The palaeoenvironmental significance of southern ... - IARC Research

Openwork block accumulations are widespread inthe summit area and vary in size from patches a fewmetres across to blockstreams measuring tens of metres inlength and covering up to 6 ha. The blockstreams originatefrom scarps, some of which have locally disintegrated.Blocks are typically angular to subangularalthough rounding and pitted surfaces indicate subsurfaceweathering in the presence of a matrix. Blockform is typically platey or elongate measuring 0.2 toin excess of 3.0 m and show distinct downslope fabricsin the blockstreams. Fabrics are transverse at lobatefronts and dip upslope in a well-developed imbrication.Downslope sorting from small to larger blocks isevident at sites, with vertical sorting to 1.5 m depths.The characteristics of the openwork block accumulationsresemble those found in other deposits in midlatitudeareas which are generally attributed to slowmass wasting processes under Late Quaternary periglacialconditions (e.g. Caine 1968; Caine and Jennings1968; Benedict 1976). In addition to the morphologicalsimilarities, the periglacial origin of the WesternCape deposits is based on two arguments; block originand movement mechanisms. Tertiary chemical solutionhas resulted in extensive pseudo-karst formsthroughout the region and increased primary and fractureporosity. This has resulted in an increased frostsusceptibility of the quartzites and rendered them moreprone to frost wedging (Boelhouwers 1996). Ubiquitousmechanical fracturing in the mountains of the WesternCape is superimposed over pseudo-karst weatheringforms. Rock scarps in the summit region are completelymechanically broken down and the coarse blockydebris intrinsically associated with the blockstreams.Recognising that no diagnostic features exist to specificallyidentify frost-weathered debris (White 1976), anorigin post-dating the Tertiary favouring frost-inducedmechanical weathering in the Late Quaternary is,nonetheless, likely.Fabrics and lobate fronts indicate slow mass movementof blockstreams and are typical characteristicsof blockstreams found throughout the world. Verticalsorting and weathering atterns indicate the presenceof matrix at the time of emplacement and movement.Deformation by slow mass flow under increased porewater pressure may account for movement wherebedrock is near the surface (e.g. Caine 1983) but wouldnot account for vertical sorting in the deposits. Sortingis accounted for by ground freezing while relativelyfast rates of clast movement in the order of 3 cm p.a. canbe achieved by frost creep on slopes of 20° (Benedict1976; Caine 1983). Rates of movement applied to theblockstreams indicate that the emplacement could haveoccurred during the Last Glacial Maximum (21–15 kaBP) (Boelhouwers 1999b).Block production, movement and emplacement issuggested as associated with the colder period of theLate Pleistocene with subsequent Holocene washingout of matrix. Environmental conditions associated withfrost penetration to at least 1.5 m suggests little insulationby snow cover. No evidence for permafrost is foundand is not required for the emplacement of the deposits.MAAT is estimated at 0°C for the summit regions; areduction of 7–8°C which corresponds well to otherproxy data for the region (Talma and Vogel 1992).2.3 KarooThe semi-arid Karoo occupies most of the interior ofSouth Africa (Figure 1) and is characterised by cooldry winters and hot summers with occasional thunderstorms.Extensive plains are interspersed with mesasand buttes formed by resistant sandstone and doleritecaprocks. The Jurassic dolerite intrusions are evidentin the landscape as dikes and sills that cover extensiveareas forming distinct ridges and plateaus. Blockysaprolitic mantles develop on these intrusions and formautochthonous blockfields which may extend intoblockstreams on steeper slopes where matrix is removed.Boelhouwers (1999a) describes doleritic autochthonousblockfields in the vicinity of Colesberg in thecentral Karoo (1212 m a.s.l.). MAAT is approximately16°C with winter and summer average temperatures9°C and 25°C in the area. Vegetation is generally sparsealthough grasses establish themselves between blockson the dolerite exposures and scattered acacia treesstand within blocky material where seedlings are protectedfrom wildfires. Joint density varies and appearsto be the main factor influencing the presence orabsence of tors and the size of blocks. Block mantlesare generally less than 1.0 m thick and may consist ofa single layer of blocks resting on tightly fitting,bedrock detached blocks. On summits, scatteredblocks rest directly on intact bedrock. On lower slopesor level surfaces a silty sediment (particle size1 mm), presumably aeolian in origin, may be presentbetween blocks.The mode of weathering is predominantly spheroidalsubsurface chemical weathering along joints, resultingin block separation and rounding with blocks remainingin situ. Weathered products are subsequently removedby wash or wind. Desert varnish on block surfacesindicates the considerable age of the blocks and theslow rate of weathering. Many blocks also have freshangular surfaces with sharp-edged spalls broken offthem. Some blocks are split in half. This fracturing iscaused by infrequent wildfires or lightening, whichappears to be the dominant cause of further disintegrationof the rounded blocks once exposed at the surface.The openwork accumulations meet several of the criteriafor autochthonous blockfields. Occurring on lowgradient slopes they are the result of in situ weathering75

of block material of sufficient thickness that any relationshipwith the underlying bedrock is lost (see White1976; Tyurin 1983). Differences may lie in the degreeof roundness of the blocks, the matrix infill whichmay be secondary, and the vegetation growth. Similarcharacteristics may be found to a greater or lesser extenthowever in block accumulations in true periglacialenvironments.The extent to which blockfields develop in theKaroo depends on the presence of dolerite outcrops,topography, subsurface moisture availability joint densityand vegetation cover. Effective removal of theweathered product matrix is determined by local topographyand vegetation characteristics which influencewash and aeolian processes. Joint density determinesblock size, the amount of matrix and the depth of theweathered zone. Secondary wind sediment may accumulatebetween blocks. A deep weathering zone allowsfor the generation of a sufficiently thick block cover,that on removal of the interstitial matrix any link withthe underlying bedrock is lost. Although formed overlong periods of time through deep weathering andsubsequent exposure, these blockfields do not specificallyrequire cold conditions for their formation andfall outside of the region associated with Pleistoceneperiglacial and glacial activity. The process of formationcan readily be perceived as continuous through variousclimatic regimes, perhaps even inhibited under colderconditions with reduced chemical activity, and activeat present.2.4 NamibiaArid to hyper-arid Namibia extends up the west coastof southern Africa (Figure 1). Mean air temperatureson the coast range between 12 and 18°, increasing to 12and 26°C in the central interior at Windhoek (1728 ma.s.l.), where winter temperatures may fall below zeroand maxima increase to around 40°C. Mean annualprecipitation at the coast is less than 20 mm, increasingto 370 mm in the central interior. Conditions in theCentral Namib Desert, characterised by extensivedunefields, have remained hyper-arid for the past 5million years (Ward et al. 1983).Although no detailed data exist on openwork accumulationsin the area, some observations can be madeon accumulations which occur inland from the coastaldunefields. In the vicinity of Helmeringhausen, eastof the dunefields, hill-summit blockfields extenddownslope into blockstreams on gradients which canexceed 20°. The openworks consist of a blocky mantleof exposed intrusive volcanics 1–2 m thick overlyingbedrock. Blocks are subrounded to subangular withno dominant preferred orientational fabric. Desertvarnish attests to the age of the exposed surfaces anddistinguishes the openworks from the adjacent matrixdominatedregolith which generally appears moremobile, extending into lobate forms on footslopes.These slope deposits are remarkably similar to theslope deposits found in other arid areas such as thosedescribed by Whitney and Harrington (1993) in southernNevada and Friend et al. (2000) in eastern California.Whitney and Harrington (1993) date the blockdeposits to the early to middle Pleistocene, note theirpresent inactivity and attributed their formation to thecolder periods of the early and middle Quaternary. Incontrast, Friend et al. (2000) describe similar depositswhich, although similar to true periglacial features,are actively forming under current desert conditions.Although further detail is required from this type ofopenwork accumulation in Namibia, given the environmentalconditions the possibility of periglacial activityas a driving force behind their formation appears highlyunlikely.3 DISCUSSIONA summary of the attributes and palaeoenvironmentalimplication of openwork accumulations in southernAfrica appears in Table 1. Used independently, theinterpretation of blockfields and blockstreams mayprovide for specific environmental conditions andthus assist in palaeoenvironmental interpretation inthe regions where few proxy data exist. Comparing theforms across the sub-continent however highlights theproblems associated with the interpretation of theseblocky features as periglacial in origin.The blockfields and blockstreams described are theproduct of a variety of weathering mechanisms undercontrasting climatic conditions. Deep chemicalweathering contributes to autochthonous (blockfield)openwork development in mountainous and in aridenvironments. Blocks in these accumulations appearto be rounded to subrounded, although modified bysubsequent weathering. Dominant mechanical weatheringincreases block production, particularly angularproducts, although directly attributing this to frostweathering is problematic (White 1976). Blocks willalso be subject to secondary weathering processes onexposure at the surface or subject to attrition duringemplacement. The development of blockfields mayspan millions of years during which numerous weatheringregimes may have existed (e.g. Rea et al. 1996).Diagnosing openwork accumulations on the basis ofweathering and resultant block form is, therefore,problematic and is manifest in the multiple origins ofblockfields in southern Africa.Notwithstanding difficulties in interpreting weatheringmodes and products, both the Western Cape andthe highlands of Lesotho show a phase of increased76

Table 1.Characteristics and palaeoenvironmental interpretation of blockstreams and blockfields in southern Africa.Age andContemporary Block origin palaeoenvironmentalLocation environment Description and emplacement interpretationLesotho Summer rainfall. Autochthonous:highlands MAAT 5–7/C est. Rounded blocks, In situ chemical Deep (Tertiary?)Marginal periglacial pitted surfaces, thin weathering of bedrock. chemical weathering.MAP 1500 mm est. weathering rinds. Upfreezing of rounded Late Pleistocene deepTriassic-Jurassic Coarse granular corestones and/or seasonal freeze.basalts. loam matrix washing out of matrix. Limited snow cover,3000–3482 m a.s.l. where present no Late PleistoceneSoil frost typicallyglaciation.0.2 m in depth. Allochthonous:Lag-type valley Mechanical weathering Late Pleistocenefloor block-streams. of valley-side scarps. block productionSubrounded, pitted Upfreezing of corestones and emplacement.blocks. Solifluction and frost Wet, colder thanThin weathering rinds. creep. present.Matrix removed byDeep seasonal freezesuffosionwithout permafrost.Western Winter rainfall. Allochthonous Frost weathering Late PleistoceneCape MAAT 7°C est. blockstreams. and wedging of scarps. block productionMountains Paleozoic quarzite Angular to subangular Slow mass movement and emplacement.1600–2249 m blocks. by frost creep. MAAT 0°C atMAP 575 mm est. Pitted block surfaces. summits.Soil frost to 0.03 m Vertically sorted. Similar winterMatrix removed by wash.precipitation.Frost penetration to1.5 m.No permafrost.Karoo Semi-arid (cool, dry Blockfields extending Rounding by in situ Quaternarywinters, hot summers). less frequently chemical weathering (contemporary?)Jurassic dolerites and into blockstreams. along joints. Non-periglacialOlder sediments. Rounded to angular Angularity from origin.Central Karoo blocks. fires and lightening.(approx): summer avg. Bedrock detached, Weathering product25/C winter avg. 9/C joint-size determined. (matrix) removed byMAP 380 mm Varnished block surfaces. wash or wind.Aeolian sediment matrixor weathering product.Namibia Arid to hyper-arid Openwork accumulations In situ weathering. Largely unknown,Coastal (interior): resembling blockfields probably polygenetic.summer 20° (26°) and blockstreams. Quaternary, perhapswinter 12°C (12°) Sub-angular to older.MAP 20 (370) mm sub-rounded. Non-periglacialNo orientational fabric.originVarnished surfaces.* Climatic data and estimates for Giants Castle (Lesotho highlands), Matroosberg (Western Cape mountains), Colesberg (Karoo) and Windhoek, Luderitzand Swakopmund (Namibia). MAAT mean annual air temperature, MAP mean annual precipitation.block production in the past. Weathering patterns andsuperimposition of blocks over and within the upperweathering mantles point to an environment wheremechanical weathering was dominant. The most likelyscenario is increased block production during the colderperiod of the Late Pleistocene when full periglacialconditions have been proposed for the mountains.Openwork development in the arid and semi-aridregions would have remained largely unaffectedalthough chemical weathering rates may have declinedand the rate of formation reduced accordingly.Although few data exists worldwide on mobilityand emplacement in actively forming blockstreams,block mobility and the subsequent development ofblockstreams are the most critical diagnostic indicatorsfor a periglacial origin. Blockstreams in theLesotho highlands and the Western Cape mountainsexhibit the characteristics pertaining to deep seasonalfreeze. This provides a stronger argument supportingincreased frost weathering of scarps rather than thereverse of using block form as an indication of environmentalconditions. In the light of the apparent dichotomythat exists between process-based weathering studiesand interpretation of relict features in palaeoenvironmentalreconstruction, further research needs to beattempted linking process and form, and in establishing77

scale linkages. In the absence of such data on weatheringand the related products, use of block formalone in palaeoenvironmental interpretation must betreated with caution.4 CONCLUSIONEarly recognition of the widespread occurrence ofblockfields in periglacial environments and assumptionsregarding their origin has led to a bias associatingthese forms with such environments. The presence ofblockfields in the Karoo and Namibia indicates that aperiglacial origin cannot be automatically assumed.Where detail are available on blockstream sedimentologyand morphology in which a mode of emplacementor movement indicates ground freezing, such withas vertical sorting, a periglacial origin may be moreobvious. The presence of both blockfields and blockstreamsshould be perceived as a function of multiplestagedevelopment, many of which require contemporary conditions for their maintenance or continueddevelopment.Future research can be directed at: (1) establishingabsolute dates for openwork accumulations from avariety of environments, (2) more detailed clay analysis(e.g. Rea et al. 1996) to reveal weathering environments,(3) debris production mechanisms associatedwith the origin of slope deposits, and (4) detailedprocess monitoring of actively forming blockstreams.REFERENCESAlexandre, J. 1962. Phenomenes periglaciaires dans leBasutoland et le Drakensberg du Natal. BuiletinPeriglac 11: 11–13.Benedict, J.B. 1976. Frost creep and gelifluction features: areview. Quaternary Research 6: 55–76.Boelhouwers, J.C. 1991. Present-day periglacial activityin the Natal Drakensberg: a short review. Permafrostand Periglacial Processes 2: 5–12.Boelhouwers, J.C. 1996. The present-day frost action environmentand its geomorphological significance in theWestern Cape Mountains, South Africa. BellevilleSouth Africa: University of the Western Cape, Schoolof Environmental Sciences, Occasional PublicationSeries no. 2.Boelhouwers, J.C. 1999a. Block deposits in southernAfrica and their significance to periglacial autochthonousblockfield development. Polar Geography 23:12–22.Boelhouwers, J.C. 1999b. Relict periglacial slope depositsin the Hex River Mountains, South Africa: observationsand palaeoenvironmental implications. Geomorphology30: 245–258.Boelhouwers, J.C., Duiker, J.M.C. & van Duffelen, E.A.1998. Spatial, morphological and sedimentologicalaspects of recent debris flow activity in the DuToitskloof Valley, Western Cape mountains. SouthAfrica Journal of Geology 101: 73–89.Boelhouwers, J.C., Holness, S., Meiklejohn, K.I. &Sumner, P.D. 1999. A blockstream in the vicinity ofSani Pass, Lesotho highlands, and its palaeoenvironmentalimplications. In Grab, S., Boelhouwers, J.,Hall, K.J., Meiklejohn, K.I. & Sumner, P.D. (eds),Quaternary Periglacial Phenomena in the Sani PassArea, Southern Africa, Field Guide to the INQUA XVCongress, Durban.Boelhouwers, J.C. & Meiklejohn, K.I. 2002. Quaternaryperiglacial and glacial geomorphology of southernAfrica: review and synthesis, South African Journal ofScience 98: 47–55.Caine, N. 1968. The Blockfields of Northeastern Tasmania.Canberra: Australian National University, Departmentof Geography, Publication G/6.Caine, N. & Jennings, J.N. 1968. Some blockstreams of theToolong Range, Kosciusko State Park, New SouthWales. Journal and Proceedings, Royal Society ofNew South Wales 101: 93–103.Caine, N. 1983. The mountains of Northeastern Tasmania.Rotterdam: Balkems, 200p.Grab, S. 1997. Thermal regime for a thufa apex and itsadjoining depression, Mashai Valley, Lesotho. Permafrostand Periglacial Processes 8: 211–217.Grab, S. 2000. Periglacial phenomena. In Partridge T.C. &Maud R.R. (eds), The Cenozoic of Southern Africa.Cape Town: Oxford, 207–216.Friend, D.D., Phillips, F.M., Campbell, S.W., Liu, T. &Sharma, P. 2000. Evolution of desert colluvial boulderslopes. Geomorphology 36: 19–45.Partridge, T.C. & Maud, R.R. 1987. Geomorphic evolutionof southern Africa since the Mesozoic. South AfricanJournal of Geology 90: 179–208.Rea, B.R., Whalley, W.B., Rainey, M.M. & Gordon, J.E.1996. Blockfields, old or new? Evidence and implicationsfrom some plateaus in northern Norway.Geomorphology 15: 109–121.Talma, A.S. & Vogel, J.C. 1992. Late Quaternary paleotemperaturesderived from a spleothem from the CangoCaves, Cape Province, South Africa. QuaternaryResearch 37: 203–213.Tyurin, A.I. 1983. Classification of rock streams. Permafrost:Fourth International Conference Proceedings.Washington DC: National Academy Press, 1283–1285.Ward, J.D., Seely, M.K. & Lancaster, N. 1983. On the antiquityof the Namib. South African Journal of Science79: 175–183.White, S.E. 1976. Rock glaciers and block fields: Reviewand new data. Quaternary Research 6: 77–97.Whitney, J.W. & Harrington, C.D. 1993. Relict colluvialboulder deposits as palaeoenvironmental indicatorsin the Yucca Mountain region, southern Nevada.Geological Society of America Bulletin 105: 108–1018.Weinert, H.H. 1961. Climate and weathered Karoodolerites. Nature 4786: 32–39.78