Paleoweathering and paleolandforms Introduction

ELSEVIER

Geomorphology

16 (1996) l-4

Paleoweathering and paleolandforms Introduction Mireille Bouchard a, Jean-Michel Schmitt b a Dkpartement de Giographie, Uniuersite’ du Quibec h Mont&al, Montr6al. Que’. H3C 3P8, Canada b Ecole des Mines de Paris, 35 rue Saint-Honorh, 77305 Fontainebleau, France

Pre-Quatemary weathering phenomena are an important aspect of continental history. The paleoweathering records may consist of more or less complete fossilized alteration profiles, sometimes of full toposequences, and of remnants of continental paleosurfaces and paleolandforms. Paleoweathering studies contribute to paleogeographic reconstitutions and global correlation of deposits. Paleoweathering features constitute a direct recording of paleoclimates, through the mineralogy, geochemistry, and geometric assemblage of weathering products, and record global changes. The recognition of ancient continental landforms is also an important aspect of paleoenvironments; moreover, it is the major tool for the reconstitution of hydrologic regimen. The analysis of ancient landforms and landscapes in relation to paleoweathering events can be approached by different ways: (1) case studies and methods of paleolandforms reconstruction, (2) historical and regional reconstitution; response of landforms to sea level changes, to tectonic events, to climatic and other environmental changes, (3) comparison of ancient landforms to present types; climatic and environmental significance of these landforms. At a mega-scale, Fairbridge (1988) mentions that weathering profiles on cratons, the most long-lived megafeatures of Planet Earth, document the major (mega-) cycles of Earth history. Hence, he stresses that a study by geomorphologists of the global distribution and chronology of paleosurfaces can ultimately lead to a broadening and strengthening of the

whole fabric of planetary history. Studies on southem continents showed the real importance of deep weathering (Thomas, 1994; Ollier, 1984) and the survival of old landscapes (Twidale, 1976). In High Latitudes, many occurrences of preserved preglacial features in once heavily glaciated areas have been reported, explained by selective erosion by Pleistocene ice sheets (Hall and Sugden, 1987). In some cases, detailed analysis of differential erosion and deep weathering permitted discrimination of these features among the other landforms. In the Lofoten-Vesteralen (Norway), differential erosion controlled the landform development within the basement rocks (Peulvast, 1988). Thus, in High Latitudes, much information on the evolution of denudation surfaces and paleolandforms can be gained by taking into account preglacial deep weathering and stripping of the profiles and also the preservation below cover rocks, followed by exhumation (Lidmar-Bergstrom, 1982, 1988). Paleoweathering profiles associated with paleosurfaces are pointed out on very old basement rocks of Precambrian age. In Canada, they are related to major unconformities in Precambrian rocks. Gall (1992) points out the presence of many widely distributed coeval paleosols which indicates that all, or parts, of Laurentia in the northwestern Canadian Shield attained relative tectonic stability and subaerial exposure subsequent to supercontinent amalgamation. The differences in the nature of paleoweathering preserved in weathering profiles developed on

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Archean granite and greenstone at the unconformity below 2300-million-year-old fluvial sediments of the Huronian supergroup. exposed mainly in mines north of Lake Huron (Canada), gave information on the evolution of the atmosphere during the Precambrian (Gay and Grandstaff, 1980; Kimberley et al., 1984; G-Farrow and Mossman. 1988). It was inferred that the atmosphere was anoxygenic in Archean and became oxygen rich during the lower Proterozoic (Roscoe, 19731. Thus, the study of paleosols could provide the nature and composition of paleoatmosphere, surface waters, paleoslopes and paleoclimates (Prasad et al., 1994). As quoted by Retallack (19901, when trying to reconstruct very old (Precambrian) landscapes, neither familiar landscapes, nor images of the Moon, Mars and Venus bear as useful information as those provided by what is known about Precambrian paleosols. Reconstructing landform evolution appears easier when dealing with more recent features, because more of them have been preserved. However, this impression is false because a good preservation of these landforms is exceptional. If topographic highs are vulnerable to erosion, lower sites in the landscapes are not really protected. For example. a marine transgression can provoke very important abrasion and, thus, can greatly modify the landscape prior to fossilization. An exact reconstruction of paleolandscapes, in terms of relief as well as climatic conditions, and a good comprehension of the evolution necessitate a sum of various benchmarks, the most useful ones being those provided by a careful study of paleoweathering. The papers in this volume were presented at a special symposium organized with this in mind by Mireille Bouchard (Montreal, Canada) and JeanMichel Schmitt (Paris, France) for the IGCP 3 17 group “Paleoweathering Records and Paleosurfaces” during the Third International Geomorphology Conference in Hamilton, Canada, August 26 to September 3, 1993.

1. Presentation

of papers

The first two papers deal with long-term evolution and paleolandforms in formerly glaciated areas, developing a specific methodology. Jean-Pierre Peul-

vast et al. present a morphological study of the Baie des Chaleurs area (northeastern Appalachians, Canada) and stress that reconstruction of landscape evolution in uplifted basement areas is difficult because of the removal of large volumes of rocks by erosion. Thus, old weathering formations that might be correlated with stages of the morphological evolution are often missing; moreover, glaciation could have partly scraped away remains of the weathering mantle. The authors’ methodological approach, however, led to the identification of several types of paleolandforms. The study of the relationships between the exhumed sub-Carboniferous paleosurface of southern Gaspe’sie and northern New Brunswick with the escarpments bordering the higher planation surfaces inland indicates recent tectonic movements, and a minor amount of differential erosion. Through a detailed analysis of the topography of Sweden, Kama Lidmar-BergstrGm examines the relation between the surface of the Precambrian basement and its cover rocks, the erosional surfaces in the Caledonian and the Precambrian surfaces and the evolution of the valley patterns. She identified and mapped three kinds of the Precambrian basement: a sub-Jotnian surface below cover rocks in downfaulted positions, an exhumed sub-Cambrian peneplain and exhumed sub-Mesozoic etch-surfaces comprising valley landscapes and undulating hilly relief. New polycyclic reliefs that have evolved after erosion of cover rocks or were never covered were also identified. The formation of valleys and polycyclic relief as well as the exhumation of the old relief is a consequence of uplift of the Scandinavian peninsula. Five main periods of denudation of importance for the present relief in Fennoscandia were identified with intervening tectonic episodes. Thus, the author considers that the shields were already denuded to low relief surfaces in the Late Proterozoic/Early Palaeozoic. Because of uplift and subsidence in the basement, different parts were exposed at different times providing the surfaces with distinct relief types, which subsequently were covered. Careful interpretation and dating of the topographies leads to conclusions on the timing of uplift and subsidence, whereas the study of polycyclic surfaces and valley patterns gives other clues to the understanding of erosional history. The next three papers address weathering profiles

M. Bouchurd, J.-M. Schmirt/GeomorpholoRy

in subtropical areas. In central Nigeria, paleoenvironmental changes are reflected in relatively datable paleosols. Reinhard Zeese shows that the investigation of relict properties in weathering profiles is one important tool for detection. Deeply weathered volcanic rocks of the Jos Plateau, capped with ferricretes and also containing intercalated remains of former soil ferricretes can be separated into a lower section without bauxitisation, but often with total bleaching of the saprolite, a middle section saprolite with bauxite and hematite and an upper kaolinitic section with hematite and goethite. A wet and hot climate can explain the bleaching by total removal of Fe2+ and bauxitisation, when groundwater is permanently high, whereas the upper section experienced well drained fluctuating groundwater conditions, that may be the result of plateau uplift. Concepts of landscape evolution in Nigeria must take into account changing environmental conditions. In India, Hema Achyuthan presents the geomorphic evolution of laterites and lateritic soils and discusses the genesis a paleo-ferricrete surface. The ferruginisation observed in a profile indicates an earlier humid hot phase. Laterisation was not continuous and has taken place when conditions were humid with a rich source of iron and good internal drainage. The ferricretisation took place under the influence of a fluctuating water table, probably associated with the Middle to Late Tertiary high sea level. Closed depressions of various sizes are a common - and somewhat enigmatic - landform in humid tropical areas on non-calcareous bedrocks, especially in Africa and South America. Heloisa Filizola and RenC Boulet show that chemical erosion is the main agent for the evolution of closed depressions and associated valleys in humid tropical Brazil, near S’?o Paulo. Datable ancient peat layers made it possible to assess the rate of sinking of these depressions. Finally, Yvonne Battiau-Queney proposes an interesting and useful classification of paleoweathering formations based on geomorphological criteria. She distinguishes and illustrates four main types of weathering formations and then considers the relation between chemical weathering and landform development, and also the climatic significance: (1) paleoweathering formations buried beneath a sedimentary or volcanic cover, (2) those trapped in

16 (1996) 1-4

karsts, (3) relict paleoweathering durated or not), and (4) polyphased tles.

3

formations (inweathering man-

2. Conclusion Paleoweathering relicts are very useful tools to reconstruct landscape development and provide insights into the evolution of major kinds of surface environments. A better understanding of weathering processes greatly helped paleosurface analysis. Clues provided by the close study of mineralogy and geochemistry of paleoweathering profiles led to the identification of climatic conditions and landscapes contemporaneous with formation. Well characterized paleoweathering profiles can then act as witnesses of the paleosurface and permit correlation of different paleosurfaces. They may also provide useful criteria for dating paleolandforms and assessing the development rate. Together with landform analysis, this approach leads to a better understanding of landscape evolution.

References Fairbridge, R.W., 1988. Cyclical patterns of exposure, weathering and burial of cratonic surfaces, with some examples from North America and Australia. Geogr. Ann., 70A(4): 277-283. Gall, Q., 1992. The early Proterozoic Thelon paleosol as part of the Matonabbee unconformity in the northwestern Canadian Shield. In: J.-M. Schmitt and Q. Gall (Editors), Mineralogical and Geochemical Records of Paleoweathering. Ecole des Mines de Paris, Mtmoires des Sciences de la Terre, 18: 163-174. Gay, A.L. and Grandstaff, D.E., 1980. Chemistry and mineralogy of Precambrian paleosols at Elliot Lake, Ontario. Precambrian Res., 12: 349-373. G-Farrow, C.E. and Mossman, D.J., 1988. Geology of Precambrian paleosols at the base of the Huronian Supergroup, Elliot Lake, Ontario. Precambrian Res., 42: 107- 139. Hall, A.M. and Sugden, D.E., 1987. Limited modification of midlatitude landscapes by ice sheets: the case of northeast Scotland. Earth Surf. Process. Landforms, 12: 53 l-542. Kimberley, M.M., Grandstaff, D.E. and Tanaka, R.T., 1984. Topographic control on Precambrian weathering in the Elliot Lake uranium district, Canada. J. Geol. Sot. Lond., 141: 229-233. Lidmar-Bergstrom. K., 1982. Pre-Quatemary geomorphological evolution in southern Fennoscandia. Medd. Lunds Univ. Geogr. Inst. Avh. 91 ./Sver. Geol. Unders. C 785, 202 pp.

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