Caves in Granitic Rocks: Types, Terminology and Origins
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Cadernos Lab. Xeolóxico de Laxe ISSN: 0213 - 4497 Coruña. 2008. Vol. 33, pp. 35 - 57 Caves in granitic rocks: types, terminology and origins TWIDALE, C. R.1 and BOURNE, J. A.1 (1) School of Earth and Environmental Sciences, Geology and Geophysics, University of Adelaide, Adelaide 5005, South Australia Tel.: +618 8303 5392; fax: +618 83034347; E-mail address: [email protected] Recibido: 1/11/2007 Revisado: 4/3/2008 Aceptado: 20/6/2008 Abstract Caves or openings of various shapes and sizes are well and widely developed in granitic rocks as well as in other lithological environments. Some are caused by preferential water-related weathering, e.g. hydration, others to sapping, but haloclasty plays a crucial role in the deve- lopment of tafoni. These are especially well represented in granitic exposures. This can be explained partly because the inherent strength of the crystalline rock permits hollowed blocks, boulders and sheet structures to remain standing. The hollows themselves owe their origin partly to the susceptibility of feldspar and mica to hydration and other forms of water-related alteration, and also to the capacity of haloclasty to rupture and break down the rock. On the other hand, dry granite is relatively stable and, particularly if it is cemented by salts concen- trated at and near the surface by lichens and mosses, it forms the crusts or enclosing visors that are an essential part of tafoni morphology. Key words: cave, niche, shelter, alveole, tafone, hydration, haloclasty 36 Twidale and Bourne CAD. LAB. XEOL. LAXE 33 (2008) INTRODUCTION produce sheet tafoni (see below) but others are spaces of roughly triangular cross-section To the layman a cave is ‘an underground left vacant by the weathering and dislocation hollow with access from the ground surface or of wedges of rock at the exposed ends of from the sea’ (HANKS, 1986, p. 253), a state- sheet structures and generated by shearing ment that is almost identical to the formal or along sheet fractures (TWIDALE et al., 1996; technical definition of a cave as ‘a natural figure 1a). Others are caused by the weather- underground open space, usually with an ing, near the present or former ground level, opening to the surface’ (BATES and JACK- of obliquely intersecting cross joints or of SON, 1987, p. 105; unless otherwise stated, weaker rocks (figure 1b). In addition they are all definitions cited in this paper are taken especially common where the fresh granite from this source). A cavern is a large cave or a underlies either an indurated veneer (figure complex of caves. In general parlance, howev- 1c) or a regolithic duricrust capping such as er, any natural shelter or overhang also is laterite or silcrete (figure 1d). Thus on referred to as a ‘cave’. plateaux near Cue and at The Granites, near Caves are of various shapes, sizes, and ori- Mt Magnet, both in the central Yilgarn Craton gins. of Western Australia, shelters are frequently formed at the base of the bluff (figures 1e and HOLLOWS ASSOCIATED WITH STEEP 1f). They are developed in kaolinitic mottled SLOPES: NICHE, SHELTER, SLOT, and pallid zones of the laterite. The location ALCOVE, NOTCH of such shelters varies according to the geom- etry of the faceted slope for they occur at A ‘niche’ is a shallow cave, recess or re- bluff and debris slope, the extent of which entrant produced by weathering and erosion varies spatially and in the long term temporal- near the base of a rock face or bluff. A ‘shel- ly. Shelters at plain level are referred to as ter’ is a long and deep niche or coalescence cliff-foot caves. of niches. The term ‘alcove’ used both in its Niches and shelters are most commonly geologic and general senses denotes a deep developed in sedimentary terrains as a result niche formed in a precipitous bluff or wall. of seepage at the base of a bluff, at the inter- ‘Notch’ used in a coastal context is compara- face of the permeable rock exposed in the rock ble to shelter but in its broader sense the face and the impermeable detritus accumulat- word can denote a small alcove or, and more ed in the so-called debris slope (so-called, commonly, a narrow passageway or slot. because by contrast with talus or scree slopes, Niches and shelters are characteristic of the debris from which it takes its name is most faceted slopes, and are typically located commonly discontinuous and/or only a few where a permeable caprock or regolith is in centimetres thick). contact with an impermeable lower forma- Though developed in Miocene Mannum tion. Rarely, where the bluff has regressed Limestone, rather than granite, bluffs bound- and simultaneously migrated upslope and ing the Murray Gorge in South Australia dis- been reduced in height as the debris slope play every gradation between slopes consist- extended upwards, the remains of former ing wholly of a bluff, to a graded slope com- shelters with indurated walls and ceilings are prising upper convexity and lower concave preserved. debris slope, distributed according to position In granitic terrains niches and shelters are with respect to the meandering river (TATE, a fairly common occurrence. Some are attrib- 1884; TWIDALE, 2000). They provide evi- utable to weathering along sheet fractures to dence of how shelters are developed. CAD. LAB. XEOL. LAXE 33 (2008) Caves in granitic rocks 37 (a) (b) (c) Figure 1. (a) Void left by the dislocation of a triangular wedge of rock, eastern flank of Ucontitchie Hill, Eyre Peninsula, South Australia. Scale provided by the late George Sved. (b) Shelter due to ground level weathering, Middle Tor, Dartmoor, south- western England. (c) Shelter high on slope developed beneath indurated crust, but in massive granite, Kokerbin Hill, southwestern Yilgarn Craton, Western Australia. 38 Twidale and Bourne CAD. LAB. XEOL. LAXE 33 (2008) (d) (e) (f) Figure 1. (d) Shelter in kaolinised zone developed beneath local silcrete (possi- bly a stream deposit in pre- dominantly lateritic carapace), The Breakaway, between Hyden and Norseman, Yilgarn Craton. (e) Shelters developed high on the slope at the base of bluffs in weathered (lateri- tised) granite near Cue, north- ern Yilgarn Craton, and (f) near plain level at The Granites, Mt Magnet, central Yilgarn Craton. CAD. LAB. XEOL. LAXE 33 (2008) Caves in granitic rocks 39 The Limestone is a calcarenite that con- Thus niches are caused by weathering and sists of some 65% CaCO3 and is more suscep- flushing of groundwaters at the permeable- tible to water attack than is granite. The outer impermeable interface. They increase in size shell of the limestone is cavernous and is rid- and coalesce to form small shelters. Thereafter dled with tubes and hollows. Where debris the elongate hollows are enlarged by the gran- slopes are developed the junction between ular disintegration of the walls and ceilings as a bedrock and detritus is marked by numerous result of water seeping through the permeable niches which in the past have merged to form country rock and dissolving the cement that shelters. From about 3000 years ago and up to binds the calcite and silica fragments. Similar perhaps the end of the Nineteenth Century the processes are responsible at suitable sites in shelters were occasionally occupied by granitic terrains, only at a slower rate, reflect- ing the greater stability of mica, feldspar and Indigenous/Aboriginal Australians (e.g. quartz as compared with calcium carbonate. HALE and TINDALE, 1930; MULVANEY et al., 1964). Archaeological excavations have revealed steepened bedrock slopes the risers ALVEOLES (OR HONEYCOMB WEA- of which display niches comparable to those THERING) developed at the back of modern shelters. The shelters were occupied periodically and the ‘Alveole’ is preferred to the term ‘honey- stepped morphology of the buried bedrock comb’ because whereas the latter implies geo- slope is related to the rates of aggradation of metric regularity and depth, the former have a the shelter floor. This varies according to random plan distribution and involves shallow whether the particular shelter was deserted, penetration. Alveoles are small hollows devel- when natural relatively slow accumulation of oped on exposed bare and essentially fresh rock debris derived from particles and fragments surfaces. They are typically a few centimetres falling from the shelter walls and ceiling diameter and a couple of centimetres deep. allowed time for backwall niches to develop They are well and widely developed in arid and and steps to form. When the site was occu- semi-arid coastal zones as well as inland sites (see MUSTOE, 1982, p. 108). They are best pied, however, natural accumulation was aug- developed on sandstone, shale, and basic crys- mented by the ash from fires, shells and other talline rocks such as dolerite, as well as on debris (the midden), the rate of accretion limestone and basalt. They are not as well increased and bedrock risers developed developed in fresh granite. Some with well- (TWIDALE, 1964). Given that these events defined septa have been noted in humid tropi- took place in a time span of hundreds of years cal north Queensland, in the monsoonal north it is clear that the backwall niches, presently of Western Australia and on The Humps near developing and evidenced on risers in the Hyden in the southwest of Western Australia excavation, develop quite rapidly considered (figure 2a). Others are developed in weathered in geological terms. granitic rocks where fractures are indurated and Shelters are optimally developed in lime- the enclosed corestones have fallen away. But stone terrains in the humid tropics. Here some alveoles are rare in fresh granite. Irregular and swamp slots or cliff-foot caves extend many ill-defined small hollows in which fresh rock is metres beneath the walls of karst towers and exposed attest the activity of haloclasty but the cupolas and extend also to depths of several margins are diffuse and the intervening septa metres.