ± 1.2 ka. Clustering of dates suggests that carbonate at this site formed during a period of ∼6 kyrs.…”

“…Dense, crystalline carbonate was also targeted specifically to minimize the incorporation of dust-size detrital carbonate and silicate material in the authigenic soil carbonate, in order to reduce their contribution to the measured carbon, oxygen, and U-series isotope ratios used for paleoenvironmental reconstruction and geochronology. Previous studies showed that soil carbonates sampled this way in the SLV are relatively low in detritus component, evidenced by successful U-series dating with low detrital 232 Th for many of the exact samples used in this study (Johnstone et al, 2018;Ruleman et al, 2019). Specific depths for each sample zone were not carefully recorded.…”

“…All U‐Th series data reported here were produced by us on the same samples used for this study, but come from previously published sources (Johnstone et al., 2018; Ruleman et al., 2019). U‐Th series dating methods followed those outlined in previous publications.…”

Climate on the Blanca Massif, Sangre de Cristo Mountains, Colorado, USA, during the Last Glacial Maximum

“…Features of alpine glacial erosion and deposition are abundant in many valleys. Valley mouths have subsequently been deeply incised by streams in response to ongoing tectonism and are fronted by well-developed alluvial fans with surfaces indicating multiple stages of development [25,26].…”

Towards Better Visualisation of Alpine Quaternary Landform Features on High-Resolution Digital Elevation Models

“…In recent decades, digital elevation models (DEMs) derived from airborne laser scanning (ALS) have been widely used for studies of Earth surface processes [14,15], and references therein]. DEMs proved to be an invaluable tool in geomorphological and natural hazards mapping, identifying geomorphological processes, detecting small-scale features, monitoring surface changes, reconstructing landscape evolution, re-evaluating the existing maps, and even determining surface age [16][17][18][19][20][21][22][23][24][25].…”