The basal rock surface on weathered granitic rocks

The Basal Rock Surface onWeathered Granitic Rocks by BRYAN P. RUXTON and LEONARD BERRY Received 27 August 1957 CONTENTS 1.

2. 3. 4.

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INTRODUCTION DEFINITIONS THE BASAL SURFACE EXHUMED BASAL SURFACES CONCLUSION REFERENCES

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ABSTRACT: Near, or at, the base of the weathering profile on granitic rocks there is often a very rapid, sometimes sudden, change to massive bedrock. The upper limit of the bedrock is designated the Basal Surface. The basal surface does not demarcate the limit of weathering. Above it more highly weathered material allows ready access to sub-surface water while below it the slightly weathered bedrock is much less permeable. When open structural surfaces are confined to near the surface, rapid or prolonged weathering may be expected to develop a well-defined basal surface, regular on sheeting structures and irregular on blocky jointing. Basal surfaces exhumed by erosion may bear evidence of their origin, e.g, the presence of well-rounded corestones in situ on their surfaces. A careful distinction should be made between exhumed basal surfaces and similar surfaces cut by surface agents of erosion.

1. INTRODUCTION the problem of tors in Britain, Linton (1955, 476) claimed that a tor is a residual mass of bedrock produced below the surface by a phase of profound rock rotting effected by groundwater and guided by joint systems. This is followed by a phase of mechanical stripping of the incoherent products of chemical action. He states that'... tors may rise sharply from a basal platform of bedrock which may be flat or inclined and is interpreted as representing the position of the water-table during the period of rock-decomposition, so that tor height cannot exceed the depth of the vadose zone of that period'. Following Linton the authors, from studies in Hong Kong (1957), concluded that 'A thin vadose zone ... would cause a concentration of weathering activity and a mature profile would be expected to develop rapidly; perhaps with a well-marked basal surface'. We also noted that ' ... many local sections indicate a definite lower limit to active weathering which is sometimes marked by a very rapid change to bedrock'. The purpose of this paper is to show that if the term 'basal platform' is changed to 'basal surface' and the latter redefined, a useful concept is introduced into geology. IN DISCUSSING

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2. DEFINITIONS A platform is 'a raised levelsurface' (Oxford English Dictionary) and the term is unsuitable for a feature which is formed under the ground surface. The adjective 'basal' makes the compound term illogical, while the term 'platform' is already widely used in its proper sense. A surface is defined as 'the limits that terminate a solid' (Oxford English Dictionary). We shall consider the solid to be a mass of solid rock inclusive of any structural surfaces whether open or not but exclusive of any weathered debris. The basal surface is then basal in respect of the main mass of weathered material and at the same time defines the upper limit of dominantly unweathered rock. The basal surface is like some other geological contacts-sometimes regular, sometimes intricately irregular, and at all times susceptible to ready generalisation. For some purposes it may be necessary to refer to the actual basal surface, and for others a generalised basal surface will suffice. The solid rock below the basal surface need not delimit weathering activity, though it may serve as a very close approximation to it. Attention is confined to basal surfaces formed by sub-surface weathering in frost-free regions, though similar conditions will probably occur elsewhere. 3. THE BASAL SURFACE

Study of the basal surface has some important applications. Firstly, the pattern of incipient weathering below the generalised basal surface reveals the structural characteristics of bedrock and to some extent predicates the future path of weathering and erosion. Secondly, a detailed knowledge of the basal surface is vital to any engineering project requiring solid bedrock for foundations. Thirdly, apart from a few closely jointed or sheared zones in granitic rocks the only important sub-surface water bodies are found in thick masses of its weathering debris overlying deeply buried basal surfaces (Enslin, 1956). Finally, specimens of solid rock collected from anywhere but a little way below the basal surface may not be completely fresh specimens. Any alteration in them mayor may not be due to weathering but cannot be automatically assumed to be exclusively the result of hypogene alteration. Wide variations in the position and characteristics of the basal surface will occur dependent on climate, rock structure, topography and geomorphic history. We shall consider three types of basal surface developed on sheet-jointed granite, blocky jointed granite, and in the area of a shatter belt (Fig. 1). Convex hills were chosen for illustration because numerous examples of this type occur in Hong Kong and excavations have supplied sections on which our diagram is based. In flatter areas and wide valley courses conditions will not be the same. Nevertheless, it seems a

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general rule that weathering profiles on granitic rocks are deeper when open joints are more numerous near the surface, and deepest along shatter and shear belts. If sheet joints are confined to, or open only near, the ground surface then a convex hill undergoing intense and rapid weathering will develop a dome-like basal surface. In this case the structure largely determines the position of any recognisable water-table which mayor may not develop as weathering proceeds. The water-table does not always determine the position of the basal surface. If rainfall is light and weathering slow the quaquaversal dips on the sheet joints tend to allow a rapid dispersal of any sub-surface water and the concept of a water-table need not be involved in a theory of the formation of a basal surface. On a convex blockily jointed hill with open joints extending some way beneath the surface, circulation of any sub-surface water will depend on the network of intercommunications between the joint-blocks. Weathering and the basal surface will be highly irregular in the early stages. Deep weathering may take place along fracture zones, or masses of greater joint intensity. Later the sub-surface network of joints is integrated owing to weathering along them and a water-table may develop in the weathered debris as in any other permeable material. After an integrated water-table has developed further weathering will be restricted mainly to the vadose zone. The resulting basal surface will be irregular in detail though in some cases it may approximate to a smooth surface. Observations in Hong Kong and the Sudan indicate that sheet jointing often underlies a zone of blocky jointing whenever this ceases at depth and this is indicated in Fig. 1. In many instances the upper sheets are thinner than the smallest diameter of the overlying joint-blocks and corestones. Any zone in which a massive granitic rock has been fractured and shattered attracts and channels sub-surface water and weathers to greater depths than the surrounding areas. The limiting factor to active weathering in this case is probably the depth to which a vigorous mixture of gas and water is maintained. The basal surface in this belt may be expected to be intricately irregular and possibly indistinct. 4. EXHUMED BASAL SURFACES Under constant conditions of climate, slope, geomorphic position, rock susceptibility and erosion, the basal surface will have a definite form and will occur at a certain depth below the ground surface. A change in the rate of erosion or in the relationship between weathering and erosion will either increase or decrease the depth of the basal surface. In some circumstances the removal of the residual debris will become more rapid than the further extension of the weathered mantle. Then lower and lower zones of weathering will be exposed until the basal surface is stripped of its overlying material and becomes exhumed.

WEATHERED GRANITIC ROCKS

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The recognition of such exhumed surfaces is important as with a little sub-aerial modification they may closely resemble other bare rock surfaces formed by sub-aerial erosion. Diagnosis is possible if the surface retains patches of the former weathering profile above it as they may retain characteristics indicating a sub-surface origin. Evidence from Hong Kong and the Sudan suggests that granite breaks up in different ways according as to whether it is exposed at the surface or buried within residual weathering debris. Beneath the surface joint-blocks are usually attacked on all sides at once from the outside inwards, giving rise eventually to well-rounded corestones by spheroidal weathering. If erosion rapidly removes the residual debris from around such a block any incipient spheroidal scaling will be completed but no more will occur. A cuboid joint-block, a sheet 'tablet' or a well-rounded corestone once on the surface may break up in a variety of ways which have never been analysed in detail. These always seem to include cross-fracturing and granular disintegration. Any bare rock surface mantled by well-rounded corestones, some of which can be proved to be in situ, is an exhumed basal surface. But one small patch of corestones on a wide rock plain is not sufficient evidence on which to base a theory for all the plain. It is only evidence of the former presence of one small area of overlying weathering mantle. A small hill on a plain could be weathered and then rapidly stripped giving rise to these conditions (Fig. 2). Thus though a large expanse of bare rock surface may be interpreted as an exhumed basal surface, this can only be proved if abundant corestones rest upon it. In both regions studied some small granite domes have been explained as exhumed basal surfaces. In Hong Kong the domes recorded are mostly

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small islands where severe marine erosion has oversteepened the coastal slopes and most of the weathering mantle has been removed by mass sliding. In the Sudan they are often near the centre of structural compartments and have been stripped by sheet and rill wash. The variety of landforms that may be exhumed from a very thick weathering mantle is considerable. In Hong Kong this mantle varies in thickness from nothing to 300 feet and complete stripping would reveal surfaces with considerable relief. Owing to the tremendous differencein the resistance of the weathered zones and the slightly weathered bedrock to erosion the basal surfaces seem to be little modified during the stripping. Many rocky headlands are clearlystripped basal surfacesjust beginning to suffer from direct marine attack. In arid and semi-arid regions the pediment has often been thought of as a cut rock surface veneered with migrating debris. It is not at all clear whether an original bare rock surface cut by the agents of erosion is variably buried under migrating debris and then variably weathered; or, whether weathering debris in situ and upper migrating material are reclining on a basal surface which is locally exposed by virtue of its own irregularities or by excessive local erosion. Both types could be present in anyone pediment area. We hope to pursue this suggestion further in a future contribution. CONCLUSION The upper limit of solid rock beneath the main mass of weathered debris on granitic rocks is here called the Basal Surface. This is usually irregular in detail but regular when generalised. For engineering excavations and detailed geological study it may be vital to delimit the actual basal surface as accurately as possible, while for most purposes a generalised basal surface will suffice. In localities undergoing excessive erosion or very slow weathering, or both, the basal surface may be exhumed. This bare rock surface may retain well-rounded corestones in situ on its surface testifying to its subsurface origin. Intense weathering accompanying or followed by severe erosion stripping the residual debris from parts of the basal surface is a common form of denudation in Hong Kong. REFERENCES J. F. 1956. Pumping Tests and the Safe Yield of Groundwater Supplies. Proc. geol. Soc. S.Afr., 59, xiii-xlvi. LINTON, D. L. 1955. The Problem of Tors. Geogr, J., 121, 470-86. RUXTON, B. P. & L. BERRY. 1957. Weathering of Granite and Associated Erosional Features in Hong Kong. Bull. geol. Soc. Amer., 68, 1263-92. ENSLIN,