6.36 Solutional Weathering and Karstic Landscapes on Quartz Sandstonesand QuartziteRAL Wray, University of Wollongong, Wollongong, NSW, Australia

r 2013 Elsevier Inc. All rights reserved.

6.36.1 Introduction 464

6.36.2 The Suite of Sandstone Karst Landforms 464 6.36.3 Chemical Weathering of Quartz Arenites 465 6.36.3.1 Silica Solubility and Chemical Kinetics 466 6.36.3.2 Naturally Occurring Silica – Forms and Solubility 466 6.36.3.3 The Effects of pH 466 6.36.3.4 Inorganic Salts and Metal Ions 466 6.36.3.5 Organic Acids 467 6.36.3.6 Bioweathering 467 6.36.3.7 Flushing Rate 468 6.36.3.8 Silica Concentrations in Streams and Groundwater 468 6.36.3.9 The Locus of Chemical Attack 468 6.36.3.10 Arenization and the Formation of Karst in Quartzose Sandstones 469 6.36.4 Large-Scale Landscapes – Ruiniform, Stone Cities, Towers, Corridors, and Grikes 470 6.36.4.1 South America 470 6.36.4.2 Australia 471 6.36.4.3 Africa 473 6.36.4.4 North America 473 6.36.4.5 Asia 473 6.36.4.6 Europe 473 6.36.5 Caves, Shafts, and Dolines 474 6.36.5.1 South America 474 6.36.5.2 Australia 475 6.36.5.3 Africa 476 6.36.5.4 Europe 476 6.36.5.5 Asia and North America 477 6.36.6 Smaller Surface Forms – Rock Basins and Runnels 477 6.36.6.1 Rock Basins 477 6.36.6.2 Runnels 477 6.36.7 Speleothems 478 6.36.7.1 South America 478 6.36.7.2 Australia 479 6.36.7.3 Africa 479 6.36.7.4 Asia 479 6.36.7.5 Europe 479 6.36.7.6 North America 480 6.36.8 Conclusions 480 References 480

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GlossaryDoline A closed depression formed by solution of the

surficial rock or subsidence collapse into an underground void.

Flushing rate The rate at which ions are removed from a

surface. It is directly related to the amount of dissolving

(solvent) fluid crossing the surface.

ay, R.A.L., 2013. Solutional weathering and karstic landscapes on quartz

dstones and quartzite. In: Shroder, J. (Editor in Chief), Frumkin, A.

d.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 6,

rst Geomorphology, pp. 463–483.

atise on Geomorphology, Volume 6 http://dx.doi.org/10.1016/B978-0-12-3747

Grike (gryke) A vertical fissure in rock developed by

dissolution along a joint, typically up to several meters

wide.

Karren or lapis Furrows, channels, or depressions on the

surface of soluble bedrock formed by the action of solution

on the rock surface.

39-6.00140-8 463

464 Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite

Pseudokarst Landforms that morphologically resemble

karst, but have formed by processes that are not dominated

by solutional weathering or solutional induced subsidence

and collapse.

Quartzite A nonfoliated metamorphic rock

(metaquartzite), or very pure sedimentary quartz sandstone

cemented by silica (orthoquartzite). Quartzite is composed

primarily of quartz, often more than 95%, with very minor

amounts of other minerals.

Quartz arenite A sandstone composed of a very high

proportion, generally greater than 90%, of quartz grains

sourced from weathered rock with only limited amounts of

other mineral grains and matrix.

Quartz overgrowth Quartz cement that has precipitated

in optical continuity during diagenesis to enclose and

cement together the grains of a sandstone.

Rate of solubility The speed at which a particular

substance will dissolve in a particular solvent.

Ruiniform Landforms having the appearance of ruins. A

term often applied to landscapes on limestone, sandstone,

or quartzite that are characterized by numerous vertical and

horizontally faced blocks related to the jointing, bedding,

and other structural features of the rock.

Solubility The quantity of a particular substance that can

dissolve in a certain volume of a particular solvent.

Abstract

Landscapes on highly quartzose bedrock that exhibit almost identical scale and morphology to those on karstified lime-

stones occur under a range of climates and on most continents. These include ruiniform towers, grikes, stone cities, caves,

dolines, smaller surface karren, and silica speleothems.

However, these rocks are much less soluble than most carbonates, and the weathering processes are quite different.However, because chemical solution is demonstratively a critical component in the genesis of these landforms, they may be

regarded as karst. This chapter summarizes the processes of karstification in quartz sandstones and then reviews the

incidence of these landforms around the world.

6.36.1 Introduction

One of the more controversial topics in modern geomorph-

ology is the definition and coverage of the terms ‘karst’ and

‘pseudokarst’. The term karst has usually been reserved for

features formed by solution on soluble rocks, typically lime-

stones, whereas pseudokarst landforms are those that look

similar, but which have formed in other rocks. Karst has been

of scientific interest for hundreds of years, but until quite re-

cently similar noncarbonate landforms were almost auto-

matically assigned to the realm of ‘pseudokarst’ – natural

curiosities that not all karst researchers thought particularly

worthy of detailed study.

However, there is still no universal agreement on the use or

boundaries of these terms, and this has been particularly

confused by the recognition during the past three decades,

especially in the last 15 years, of a range of similar forms on

rocks of markedly different composition to the limestones

where the term karst was first applied. Thus, a revolution has

occurred in our understanding of the importance of solution

to the development of some highly quartzose sandstones and

quartzites, where it is now recognized that solution can lead to

the formation of karstic landforms in quartzose rocks very

similar to those in limestones, such as those in the Roraima

region of tropical Venezuela or parts of northern Australia.

However, more than just the presence of solutional wea-

thering is required for sandstone karst development; the so-

lution must act in a trigger role (Ford, 1980) or be ‘‘critical

(but not necessarily dominant)’’ (Jennings, 1983: 21) in the

way that it prepares the rock for the later development of

landforms that otherwise would not have formed. Physical

erosion of weathered material is very important in the

formation of karstic landforms on quartz arenites, but the

common factor that sets them apart from sandstone wea-

thering in general is the ‘‘critical (but not necessarily domin-

ant)’’ preparatory action of chemical solution (Jennings, 1983;

Young et al., 2009).

This necessity of solution, even though arenites are much

less soluble than limestones, has nonetheless prompted a

number of influential scholars to call for recognition of these

landforms as true karst, not just pseudokarst (Mainguet, 1972;

Martini, 1979; Jennings, 1983; Jennings, 1985; Young, 1986;

Wray, 1997a, 1997b; Doerr, 1999; Martini, 2000b; Doerr and

Wray, 2004; Young et al., 2009). They have argued that karst

should be seen as the ‘‘process, solution, which is thought to

be critical (though not necessarily dominant) in the devel-

opment of the landforms and drainage characteristics’’ of an

area (Jennings, 1983: 21).

6.36.2 The Suite of Sandstone Karst Landforms

Examination of the recent geomorphic literature shows that

virtually all karst forms occurring in limestones can be identi-

fied in quartz arenites, in most climates and continents (see

Figure 1), with comparable form, and at compatible scale

(Wray, 1997b; Young et al., 2009). These scales are each roughly

an order of magnitude different in size, and range from

• large bedrock towers and pinnacles, joint-controlled stone

cities, corridors, and grikes, which are all tens to hundreds

of meters in size;

• dolines, shafts, and caves with dimensions also at tens to

hundreds of meters;

1

43

25

6

87

9

14

13

15

1617

18

1923

25

2426

27

2829

333231

30 34 3536

3738

2221

20

1011

12

Figure 1 Location map of solutional weathering and karstic landscapes referred to in text. North America: 1 Mackenzie Mountains, 2Canyonlands National Park, 3 Colorado Plateau, 4 Minnesota, 5 Virginia. South America: 6 Roraima, Venezuela, 7 Mato Grosso State, Brazil, 8Chapada Diamantina National Park, 9 Vila Velha, 10 Minas Gerais, 11 Rio Claro region, 12 Parana State. Africa: 13 Mauritania, 14 Atlas Mts.Morocco, 15 Tassili, Algeria, 16 Niger, 17 Nigeria, 18 Tibesti, Tchad, 19 Egypt, 20 Cape Town and Table Mountain, South Africa, 21 Transvaal,22 Zimbabwe, 23 Jordan. England and Europe: 24 England and Wales, 25 Czech Republic, 26 Poland. Russia: 27 Urals region, Russia. China: 28Wulingyuan, 29 Mt. Danxia. Australia: 30 Purnululu, 31 Keep River, 32 Kakadu, 33 Bunju, east Arnhem Land, 34 Limmen and Abner Range, 35Lawn Hill Gorge, 36 Carnarvon Range, 37 Sydney Region, 38 Grampians Range.

Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite 465

• smaller surface karren or lapies, including runnels and

basins, which are typically tens of centimeters to several

tens of meters in size;

• speleothems normally measured in millimeters to centi-

meters; and

• chemical etching features generally only micrometers to

millimeters in size.

The largest features typically combine to form complex ruin-like

landscapes. These sharply dissected ‘ruiniform’ assemblages

(Mainguet, 1972) occur mainly on sandstones and character-

ized by numerous vertical and horizontally faced blocks related

to the jointing, bedding, and other structural features of the

rock (Wray, 1997b; Grimes et al., 2009a; Young et al., 2009).

Their erosion is driven by the hydraulic gradient, and they are

most common in steeper topography and near the dissected

margins of plateau where joints are most open and surface

runoff velocities are highest (Wray, 1997b; Young et al., 2009).

Ruiniform landscapes commonly form a complex but

progressive and gradational sequence. On relatively flat-

bedded sandstones, this typically begins with a high-level

sandstone pavement that is then dissected into minor joint-

controlled grikefield closer to the plateau margins. The mar-

gins are then further dissected into giant grikefield, and in

areas of more erosion, into a more complex stone city, then

into stone forest and isolated remnant pinnacles on pedi-

ments, and, finally to relatively flat pavement once again at a

lower level.

However, it is important to note two points emphasized by

Grimes et al. (2009a, 2009b). First, that not all components of

sandstone ruiniform landscapes are karstic, because other

erosive agents will be operating, and solutional processes may

not have influenced some of these. It could also be said that

carbonate karst landforms can mimic ruiniform ones – the

important factor being a strong structural control of localized

weathering and erosion, not the presence or absence of solu-

tion (Grimes et al., 2009a).

These larger structurally controlled landscapes are generally

covered by smaller surface lapies forms (small grikes, basins,

runnels, etc.). These are more controlled by the manner in

which water contacts the rock surface (Bogli, 1960; Jennings,

1985) and are generally independent of the larger genetic se-

quence. However, they are just as important in the landscape

as the largest elements because, as they are directly related to

surface erosion, they commonly provide a key to under-

standing the development of the larger landforms (Hettner,

1928). At an even smaller scale, but arguably equally im-

portant, are speleothems and microscopic etch forms that are

clearly related to localized hydro-chemical conditions and the

way in which water moves through the rock.

Before examining the global range of karstic landscapes on

quartz sandstones, it is first necessary to briefly review the

basics of the chemical weathering of silica. Some of the factors

that influence or complicate this critical weathering process

also need to be highlighted. Following this, an examination of

karstic forms at a range of scales and location is presented.

6.36.3 Chemical Weathering of Quartz Arenites

Most quartz sandstones are around 10 times more resistant to

chemical attack than carbonates (Meybeck, 1987), and some

466 Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite

very pure quartzites may be even more resilient. However,

although their susceptibility may be quite low, quartz sand-

stones are not virtually chemically inert as was once suggested

(Tricart and Cailleux, 1972), and there is now much clear and

irrefutable evidence for solutional landforms on quartz

sandstone and quartzites. For a more detailed discussion, see

the more comprehensive review of Young et al. (2009).

6.36.3.1 Silica Solubility and Chemical Kinetics

The solubility of limestone may reach several hundred mg l�1,

depending upon temperature and partial pressure of carbon

dioxide (pCO2) (Jennings, 1985), but the equilibrium solu-

bility of quartz is quite low, generally less than 20 mg l�1

(ppm) at normal temperatures. Therefore, the rate of chemical

weathering of quartzose rocks is generally slow.

In its simplest form, the congruent dissolution of pure

silica can be written (Henderson, 1982; Brady and Walther,

1990; Martini, 2000b) as

2H2Oþ SiO2ðqtzÞ-SiðOHÞ04 log K ¼ �3:7ð25 1CÞKEaH4SiO4ðaqÞ ¼ 1:1� 10�4 ½1�

This Si(OH)4 monomer is the main form of dissolved silica

(Iller, 1979), and in near-neutral pH conditions it generally

exists as uncharged silicic acid, H4SiO4. This is probably the

main reaction in very quartzose sandstones and quartzites

with only very small amounts of accessory minerals or matrix

clay.

However, the weathering of silicate minerals generally

leads to the formation or precipitation of new solid phases,

and the weathering of potassium feldspar, for example, results

in the formation of silicic acid in solution and kaolinite clay

(Aston, 1983);

4KAlSi3O8 þ 22H2O-4Kþ þ 4OH� þ Al4Si4O10ðOHÞ8

þ8H4SiO4ðaqÞ ½2�

The monosilicic acid released in aqueous solution will not

necessarily remain in solution, but may precipitate as

amorphous silica or opal-A, or participate in the neoformation

of other clays (Velbel, 1985). These processes are reversible,

and over time opal can be transformed to a more ordered

form of silica – to cristobalite–tridymite, thence chalcedony,

and finally to quartz (Wray, 1999). Thus, silica is present in the

weathering environment not only as low-solubility crystalline

quartz but also as far more soluble amorphous and poorly

crystallized forms.

6.36.3.2 Naturally Occurring Silica – Forms and Solubility

Silica occurs naturally in eight pure forms: five are crystalline

(quartz, tridymite, cristobalite, coesite, and stishovite), and

three are amorphous (amorphous silica, opal-A, and lechate-

lierite) (Krauskopf, 1956; Jones and Segnit, 1971; Yariv and

Cross, 1979). However, only quartz, opal-A, and amorphous

silica are significant in the study of sedimentary rocks (Siever,

1962).

The silica polymorphs have quite different solubility

(Krauskopf, 1956). For most natural pH levels, the equi-

librium solubility of amorphous silica ranges from

60–80 mg l�1 at 0 1C to 100–140 mg l�1 at 25 1C, and about

300–380 mg l�1 at 90 1C (Krauskopf, 1956; Siever, 1962; Yariv

and Cross, 1979). However, the rate of dissolution of crystal-

line quartz is extremely slow at normal temperatures (around

10�17 mol cm�2 s�1 at 25 1C (Bennett, 1991)), and is con-

trolled by the rate of bond breaking and silica hydration at the

mineral surface (Dove and Rimstidt, 1994; Martini, 2000b).

The equilibrium solubility of quartz at neutral pH is between

6–14 mg l�1 at around 25 1C (Yariv and Cross, 1979) and

about 20 mg l�1 at 50 1C (Adamovic, 2005). The concen-

tration of dissolved silica in cool, neutral-pH groundwater

therefore does not usually exceed 20–30 mg l�1, but in some

hot springs it may reach thousands of mg l�1 (Serezhinikov,

1989).

Therefore, the form of silica and temperature immediately

influence its solubility. However, a range of other factors, in-

cluding pH and the presence of other reactive species, can

influence either the total solubility or rate of dissolution of

silica (Siever, 1962; Yariv and Cross, 1979).

6.36.3.3 The Effects of pH

In the laboratory, silica solubility is greatly influenced by pH.

The solubility of amorphous silica is independent of pH be-

tween pH 2 and about 9.5 (Siever, 1962) (see Figure 2), but

then it rapidly increases into the range of 600–1000 mg l�1

(Yariv and Cross, 1979) as a result of ionization of H4SiO4

(Martini, 2000b).

The solubility of quartz is essentially stable between pH 3

and pH 8, but rises very rapidly with increasing alkalinity (see

Figure 2). Above pH 9.83, it increases to more than 20 mg l�1

at 25 1C (Adamovic, 2005). At highly acid pH, solubility may

also rapidly increase but is poorly understood (Serezhinikov,

1989).

6.36.3.4 Inorganic Salts and Metal Ions

Like many geochemical systems, the total equilibrium

solubility concentrations and the rate of solution of the silica-

water system are influenced by a number of naturally occur-

ring chemical species. For example, in some environments,

mechanical breakdown of many stones is dominated by

physical salt crystal expansion (e.g., Goudie, 1974), but some

salts and metal ions also influence silica solution.

Solutions of salts can have complex chemical effects on

silica solubility. Most inorganic salts in solution tend to de-

crease silica solubility (Okamoto et al., 1957; Iller, 1979), and

the anion species is not particularly important. Thus, in high

concentrations of most salts, the equilibrium solubility of

silica will be very low, particularly if there is little carbonate or

phosphate to provide high pH.

Sodium chloride, however, strongly accelerates the dis-

solution rate of quartz (Yariv and Cross, 1979) and amorph-

ous silica (Kastner, 1981), and quartz solubility appears to be

increased by seawater (about 0.5 M NaCl; von Damm et al.,

1991; Dove and Elston, 1992). Group 1A salts in the solute

2000

1000

500

200

100

mg

l−1

50

20?

?

10

5

0 2 4 6

pH

8 10 12

Quartz

Amorphoussilica(opal)

Figure 2 The relationship between pH and the solubility ofamorphous silica and crystalline silica (Quartz). Reproduced fromKrauskopf, K.B., 1956. Dissolution and precipitation of silica at lowtemperatures. Geochimica et Cosmochimica Acta 10, 1–26; Iller, R.K.,1979. Chemistry of Silica. Wiley, New York, NY, pp. 21–28, andSerezhinikov, A.I., 1989. Silica in acid natural solutions. Transactions(Doklady) of the USSR Academy of Sciences: Earth Science Section298, 134–138.

Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite 467

may also change either or both the solubility and the rate of

solution of quartz (Dove and Crerar, 1990; Bennett, 1991;

Dove and Elston, 1992).

Some metal ions may also influence silica solubility. Yariv

and Cross (1979) and Reardon (1979) suggested that mono-

silicic acid solubility is affected by multivalent (but not

monovalent) metallic cations, particularly in acidic waters.

Iron-silicate complexing, for example, the formation of

FeH2SiO4, markedly increases quartz solubility under oxi-

dizing conditions, potentially by a factor of 10 over that of

amorphous silica (Reardon, 1979; Morris and Fletcher, 1987;

Serezhinikov, 1989).

However, the effects of dissolved aluminum seem to be

different; with Beckwith and Reeve (1969) and Mullis (1991)

finding that low concentrations of Al3þ strongly inhibit quartz

dissolution. Bennett et al. (1988) found no apparent correl-

ation between dissolved aluminum and quartz dissolution.

6.36.3.5 Organic Acids

Organic compounds, particularly organic acids, also influence

silica–water interactions. Siever (1962) thought that some

organic compounds lowered the solubility of amorphous

silica, but other organic compounds form organic-silica

complexes, which increase the solubility of both amorphous

and crystalline silica (Huang and Keller, 1970; Jackson et al.,

1978; Yariv and Cross, 1979). Bennett (1991) also noted that

several organic polar anions chelate silicic acid in solution,

thereby increasing both the apparent equilibrium solubility

and rate of quartz dissolution.

Bennett et al. (1988) showed that dissolved silica concen-

trations correlate with dissolved organic carbon, and noted

that the reactivity of quartz with several organic acids could be

defined by the reaction series: citrate4oxalate4salicylate4acetate (see also Bennett, 1991). Huang and Keller (1970)

found that a similar system complexes aluminum to accelerate

the dissolution of alumino-silicates at acidic pH. Multi-protic

acids (oxalate and citrate) or multifunctional acids (salicylic

acid) accelerated quartz dissolution, whereas acetic acid, a

mono-functional mono-protic acid, does not (Bennett et al.,

1988). Ghosh (1991) thought that multifunctional organic

acids accelerate the dissolution of quartz by decreasing the

activation energy of the reaction.

However, in this complex area of silica chemistry, field

measurements do not always correspond with results from

controlled laboratory conditions. For example, in the Brazilian

field studies of Wiegand et al. (2004), silica-organic com-

plexing did not seem to indicate increased quartz solubility,

but laboratory work did show accelerated dissolution when

organic concentrations were high. Martini (2000b) also noted

that waters off most quartzites have a low pH because of or-

ganic acids, and thought these acids may be responsible for

increasing the rate of dissolution, but he also noted that or-

ganic complexing with silica is only effective at high pH.

6.36.3.6 Bioweathering

It has been known for a long time that biological agents, most

notably plants and fungi, also contribute directly to the

physical and chemical breakdown of most rocks; however,

interest in the nature of bioweathering on both natural and

man-made stone surfaces has increased in recent years (e.g.,

Kumar and Kumar, 1999; Papida et al., 2000; Warscheid and

Braams, 2000; Stretch and Viles, 2002; Burford et al., 2003;

Hoffland et al., 2004; Turkington and Paradise, 2005). It is

now accepted that the destruction of quartz sandstone surfaces

may be accelerated (or even temporarily slowed) by the

growth and actions of biological agents, and dissolved silica

may be released into the environment.

Low-order biological organisms generally degrade sand-

stone surfaces (e.g., Cooks and Pretorious, 1987; Hale, 1987;

Wessels and Schoeman, 1988; Cooks and Otto, 1990; Robin-

son and Williams, 2000; Turkington and Paradise, 2005). Apart

from the obvious physical effects of penetration of plant, li-

chen, etc., parts between grains of arenites (Paradise, 1997;

Duane, 2006), some fungi and bacteria also produce organic

acid solutions that may react with silica (Henderson and Duff,

1963; Paradise, 1997; Duane, 2006). Over long periods, the

cumulative effects of such small-scale reactions could contrib-

ute significantly to the weathering of many minerals including

quartz. Some lichens while actually degrading the rock also

protect and bind the rock surface while they live, but when

they die, they expose the weathered rock to erosive processes

468 Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite

(Viles and Pentecost, 1994; Hoatson et al., 1997; Kurtz and

Netoff, 2001; Bjelland and Thorseth, 2002; Goudie et al., 2002;

Mottershead et al., 2003; Paradise, 2003).

6.36.3.7 Flushing Rate

The rate at which ions are removed from a surface is directly

related to the amount of dissolving fluid crossing the surface –

the ‘flushing’ rate (Douglas, 1978: 230). The importance of

this flushing rate to the silica–water system was emphasized by

Rimstidt and Barnes (1980) and by Doerr (1999), who argued

that in the hot and extremely wet conditions (up to around

7500 mm p.a.) of tropical Venezuela, high water throughput

has countered the very low total solubility to produce the

spectacular karst landscapes within the intensely weathered

Roraima Quartzites.

Therefore, for a given solubility regime, higher rates of

flushing will increase silica loss. However, Berner (1978) em-

phasized that increased flushing only increases dissolution up

to a limiting amount, controlled by the rate of mineral re-

activity, beyond which additional flushing has virtually no

effect.

6.36.3.8 Silica Concentrations in Streams andGroundwater

Analysis of surface and groundwaters demonstrates the low

solubility of quartz in natural environments. Pure arenites and

quartzites yield waters with quite low dissolved loads and

silica the dominant solute.

For example, waters draining from the Roraima quartzites

in tropical high-rainfall Venezuela, the area of best-developed

solutional landforms, have very low dissolved silica concen-

trations. Chalcraft and Pye (1984) measured o1 mg l�1 dis-

solved silica in table-mountain waters, and 5–7 mg l�1 in

surrounding rivers. Mecchia and Piccini (1999) measured

0.18–0.52 mg l�1 in surface runoff and underground fractures

to 80 m depth, and 0.92–1.30 mg l�1 in deep (300–350 m)

fractures at the Aonda Cave System. Aubrecht et al. (2008b)

found low dissolved silica between about 0.25 and

1.54 mg l�1 on the Chimanta Massif and Mt. Roraima.

In cooler and dryer Brazil, Wiegand et al. (2004) found

much higher values, but maximum dissolved silica was less

than 8 mg l�1. In cool-temperate Germany, Striebel and

Schaferjohann (1997) found around 8 mg l�1 in a main

stream and slightly higher 9–16 mg l�1 in smaller lateral tri-

butaries, and similar values have been found by Wray and

others for surface and groundwaters on highly quartzose

sandstones near Sydney in southeastern Australia. These

higher concentrations may reflect less dilution in these lower

precipitation areas compared with the very wet Roraima.

In temperate South Africa, Martini (2000b) found that

dissolved silica from the quartzites of the Table Mountain

Formation averaged 6.3 mg l�1. However, Grimes et al.

(2009a) found that dry-season sandstone surface waters in

northern Australia showed much higher dissolved silica values

between 1.75 and 24 mg l�1, with most readings close to the

average of 13.4 mg l�1.

Interestingly, Mainguet (1972) also found that silica con-

centrations of some southern African streams are highest at

the end of the dry season and declined by a few mg l�1 during

the wet season. She considered that this was due to the waters

issuing at the end of the dry season having had the longest

residence time in the rock. This probably accounts for the high

values found by Grimes et al. (2009a), but wider studies are

needed.

6.36.3.9 The Locus of Chemical Attack

The actual process of solution is occurring at an atomic or

molecular level and there is much microscopic evidence for

attack and etching of the surface of quartz grains and over-

growths (quartz cement that has precipitated in optical con-

tinuity during diagenesis to enclose the grains).

White et al. (1966) argued that in the Roraima quartz is

hydrated to much more soluble opal, then dissolved. How-

ever, Martini (1979) rejected this mechanism because at sur-

face conditions the transformation of quartz to opal is

thermodynamically not possible. Chalcraft and Pye (1984)

also rejected hydration, and provided clear thin-section and

scanning electron microscope (SEM) evidence for direct so-

lution of quartz from grains, overgrowths, and cement, lead-

ing to widening of grain-to-grain contacts and the freeing of

individual detrital grains (Chalcraft and Pye, 1984).

Ghosh (1991) also found microscopic evidence that in

surface outcrops of weathered Roraima quartzites dissolution

of quartz cement along overgrowth boundaries has led to a

clear network of lamellar porosity, whereas unweathered sub-

surface samples show abundant welding of grains by syntaxial

quartz cement and sutured grain-to-grain contacts. Ghosh

(1991) therefore argued that this variation shows that quartz

dissolution is a surface phenomenon, and did not originate at

depth in a process recognized by petroleum geologists in the

formation of similar secondary sandstone porosity (Pye and

Frinsley, 1985; Burley and Kantorowicz, 1986; Hurst and

Bjorkum, 1986; Shanmugam and Higgins, 1988).

Crook (1968) had argued that only chemical solution

could explain the embayments and smoothed surfaces visible

on detrital quartz grains, and Burley and Kantorowicz (1986)

provided photographic evidence for solutionally produced

quartz grain surface features including tiny pits, notches, and

embayments. They found that these corrosion features tended

to be more intense on the surfaces with high free-surface en-

ergy such as grain peripheries, along fractures and between

crystal boundaries.

Burley and Kantorowicz (1986) proposed two mechanisms

of quartz corrosion:

• Transport-controlled dissolution controlled by the rate of trans-

port of ions to and from the reaction surface. This results in

rapid, nonspecific corrosion at all available sites and is

typical of attack by strongly concentrated solutions.

• Surface-reaction-controlled dissolution controlled by the reaction

rate at the solid–fluid interface. This is generally a slow and

more specific dissolution, common of relatively insoluble

minerals in solutions of low chemical reactivity. It tends to

produce distinct crystallographically controlled features

such as well-defined notches or ‘v-pits’.

(a)

(b)

Figure 3 SEM images of dissolution within the sandstone at Bunju inArnhem Land Australia. (a) Most intense dissolution at areas ofhighest free-surface energy, namely grain surfaces and edges of quartzovergrowths. Overgrowth surfaces are relatively uncorroded. Also notethat despite the high degree of quartz overgrowth development, thereare still pathways between grains, imparting a high primary porositythat has allowed easy penetration of weathering solutions. (b)Crystallographically (surface-reaction) controlled v-shaped etch pits(indicated by arrows) on detrital grains and overgrowths.

Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite 469

Hurst and Bjorkum (1986) challenged Burley and Kantor-

owicz (1986), arguing that quartz dissolution rates are too low

for transport-controlled etching, and emphasized that etching

will concentrate at sites with the highest free energy, and,

because quartz overgrowth lowers surface free energy, dis-

solution will be most intense on detrital grain surfaces, face

corners, and edges of overgrowths. Brady and Walther (1990)

and Withe and Peterson (1990) also supported this idea of

preferential solution at high-energy surface sites.

SEM studies from a number of areas provide some evi-

dence in support of these contentions, and also some con-

tradictory ideas. Young (1988) provided graphic SEM evidence

of both surface-reaction and transport-controlled dissolution

from the east Kimberley of Western Australia. He found

widespread dissolution at surface high-energy locations, and

also found that the intensity of quartz etching in the Kim-

berley was variable and closely related to the primary porosity

of the host rock; the more porous sandstones allowed greater

penetration of corrosive solutions and are typically more

deeply weathered than those sandstones with very low pri-

mary porosity and lacking interconnected pathways for fluid

penetration. Dissolution could also increase primary porosity.

Similar results were found by Grimes et al. (2009a) from

other northern Australian quartz sandstones, where clear evi-

dence was apparent for the effects of primary porosity, surface

free energy, and both nonspecific, transport-controlled dis-

solution and small crystallographically controlled v-pitting

(Figure 3).

There is also clear evidence for solutional etching of quartz

in sandstone areas outside the humid tropics. In the cool-

temperate Sydney area of southeastern Australia, Wray (1997a,

1997b) found similar variability linked to the degree of pri-

mary porosity, and both surface- and transport-controlled

dissolution features. Detrital quartz grains, grain-to-over-

growth boundaries, overgrowth-to-overgrowth contacts, and

other similar discontinuities or defects are generally far more

corroded than most overgrowth faces. Etching of quartz

overgrowths is most intense on the rhombohedral faces and

edges, especially in the most weathered sandstones, whereas

less attack occurs on the overgrowth prism faces.

In South Wales, Wilson (1979) presented SEM images of

small v-shaped chemical etch pits on the quartz grains of the

Millstone Grit that he attributed to slow solution by high pH

fluids, and Battiau-Queney (1984) suggested that a long per-

iod of subaerial weathering, possibly under a hot wet tropical

climate, was responsible for this quartzite being so extensively

weathered.

Solutional weathering is thus a highly significant and

critical stage in the silica karst process. However, although the

focus of attack within the rocks is generally clear, some of the

detailed impacts of this process are only beginning to be

understood.

6.36.3.10 Arenization and the Formation of Karst inQuartzose Sandstones

In many carbonates, around 80% or more of rock bulk is

soluble (Jennings, 1985), but in the purer quartz arenites the

dissolvable fraction is no more than 10–20% (Martini, 1979),

and silica may be dissolved not only from cement but also

directly from quartz grains and overgrowths. Therefore, in

most arenites, solution does not remove the majority of rock

bulk, and the landscapes are shaped by a combination of silica

solution assisting later mechanical erosion.

Martini (1979, 1987, 2000a) studied quartzite karst in

southern Africa and introduced the term ‘arenization’ for the

interaction of chemical solution and mechanical erosion that

can lead to quartzose karst development. He suggested that

slow chemical dissolution of quartz, especially along crystal

boundaries, frees individual grains making the rock less co-

herent, mechanically weaker (particularly in shear and ten-

sion), and more susceptible to physical erosion than it

otherwise would have been. Mechanical removal of sand grains

and general rock bulk follows by piping and fluvial processes.

Thus, while processes other than true solution largely erode

the landscapes, critical (though not necessarily dominant)

470 Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite

solution (Jennings, 1983) has played an essential precursor or

trigger role (Ford, 1980: 345). Un-arenized rock would not

erode in the same manner or form the same landforms as

arenized rock (Jennings, 1983; Martini, 2000a).

Total rock solubility is, however, not the only controlling

factor in arenization. Reaction rates between silica and water

near the Earth’s surface are slow enough to often be out of

equilibrium (Brady and Walther, 1990), and, if equilibrium is

not attained, many geological processes can only be under-

stood in terms of reaction kinetics (Rimstidt and Barnes,

1980). Martini (2000b) therefore argued that in the formation

of silica karst the thermodynamics and, more particularly, the

complicated kinetics, or rate of reaction, are just as important

as total solubility.

The faster the reaction rate, the shorter is the distance a

solution can penetrate before attaining saturation and the

cessation of the reaction. Martini (2000b) therefore believed

that a fast reaction rate results in near-surface arenization and

general surface lowering and precludes deep karstification.

Slower rates allow deeper penetration of solutions and joint

widening without general surface lowering; and slower rates

still allow crystal boundary solution with a deep chemical

weathering of the rock. Voids along crystal boundaries are

generally very thin and water circulation is very slow; hence,

unless the reaction kinetics are very slow, saturation will be

reached only after a very short distance into the rock. Martini

(2000b) therefore argued the somewhat perplexing idea that if

the total solubility of silica remained the same, but the rate of

silica solution were even slower than its present very slow rate,

karst on quartzite would be much more common.

These ideas for arenization are supported by numerous

observations from the Roraima by Piccini (1995), Doerr

(1999), Mecchia and Piccini (1999), Galan et al. (2004), and

others, who all noted that the deeply weathered (arenized)

quartzite easily breaks down to sand that is removed by

flowing water. Most observers agreed that arenization acts

most effectively along joints, where water most easily pene-

trates and has a greater time for reaction compared to surface

water. Surface runoff has little time to dissolve silica cement,

and the alternating wet and dry conditions may actually

form hard crusts of silica and iron oxide that protect the rock

surface.

Briceno and Schubert (1990), Piccini (1995), and Doerr

(1999) have also noted that the very stable and long-term

environmental conditions have limited mechanical wea-

thering in the Roraima, allowing the strong development of

solution forms. Mechanical processes are active, but their

effects are mostly concentrated along the streams, especially

near the border of the plateau, and inside the active caves.

Doerr (1999) listed a combination of factors that he con-

sidered to have promoted the intense karstification of the

Roraima quartzites:

• the unweathered rock is of very low porosity, but dense

joints and microfissures have allowed water to infiltrate

deeply and initiate corrosion;

• the arenite is very pure, and so fissures enlarged by cor-

rosion are not clogged by excess weathering residue;

• the low total solubility of silica is offset by very high

precipitation;

• there is little soil or other sediment to block water ingress

and thus prevent karstification; and

• very long periods of stable subaerial exposure and wea-

thering have been available with no other processes such as

glaciation, periglaciation, submergence under the sea, or

frost weathering to interfere with karstification.

The arenization process also operates outside Roraima in drier

regions. Mainguet (1972) described undoubtedly arenized

sandstone landscapes from central Africa, and Martini (1979,

1982, 2000a) gave examples from southern Africa. Many of

the quartz sandstone of northern Australia also show excellent

evidence for deep arenization and Jennings (1979), Young

(1986, 1987), and Grimes et al. (2009a) all reported wide-

spread examples of complex ruiniform landscapes formed

where poorly bound sand is clearly eroding from deeply are-

nized sandstones.

An alternate hypothesis for the formation of many surface

and underground features in the Roraima has recently been

proposed by Aubrecht et al. (2008b). They suggested that

arenization is not totally responsible for the formation of

poorly bound quartzite, but rather suggested that certain beds

within the orthoquartzite have only ever been poorly lithified

or even never actually lithified. These ideas are supported by

observations of numerous well-lithified pillars of rock pene-

trating surrounding poorly lithified sands, which they believed

indicate localized, rather than all-encompassing flow of the

cementing solutions. They believed that it is the erosion of

poorly lithified beds rather than arenized rock that leads to the

formation of many surface and underground features. These

are interesting ideas and worthy of further consideration,

particularly as they may be supported (to a lesser degree) by

observations of similar pillars by Grimes et al. (2009a) in

some northern Australian sandstones. Further consideration is

certainly needed of this idea.

The global inventory of landforms inexorably linked to the

solution of silica has been growing rapidly during the last few

decades. They are now identified on most continents, under

most climates, and, as already noted, at a range of scales. Next,

representative examples of these landforms are briefly dis-

cussed. Further recent and detailed analysis can be found in

Young et al. (2009).

6.36.4 Large-Scale Landscapes – Ruiniform, StoneCities, Towers, Corridors, and Grikes

At the largest scale, the preferential erosion of arenized joints

and other structural features, such as bedding, produces the

largest ruiniform landforms. These complex, generally angular

or rounded, assemblages are tens to hundreds of meters or

more in size, and commonly appear very similar to the larger

forms occurring in limestones.

6.36.4.1 South America

The world’s best-documented and best-developed ruiniform

landforms are the flat-topped tepui mesas or table mountains

cut in the Precambrian Roraima Group orthoquartzites of the

Venezuelan Guyana Shield. These landscapes are undoubtedly

Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite 471

some of the most spectacular on the planet (Wray, 2010), and

have unique biological and geomorphological characters rec-

ognized for World Heritage. They are also some of the least

explored landscapes – most tepui are very rugged and almost

inaccessible except by helicopter.

The Roraiman tepuis are generally fringed by precipitous

cliffs up to a kilometer high (George, 1989), and their sum-

mits are eroded into intensely complex networks of ruiniform

stone city, isolated towers, joint-controlled grikes, and residual

blocks. The summits are often shrouded by mist and precipi-

tation is very high, around 2800–7500 mm annually (Chal-

craft and Pye, 1984; Doerr, 1999). This high runoff cascades

over the cliffs as high waterfalls or drains via dolines and

shafts into large cave systems to resurge hundreds of meters

lower.

On the tepui summits are complex collections of residual

quartzite towers and smaller pinnacles, chaotic block accu-

mulations, walls, arches, depressions, caves, and many smaller

residual forms resulting from deep solution and arenization

progressively widening joints and other fractures (Tate, 1938;

White et al., 1966; Zawidzki et al., 1976; Urbani, 1977;

Chalcraft and Pye, 1984; Pouyllau and Seurin, 1985; Briceno

and Schubert, 1990; Briceno et al., 1990; Yanes and Briceno,

1993; Piccini, 1995; Doerr, 1997; Doerr, 1999; Aubrecht et al.,

2008b). Piccini (1995) noted that there are two common

types of rock towers or pinnacles on Auyun tepui. Those near

the tepui rim are from 10 to 4100 meters high, and are due to

solution-erosion processes widening fractures opened by

plateau-edge stress release. Back from the rim, the towers are

smaller (up to several tens of meters) and form by solution

and erosion along a regular network of two or more joint sets

near the edges of small step-like, bedding-related, scarps on

the plateau surface.

The Roraima region has been geologically stable for a very

long period (Briceno and Schubert, 1990; Briceno et al.,

1990), and this long exposure to subaerial weathering and the

high precipitation are undoubtedly the key factors in the re-

markably strong development of this karst (Briceno and

Schubert, 1990; Briceno et al., 1990; Doerr, 1999; Wray, 2010).

Karstification has probably been occurring uninterrupted for

at least 70 million years (Yanes and Briceno, 1993), if not

since the mid-Mesozoic (Jurassic) (Briceno and Schubert,

1990; Briceno et al., 1990).

Ruiniform tower assemblages are also reported elsewhere

in South America. For example, Twidale (1987), De Melo and

Coimbra (1996), and De Melo et al. (2004) described com-

plex, stone city landscapes of towers, grikes, corridors, and

labyrinth networks in Palaeozoic quartz sandstones from the

Vila Velha region of southern Brazil. Similar landscapes occur

around Minas Gerais (Correa Neto, 2000) and near Rio de

Janeiro (Avelar et al., 2009).

6.36.4.2 Australia

Hot, humid, tropical environments are not mandatory for the

formation of complex sandstone karst landscapes. De-

scriptions have been published of clearly karstic sandstones in

a number of seasonally arid savannah and even cool tempe-

rate environments (Wray, 1997b; Young et al., 2009), and

Australia is a good case in point; complex solutional wea-

thered sandstones and quartzite landforms occur from hot,

seasonally dry savannah at around 151 S latitude to cool,

moist temperate areas around 35–401 S latitude.

Although smaller than the enormous features of the Ror-

aima, some of the most widespread towered ruiniform sand-

stones in the world are scattered across more than 2000 km of

the seasonally arid tropical savannas of northern Australia,

from the Kimberley region of Western Australia to northern

Queensland. Difficult access in these very remote areas

has limited detailed geomorphological studies, but very

well-developed ruiniform landforms are known from several

Proterozoic and Palaeozoic quartz sandstones.

In the eastern Kimberley Region of Western Australia, the

Devonian quartz sandstone of Purnululu (the Bungle Bungle

Range) has been eroded into thousands of world-famous

beehive-shaped towers and cones (see Figure 4(a)). The area is

ascetically unusual, and distinct, regular horizontal bands of

dark-gray cyanobacteria crust that is not directly related to

bedding characteristically mark the steeply sloping tower sur-

faces. Young (1986) demonstrated microscopically that in-

tense etching and arenization of silica cement and grains were

critical in the formation of these towers. Solution has been so

intense along joints that blocks of the rock can be easily

sheared by hand pressure alone; however, the interlocking

network of grains allows the sandstone to retain a high

compressive strength, allowing it to stand in steep faces, tur-

rets, and sinuous aretes (Young, 1986, 1988). Because of the

critical nature of the solution in imparting the mechanical

properties of the sandstone, Young (1986) argued that despite

a lack of obvious subsurface drainage Purnululu should be

considered as tower karst. In 2003, Purnululu was inscribed as

World Heritage for its importance in demonstrating the pro-

cess of cone karst formation on sandstone, and is ‘‘by far, the

most outstanding example of cone karst in sandstones any-

where in the world’’ (UNESCO, 2009b).

Ruiniform, towered, sandstones with similar morphology

occur in many other northern Australian sandstones, but, until

recently, information has been scarce. In northern Queens-

land, Twidale (1956) described unusual beehives or small

towers (about 6 m in height and 3 m in diameter) in hori-

zontally bedded quartz sandstone, and Jennings (1979, 1983)

described the large Ruined City in Proterozoic quartz sand-

stones of the Arnhem Land region of the Northern Territory.

Johns (1994) also presented photographs of several other ‘lost

cities’ several hundred kilometers further south in the Limmen

and Abner Range areas, and west in the Litchfield area near

Darwin. Recent field studies by Grimes et al. (2009a) exam-

ined each of these areas and confirmed that ruiniform sand-

stones are widespread at these and other areas across much of

north Australia.

Undoubtedly, the finest example of sandstone karst in

northern Australia must be the Ruined City in Arnhem Land

(Figure 4(b)); it rivals Purnululu for esthetic beauty, but dis-

plays a much wider range of sandstone karst forms across its

30 km2 or more of near-horizontal and deeply arenized white

Proterozoic quartz sandstone. Jennings (1979) first described

the Ruined City and gave its aboriginal name as Bourlungu;

however, the local aboriginal owners actually call it Bunju

(Grimes et al., 2009a). Jennings (1983: 21) described Bunju as

(a) (b)

(c) (d)

Figure 4 Ruiniform sandstone landscapes from Australia. (a) The beehive-shaped towers and cones of Purnululu (the Bungle Bungle Range) inthe east Kimberley, Western Australia. Towers in the middle-left foreground rise to around 15–30 m above the dry white sandstone stream bed,whereas towers on the right have formed up the hillside to over 150 m above the stream. The sandstone is deeply weathered, very friable, and isextremely weak in shear and tension, but retains sufficient strength in compression to allow it to stand in steep faces. The horizontal bands ofdark-gray cyanobacteria crust are not directly related to the bedding of the white sandstone. (b) An aerial view of a small part of Bunju (TheRuined City) eroded in white friable sandstone in east Arnhem Land, Northern Territory. The sandstone is gently dipping toward the observer.The upper sandstone units are eroded into thousands of towers around 10–15 m high. These sit on structural pavements cut by long grikesystems around 10–15 m deep. Below this are remnant towers sitting on a lower still sandstone structural pavement. (c) An aerial view of the‘Lost City’ in deeply arenized sandstone of the Abner Range, Northern Territory. Erosion of joints into a plateau margin has left hundreds closelyspaced, slender, vertical pinnacles about 10–30 m high. (d) Eroded giant grike in the moderately dipping sandstones at Wonderland in theGrampians Range of western Victoria. The sandstone dips toward observer at around 251, and the grike is around 15 m deep, 10 m wide, and500 m long. Note person in lower left for scale.

472 Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite

being ‘‘chopped up by meshes of corridors and canyons, and

in parts reduced to towers jumping out of the plain,’’ and this

is an apt description of this very complex landscape. The

sandstone is dominated by bedding and several roughly per-

pendicular joint sets; two are expressed strongly in the upper

sandstone units and erosion has given rise to a suite of hun-

dreds of towers or cones up to several tens of meters high.

These towers are separated by grikes or box canyons, forming a

very complex stone city consisting of parallel walls and towers

separated by networks of streets and open spaces. The lower

sandstone units only have one dominant joint direction, and

this is expressed in broad structural pavements dissected by

long parallel grikes. Cliffs and isolated towers around 10 m

high edge the different sandstone units, and towers isolated by

erosion are common on the pavements and the surrounding

sand-covered plain. Like Purnululu, loose, arenized, sand is

clearly being eroded from the joints and transported by

intermittent streams out onto the surrounding sand plains.

Arches, dolines, and small caves also occur.

Other ruiniform stone cities (lost cities) are found in the

Limmen National Parks and in the Abner Range (Figure 4(c))

several hundred kilometers further south (Johns, 1994;

Grimes et al., 2009a). These consist of gently dipping dissected

plateau edges with many closely spaced, slender, vertical pin-

nacles about 10–30 m high, many of which have height-to-

width ratios of about 5:1, and even 10:1. Although weathering

has also penetrated deeply into these sandstone masses, the

strong rectilinear alignment of the narrow grikes between the

pinnacles, and the influence of the dip forming slightly tilted

pinnacles, shows that weathering and subsequent erosion

have clearly been concentrated down vertical joints (Grimes

et al., 2009a).

Other areas of ruiniform sandstone occur in Kakadu and

Litchfield National Parks close to Darwin, and at Keep River

Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite 473

National Park on the West Australian border. At each of these

areas are complex assemblages of towers and pinnacles and

stone city cut-up by joint-controlled grikes, but these areas

are all much smaller in area than Bunju (Grimes et al.,

2009a). Grimes et al. (2009a, 2009b) proposed a descriptive

and classification system for these very complex ruiniform

stone cities.

SEM imagery of sandstone from all these areas shows in-

tense arenization with both transport-controlled and surface-

reaction dissolution of silica from grains and overgrowths.

Variation in free-surface energy also controls the location of

dissolution.

Australian quartz sandstone karst is not limited to the

tropics. In the Permo-Triassic quartz sandstones near Sydney,

in temperate southeastern Australia (Young and Young,

1988; Wray, 1997b; Young et al., 2009), are large numbers

of near-vertical walled towers and beehive-shaped towers

and rectilinear joint-controlled networks of narrow, deep

grikes. None are as well developed as the northern Australian

stone cities, but strong lithologic control and the preferential

erosion of joints have controlled the development of

these towers in weathered quartz sandstone. The quartz

sandstones of the Grampians Range in drier western Victoria

also host an impressive array of solutionally influenced

forms (Young et al., 2009). Most notably are the large

corridors and grikes of the Wonderland area (see Figure 4(d)),

where the weathering of joints has produced intersecting

joint corridors several hundred meters long and tens of

meters deep.

6.36.4.3 Africa

Mainguet (1972) described a number of ruiniform landscapes

from presently arid Africa, where solution is believed to

have played a critical role. These include the Ennedi region of

Chad, central Africa, and she gave as a type example the in-

tensely dissected surfaces of the Gara sandstones in Mauri-

tania. Other examples of similar landscapes occur extensively

in sandstones across the Sahara but not all authors have

confirmed the importance of solution: through Tassili and the

Plateau du Djao (Busche and Erbe, 1987) in Algeria, or in the

Nubian sandstones of Egypt (Osborn, 1985). Some of these

assemblages are truly astonishing, notably the incredible

arrays of towers cut in Cambrian sandstone that jut up ab-

ruptly from extremely flat erosional surfaces in the Rui-

nenlandschaft of the Tibesti of the central Sahara (Furst, 1966).

Marker (1976) and Marker and Swart (1995) also described

similar sandstone and quartzite landscapes in the southern

Western Cape and Table Mountain areas, but referred to these

areas as ‘pseudokarst’ and did not argue for their recognition

as true karst.

6.36.4.4 North America

Fine examples of ruiniform landscapes also occur in the

southwestern United States, especially in the Cedar Mesa

Sandstone of Canyonlands National Park, Utah (Young et al.,

2009). Ford and Williams (1989) also mention sandstone

grikelands in US deserts, and the sandstones along crest lines

in the subpolar periglacial Mackenzie Mountains of Canada,

but few further details were given.

6.36.4.5 Asia

Osborn (1985) described rugged ruiniform sandstone terrain

in Jordan that is very similar to that of the Algerian and

Egyptian sandstones across northern Africa. Sandstone and

quartzite tower karst landscapes are also recognized in

southeastern China (Hamilton-Smith, 2006). The towers and

cones of Zhangjiajie in the World Heritage-listed Wulingyuan

Scenic and Historic Interest Area in Hunan Province are

dominated by more than 3000 narrow sandstone and

quartzite pillars and peaks, many over 200 m high (Hamilton-

Smith, 2006; UNESCO, 2009a). The low-dip quartzite is fairly

pure, and prominent vertical jointing ensures strong structural

control for the pillars (Yang et al., 2011).

Numerous other areas of complex tower and tower-cluster

landscapes occur across much of China. These spectacular

Danxia landforms, named after Mt. Danxia in Guangdong

Province, have formed on low-dip Mesozoic quartzose red-bed

sandstones and conglomerates and are particularly well de-

veloped in the southeast. Danxia landscapes are complex, with

considerable variability, but are dominated by joint-controlled

cliffs, canyons, and towers up to several hundreds of meters

high (Peng, 2000, 2007; Danxia, 2009). These Danxia land-

forms are believed to have developed during the last 6 million

years in response to regional neo-tectonic uplift and faulting,

with erosion widening major joints (Peng, 2000), but whether

solutional weathering has been critical to the degree seen in

other unmistakably arenized sandstone landscapes elsewhere

is yet to be fully determined.

6.36.4.6 Europe

Eastern Europe has many rock cities, and the size and shape of

these towers depend mainly on lithological features of the

sandstones and the density of the jointing. The most famous

are near Ardspach and Teplice in the Czech Republic (Migon

et al., 2002; Urbanova and Prochazka, 2005) and the Stolowe

Mountains of Poland (Latocha and Synowiec, 2002; Alexan-

drowicz and Urban, 2005).

Cilek (2002) believed that the sandstone cities of Bohemia,

which are a maze of narrow joint-controlled gorges or ver-

tically walled fissures commonly only 1 m wide, have been

formed by the excavation of sandstone from arenized joints

(aided by freeze/thaw). Biological and salt weathering is

modifying continued erosion, and case hardening by silica has

aided surface stability. The action of these processes is con-

tinuing to an ultimate reduction of the towers. Similar pro-

cesses are also shaping Polish sandstone rock cities in fine to

coarse sandstone and conglomerate (Alexandrowicz, 2006;

Urban et al., 2006a).

England has no comparable rock cities, but many examples

of narrow joint-controlled fissures in various sandstone

pavements occur. However, Robinson and Williams (1994)

have argued that most of these joints could not have been

opened by solution, and suggested that stress release and mass

movement are most likely responsible. Nevertheless, they

474 Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite

also noted that there are cases where the joint faces have

weathered back, rather than moved apart, and that therefore

chemical weathering along closely spaced joints may have

been important.

6.36.5 Caves, Shafts, and Dolines

6.36.5.1 South America

There are larger quartzite caves in the Roraiman tepuis than in

any other region in the world. The high runoff that has infil-

trated underground via fractures and sinkholes has formed

many large and intricate, commonly joint- and bedding-

controlled cavern systems that reemerge on the vertical walls

of the tepuis, up to several hundred meters below the sum-

mits. Virtually, every author has commented that these caves

have been formed by the mechanical removal of sand from

the arenized quartzites over a remarkably long period of time

(Briceno and Schubert, 1990; Piccini, 1995; Doerr, 1999;

Urbani and Carreno, n.d.).

Coordinated speleological exploration in this remote area

only began in the early 1970s. One of the first reports was of a

complex cave system 150 m below the surface of Cerro Autana

that was over 400 m long with ancient phreatic tubes up to

20 m in diameter (Colvee, 1973; Urbani and Szczerban, 1974;

Table 1 The longest and deepest sandstone caves and shafts of the world2004; Smıda et al., 2005; Urbani and Carreno, n.d.)

Cave system Length (m)

Sistema Roraima Sur 10.8 kmCueva Ojos de Cristal 5.3 kmCueva Charles Brewer 4.8 kmAbismo Guy Collet –Gruta do Centenario 3.8 kmGruta da Boccaina 3.2 kmSima Auyan-tepui

Noroeste2.95 km

Cueva Chimanta 2.90 kmGruta das Bromelias 2.7 kmCueva del Diabolo 2.3 kmSima Aonda Superior 2.13 kmMagnet Cave 2.49 kmSima Aonda 1 1.88 kmSima Acopan 1 1.38 kmSima de la Lluvia 1.35 kmLapao 1.2 kmSima Menor 1.16 kmSima Aonda 2 1.05 kmMawenge Mwena –Gruta Allaouf –Sima Aonda Este 2 0.82 kmSima Auyan-tepui

Norte 10.63 km

Sima Auyan-tepuiNorte 2

0.54 km

Sima Aonda Sur 1 0.43 kmSima Aonda 3 0.42 kmSima Mayor de

Sarisarinama0.40 km

Urbani, 1976; Urbani and Carreno, n.d.). This cave is un-

questionably very old, and Colvee (1973) thought that these

huge phreatic passages may have formed during the Pre-

cambrian. Urbani and Szczerban (1974) also described several

active river caves, one in Territorio Amazonas which passed

800 m through Guanay Mountain, and another resurging over

1000 m from its sink. Large dolines, caves, and vertically

walled collapse shafts have formed in the quartzites of the

Sarisarinama Plateau in nearby Bolivar State (Zawidzki et al.,

1976; Urbani and Carreno, n.d.). Szczerban et al. (1977) also

reported numerous caves in Bolivar State, with the caves and

dolines of the Meseta de Guaiquinima undoubtedly con-

nected underground by a stream nearly 2 km long (see also

Urbani and Carreno, n.d.). Other active and dry underground

drainage systems are known from the area (Chalcraft and

Pye, 1984; Pouyllau and Seurin, 1985; Briceno and Schubert,

1990).

Reports of longer and larger-volume caves within quartzites

have almost constantly been forthcoming during the last two

decades, and now at least 16 caves are known in South

America and Africa, with passage length in excess of 1000 m,

and depths of up to 671 m. Table 1 shows the longest and

deepest reported sandstone and quartzite caves.

The longest cave so far reported from the Roraima region

is the Sistema Roraima Sur (recently joined to the adjacent

Cueva Ojos de Cristal). This cave system only attains a

(Martini, 2000; Auler, 2002; Carreno and Blanco, 2004; Galan et al.,

Depth (m) Location

72 Venezuela, Roraima72 Venezuela, Roraima

110 Venezuela, Chimanta671 Brazil, Barcelos Amazon481 Brazil, Minas Gerais404 Brazil, Minas Gerais370 Venezuela, Roraima

150 Venezuela, Chimanta– Brazil, Minas Gerais– Venezuela, Chimanta320 Venezuela, Roraima– South Africa, Transvaal383 Venezuela, Roraima90 Venezuela, Roraima

202 Venezuela, Roraima– Brazil, Chapada Diamantina248 Venezuela, Roraima325 Venezuela, Roraima305 Zimbabwe, Chimanimani294 Brazil, Minas Gerais295 Venezuela, Roraima320 Venezuela, Roraima

297 Venezuela, Roraima

290 Venezuela, Roraima335 Venezuela, Roraima314 Venezuela, Roraima

Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite 475

depth of 72 m below the tepui surface, but within this depth

are more than 10.8 km of surveyed dry and active stream

passage (Galan et al., 2004). Volumetrically, the largest known

quartzite cave is Cueva Charles Brewer within the Chimanta

massif. This 4.8-km-long cave is around 110 m deep and

drains an extensive area of the mountain surface. Many of the

passages contain active streams, which even during the dry

season carry an estimated 200–300 l s�1. The sheer under-

ground volume and size of this cave exceed all other known

quartzite caves; passages are typically 40 m wide, but some

reach 60 m, and the biggest chamber is the Gran Galerıa Karen

y Fanny, with a domed chamber 70 m wide, up to 40 m high,

and more than 355 m long (Smıda et al., 2005).

In addition to subhorizontal cave systems, Venezuela,

particularly the Sarisarinama Plateau, has a number of huge

vertically walled quartzite shafts or simas. The most spec-

tacular are 150 to nearly 400 m deep and 100–400 m wide

(Urbani and Szczerban, 1974; Pouyllau and Seurin, 1985;

Mecchia and Piccini, 1999; Urbani and Carreno, n.d.); similar

shafts are found in Guyana (Urbani, 1977). In cave explor-

ation on Auyan-tepui, Mecchia and Piccini (1999) found cave

linkages between several similar shafts, and proposed that

these shafts probably form by roof collapse over underground

voids enlarged by basal erosion by underground flow.

Recent explorations of caves within quartzite tepui in the

very remote Araca Amazon region near the Brazil–Venezuela

border have found the 671-m-deep Abismo Guy Collet (Ras-

teiro, 2007; Urbani and Carreno, n.d.). Little has yet been

published about this apparently dominantly vertical cave,

but it currently holds the world’s depth record for quartzite,

and is one of the deepest caves in any rock in the Southern

Hemisphere.

Various regions of Brazil also contain a number of very

significant sandstone and quartzite caves. Minas Gerais, north

of Rio de Janeiro, has possibly the highest localized concen-

tration of long quartz sandstone and quartzite caves known

anywhere. Here, there are at least 35 dry or active stream caves

with passage lengths of more than 500 m and considerable

depths (Correa Neto, 2000). The most significant is the 3790-

m-long and 481-m-deep Gruta do Centenario (Auler, 2002),

and close by are the 404-m-deep Gruta da Bocaina and Gruta

Allaouf that reaches a depth of 294 m (Auler, 2002). Quartzite

rocks at Ibitipoca and municipalities of Luminarias, Carran-

cas, and Sao Tome das Letras host several important quartzite

caves, including the 2.7-km Gruta das Bromelias, with po-

tential for other long and deep caves in each of these areas

(Auler, 2002).

Nearby in the Preto River basin, on the border of Rio de

Janeiro and Minas Gerais states, Avelar et al. (2009) reported

smaller quartzite caves, and De Melo and Coimbra (1996)

stated that caves and collapse shafts called furnas (De Melo

et al., 2004) are also found in the Palaeozoic quartz sand-

stones of the Vila Velha region of southern Brazil. Further

north, Wiegand et al. (2004) noted major sandstone caves in

the Chapada Diamantina National Park, and Auler (2002)

reported the Lapao as a 1.2-km-long quartzite conglomerate

cave here. Other caves over 1 km long are found in the Cha-

pada dos Guimaraes region of Mato Grosso State (Auler,

2002). Other vertically walled furnas have been described

from the Rio Claro area (Wernick et al., 1977), and by Bret

(1962) from Parana State, which can be 50 m in diameter and

112 m deep with the lower 48 m filled with water.

6.36.5.2 Australia

The number and size of caves and associated karst features in

the Roraima and nearby regions is exceptional. However,

smaller numbers of active and relict sandstone and quartzite

caves have been described from most continents. Australia also

displays fine examples of caves and other underground water

movement through smaller solutionally influenced conduits.

Jennings (1979, 1983) found water seeping from small

tubes in the sandstones at Bunju, and other similar examples

have been discovered in recent years. Wray (2009) described

thousands of rounded conduits (5–150þ cm diameter,

10þ m length) within the Jurassic quartz sandstones of the

temperate savannah Carnarvon Range in central Queensland.

He suggested that water that had proceeded down the prom-

inent vertical joints was concentrated into more permeable

zones through the sandstone in the epi-phreatic zone,

dissolving (arenizing) intergranular silica cement. Later

valley erosion increased the hydraulic gradient and initiated

outflow at springs that induced vadose piping that removed

sand grains, forming conduits. Most of these conduits are

now relict, but a number of large regional springs suggest

groundwater flow may still use such conduit networks.

Very well-developed groundwater conduits also occur in

the northern Australia sandstones most conducive to stone-

city formation, where there are thousands of small (1–5 cm)

branching and anastomosing phreatic tubelets, and even lar-

ger pipes several tens of centimeters in diameter and several to

many tens of meters or more in length (Grimes et al., 2009a).

Only a few of these tubes have been seen to carry water during

the dry season, but the sheer numbers of these conduits clearly

prove solutional conduit formation and channelized ground-

water movement through these sandstones in a manner very

similar to limestone karst.

Several larger sandstone caves are known in northern

Australia (Wray, 1997b; Grimes et al., 2009a; Young et al.,

2009). Whalemouth Cave is a large vertical, joint-guided cave

formed in Proterozoic quartzite just north of Purnululu. The

cave drains from a plateau surface where an intermittent sur-

face stream sinking in a blind valley feeds it. It resurges around

120 m lower at the base of a cliff (Jennings, 1983; Grimes,

2007; Grimes et al., 2009a). Widallion Cave near Lawn Hill

Gorge in western Queensland is similar and around 90 m

deep (Grimes et al., 2009a). Both these caves take considerable

water flows during the wet season.

Yulrienji Cave, south of Arnhem Land (Jennings, 1983;

Wray, 1997b; Grimes et al., 2009a; Young et al., 2009), is a

horizontal 50-m-long and 8-m-wide arched tunnel piercing a

sandstone stone city block. It is 13 m above the level of the

nearby stream. This cave is undoubtedly very old, and of un-

certain genesis, bearing little relationship to the modern

stream pattern.

Several joint-controlled, sandstone maze caves have also

recently been discovered (Grimes et al., 2009a), and the most

extensive of these is estimated to contain more than 1.5 km of

passage and flat-roofed chambers showing clear phreatic and

Figure 5 Doline (center) in sandstone at Bunju, east Arnhem Land,Australia. This doline is about 20 m across and about 10 m deep.Trees are growing in the doline bottom and it appears to beconnected to the nearby 20-m-high cliff via a cave through theorange sandstone.

476 Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite

structural features. These caves have so far received little study,

but they are undoubtedly of international significance.

Dolines have formed in the sandstones at Bunju (Figure 5),

and a smaller number also occur on the sandstone plateau

east of Kakadu. The Kakadu dolines are steep sided and prob-

ably collapse type, and Bunju shows a variety of types ranging

from fields of shallow elongated hollows (10 m wide, but up

to 50 m long) to less common but larger deep steep-sided

holes up to 10–30 m across and about 10 m deep. Many of

these are very likely connected by caves to nearby cliffs. Large

collapse dolines also occur in sandstone at Bath Range and

Nhumby Nhumby east of Bunju, but they remain of uncertain

origin. Detailed ground investigations are required in these

remote areas.

Southeastern Australia has only one known significant

sandstone cave. This active stream cave formed through quartz

sandstone near Sydney is 80 m long, 1–4 m high, and around

10 m wide (Pavey, 1972; Wray, 1997b; Young et al., 2009).

6.36.5.3 Africa

Active stream caves have formed in thick-bedded Upper

Cretaceous quartz sandstones in humid tropical southern

Nigeria. The longest is Ogbunike Cave, a 350-m complex and

multilevel sandstone maze, but Egboka and Orajaka (1987),

Mbanugoh and Egboka (1988), and Szevtes (1989) have de-

scribed more than 16 horizontal caves, some associated with

deeply incised canyons, and in most bedding planes and

joints have influenced cave development.

Mainguet (1972) found so many caves and tubes in the

sandstone terrain of the arid Tibesti region of Tchad that she

stated that they are as common in sandstone as in limestone.

However, she also considered them to be essentially relicts of a

once more humid climate. The same is true of the many small

sandstone caves in the nearby Saharan region of eastern,

northeastern, and western Niger (Busche and Erbe, 1987;

Sponholz, 1989; Busche and Sponholz, 1992; Willems et al.,

1996; Willems et al., 1998; Sponholz, 2003). These authors

noted that water gushes after heavy rain from many small

phreatic caves and tubes a few centimeters in diameter. These

are unquestionably formed by silica solution and prove a well-

developed subterranean network of karst passages in the

sandstones. However, they believed these tubes formed during

wetter periods in the mid-Tertiary, and closed scarp-foot

drainage depressions might have been active until the Plio-

cene. Although the total number of accessible caves is small,

the sandstones of this area are completely riddled with small

inaccessible passages (Busche and Erbe, 1987; Sponholz,

1989; Busche and Sponholz, 1992; Willems et al., 1996;

Willems et al., 1998; Sponholz, 2003). Large sandstone caves

that may be of solutional origin also occur in the presently

hyper-arid regions of Egypt and Libya, but are only poorly

described (Halliday, 2003).

In the eastern Transvaal, the large Berlin Cave has formed

within the deeply weathered Black Reef Quartzite (Martini,

1979; Martini, 1982; Martini, 2000a). The southern system

consists of at least 17 caves beneath two large complex do-

lines. Nearly all of the caves contain active streams, and bed-

ding is the dominant structural control on cave development.

The northern system has smaller caves, but these have the

same general morphology.

Southern Africa’s longest known quartzite cave is Magnet

Cave in the Transvaal Daspoort Quartzite (Martini,

2000a). With a length of 2.49 km, this was at one time one

of the longest known quartzite caves in the world (Martini,

2000a). Several cave systems and dolines occur here, and the

caves are formed by removal of arenized quartzite by flowing

water.

Although the Transvaal has the longest quartzite caves in

Africa, Zimbabwe has the deepest. In the cool temperate

Chimanimani Highlands, on the border of Zimbabwe and

Mozambique, many large jungle-filled dolines lead to

several deep caves formed in the faulted and deeply weathered

Umkondo Group quartzite (Aucamp and Swart, 1991).

Bounding Pot reaches a depth of 190 m, while in the adjacent

doline the series of chambers and shafts in Jungle Pot plunges

250 m vertically, and the 305-m-deep Mawenge Mwena is

probably Africa’s deepest known cave (Aucamp and Swart,

1991; Martini, 2000a).

6.36.5.4 Europe

In northern England, Mullan (1989) and Self and Mullan

(2005) have noted 10 small caves in the quartz Fell Sandstone.

The largest of these caves is only 9.6 m long and apparently

formed by piping through arenized sandstone. This area was

recently glaciated and so these caves probably developed quite

quickly.

Vertically walled collapse dolines or shafts with closed in-

ternal drainage also occur in the Millstone Grit of Wales

(Battiau-Queney, 1984). They average around 100 m in length

and were not glacially carved, nor have they collapsed into

underlying limestone. Battiau-Queney believed that seepage

water has been concentrated by regular jointing and the

quartzite has been deeply weathered, perhaps under a prior

tropical climate. These collapses may therefore reflect collapse

along vertical joints into extensively weathered beds within

the quartzite (Battiau-Queney, 1984).

Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite 477

Many caves are reported from Eastern Europe, some con-

taining silica speleothems (Urban et al., 2006b), but these are

mostly tectonic caves formed by stress release or block

movements (Young et al., 2009).

Although information in English is scarce, karst in siliceous

sediments seems to be widespread in Russia (Lyakhnitsky and

Khlebalin, 2006). Caves, cones, depressions, and holes have

formed in the conglomerates of the Yuryuzan–Sylvino de-

pression of the pre-Uralian trough, and almost 50% of the

area around Bashkiria contains sandstone karst. There are

numerous caves in conglomerate and sandstone in the Kama-

Middle Volga, the Caucasus, the Lena-Enisei, and the Sayan

areas, as well as in the Ural Mountains (Lyakhnitsky and

Khlebalin, 2006).

6.36.5.5 Asia and North America

Apart from the report of Shade et al. (2000) for a series of

short caves, springs, and closed depressions in the Pre-

cambrian Hickey Sandstone of Minnesota, there are no known

reports of sandstone caves from the Asian or North American

continents.

6.36.6 Smaller Surface Forms – Rock Basins andRunnels

Smaller surface karren or lapies are typically tens of centi-

meters to several tens of meters in size and are generally

controlled by the manner in which water contacts the rock

surface (Bogli, 1960; Jennings, 1985). The most common of

these forms are rock basins and drainage runnels, and al-

though aspects of their formation remain obscure, there is

little doubt that solutional weathering is aided by standing

and/or channelized water.

There are a number of scattered reports of karren and lapies

on sandstones and quartzites, but their distribution, morph-

ology, and formative aspects have not been as well studied as

those on limestones. They are often also very common on

granites (Hedges, 1969; Twidale, 1982; Twidale, 1987). A

general summary will therefore be given, rather than by

continent.

6.36.6.1 Rock Basins

Rock basins (also called pits or pans) occur in South America

(White et al., 1966; Chalcraft and Pye, 1984; Pouyllau and

Seurin, 1985; George, 1989; Briceno and Schubert, 1990),

Australia (Twidale, 1980; Wray, 1997b), Africa (Marker, 1976;

Busche and Erbe, 1987; Cooks and Pretorious, 1987), England

(Robinson and Williams, 1994), Europe (Franzle, 1971;

Backhaus, 1972; Vıtek, 1979a, 1979b; Alexandrowicz, 1989;

Varilova, 2002; Mertık and Adamovic, 2005; Thiry, 2005), and

the USA (Schipull, 1978; Howard and Kochel, 1988). Most

authors agree that the solutional action of standing water,

perhaps aided by decomposing organic material, plays a major

component in the formation of these basins by aiding the

detachment of sand grains that are then removed by over-

flowing water or wind.

Basins are commonly near circular to oval in shape, but

may also be of irregular shape, their floors are flat, gently

hemispherical or irregular, and they are bordered by walls that

vary from gently sloping to vertical or undercut. In the bottom

of many basins is retained a residuum of weathered material

or organic litter. Several adjacent basins may coalesce to form

a larger, irregular-shaped basin (see Figure 6(a)), or may even

nest inside one another. On gently sloping rock surfaces,

basins occur in some places in chains downslope, and are

commonly connected by a runnel or spillway.

Basins generally range in size from several centimeters to

several meters in diameter, and may be up to several tens of

centimeters deep (Franzle, 1971; Schipull, 1978; Cooks and

Pretorious, 1987). Smaller basins are by far the more com-

mon, but ones of several tens of meters in size occur in some

places (Netoff and Shroba, 1993; Netoff and Shroba, 2001).

Field observations by the author suggest that the degree of

basin and runnel development is linked to surface resistance.

Observations from southern and northern Australia, New

Zealand, and China indicate that well-developed basins and

runnels typically only form on relatively resistant sandstone

surfaces.

6.36.6.2 Runnels

Bogli (1960) classified two limestone runnel types: those

formed from the sheetflow of rainwater (rillenkarren) and the

larger group formed by surface water flowing from an external

source (rinnenkarren, rundkarren, and decantation forms).

Rillenkarren apparently do not form on quartz sandstone

or quartzite, and Robinson and Williams (1992) provided the

only known description from the Atlas Mountains in Morocco

of small ridges and hollows that bear ‘‘a somewhat similar

appearance’’ to rillenkarren (Robinson and Williams, 1992:

426). However, they are equivocal as to the origin of these

features, and because rain beat is not high Robinson and

Williams questioned whether they formed in the same man-

ner as rillenkarren, and suggested that they may have de-

veloped beneath snow or a former soil cover. The lack of

rillenkarren on most sandstones is most probably not due to

any lack of sheetwash, but rather as a consequence of the

coarse granular nature of most quartz arenites; rillenkarren

commonly do not form on coarse-grained limestones, mar-

bles, or dolomites either (Ford and Lundberg, 1987).

Larger runnels are relatively common on many sandstones

and quartzites. These channels are similar to limestone rin-

nenkarren (formed subaerially) and the more rounded rund-

karren (formed beneath a soil cover) (Bogli, 1960; Ford and

Lundberg, 1987). They begin where surface flow breaks into

linear streams and commonly display dendritic or meandering

plan forms on low-angle surfaces. Runnels are typically

10–50 cm or more wide, have sharp or rounded channel rims,

and rounded or v-shaped bases. They may increase in depth

and width downslope with increasing catchment area, and

their lengths are variable and dependent on the volume of

water available, length and gradient of slope, rock texture, and

amount of cover removed (Ford and Lundberg, 1987).

In the Roraima, White et al. (1966), Pouyllau and Seurin

(1985), and Piccini (1995) all reported frequent 25–50-cm-

(a) (b)

Figure 6 (a) Rainwater filled rock basin at Mt. Stapylton in the Grampians Range of western Victoria, Australia. This flat-floored basin, about2.5 m long, has formed from the joining of two smaller basins. The walls are undercut and around 20 cm high. Decomposing organic material inthe basin may aid the solutional process; loosened sand grains are blown from the basin when dry. (b) Part of a 25þ m long runnel near thebasin in (a). This runnel is about 30 cm across, has a mostly flat base, and is fed by water overspilling an upslope basin. The entire runnel is20 m long.

478 Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite

deep grooves or runnels. At the Vila Velha State Park in

southern Brazil, De Melo et al. (2004) also noted the wide-

spread occurrence of sandstone lapies or runnel forms. In

Morocco, Robinson and Williams (1992) also described

shallow channels or gutters up to 100 mm wide and 10 mm

deep in sandstone pavements, and Marker (1976) found nu-

merous runnels draining basins in the Transvaal. The Navajo

Sandstone of the US Colorado Plateau also displays extensive

systems of runnels (Howard and Kochel, 1988).

Fine runnel systems also occur at many places in Australia

and are commonly associated with rock basins. They are well

formed at Kakadu near Darwin (Grimes et al., 2009a), near

Sydney (Wray, 1996), at Wonderland and Mt. Stapylton in the

Grampians Range of western Victoria (see Figure 6(b)), and at

many other locations (Young et al., 2009).

On higher-angle surfaces, runnels tend to be subparallel

and resemble limestone flutes or wandkarren. Good examples

are seen on many steeply sloping sandstone in the Roraima

(Pouyllau and Seurin, 1985; Piccini, 1995), Australia (Wray,

1996; Young et al., 2009), and also in Britain and Europe, such

as those on the Millstone Grit and Fell Sandstone (Robinson

and Williams, 1994). Vertical flutes also groove many

European Neolithic standing stones, and suggest a relatively

rapid formation, probably within the last 3200–400 years

(Robinson and Williams, 1994; Self and Mullan, 2005).

6.36.7 Speleothems

Speleothems of silica are generally smaller than those of car-

bonate, are much less studied, and are most commonly de-

scribed from sandstone and quartzite caves.

6.36.7.1 South America

The largest and most highly developed silica speleothems

occur in caves at Roraima, with White et al. (1966) publishing

the first reports from this area of thin opal flowstones

and small stalactites. A decade later, Urbani and Szczerban

(1974) and Zawidzki et al. (1976) described numerous

speleothems, including opal stalactites up to 10 cm long,

flowstones, and different types of crusts and coralloid spe-

leothems in Cerro Autana Cave and other caves of the

Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite 479

Sarisarinama Plateau. These speleothems were clearly con-

centrically banded and showed a composition of opal-A,

length-fast chalcedony, and calcite. These authors believed that

these speleothems were formed by direct precipitation of the

opal and calcite from alkaline waters at ambient temperature,

with the chalcedony representing subsequent recrystallization

of the opal.

Chalcraft and Pye (1984) found flowstones and stalactites

on Mount Roraima with a porous texture formed of detrital

sand grains cemented by cristobalite, tridymite, and authigenic

quartz instead of opal-A. Briceno and Schubert (1990) also

reported many small amorphous silica stalactites within the

many orthoquartzite caves on the Roraiman tepui.

However, these silica speleothems are not only composed

of pure silica. The reviews of Urbani (1990, 1996) showed

that a range of speleothem and other cave minerals have

been identified, including carbonates, silica, nitrates, oxides/

hydroxides, phosphate, and sulfates.

Highly detailed analyses have recently been made on spe-

leothems from some of the largest of the region’s caves. In the

Roraima Sur cave system, Carreno and Blanco (2004) and

Carreno and Urbani (2004) found widespread amorphous

opal-A stalactites, but the great length and complexity of this

cave have formed microclimates, which affect airflow patterns,

condensation, and capillary action. This zonation between the

entrance and deeper parts of the cave has modified air

movement, and this has observably influenced the dimension,

concentration, spatial distribution, and rotation angles of

speleothem growth.

Recent studies from the very sizable Cueva Charles Brewer

cave on the Chimanta Plateau (Smıda et al., 2005; Aubrecht

et al., 2008a; Aubrecht et al., 2008b) also strongly suggest a

biological component in some silica speleothem formation.

This detailed analytical evidence shows that a range of unusual

opal-A and chalcedony speleothems are in fact columnar

stromatolites composed of fine-laminated silicified cyano-

bacteria or peloidal cyanobacteria stromatolites, where the

microbes have mediated silica precipitation. This unusual

formative mechanism has prompted these authors to adopt

the terms ‘biospeleothems’ and ‘microbialites’. Although

microbiological action has been suggested in influencing

speleothem formation elsewhere (Willems et al., 1998; Wray,

1999), these studies provide some of the most comprehensive

studies to date for this unusual and possibly important

process.

Silica speleothem deposits also occur in Brazil. Wiegand

et al. (2004) noted opaline silica coralloids, crusts, flowstones,

stalactites, and stalagmites from caves in a number of areas

of eastern Brazil, including the Chapada Diamantina National

Park.

6.36.7.2 Australia

Silica speleothems also occur across the Australian continent

from the tropical north to the cooler south. Jennings (1979)

described coralloids and flowstones at Bunju in tropical Arn-

hem Land, but most published accounts from northern Aus-

tralia have concentrated on flowstones (silica skins) because of

their association with aboriginal rock art. In Kakadu,

Watchman (1992, 2007) found silica skins composed of opal-A

and a wide range of other minerals of calcium, sulfur, alu-

minum, magnesium, and potassium. Opaline coralloids and

flowstones were also described in Kakadu, and within many of

the stone cities by Grimes et al. (2009a). Wray (2009) and

Grimes et al. (2009a) also found small stalactites and flow-

stones inside many of the drainage conduits throughout the

sandstones of the Carnarvon Gorge region of central Queens-

land. Many unstudied examples of opaline flowstones and

stalactites are also known from numerous places in the Kim-

berley region (R. Wende, 1996, personal communication).

A variety of silica-based speleothems, including coralloids,

stalactites, and flowstones, were described from the cooler

Sydney region by Young and Young (1992) and Wray (1997c,

1999), and Young et al. (2009). The irregular branching cor-

alline forms range from less than 1 to over 75 mm in length,

with branches from 0.25 to over 12 mm in diameter. There are

also more regular conical or tapering stalactites typically

1–50 mm long and 0.5–5 mm diameter with no or few

branches and no central drip-water hole, but the most com-

mon form is a highly irregular and highly porous, bulbous,

coralline popcorn around 5–50 mm in diameter that is nor-

mally found as bunches in association with other silica stal-

actites (Wray, 1997c, 1999). Silica stalagmites are quite rare,

but silica flowstones with a thickness of up to 10 mm or so are

fairly common. Microscopic analysis has shown each of these

forms to be composed dominantly of banded opal-A and

chalcedony, with some kaolinite clay but no calcite. The

chalcedony is believed to form from a later recrystallization of

opal-A (Wray, 1997c, 1999). Wray (1999) also thought it

likely that microorganisms played some role in the formation

of silica speleothems in southeastern Australia, but no further

studies have yet been published.

6.36.7.3 Africa

Few descriptions of silica speleothems have so far been made

from the African continent. Martini (1979, 1982) found opal

popcorn and small stalactites containing limonite, as well as a

dark, partly organic, flowstone in the caves of the Black Reef

Quartzite in the Eastern Transvaal. Willems et al. (1998)

conducted detailed laboratory studies on speleothems from a

formerly deep phreatic sandstone karst in eastern Niger and

argued that bacteria may have played some role in the for-

mation of these speleothems.

6.36.7.4 Asia

There are no known reports of silica speleothems from the

Asian continent.

6.36.7.5 Europe

In Europe, most reports of speleothems come from small

flysch sandstone caves in the Polish Beskidy Mountains

(Urban et al., 2006b). Most speleothems in the region are

calcite based, but noncalcite speleothems occur in numerous

fissure caves formed in sandstones high in carbonate. In

the Roznow foothills, another cave in quartz sandstone

480 Solutional Weathering and Karstic Landscapes on Quartz Sandstones and Quartzite

has thin crusts and botryoidal forms of kaolinite, quartz,

and amorphous material with admixture of chlorites. In

other Polish caves, 10-cm-long light-brown stalactites are

reportedly formed externally of thin laminates of an unknown

amorphous material with silica in various stages of crystal-

lization (opal-quartz) (Urban et al., 2006b).

6.36.7.6 North America

Like larger sandstone solutional forms, reports of speleothems

from the North American continent are very few. The only

known report is of opaline coralloids and stalactites on sand-

stones of the Lee Formation in Virginia, USA (Porter, 1979).

6.36.8 Conclusions

The solution reactions involved in the weathering of quartzose

sandstones are complex and variable, and influenced by a

range of factors including pH, organic acid concentrations,

presence of various anions and cations, water throughput, and

temperature. The intensity of solution is perhaps most affected

by the volume of water flowing through the sandstone, and

the length of time this occurs.

The process of arenization – dissolution, granular disinte-

gration, and erosion – is critical to the development of sand-

stone landscapes. This arenization plays a critical trigger role

and changes the mechanical properties of the sandstone;

arenized quartz sandstone will have different mechanical

properties compared to un-arenized rock and later erosion will

proceed differently. The resultant landforms are not the same

as what would otherwise have formed. Because of the im-

portance and necessity of solution, even though it may not

have been the dominant erosive process, the use of the term

‘karst’ in some quartz sandstones and quartzites is justifiable.

At the largest scale, structural features of the bedrock

(joints, bedding, etc.) are major controls on landscape for-

mation, and the preferential erosion of these structural elem-

ents leads to complex ruiniform landscapes. Underground

water movement also exploits these structural elements and

focuses chemical weathering and later mechanical erosion

with the formation of caves or collapse features. Smaller sur-

face karren develop on the surfaces of the larger forms, and are

important as they are directly related to the ongoing surface

erosion of the landscape. Speleothem formation demonstrates

the importance of the solution and movement of dissolved

silicon in waters seeping through sandstone.

There are still many aspects of silica karst that are poorly

understood, and it is therefore hoped that continued labora-

tory and field investigations will continue to expand our

understanding of these generally spectacular landscapes.

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Biographical Sketch

Dr. Robert A.L. Wray is a geomorphologist and Principal Fellow in the School of Earth and Environmental Sciences,

University of Wollongong, Wollongong, Australia. His interests are in sandstone geomorphology, sandstone and quartzite

karst, long-term landscape evolution, and geo-heritage conservation and management.