Dec 23, 2008 - significance to periglacial autochthonous blockfield development, Polar ..... Potter and Moss (1968) described blockstreams built of angular ...
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Block deposits in southern Africa and their significance to periglacial autochthonous blockfield development Jan Boelhouwers
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Department of Earth Sciences , University of the Western Cape , Private Bag XI7, Bellville, 7535, South Africa Published online: 23 Dec 2008.
To cite this article: Jan Boelhouwers (1999) Block deposits in southern Africa and their significance to periglacial autochthonous blockfield development, Polar Geography, 23:1, 12-22, DOI: 10.1080/10889379909377662 To link to this article: http://dx.doi.org/10.1080/10889379909377662
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BLOCK DEPOSITS IN SOUTHERN AFRICA AND THEIR SIGNIFICANCE TO PERIGLACIAL AUTOCHTHONOUS BLOCKFIELD DEVELOPMENT
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Jan Boelhouwers Department of Earth Sciences, University of the Western Cape, Private Bag XI7, Bellville 7535, South Africa Abstract: Autochthonous blockfields are generally considered landforms indicative of periglacial environments. This paper presents observations from the semi-arid Karoo and afro-montane Drakensberg and Lesotho highlands, southern Africa, on nonperiglacial autochthonous block mantles on dolerite sills and basalt. Subsurface weathering in resistant lithology with subsequent removal of fines is argued to result in autochthonous blockfields. It is proposed that blockfields may develop under a wide range of environments wherever resistant lithologies are present. The paleoenvironmental significance of blockfields depends entirely on establishing a more reliable analysis of weathering environments under which blocks developed and, where fines are produced, subsequent block concentration.
INTRODUCTION Openwork block deposits are predominantly described for the high-altitude and high-latitude environments of the world. They are referred to as blockfields when forming sheets of blocks on slopes of angles less than 10°, and block slopes when on slopes steeper than this (White, 1981). Autochthonous blockfields refer to those developed by in situ weathering, whereas allochthonous blockfields comprise material that moved downslope. A blockstream is defined as "an elongate body of rocks extending farther downslope than across the slope" (White, 1981). These types of block accumulations are generally thought to be genetically associated with active frost environments and have frequently been used as relic indicators of past periglacial conditions. This understanding is based largely on the early association of blockfields with cold regions and the perceived dominance of frost weathering in such environments. In contrast, there is a growing body of literature that points toward a non-periglacial origin of blocks found in autochthonous blockfields in arctic and alpine environments. Working on blockfields in northeastern Tasmania, Caine (1968) was one of the first to postulate a block origin from Tertiary chemical weathering mantles. Ives (1974) and Sugden and Watts (1977) argued for survival of tors and felsenmeer beneath the Laurentide ice sheet in Canada. Kleman and Borgstrom (1990) provided further evidence for landform preservation beneath cold-based ice-sheets and suggested a pre-glacial origin of blockfields in parts of Scandinavia. The clay-mineral analyses by Roaldset et al. (1982) and Rea et al. (1996) provided further strong indications of the preservation of pre-Pleistocene weathering mantles to the present. The latter proposed a block origin from chemical weathering mantles, with frost heaving 12 Polar Geography, 1999,23, No. I, pp. 12-22. Copyright © 1999 by V. H. Winston & Son, Inc. All rights reserved.
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Fig. 1. Location of South African sites discussed in text. of corestones from the regolith to the surface, resulting in the development of blockfields. Despite their strictly morphological definition, the early association of blockfields with periglacial environments and the resurgence of a discussion on their origin invites a consideration of block weathering mantles outside cold-climate regions. Such discussion is particularly relevant where blockfields are used as indicators of paleo-periglacial environments. In southern Africa, blockfields have been reported for the Lesotho highlands (Sparrow, 1971; Hastenrath and Wilkinson, 1973; Boelhouwers, 1994), Amatolas (Marker, 1986; P. Sumner, pers. comm., 1998), and Western Cape Mountains (Hagedorn, 1984; Borchert and Sanger, 1988; Boelhouwers, 1998). These have mostly been attributed to Pleistocene periglacial origins by frost weathering, although Boelhouwers (1998) noted the development of openwork block accumulations on slopes by washing out of matrix from debris mantles. Observations are here presented to highlight the widespread occurrence of autochthonous block covers of non-periglacial origin, in a warm semi-desert and the more humid afromontane environment of the Drakensberg and Lesotho highlands in southern Africa. THE SEMIARID KAROO The semiarid Karoo occupies most of the interior of South Africa (Fig. 1). It forms extensive plains interspersed with mesas and buttes formed by resistant caprocks of sandstone or dolerite. The Jurassic dolerite intrusions are manifest in the form of dikes and sills that cover extensive areas of the Karoo and which form distinct ridges and plateaus in the landscape. Blocky saprolitic mantles develop in these resistant
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Fig. 2. Mean monthly temperature and rainfall data for the Karoo at Colesberg (1212 m elev.) and the Lesotho highlands at Mokhotlong (2377 m elev.) (Tyson et al., 1976; SAWB, 1986). lithologies and form autochthonous blockfields on level surfaces as well as blockslopes and blockstreams on slopes where the matrix is removed. Observations are here presented on autochthonous block mantles near Colesberg. Climate of the study area is characterized by cool dry winters and hot summers with occasional highintensity thunderstorms. Monthly temperature and rainfall characteristics for Colesberg are presented in Figure 2. Vegetation is generally sparse, but grasses establish themselves between blocks on the dolerite surfaces. At a site of extensive dolerite outcrops, dolerite slopes, level elevated surfaces, and residual tors are separated by sandy depressions containing flat channel beds (Fig. 3). Soils are thin or absent and bedrock is exposed in many places on the slopes and summit areas. Fine sediment (
