Debris-Bed Friction of Hard-Bedded Glaciers

Field measurements of debris-bed friction on a smooth rock tablet at the bed of Engabreen, a hard-bedded, temperate glacier in northern Norway, indicated that basal ice containing 10% debris by volume exerted local shear traction of up to 500 kPa. The corresponding bulk friction coefficient between the dirty basal ice and the tablet was between 0.05 and 0.08. A model of friction in which nonrotating spherical rock particles are held in frictional contact with the bed by bed-normal ice flow can account for these measurements if the power law exponent for ice flowing past large clasts is 1. A small exponent (n \u3c 2) is likely because stresses in ice are small and flow is transient. Numerical calculations of the bed-normal drag force on a sphere in contact with a flat bed using n = 1 show that this force can reach values several hundred times that on a sphere isolated from the bed, thus drastically increasing frictional resistance. Various estimates of basal friction are obtained from this model. For example, the shear traction at the bed of a glacier sliding at 20 m a−1 with a geothermally induced melt rate of 0.006 m a−1 and an effective pressure of 300 kPa can exceed 100 kPa. Debris-bed friction can therefore be a major component of sliding resistance, contradicting the common assumption that debris-bed friction is negligible

Debris-Bed Friction of Hard-Bedded Glaciers

Field measurements of debris-bed friction on a smooth rock tablet at the bed of Engabreen, a hard-bedded, temperate glacier in northern Norway, indicated that basal ice containing 10% debris by volume exerted local shear traction of up to 500 kPa. The corresponding bulk friction coefficient between the dirty basal ice and the tablet was between 0.05 and 0.08. A model of friction in which nonrotating spherical rock particles are held in frictional contact with the bed by bed-normal ice flow can account for these measurements if the power law exponent for ice flowing past large clasts is 1. A small exponent (n n = 1 show that this force can reach values several hundred times that on a sphere isolated from the bed, thus drastically increasing frictional resistance. Various estimates of basal friction are obtained from this model. For example, the shear traction at the bed of a glacier sliding at 20 m a−1 with a geothermally induced melt rate of 0.006 m a−1 and an effective pressure of 300 kPa can exceed 100 kPa. Debris-bed friction can therefore be a major component of sliding resistance, contradicting the common assumption that debris-bed friction is negligible.This article is from Journal of Geophysical Research: Earth Surface 110 (2005): no. F02007, doi:10.1029/2004JF000228. Posted with permission.</p

Gondwana Glacial Paleolandscape, Diamictite Record of Carboniferous Valley Glaciation and Preglacial Remnants of an Ancient Weathering Front in Northwestern Argentina

A record of glacier advance and retreat is preserved in Carboniferous strata exposed in an exhumed glacial paleovalley on the eastern side of the Paganzo basin. Previous investigations have focused on the sandstones in the paleovalley and inferred a glacial lacustrine history. New observations have demonstrated that remnants of a preglacial, ancient weathering front, developed under wet tropical conditions and composed of corestones, are found underneath the glaciogenic deposits. Delta and alluvial fan deposits were also recognized, but no inferences were made from the diamictites in the paleovalley regarding glacial events (Andreis et al., Bol Acad Nac Cienc Cordoba 57:3–119, 1986; Buatois and Mángano, J Paleolimnol 14:1–22, 1995; Sterren and Martínez, El Paleovalle de Olta (Carbonífero): Paleoambiente y Paleogeografía. 13º Congreso Geológico Argentino and 3º Congreso de Exploración de Hidrocarburos, Actas, 2, 89–103, 1996). This chapter focuses on the diamictites and provides a link between the sediment infill and the glacial origin of the paleovalley. We describe diamictites and associated sediments at three main locations: at La Chimenea, near the mouth of the paleovalley; at Mid-Valley, near the middle of the paleovalley; and at the Campsite near the head of the valley. We interpret some of the diamictites exposed at La Chimenea and at Mid-Valley to be subglacial tillite. Deformation in the sandstone underlying the tillite indicates warm-based conditions as the glacier advanced over soft deformable sediment. At the Campsite location, a diamictite bed, which is about 1.5 m thick, lies within a sequence of alternating sandstone and siltstone beds. The diamictite bed is interpreted to represent an ice-front readvance during a period of ice retreat. The diamictite may be a debrite originating off the ice front, or a subglacial deposit, i.e., a tillite, or a combination of both. Two additional diamictite beds, exposed higher in this sequence of alternating sandstone and siltstone beds, may also record minor ice-front advances into the flooded valley. Evidence of an ancient, preglacial weathering front (Late Devonian?–Earliest Carboniferous?) has been found in the granitic basement rocks which underlie the glaciogenic deposits, as large corestones included in a weathered regolith. This weathering front was developed under wet tropical conditions, before the onset of Carboniferous glaciations. The tillite and other diamictites overlying the corestones are composed largely of locally derived granitic basement rock. Features observed in the tillite and other diamictites are attributed to rapid rates of deposition, depositional processes, and the susceptibility of pre-weathered granitic basement rock to glacial and other erosional processes. Processes other than glacial erosion and deposition, including mass transport (slumping, rafting, sliding, and debris flow), also operated in the steep-sided valley and contributed large amounts of diamictite and other sediment to the valley fill. Corestones, weathered from the basement rock during a pre-Carboniferous period of intense weathering, constitute the larger clasts in the diamictite and associated deposits. The glacial paleolandscape is very well preserved in detail, after being buried during the Permian and later exhumed in the Cenozoic. The glacial valley was likely a transitional (fjord) environment, as micropaleontological material (Gutiérrez and Limarino, Ameghiniana 38:99–118, 2001) and clay mineral assemblages (Net et al., Sediment Geol 152:183–199, 2002) indicate a marine transgression into the area during the Middle Carboniferous.Fil: Socha, Betty. No especifíca;Fil: Carignano, Claudio Alejandro. Universidad Nacional de Córdoba; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rabassa, Jorge Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Austral de Investigaciones Científicas; Argentina. Universidad Nacional de Tierra del Fuego; ArgentinaFil: Mickelson, Dave. University of Wisconsin; Estados Unido