GPR survey of a lobate rock glacier in Yankee Boy Basin, Colorado, USA

Abstract

The internal structure of a lobate rock glacier located in the San Juan Mountains of southwest Colorado was investigated using ground penetrating radar (GPR). A 440 m, 25 MHz longitudinal profile oriented along the central axis of the rock glacier shows moderate to strongly coherent reflection horizons or layers that can be recognized clearly to a depth of 30-35 m. The layers are interpreted as representing ice-supersaturated sediments and coarse, blocky rockslide debris that are the result of flow, perhaps generated by seasonal snow pack covered by episodic debris flows or high-magnitude discharges of talus from the cirque headwall. Profiles collected at 50 MHz indicate that, in the upper 20 m thickness of the rock glacier, many of these layers are laterally continuous, The total depth of penetration (-40 m at 25 MHz) was sufficient to detect the rock glacier-cirque-floor contact, which is composed of underlying moraine. Several prominent reflection events that subdivide the profile into broad 10-15 m-thick layers represent contacts between major depositional units. These units are believed to be individual flow lobes that were initiated at various cirque-headwall locations. We interpret this rock glacier to be a composite feature that formed by a process involving the development and subsequent overlap of discrete flow lobes that have over-ridden older glacial moraine and protalus rampart materials. The latter materials have been incorporated into the present flow structure of the rock glacier.

Structure and genesis of the Thabor rock glacier (Northern French Alps) determined from morphological and ground-penetrating radar surveys

“…For example, 1) Straight and parallel reflectors concordant with a basal one express low longitudinal deformation and/or vertical compaction; 2) Chaotic reflectors may reflect local downwasting and perturbation of the shallow structure; 3) Undulating and toplapping reflectors; and 4) upward-or rock glacier center-dipping reflectors rather evocate the occurrence of compressive stresses and thrusting. Indeed, undulating and toplapping reflectors are well known in rock glacier deformation structures (e.g., Degenhardt et al, 2003;Monnier et al, 2008), and the dipping reflectors observed here are analogous to GPR signatures given by thrust planes in glaciotectonized sediments (Overgaard and Jakobsen, 2001;Sadura et al, 2005) or by debris inclusions in glacier ice (Woodward et al, 2003) and debris-covered glacier ice structures in rock glaciers (Fukui et al, 2008).…”

Can a simple Numerical Model Help to Fine-Tune the Analysis of Ground-Penetrating Radar Data? Hochebenkar Rock Glacier as a Case Study

“…While penetration depth increases at low frequencies, the depth resolution decreases. Frequencies between 6.4 and 250 MHz have been used to measure thickness of up to about 40 m and internal properties of rock glaciers in the Alps, the polar regions, and the United States (Fukui et al, 2007;Degenhardt, 2009;Degenhardt et al, 2003;Nickus et al, 2015).…”

Development of tongue-shaped and multilobate rock glaciers in alpine environments – Interpretations from ground penetrating radar surveys

“…Data collection typically involves centerpoint antenna frequencies of 25, 50, or 100 MHz (e.g., Isaksen et al, 2000;Degenhardt et al, 2003;Berthling et al, 2003). This range usually provides the best optimization between resolution and depth of penetration when antennas are used in broadside reflection mode with constant sourcereceiver offsets (Jol and Bristow, 2003).…”

“…9A). Given suitable magnitude, volumes and discharge frequency, debris lobes can coalesce or stack to form a "composite" rock glacier such as the one at Gilpin Peak (Degenhardt et al, 2003). In contrast, if debris is being supplied primarily from a narrow zone along the cirque wall over an extended period of time, a tongue-shaped rock glacier will likely result.…”