Rock control on microweathering of bedrock surfaces in a periglacial environment

Abstract

Microweathering of ice-smoothed bedrock surfaces was investigated in the Røldal area of Hardanger Plateau (60°), southern Norway. Postglacial rates of weathering were determined from surface lowering using quartz veins as reference surfaces. Weathering processes are inferred from assessment of weathering rind formation, surface hardness, and the preservation of small-scale glacial erosional features. Surface lowering rates for a range of metamorphic rocks vary from 0.05 to 2.20 mm ka − 1 and are broadly comparable with those obtained from crystalline rocks in other periglacial environments. The mean rate of surface lowering at 0.55 mm ka − 1 is low and demonstrates the relatively small impact of microweathering on postglacial landscape evolution. Variations in bedrock microweathering can be explained by lithological variation. Amphibolite and mica-rich bedrock surfaces experience greater denudation and weakening, least weathering rind formation, and abundant preservation of glacial striae, despite greater surface lowering. Conversely, quartz-rich bedrock surfaces are most resistant to denudation and weakening, but have greater weathering rind formation and fewer preserved striae. Postglacial microweathering is achieved primarily through granular disintegration involving detachment and removal of mineral grains and weakening from increased porosity. Granular decomposition is manifest in the formation of weathering rinds. Analysis of interactions between weathering indices indicates that rind accumulation is limited by microerosion. A conceptual model is proposed that illustrates the temporal interrelationships between in situ and erosional facets of microweathering in two contrasting mineral assemblages. The model proposes that cyclic processes of in situ disintegration, decomposition, and erosion are at work. The relative balance between these processes varies with lithology so that in more resistant quartz-rich rocks the net effect is minimal surface lowering and accumulation of weathering rind. In weaker, amphibolitic and micaceous rocks, the net effect is greater surface lowering and minimal accumulation of weathering rind. The results of the research demonstrate the important influence of rock properties, notably mineral composition, in postglacial microweathering of crystalline bedrock in a periglacial environment.

Weathering rinds as mirror images of palaeosols: examples from the Western Alps with correlation to Antarctica and Mars

“…Mean maximum rind thicknesses reported here are comparable with thicknesses reported elsewhere from much shorter time since at University of Iowa on August 10, 2015 http://jgs.lyellcollection.org/ Downloaded from WEATHERING RINDS: MIRROR IMAGES OF PALAEOSOLS 841 deposition, particularly those reported by Nicholson (2008) in southern Norway, André (1996) in northern Sweden and Etienne (2002) in Iceland. Assuming those workers did not measure fracture faces that weather inordinately fast, the reported differences may well reflect different microenvironments, lithological differences or loss by erosion (Mahaney et al 2012a).…”

“…Apart from recent work in direct rind dating using U-series nuclides (Pelt et al 2003), which greatly refines the method of using rind thickness alone as a chronometric tool, little attention has been paid to the wealth of palaeoenvironmental information recorded in terrestrial rinds developed in various lithological species. Some researchers have focused recently on moisture as a mechanism of microweathering (Thorn 1975), as a control of oxidation and hydrolysis leading to rind growth between rock walls and near planar bedrock (Nicholson 2008(Nicholson , 2009, and as a significant factor in developing diffusion models to explain rates of basalt or andesite weathering (Oguchi & Matsukura, 2000;Oguchi 2004;Sak et al 2004;Navarre-Stichler et al 2011). Mahaney et al (2012aMahaney et al ( ,b, 2013a studied rind growth using high-resolution scanning electron microscopy (SEM) to image not only country rock but also allochthonous materials, including organic compounds, with chemistries determined by energy-dispersive spectrometry (EDS), and thereby discovered that detailed palaeoenvironmental archives lay buried in rind successions on Mount Kenya.…”

Improved Schmidt-hammer exposure ages for active and relict pronival ramparts in southern Norway, and their palaeoenvironmental implications

“…The second assumption was tested by Shakesby et al (2011) on raised beaches in northern Sweden, where many control points of Holocene age were available. Although weathering rates are likely to slow down over longer timescales (Colman, 1981;Colman and Dethier, 1986), linear rates can be expected over the Holocene timescale where relatively resistant lithologies are subject to relatively slow rates of subaerial chemical weathering in periglacial environments (André, 1996(André, , 2002Nicholson, 2008Nicholson, , 2009Fig. 4.…”

The use of the Schmidt Hammer and Equotip for rock hardness assessment in geomorphology and heritage science: a comparative analysis

“…Spatial variations in hardness can also be used to investigate environmental infl uences on weathering, such as aspect (Hall, 1993;Waragai, 1999;Burnett et al, 2008) and seasonal snow patches (Ballantyne et al, 1989(Ballantyne et al, , 1990Benedict 1993: Grab et al, 2005. Furthermore, hardness can be used to help infer the role of rock control in determining weathering rates and processes, as for example reported by Nicholson (2008Nicholson ( , 2009) from periglacial sites in southern Norway.…”