Seismic Reflection and Electrical Resistivity Imaging Support Pre‐Quaternary Glaciation in the Rocky Mountains (Unaweep Canyon, Colorado)

Abstract

Landscape shapes record processes of formation. In particular, fluvial processes typically exhibit V-shaped profiles with steep slopes, whereas glacial processes carve U-shaped valleys with variable slopes and overdeepenings (MacGregor et al., 2000;Prasicek et al., 2014). Overdeepened valleys are those with depths extending below fluvial base level (Preusser et al., 2010) and are key elements of glacial landscapes where warm-bed conditions exist(ed) (e.g., Benn & Evans, 1998;Cook & Swift, 2012). Such valleys are characterized by closed topographic depressions and are commonly eroded several hundred meters below the base level (Fiebig et al., 2010;Menzies, 1995;Preusser et al., 2010). Overdeepening on basement is unknown in canyons carved exclusively by fluvial processes (Cook & Swift, 2012;Huuse & Lykke-Andersen, 2000;Linton, 1963); rather, overdeepenings are considered "unambiguous features of glacially sculpted landscapes" (Pomper et al., 2017). Overdeepening occurs in cirques, valley outlets of alpine glaciers, and fjords and valleys draining continental ice sheets, related to perturbations in the bed that amplify the action of high-pressure meltwater (Hooke, 1991).Seismic imaging has been widely used to characterize Quaternary glacial valleys and, together with drilling and other subsurface data, has demonstrated the distinctive propensity for glacial processes to produce not Abstract Unaweep Canyon (Uncompahgre Plateau, Colorado) represents an enigmatic landscape with a complex evolution. Interpretations for its origin have ranged from ancestral fluvial erosion in the late Cenozoic to glacial erosion in the Paleozoic, or some combination thereof, with significant implications for global climatic and large-scale tectonic reconstructions. To address the conflicting interpretations, we acquired a high-resolution seismic reflection profile to investigate the depth, structure, and sedimentary infill in the canyon. The data set is further complemented with an electrical resistivity survey. Integrated with other geophysical and geological data, the results show an overdeepened Precambrian basement with transverse U shape and support the hypothesis of a pre-Quaternary glacial origin. Our data constitute the first detailed image of a buried pre-Quaternary glacial valley in North America; if substantiated with core studies, these results have far-reaching implications for our understanding of global ice houses as well as the tectonic conditions, enabling preservation of such systems. Plain Language SummaryThe shape of valleys depends on the processes that created them.Broad valleys with the transverse shape of a "U" are distinct signs of glaciers, which carved the valley in the bedrock. Since old valleys are filled up with sediments, we need geophysical tools to image their structure in the buried subsurface. Using such tools, we have found a buried U-shaped valley in western Colorado. Since we know from other observations that there were no glaciers in this specific location in the last ice age, the valley must have ...

Detecting upland glaciation in Earth’s pre-Pleistocene record

“…The Cutler Formation that crops out near Gateway, Colorado (United States; Figure 2) has been hypothesized to record proglacial deposition based on the presence of an inferred late Paleozoic glacially carved valley within the uplands (Soreghan et al, 2007;Soreghan et al, 2008a;Patterson et al, 2021), inferred ice-contact facies in the most proximal deposits onlapping the paleo-upland (Soreghan et al, 2008b;Soreghan et al, 2009), microstriae on quartz grains (Keiser et al, 2015), and inferred proglacial fluvial deposition (see discussion in 3.2.5. Case study below; Soreghan et al, 2009;Sweet, 2017).…”

Combining geophysical prospection and core drilling: Reconstruction of a Late Bronze Age copper mine at Prigglitz‐Gasteil in the Eastern Alps (Austria)

“…Archaeological excavations at prehistoric mines are extremely challenging and expensive, and this often severely limits the area that can be investigated. An alternative approach is to use geophysical methods to gain information about variations in the subsurface in a non‐invasive manner, permitting the mapping of extensive areas while also reducing time and costs (e.g., Aigner et al, 2021 ; Bücker et al, 2021 ; Gallistl et al, 2018 ; Katona et al, 2021 ; Patterson et al, 2021 ). For instance, geomagnetic and georadar surveys have successfully delineated the full extent, number and depth of Neolithic flint‐mining shafts at Arnhofen in southern Germany (Faßbinder, 2003 ; Leopold & Völkel, 2003 , 2004 ).…”