Using Geophysical Methods to Study the Shallow Subsurface of a Sensitive Alpine Environment, Niwot Ridge, Colorado Front Range, U.S.A

Abstract

Shallow seismic refraction (SSR) and ground penetrating radar (GPR) are noninvasive geophysical techniques that enhance studies of the shallow subsurface deposits which control many geomorphic and biogeochemical processes. These techniques permit measuring the thickness and material properties of these deposits in sensitive alpine areas without using extensive pits and trenches that can impact current biogeospheric processes or distort them for future research. Application of GPR and SSR along 1.5 km of geophysical lines shows that layers of fine to coarse, blocky deposits of periglacial origin underlie alpine slopes in the vicinity of Niwot Ridge, Colorado Front Range. Interpretation of geophysical and drilling data shows that depth to bedrock ranges from 4 to .10 m and is not simply related to local slope. Our measurements suggest that ice lenses form seasonally beneath solifluction lobes; ice was not present in adjacent areas. Ice lenses are associated with local ponded water and saturated sediments that result from topographic focusing and low-permeability layers beneath active periglacial features. Geophysical interpretations are mainly consistent with data derived from nearby drill cores and corroborate the utility of GPR in combination with SSR for collecting subsurface data required by different landscape models in sensitive alpine environments.

Influence of glacier advance on the development of the multipart Riffeltal rock glacier, Central Austrian Alps

“…Maurer and Hauck, 2007;Hausmann et al, 2007b;Degenhardt, 2009;Leopold et al, 2011). Hausmann et al, 2007b;Leopold et al, 2008;Degenhardt, 2009). Here we present a line collected with the 50 MHz antennae due to best results with this frequency.…”

Thawing glacial and permafrost features contribute to nitrogen export from Green Lakes Valley, Colorado Front Range, USA

“…Bedrock in GLV is composed of Precambrian schists and gneisses, the Silver Plume quartz monzonite, and Audubon-Albion stock . Permafrost has been verified above 3,500 m on Niwot Ridge (Ives and Fahey 1971) and more recently near Green Lake 5 (GL5; Leopold et al 2008) and in the GL4 watershed (Janke 2005). In addition to ARK, three rock-glacier/blockfield sites have been sampled: an 8-ha lobate rock glacier (RG5) at the foot of the north-facing side of Kiowa Peak and two blockfield sites above GL4 (EN.4L and KIO.SW, Fig.…”

Stratified slope deposits: periglacial and other processes involved

“…Additional data may be obtained from (macro) fabric analysis and micromorphological observation (Van Vliet-Lanoë 1987;Bertran et al 1997;Harris 1998;Bertran & Texier 1999). Recently, geophysical methods have been used to recognize stratification within slope deposits, although at a relatively low resolution Völkel & Mahr 2001;Leopold & Völkel 2003;Leopold et al 2008). Radiocarbon dating is often applied to older deposits (Blikra & Nemec 1998;Nemec & Kazanci 1999;García-Ruiz et al 2001;Eaton et al 2003) whereas dendrochronology and lichenometry may be applied to relatively young material (McCarroll 1993;McCarroll et al 1995;Jomelli 1997;Lafortune et al 1997;Hétu & Gray 2000).…”