Scholarly Work - Earth Sciences
Permanent URI for this collectionhttps://scholarworks.montana.edu/handle/1/8747
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Item Morphology, timing, and drivers of post-glacial landslides in the northern Yellowstone region(Wiley, 2024) Dixon, Jean L.; Nicholas, Grace E.; Pierce, Kenneth L.; Lageson, DavidThe withdrawal of glaciers in mountainous systems exposes over-steepened slopes previously sculpted by ice. This debuttressing can directly trigger mass movements or leave slopes susceptible to them by other drivers, including seismogenic shaking and changing climate conditions. These systems may pose hazards long after deglaciation. Here, we investigate the drivers of slope failure for landslides at the northern entrance to Yellowstone National Park, a critical conduit traversed by ~1 million visitors each year. Through field mapping and analyses of LiDAR data, we quantify the spatial and temporal relationships between eight adjacent slides. Stratigraphic relationships and surface roughness analyses suggest initial emplacement 13–11.5 ka, after a significant delay from Deckard Flats glacial retreat (15.1 ± 1.2 ka). Thus, rapid glacial debuttressing was not the direct trigger of slope failure, though the resultant change in stress regime likely had a preparatory influence. We posit that the timing of failure was associated with (1) a period of enhanced moisture and seismicity in the late Pleistocene and (2) altered stress regimes associated with ice retreat. Historical archives and cross-cutting relationships indicate portions of some ancient slides were reactivated; these areas are morphologically distinguishable from other slide surfaces, with mean topographic roughness 2 times that of non-active slides. Stream power analysis and archival records indicate Holocene incision of the Gardner River and human disturbances are largely responsible for modern reactivations. Our findings highlight the importance of combining archival records with stratigraphic, field and remote sensing approaches to understanding landslide timing, risk, and drivers in post-glacial environments. This study also provides a valuable baseline for geomorphic change in the Yellowstone system, where a 2022 flood incised streams, damaged infrastructure and further reactivated landslide slopes.Item Capturing the complexity of soil evolution: Heterogeneities in rock cover and chemical weathering in Montana's Rocky Mountains(Elsevier BV, 2022-05) Benjaram, Sarah S.; Dixon, Jean L.; Wilcox, Andrew C.We investigate the relationship between chemical weathering, persistence of soil cover, and topography in two neighboring mountain ranges in the northern Rockies of western Montana, USA. We augment existing tools for measuring chemical weathering with adjustments for both local and landscape-scale contributions from unweathered rock fragments, boulders, and bedrock exposure. Adjusted weathering intensities recognize that quantifying weathering in mountainous systems should account for rock exposure, rather than focusing solely on fine-grained soil mantles. Our study systems' distinct morphologies are shaped by their unique climate histories. The previously glaciated Bitterroot Mountains consist of steep hillslopes with abundant rock cover, while the neighboring unglaciated Sapphire Mountains display convex, soil-mantled hillslopes. Over 380 soil thickness measurements, 118 analyses of soil and rock geochemistry, and digital terrain analysis reveal that patchy soils in the bedrock-rich system are roughly half as thick as those in the continuously-soil-mantled landscape, and ~45% less weathered, despite wetter conditions that would be expected to enhance weathering. These disparities increase when accounting for coarse rock fragments in soils and bedrock cover across the study catchments. The near continuously soil-mantled Sapphire system experiences ~1.5 times greater weathering intensity at a catchment scale compared to the bedrock-rich Bitterroot system. Rock exposure across the mountainous study system increases with increasing slope gradient. However, we find no clear threshold at which soils decrease in abundance or weathering intensity, and soils are surprisingly resilient even at the steepest hillslopes (comprising ~60% of the landscape area at slopes >30°). Our new data quantify soil abundance and chemical weathering intensity at both local and landscape scales. This work highlights how measurements of soil and rock cover need to be incorporated into studies quantifying chemical weathering, as traditional approaches may significantly overestimate and mischaracterize weathering regimes in mountain environments.