Geomorphology 131 (2011) 14–27
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Geomorphology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h
Rock doughnut and pothole structures of the Clarens Fm. Sandstone in the Karoo
Basin, South Africa: Possible links to Lower Jurassic fluid seepage
Stefan Grab a,⁎, Henrik Svensen b
a
b
School of Geography, Archaeology & Environmental Studies, University of the Witwatersrand, P/Bag 3, WITS 2050, South Africa
Physics of Geological Processes, University of Oslo, PO Box 1048, Blindern, 0316 Oslo, Norway
a r t i c l e
i n f o
Article history:
Received 15 November 2010
Received in revised form 14 April 2011
Accepted 17 April 2011
Available online 22 April 2011
Keywords:
Rock doughnuts
Potholes
Morphology
Process origins
Clarens Fm. Sandstone
a b s t r a c t
South Africa has a wealth of sandstone landforms, yet many of these have not been examined in detail to
expand knowledge on their morphology and process origins. Here we present data on primary morphological
statistics, rock hardness, surface roughness and petrographic investigations of rock doughnuts and associated
pothole structures in Golden Gate Highlands National Park (GGHNP) and in the Witkop III complex, with the
aim of using such data and field observations to argue their likely origins. Schmidt hammer R-values indicate
consistently harder doughnut rims (mean = 48.7; n = 150) than the enclosed potholes (mean = 37.8;
n = 150) and surrounding sandstone platform (mean = 39.7; n = 250). The petrography of Clarens Fm.
Sandstone shows that the typical whitish sandstone is affected by intense chemical weathering. Pothole rims
and the irregular reddish crust typical of the Witkop III outcrops show a secondary cementation by
microcrystalline silica. Although preservation of old land surfaces is difficult to prove, small and circular pipe
structures filled with calcite-cemented sand are present locally surrounding the Witkop III hydrothermal
complex, and represent conduits for fluidized sand. Based on the morphologies of the Witkop III summit with
the associated potholes and pipes, we hypothesize that they are remnants of morphologies created by Jurassic
fluid seepage, with a superimposed and secondary silica cementation. Given that fluidization structures
evidently occur in Clarens Fm. Sandstone, as is the case at Witkop, such mechanisms could possibly have
contributed to the observed rock doughnut structures elsewhere on Clarens Fm. Sandstones, such as at the
GGHNP where the rock doughnut morphological attributes are typical to landforms originating from fluid
venting.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Southern Africa is well known for its diversity of sandstone
landscapes and cultural stone heritage (Meiklejohn et al., 2009;
Young et al., 2009; Grab et al., 2011). Past sandstone research in
southern Africa has focused particularly on the sedimentology (e.g.
Erickson, 1981, 1984, 1985; Holzförster, 2007) and paleontology (e.g.
Bordy et al., 2004) that have contributed to the development of the
contemporary landscape. In contrast, geomorphologists have placed
emphasis on understanding contemporary micro- spatio-temporal
scales of sandstone weathering in southern Africa, with particular
application to understanding the controls on San rock art deterioration (e.g. Hoerlé, 2006; Hall et al., 2007; Denis et al., 2009;
Meiklejohn et al., 2009). However, relatively few studies on the
sub-continent have examined specific sandstone landforms and their
associated process origins, and even less so, made linkages between
⁎ Corresponding author. Tel.: + 27 11 717 6512; fax: + 27 11 717 6529.
E-mail addresses: Stefan.grab@wits.ac.za (S. Grab), Henrik.svensen@matnat.uio.no
(H. Svensen).
0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2011.04.015
ancient deformation processes as a control on modern landscape
development.
A variety of conical- or cone-shaped structures in sedimentary
rock have been described in the literature and may include mud and
sand volcanoes (Aslan et al., 2001; Dimitrov, 2002), carbonate seep
structures (Aiello et al., 2001) and other related hydrothermal/fluid
expulsion structures (Kulm and Suess, 1990; Orange et al., 1999;
Svensen et al., 2006), fluidized sandstone (clastic pipe) structures
(Netoff and Shroba, 2001) and doughnut and font assemblages
(Twidale and Campbell, 1998; Netoff and Shroba, 2001). Broadly, the
ensuing landforms may represent active or fossil sandstone structures
of neotectonic and/or fluid expulsion origins and/or products of
differential weathering and erosion unrelated to seepage. To this end,
we review current knowledge on the morphological attributes and
suggested process origins of relatively small-scale conical- and
associated pothole-shaped sandstone structures; and in addition,
present the first geomorphological and sedimentological information
of such landforms from southern Africa through the provision of
morphological and petrographic data on rock doughnut (i.e. conical
forms) and pothole assemblages from two Clarens Fm. Sandstone
sites. Ultimately, the observations and data are used to help ascertain
S. Grab, H. Svensen / Geomorphology 131 (2011) 14–27
15
2. Conical- and pothole-shaped weathering/erosion
bedrock forms
height (Twidale and Campbell, 1998). Yet elsewhere, rock doughnuts
reported from quartzose sandstone in New South Wales, Australia, are
only 0.05–0.1 m in height. In contrast, megafonts are substantially
higher (5–35 m) and have deep summit pits (up to 16 m) (Netoff and
Shroba, 2001).
2.1. Rock doughnuts
2.2. Potholes (basins, gnammas, and weathering pits)
The term ‘rock doughnut’ was introduced by Blank (1951) to
describe ‘rounded annular ridges encircling weather [weathering]
pits’ (p822), based on observations undertaken on granite domes in
central Texas (Blank, 1951). Rock doughnuts, also referred to as
‘elevated pits’ (Hills, 1971) and ‘cones’ (Young et al., 2009), have
predominantly been reported from granite (Blank, 1951; CoudéGaussen, 1981; Twidale and Bourne, 2000; Twidale and Vidal Romaní,
2005) and sandstone (Hills, 1971; Twidale and Campbell, 1998;
Young et al., 2009) outcrops, but have also been recorded in
calcarenite (Fairbridge, 1947–8; Twidale and Campbell, 1998), chalk
(Burnaby, 1950) and basalt (Cotton, 1963).
Rock doughnuts are circular/oval in plan, and many are conical in
cross-section with a pothole in the center (Fig. 1); in most cases these
are summit potholes, while less frequently potholes are incised to
below the level of the surrounding bedrock platform. The potholes
develop singly or in clusters, are flat floored and contain weathered
detritus (Netoff and Shroba, 2001). In some instances the doughnut
rims have been breached to produce ‘half doughnuts’ (Blank, 1951;
Twidale and Vidal Romaní, 2005) or ‘armchair’ forms (Netoff and
Shroba, 2001), which represent water exit points on the downslope
side of the doughnuts. The rock doughnuts first described by Blank
(1951) had pothole diameters of 0.15–1.8 m, surrounded by relatively
low but wide rims (mean height: 0.15 m; mean width: 0.46 m), which
would have resembled doughnuts, hence their name sake. Coastal
rock doughnuts in South Australia are typically tens of cms high and
wide, with maximum dimensions of 1.5 m in width and 0.8 m in
While potholes are common to many rock surfaces, conical
landforms with summit pits are rare, and thus rock doughnut-pothole
or (mega)font-summit pit assemblages are relatively unique landforms. Potholes (Johnsson, 1988; Chan et al., 2005; Graham and
Wirth, 2008), which are also commonly referred to as ‘rock basins’
(Twidale, 1997; Twidale and Campbell, 1998; Twidale and Vidal
Romaní, 2005), ‘weathering pits’ (Goudie and Migón, 1997; Hall
and Phillips, 2006) and ‘gnammas’ (Twidale and Corbin, 1963;
Domínguez-Villar et al., 2009), are a fundamental morphological
attribute of rock doughnuts. Potholes are particularly common
bedrock surface forms on near-horizontal granite outcrops, but have
been reported from a variety of other lithologies including sandstone,
granodiorite, pegmatite, schists, gneiss, quartzite and basalt (c.f.
Goudie and Migón, 1997). Although potholes are typically 1 m in
diameter and 0.15–0.3 m deep (Goudie and Migón, 1997), they may in
some instances coalesce to form larger varieties, and have been
reported to reach 11 m in length (Goudie and Migón, 1997) and almost
1.5 m in depth (Hall and Phillips, 2006). Potholes are variable in plan
view but are commonly circular to ovoid. However, elliptical forms
are also common, especially where potholes have coalesced (Goudie
and Migón, 1997; Domínguez-Villar, 2006; Hall and Phillips, 2006).
a likely linkage between ancient deformation processes and contemporary landform development.
2.3. Process origins
The somewhat unusual morphology of rock doughnuts has
intrigued scientists and stimulated considerable debate concerning
Fig. 1. A rock platform at GGHNP hosting rock doughnut and pothole structures. The rock doughnut in the foreground displays the typical cone-shaped morphology with a summit
pothole (various dimensional measurements undertaken are specified).
16
S. Grab, H. Svensen / Geomorphology 131 (2011) 14–27
their process origins. Numerous hypotheses for rock doughnut
formation have been proposed, several of which are critically
reviewed by Twidale and Campbell (1998). Given the considerable
morphological and dimensional variability of rock doughnuts and
(mega) fonts, it is conceivable that geological and geomorphic process
mechanisms would differ across the wide variety of environmental
contexts where such phenomena have been observed. In short, some
of the hypotheses include differential water-level weathering/erosion
due to water/moisture contrasts near the surface (Cotton, 1963; Hills,
1971; Twidale, 1976, 1988; Twidale and Bourne, 1998, 2000; Twidale
and Vidal Romaní, 2005), induration through solute precipitation and
oxidation (Blank, 1951; Twidale and Bourne, 1998), or due to
lithological variations, liquefaction and fluidization which produce
clastic pipes (Netoff and Shroba, 2001). We discuss possible process
mechanisms in more detail in the discussion/conclusion section.
3. Regional setting
We report on measurements and observations undertaken in two
Clarens Fm. Sandstone areas of southern Africa, one in the Golden
Gate Highlands National Park (GGHNP) in the northeastern Free State
Province, and the second in the Witkop III region of the Eastern Cape
Province (Fig. 2).
3.1. Regional geological setting
The Karoo Basin (Karoo Supersequence) of southern Africa is a
product of in-fill from the Late Carboniferous to Early Jurassic and
attains a maximum thickness of about 12,000 m (Johnson, 1976;
Johnson et al., 1997; Turner, 1999). The uppermost and youngest
(Late Triassic to late Lower Jurassic) successions in the Karoo Basin are
collectively known as the Stormberg Group, which includes the
Molteno, Elliot and Clarens Formations. The Stormberg Group
sandstones originate from fluvial and aeolian deposition during
warm, semi-arid (Molteno/Elliot Fms.) to desert (Clarens Fm.)
conditions (Johnson et al., 1996). Large Igneous Province (LIP)
volcanism, which is argued to have had a dramatic impact on landscapes and the sedimentary system (Holzförster, 2007; McClintock
et al., 2008), formed at ca. 183 Ma, with remnant thicknesses of
N1500 m centered around Lesotho and surrounding regions (Duncan
et al., 1997; Riley and Knight, 2001; Jourdan et al., 2005). The initial
phase of magmatic activity coincided with the final stages of
sedimentation in the basin, and thus some of the earlier lava flows
interfinger with Clarens Fm. siliciclastic sediments (Botha and Theron,
1967; Schmitz and Rooyani, 1987; McClintock et al., 2008). The major
stage of the Karoo LIP is characterized by lava flows, dolerite sill
emplacement, diatreme-like hydrothermal vent complexes (Jamtveit
et al., 2004), minor andesite to dacite dome complexes (Marsh and
Eales, 1984), and saucer-shaped sills and injected sands (Svensen
et al., 2006; Polteau et al., 2008).
3.2. The GGHNP region, northeastern Free State Province
The GGHNP is located in the northeastern Free State Province of
South Africa (28°31′S; 28°37′E) and is topographically represented by
the Little Caledon Valley (ca 1900 m a.s.l.) and Rooiberg Mountain
Range (up to 2837 m a.s.l.). The region is characterized by a variable
mean annual rainfall ranging from 700 to 1000 mm (Nicol, 1973),
most of which falls during the summer months between November
and April. Summers are mild (mean temperature = ca. 18 °C),
while winters are cold (mean temperature = ca. 7 °C) with ca. 60
frost days per annum (Schulze, 1997). The dominant vegetation is
Fig. 2. Localities of the two study sites within the Stormberg Group sandstones and adjacent Drakensberg Group volcanics.
S. Grab, H. Svensen / Geomorphology 131 (2011) 14–27
Highlands Sourveld and Austro-Afro Alpine Grassland (Du Preez and
Bredenkamp, 1991; Kay et al., 1993).
The dominant sedimentary formations in the GGHNP include the
Molteno, Elliot and Clarens Formations, which are overlain by the
basaltic Drakensberg Group (Groenewald, 1986) (Fig. 3). The rock
doughnuts are located at 1960 m a.s.l. (28°35′00″E; 28°29′41″S) on an
upper Clarens Fm. Sandstone platform, where the formation thickness
is 150–180 m. The exposed sandstone platform is located at the
contact zone with Drakensberg flood basalts, which form a ca. 200–
400 m thick capping in the region (Figs. 1 and 3). The Clarens Fm.
Sandstones at the site are fine- to very fine-grained, light yellowbrown, massive to crudely bedded, and predominantly quartzitic. A
small dike intrusion is located adjacent to the platform where rock
doughnuts have developed, which is also accompanied by red
oxidation of the sandstones, and may reflect past heating. In addition,
the Clarens Fm. Sandstones have, in patches, been hornfelsed to a
gray, flinty structure closer to the intrusion.
17
The wide assortment of sandstone surface phenomena in the
GGHNP are summarized by Grab et al. (2011); these include case
hardening, surface spalling, cliffs and pillars, arches, pedestal rocks,
rock doughnuts, cavernous weathering forms (honeycombs, tafoni,
caverns and alcoves), potholes, polygonal cracking, bedrock microdrainage channels (fluted, guttered and karren forms) and lichen
induced weathering forms. Of these, only potholes (‘weathering
basins’; Cooks and Pretorious, 1987) and the geomorphic role of
endolithic lichen (e.g. Wessels and Schoeman, 1988; Wessels and
Wessels, 1995; Büdel et al., 2004) have been investigated in detail.
3.3. The Witkop III region, Eastern Cape Province
The Witkop III area is located in the northern parts of Eastern Cape
Province of South Africa (31°12′S; 27°13′E), about 10 km west of the
main area with outcropping Drakensberg group flood basalts. The
elevation is ca. 1900 m a.s.l., and the Clarens Fm. Sandstone is well
Fig. 3. Site location and geological setting of rock doughnuts and pothole structures in the GGHNP, eastern Free State Province. The inset map shows the spatial distribution of rock
doughnuts across the sandstone platform.
18
S. Grab, H. Svensen / Geomorphology 131 (2011) 14–27
exposed on a 2–4 km wide mountain area with a gentle relief (Fig. 4).
The whole succession of Elliot Fm. strata and Clarens Fm. Sandstone is
exposed in the area, and the Clarens Fm. Sandstone is about 140 m
thick with two main aeolian cross-bedded dune unit horizons
(Svensen et al., 2006). Thus, both the thickness and the stratigraphy
of the Clarens Fm. resemble that of the GGHNP area, although no
detailed studies of the Clarens Fm. from these localities are available.
The upper parts of the Clarens Fm. are characterized by a massive unit
overlain by finely laminated strata and occasional volcaniclastic
horizons (Svensen et al., 2006). We have studied potholes both in
the Clarens Fm. on top of a massive sandstone unit at the base of the
formation (Locality A in Fig. 4), and within the Witkop III
hydrothermal vent complex (Locality B in Fig. 4).
The Witkop III structure is a so-called hydrothermal vent complex
that formed as a result of vigorous gas venting in the Lower Jurassic
(Jamtveit et al., 2004; Svensen et al., 2006). The structure formed as a
result of contact metamorphism around igneous sill intrusions at
depth in the Karoo Basin, when the sills were emplaced as a part of the
Karoo large igneous province about 182–183 million years ago
(Svensen et al., 2008). The Witkop III complex is an erosional remnant
of what used to be a bigger crater filled by sediment breccias and
aeolian sand. The present day morphology of the crater fill is
dominated by ridges and small crests, where potholes are present at
the apexes of font-like structures (Fig. 5). Moreover, the surface of the
sandstones is covered by a red-brown crust, resembling the crust on
the rims of the potholes from locality A. In this study we compare the
potholes from within the Witkop III complex to those from the Clarens
Fm. to the north, as the two sites represent different morphological
settings. In addition, some comparisons are made between the
features examined in the Eastern Cape with those from the GGHNP
area. Two additional samples of hard and well cemented crusts on the
Clarens Fm. Sandstone were collected from the Witkop II hydrothermal vent complex for petrographic analysis (see Svensen et al., 2006).
4. Methods
Nineteen rock doughnuts located on a slight convex rock platform
in the GGHNP were measured for their size dimensions (a/b-axis basal
diameters, N–E–S–W aspect rim heights, max./min. rim widths, a/baxis pothole diameters, and pothole depths), with primary
Fig. 4. Site location and geological setting of rock doughnut and pothole structures in the Witkop III area, Eastern Cape Province, with a general stratigraphic overview of the Karoo
Basin (right). The star next to the Witkop III locality shows the Clarens sandstone locality. Note that the hydrothermal vent complexes in the Karoo Basin are present in the Stormberg
Group (i.e. close to the paleo-surface in the Early Jurassic) (modified from Svensen et al., 2006).
S. Grab, H. Svensen / Geomorphology 131 (2011) 14–27
19
Fig. 5. Summit view from the Witkop III hydrothermal vent complex. The sandstones and sediment breccias of the complex are covered by a reddish and rough weathering crust.
Note the potholes (shown by arrows) at the top of font-like structures.
morphological statistics calculated (mean, standard deviation, ratios)
(Fig. 1, Table 1). In addition, the diameters of 160 potholes and
doughnuts were measured at the Eastern Cape sites, and their GPS
coordinates recorded using a Garmin Etrex GPS. We identified 107
potholes from the Witkop III complex, measured from the edges of the
holes, and 53 doughnuts from the surrounding Clarens Fm. Sandstone,
measured from the top of the rims (Table 2).
Rock hardness values were obtained for two calcareous concretions and adjacent Clarens Fm. Sandstone, and for two rock doughnut
pothole floors, corresponding rims, and the adjacent bedrock
platform. After cleaning rock surfaces with carborundum, a classic
N-type Schmidt hammer was used to obtain 50 hardness readings per
micro-site.
Surface roughness was measured for a doughnut pothole floor, rim
top, north and south aspects of the doughnut sides, and adjacent
sandstone. Surface roughness was measured using a profile-gage,
which is pressed against the rock surface. The profile is then
transferred and traced onto millimeter graph paper, from which
deviograms are produced, as suggested by McCarroll (1992) and
McCarroll and Nesje (1996).
Samples from the red-brown hard crusts from the Clarens Fm. at
both Witkop II and Witkop III were sampled specifically for
petrographic investigations (Fig. 4). The petrography was studied
using a JEOL JSM 6460LV scanning electron microscope at the
Department of Geosciences, University of Oslo. Both thin sections
and mounted rock pieces were used. Given that the rock doughnuts at
Table 1
Size dimensions (a/b-axis basal diameters, N–E–S–W aspect rim heights, max/min rim widths, a/b-axis pothole diameters, and pothole depths) for rock doughnuts at GGHNP, with
primary morphological statistics (mean, STD, ratios) (n = 19).
Rock
doughnut
Basal diameter
Rim width
Pothole diameter
Max
Min
Max/Min ratio
North
Height
South
West
East
mean
Max
Min
Max
Min
Max/Min ratio
Pothole
depth
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
STDEV
MEAN
131
162
111
101
63
129
143
122
155
101
162
202
346
98
82
195
131
122
112
60.8
140
131
154
106
101
60
119
135
111
152
91
148
194
297
94
73
188
123
116
95
52.2
130
1.00
1.05
1.05
1.00
1.05
1.08
1.06
1.10
1.02
1.11
1.09
1.04
1.16
1.04
1.12
1.03
1.07
1.05
1.18
0.05
1.07
25
31
20
19
9.5
22.5
31
32
28
12
16
40
59
10
8.5
40
24
15
8.5
13.2
23.7
32.5
26
19
24.5
10
22
27
21
29
8
32
58
72
36
25.5
21
27
26.5
26.5
14.7
28.6
27.5
30
18
12
15
28
34
36
28.5
9
26
15
64
16
19
39
35.5
23.5
18
12.7
26
39
38
17
21
8.5
19
33
22
42.5
14
47.5
66
59
16
20
34
16
16.5
19
16.2
28.8
31
31
19
19
11
23
31
28
32
11
30
45
64
20
18
34
26
21
18
12.3
27
22
26
27
29
17
29
28
28
35
24
42
24
62
23
23
24
25
20
23
9.9
28
16
22
17
17
12
15
21
14
25
14
21
9
20
15
14
21
12
14
4
5
16
62
54
42
29
22
39
50
29
53
51
53
62
119
23
31
90
66
43
38
23.7
50
53
50
39
28
16
38
46
26
52
34
45
58
113
19
27
75
44
41
34
21.9
44
1.17
1.08
1.08
1.04
1.38
1.03
1.09
1.12
1.02
1.50
1.18
1.07
1.05
1.21
1.15
1.20
1.50
1.05
1.12
0.15
1.16
8.0
13.1
11.8
7.5
6.5
9.0
7.9
5.5
12.5
5.2
7.3
14.0
48.7
6.0
3.5
9.0
7.2
6.3
4.9
9.8
10.3
20
S. Grab, H. Svensen / Geomorphology 131 (2011) 14–27
Table 2
Coordinates and diameters of potholes and rock doughnuts in the Witkop III area.
Clarens Fm. doughnuts
S
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
11.613
11.614
11.614
11.614
11.614
11.618
11.620
11.620
11.621
11.621
11.621
11.619
11.619
11.619
11.620
11.621
11.621
11.618
11.618
11.619
11.618
11.615
11.612
11.608
11.607
11.606
11.606
11.608
11.607
11.605
11.602
11.602
11.601
11.597
11.596
11.594
11.595
11.593
11.594
11.594
11.590
11.585
11.587
11.594
11.596
11.602
Witkop III potholes
E
Diameter (cm)
S
27 13.923
27 13.927
27 13.926
27 13.928
27 13.927
27 13.926
27 13.925
27 13.926
27 13.926
27 13.926
27 13.924
27 13.921
27 13.922
27 13.921
27 13.918
27 13.919
27 13.919
27 13.916
27 13.917
27 13.916
27 13.916
27 13.916
27 13.919
27 13.919
27 13.919
27 13.920
27 13.919
27 13.914
27 13.913
27 13.913
27 13.915
27 13.920
27 13.920
27 13.920
27 13.922
27 13.924
27 13.923
27 13.923
27 13.922
27 13.918
27 13.915
27 13.922
27 13.921
27 13.926
27 13.926
27 13.926
93
170
220
75
130
95
75
43
95
58
45
70
50
90
115
60
60
35
100
100
45
35
85
15
62
60
70
40
65
90
70
40
190
400
50
90
30
83
35
150
50
45
12
65
120
50
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
GGHNP represent rare and unique sandstone features and are located
in a National Park and World Heritage Region where emphasis is
placed on geo-heritage conservation, it was not possible or appropriate to extract rock cores for laboratory analysis.
5. Results
5.1. Northeastern Free State region
The GGHNP rock doughnuts are conical in cross-section and oval/
circular in plan (Fig. 1). A characteristic not previously reported for
rock doughnuts are occasional subsidiary or ‘parasitic’ cones (i.e.
‘parasitic’ rock doughnuts) which flank the lower sides of the primary
doughnuts (Fig. 6). While the primary doughnuts have an apparent
vertical cylindrical-axis orientation, those of parasitic doughnuts have
recorded orientations of ca. 10° from the vertical. The primary
(central) doughnut pothole floors are near horizontal while those of
subsidiary potholes and ‘parasitic’ doughnuts have slightly inclined
floors.
A total of 20 rock doughnuts occur over an 82.8 m2 area on a rock
platform at GGHNP (Figs. 1 and 3), with a Nearest Neighbor Index
12.104
12.107
12.108
12.110
12.110
12.111
12.110
12.110
12.110
12.110
12.109
12.108
12.105
12.106
12.108
12.108
12.115
12.117
12.141
12.134
12.135
12.134
12.131
12.131
12.125
12.120
12.129
12.128
12.131
12.139
12.139
12.140
12.140
12.149
12.156
12.157
12.159
12.152
12.154
12.154
12.153
12.180
12.178
12.177
12.177
12.177
E
Diameter (cm)
S
27 13.805
27 13.806
27 13.806
27 13.808
27 13.808
27 13.809
27 13.809
27 13.809
27 13.811
27 13.810
27 13.811
27 13.811
27 13.816
27 13.816
27 13.812
27 13.810
27 13.791
27 13.792
27 13.761
27 13.768
27 13.769
27 13.772
27 13.773
27 13.769
27 13.769
27 13.762
27 13.753
27 13.752
27 13.764
27 13.760
27 13.761
27 13.750
27 13.753
27 13.741
27 13.746
27 13.752
27 13.753
27 13.737
27 13.732
27 13.731
27 13.732
27 13.740
27 13.746
27 13.748
27 13.749
27 13.748
60
60
25
25
30
50
25
30
50
40
50
45
45
30
10
35
30
30
20
30
110
30
25
15
30
15
120
30
40
15
5
15
20
40
15
20
20
30
30
15
20
25
30
15
30
30
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
31
12.195
12.200
12.207
12.207
12.207
12.207
12.203
12.210
12.21
12.210
12.213
12.213
12.212
12.215
12.216
12.217
12.214
12.216
12.216
12.217
12.216
12.222
12.222
12.221
12.222
12.223
12.224
12.229
12.229
12.229
12.230
12.225
12.224
12.222
12.221
12.221
12.220
12.220
12.220
12.219
12.221
12.221
12.221
12.221
12.197
12.197
(NNI) of 0.3672 (mean distance = 1.48 m, ±0.142). Although this
indicates a relatively random distribution, 70% (14) of the doughnuts
are located in clusters of 3 to 4, where the mean distance between
doughnuts is 0.2–0.4 m. The remaining 30% (6) are more ‘isolated’ and
randomly distributed (1.5–4.4 m apart). In addition, numerous
potholes without raised cones or rims occur across the platform.
The 19 rock doughnuts measured in the field have basal diameters
ranging between 0.73 m and 3.46 m (mean = 1.35 m), and average
overall heights varying from 0.11 m to 0.64 m (mean = 0.28 m)
(Table 1), making them comparable in size to those reported
elsewhere (Blank, 1951; Twidale and Campbell, 1998). However,
the mean rim heights may be highly variable for individual doughnuts
and are higher on average on south- (0.247 m) and east- (0.232 m)
facing sides than north- (0.178 m) and west- (0.188 m) facing sides,
given that the rock platform dips slightly toward the southeast. The
rock doughnut rims average 0.22 m in width, but these too may be
highly variable in diameter for individual doughnuts, which is
primarily due to greater erosion at water exit points (where these
exist) and a function of differential (aspect controlled) weathering
along the pothole edges and doughnut sides. The potholes average
0.47 m in diameter and 0.1 m in depth.
S. Grab, H. Svensen / Geomorphology 131 (2011) 14–27
21
Fig. 6. A primary (central) rock doughnut flanked by an inclined ‘parasitic’ doughnut at the GGHNP site (Hammer length [handle + head] = 0.31 m).
Schmidt hammer R-values indicate consistently harder doughnut
rims (mean = 48.7; n = 150) than the enclosed potholes (mean =
37.8; n = 150) and surrounding sandstone platform (mean = 39.7;
n = 250) (Fig. 7). Only calcareous concretions have considerably
higher R-values (mean = 54.9; n = 100) than the doughnut rims.
Similarly, Whitlow and Shakesby (1988) provide R-values for gutters,
Fig. 7. Comparative Schmidt hammer R-values for sandstone surfaces, concretions and rock doughnut rims and pothole floors at the GGHNP site.
22
S. Grab, H. Svensen / Geomorphology 131 (2011) 14–27
Fig. 8. Roughness indices (deviograms) for various micro-morphological rock doughnut localities and the adjacent sandstone platform. Within each deviogram the bars represent
measurement intervals of, from left to right, 5, 10, 15, 20, 25 and 30 mm.
rims and surrounding bedrock on a granite outcrop in Zimbabwe, and
demonstrated that the silica coated rims were substantially harder
(47.1) than the gutter floors (26.9) or surrounding bedrock (39.7).
Fig. 8 presents standard deviations of the differences between
adjacent rock surface profile gage values, according to methods
proposed by McCarroll (1992) and McCarroll and Nesje (1996). Such
rock surface roughness values may be used to assist in determining
the relative degree of rock surface weathering at particular sites
(McCarroll and Nesje, 1996). Within each deviogram, the bars
indicate horizontal surface measurement intervals of 5, 10, 15, 20,
25 and 30 mm. Given that there is little variation between the
measurement intervals used, we calculated mean deviations across all
6 measurement intervals for each sampling site. Our findings indicate
that the sandstone surfaces surrounding doughnuts are considerably
rougher (mean SD = 6.385 mm) than those on rock doughnut sides
(mean SD = 3.331 mm), rim tops (mean SD = 4.256 mm) and pothole
floors (mean SD = 0.805 mm). Although the north (SD = 3.403 mm)
and south (SD = 3.260 mm) aspects of doughnut sides have similar
roughness values, the rim tops seem to be weathering at a preferential
rate to the sides, possibly due to greater exposure.
5.2. Eastern Cape region
The average Witkop III pothole diameter is 0.42 m (±0.24 m),
whereas the average Clarens Fm. doughnut rim diameter is 0.83 m
(±0.63 m) (Fig. 9). The Clarens Fm. potholes are characterized as
being circular with a smooth rim. Although no surface hardness and
roughness tests were undertaken at the Eastern Cape sites, the rock
comprising the rims are notably harder and darker compared to the
whitish and smooth weathering surface of the Clarens Fm. Sandstone.
In contrast, the Witkop III potholes do not have a pronounced rim, and
appear more as circular holes in the otherwise very rugged and
irregular rock surface (Fig. 5). The smooth whitish weathering surface
which is typical for the Clarens Fm. is absent around such potholes.
The petrography of Clarens Fm. Sandstone shows that the typical
whitish sandstone is affected by intense chemical weathering (see
photos A and B in Fig. 10). There is no textural and mineralogical
difference between the Clarens Fm. samples from Witkop II and
Witkop III. The feldspars are partly dissolved and kaolinite is locally
present, as observed in photos B and F of Fig. 10. The pothole rims and
the irregular reddish crust typical of the Witkop III outcrops show a
Fig. 9. Frequency histograms of measured doughnut rims in Clarens Fm. Sandstone and pothole diameters from the top of the Witkop III hydrothermal vent complex.
S. Grab, H. Svensen / Geomorphology 131 (2011) 14–27
23
Fig. 10. SEM petrography of selected samples from the Witkop III and Witkop II areas. A) Partially dissolved albite with clay mineral coating from a sandstone of the outer zone of
Witkop III. B) Albite dissolution in reddish sandstone from the summit of Witkop III. The porosity is given in black. C–F) Secondary silica coating in sandstone from Witkop II with
overgrown kaolinite and barite crystals in the pores.
secondary cementation by microcrystalline silica (Photo C–F; Fig. 10).
The silica is forming a ~ 5 μm thick coating around the porosity of the
sandstones. Barite are also occasionally present in the pothole rim
samples, presumably originating from feldspar weathering, as barium
is observed in feldspar from the same rocks. Unpublished observations and data from other Clarens Fm. sites in the Eastern Cape
Province indicate that both the reddish hard crusts and the secondary
silica cement are common and widespread.
6. Discussion and conclusions
The GGHNP has a rich diversity of sandstone landforms which
provide much potential for geo-tourism (Grab et al., 2011). In
addition, over half of the proclaimed World Heritage Sites in southern
Africa are based on natural geological heritage (Reimold et al., 2005).
To this end, new data on rock doughnuts (i.e. conical forms) and
associated pothole assemblages from the Clarens Fm. Sandstone
should provide valuable environmental information for the development of regional geo-tourism.
6.1. Doughnut and associated pothole characteristics
The GGHNP rock doughnut potholes are similar in diameter
(mean = 0.47 m) to those measured elsewhere in northeastern Free
State Clarens Fm. sandstone (mean = 0.5 m) by Cooks and Pretorious
(1987) and those we report from the Eastern Cape (mean = 0.42 m).
The GGHNP rock doughnut potholes have an average a/b-axis ratio of
1.16, which are the most rounded forms yet documented in the
literature (Fig. 11). Mean pothole length:width ratios reported
elsewhere include 1.76 and 1.52 for two Namibian sites (Goudie and
Migón, 1997), 1.35 for a southern Patagonian site (Domínguez-Villar,
2006), 1.22 for potholes in the Cairngorms (Hall and Phillips, 2006),
and 1.25 for Clarens Fm. Sandstone potholes reported by Cooks and
Pretorious (1987). Thus, while pothole populations elsewhere in the
24
S. Grab, H. Svensen / Geomorphology 131 (2011) 14–27
Fig. 11. Length/width ratios for potholes, based on data from various international and southern African sites. [Data sourced from Goudie and Migón (1997) (Namib); Hall and
Phillips (2006) (Cairngorm); Domínguez-Villar (2006) (southern Patagonia); Cooks and Pretorious (1987) (Clarens)].
world and on Clarens Fm. Sandstone tend toward rounding, these
populations also host occasional elliptical forms, yet the GGHNP rock
doughnut potholes are all exceptionally round. Notably, the mean rock
doughnut basal a/b-axis ratios (1.07) tend to near perfect rounding
(Fig. 11). Such near perfectly rounded lithology of rock doughnut
cylindrical cone structures (in plan view) is more likely to have
influenced the well rounded (a/b-axis ratio = 1.16) potholes, than the
reverse. Firstly, potholes are typically less well rounded on Clarens Fm.
Sandstone and include more elliptical shapes, which are not observed
on the rock doughnuts from our two study regions, and secondly,
should the doughnuts have formed around the potholes and have been
influenced by the degree of pothole rounding, it would be expected
that the doughnut basal morphology would have a higher length/
width ratio as they are located on slightly convex rock platforms.
Most notable are the smooth surfaces of pothole floors (Fig. 8),
which are more likely a product of water and aeolian scour (abrasion),
rather than weathering alone. However, the much rougher sandstone
surrounding the rock doughnuts are undergoing substantial spalling,
and are evidently weathering at an accelerated rate, relative to the
harder and smoother doughnut surfaces. Using the doughnut rims as a
benchmark, denudation rates on the surrounding sandstone surfaces
are on average 2.04 times faster over unit time, than that for the
doughnut pothole floors, which represent transport limited micromorphological sites.
6.2. Hypotheses for Clarens Fm. rock doughnut–pothole forms
Blank (1951) proposed two hypotheses for the rock doughnuts
examined in central Texas. Firstly, it is considered that dissolved solids
contained in standing water within the potholes may penetrate the
immediate surrounding bedrock, forming a ‘halo of saturated’ bedrock
around the potholes. Consequently, it is argued, the ‘halo’ becomes
indurated through solute precipitation and oxidation, thus providing
for a more resistant surface to erosion. Secondly, it is hypothesized
that the potholes rapidly overflow during heavy rains, thus reducing
runoff velocity through and over the potholes, consequently producing eddies around the border of the potholes, which in turn would
limit water flow and erosion in this zone. The physics of such a
hypothesis has been challenged by Twidale and Campbell (1998), and
in any case cannot account for the predominant occurrence of rock
doughnuts on near-level summit platforms. Other early suggestions
for rock doughnut formations are also closely associated with the
potholes; these include differential water-level weathering/erosion
(Cotton, 1963), variability in wetting/drying cycles (Hills, 1971), and
protection through continuous wetting (Twidale, 1976).
More recently, Twidale and co-authors have proposed that rock
doughnuts are a product of differential weathering and erosion,
primarily due to drainage and moisture contrasts at the rock surface
(e.g. Twidale, 1988; Twidale and Bourne, 1998, 2000; Twidale and
Vidal Romaní, 2005). The first stage is represented by a pothole which
is surrounded by a thin regolith cover. It is hypothesized that moisture
from the surrounding regolith is preferentially drained into the
pothole, consequently desiccating such regolith more rapidly than
that some distance from the pothole. It is further postulated that the
circular rim of desiccated regolith would retard the rate of rock
weathering beneath such vegetation, hence producing the raised
bedrock rims flanking the potholes.
In alliance with Blank's (1951) first hypothesis, Twidale and
Bourne (1998) also support the idea that ‘some’ doughnuts may be a
product of induration (silica and iron oxides) due to precipitates from
pothole overflow/spillage onto the surrounding bedrock, or due to
preferential weathering of coastal platforms surrounding the more
protected water-filled potholes (i.e. beach etching). Similarly, Young
et al. (2009) propose that the upraised rims of observed ‘cones’ in the
southern Sydney Basin represent hardened bedrock due to possible
silica induration, but provide no supporting evidence.
Finally, Netoff and Shroba (2001) consider the origin of clastic pipe
structures (including rock doughnuts) near Glen Canyon, southeastern Utah, and suggest that the features may predominantly owe their
origin to lithological variations with consequential differences in
weathering and erosion. It is suggested that combinations of
liquefaction, fluidization, collapse, and faulting produces clastic
pipes, which in many instances provides for the selective depletion
and enrichment of carbonate within the pipe structures, and in turn
controls weathering and erosion to produce the observed doughnut,
megafont and armchair megafont morphologies.
To summarize, we simplify the available hypotheses for rock
doughnut and pothole formation in two groups: 1) local induration
and weathering of originally homogenous rock strata, and 2) local
induration and weathering controlled by pre-existing structures, such
as pipes or other gas and water seepage structures. In the following
we discuss how the various hypotheses fit with our new observation
and data from the Karoo Basin.
The first hypothesis implies that doughnuts and potholes can be
formed as a consequence of the local water flow and mineral
precipitation environment in a manner similar to for instance
travertines (e.g., Hammer et al., 2010). We argue that the hypothesis
of induration (1) (Blank, 1951; Twidale and Bourne, 1998) cannot
adequately explain the regularity of rock doughnut and pothole
diameters at the GGHNP site, as water flow and wetting around
potholes would be routed preferentially in the flow direction or
according to dominant westerly winds. It would thus be expected that
on sloping rock platforms, such as that at the GGHNP and Witkop
sites, doughnut diameters would tend toward downslope elongation,
which however is not the case. In addition, while such ideas may hold
true for particular sites, they cannot explain why the majority of
S. Grab, H. Svensen / Geomorphology 131 (2011) 14–27
potholes observed in Clarens Fm. Sandstone do not have a circular
bedrock rim (i.e. doughnut). Although potholes are very common on
Clarens Fm. Sandstone and on the platforms where the rock
doughnuts occur, raised rims are uncommon around the potholes
and upraised cone- or dome-shaped forms (i.e. the rock doughnuts)
are particularly rare. Further to this, the clustered pattern of rock
doughnut distribution across the bedrock platform at GGHNP, the
occurrence of ‘parasitic’ doughnuts projecting from ‘primary’ doughnuts (which has not previously been reported from rock doughnuts),
and the occurrence of inclined pothole floors of ‘parasitic’ doughnuts,
is more akin to secondary conduits and doughnut structures noted in
carbonate chimneys originating from fluid venting (Kulm and Suess,
1990). Similarly, Netoff and Shroba (2001) report the occurrence of
branching pipe structures and suggest the possible enrichment of
precipitates vertically up the ascending water columns and enhanced
precipitation of cementing agents along clastic pipe margins. This
brings us to Hypothesis 2.
Based on the morphologies of the Witkop III summit with the
associated potholes and pipes, and of the rock doughnuts at GGHNP,
we hypothesize that they are remnants of morphologies created by
Jurassic fluid seepage, with a superimposed and secondary silica
cementation. The seepage could involve local fluidization and even
small eruptions in a similar manner as on dormant mud volcanoes (cf.
Mazzini et al., 2009), with surface manifestations as springs or small
mounds that eventually were leveled by the ongoing erosion. The
seepage would include gas and water, and could result from basinscale heating and contact metamorphic devolatilization of the Karoo
supersequence following the emplacement of igneous sills and dikes
(e.g., Chevallier and Woodford, 1999; Svensen et al., 2007). The
absence of significant gas accumulations in the Karoo Basin suggests
that most of the generated gas escaped to the atmosphere, either
through pipe structures or via diffuse seepage.
Around the Witkop III hydrothermal vent complex, the landscape
gives the impression of representing a landscape that existed when
the complex was formed in the Lower Jurassic. Although preservation
of old land surfaces is difficult to prove, small and circular pipe
structures filled with calcite-cemented sand are present locally
surrounding the complex (Svensen et al., 2006; see Fig. 12), and
represent conduits for fluidized sand. Moreover, sandstone pipes of
several meters in diameter are present within the Witkop III complex,
interpreted as fluidization pipes. The resemblance to modern mud
volcano gryphons is striking. The various fluidization structures are
shown schematically in Fig. 13, and all relate to the formation of the
vent complex and water and gas expulsion in the Early Jurassic.
Besides fluidization as a result of degassing from heated sedimentary
sequences, dewater of the Clarens Fm. Sandstone due to the loading
from the flood basalts represent a likely mechanism for fluidization,
and would clearly result in structural modifications of the Clarens Fm.
on a basin-scale.
Still, the question remains whether wider portions of the Clarens
Fm. Sandstone surrounding the hydrothermal vent complexes at
Witkop were affected by Jurassic fluid seepage, and if the doughnutpothole structures in other areas of Clarens Fm. outcrops, such as at
GGHNP, represent former zones of seepage. Without structural or
borehole evidence (petrography and grain size distribution), the
hypothesis of a fluid seepage origin for the GGHNP rock doughnuts
cannot be confirmed. However, given that fluidization structures
evidently occur in Clarens Fm. Sandstone, as is the case at Witkop (cf.
Svensen et al., 2006), such mechanisms could possibly have
contributed, at least in part, to the observed rock doughnut structures
elsewhere on Clarens Fm. Sandstones. Moreover, the study by
Holzförster (2007) suggests that the sedimentology of the Clarens
Fm. Sandstone was more strongly influenced by phreatic eruptions
and fluidization than previously thought. Large-scale disturbance of
the Clarens Fm. Sandstone by seepage has however not been
investigated in detail across the Karoo Basin. Where localized over-
25
Fig. 12. A) Circular potholes in font-like structures and sandstone pipes (see arrows)
from the summit of the Witkop III hydrothermal vent complex. The pipes have
diameters typically less than 20 cm and are associated with flow structures in
sandstone, suggesting that the pipes formed near the paleo-surface and hence close to
the contemporary summit. B) Vertical calcite-cemented dewatering pipes in the
Clarens Fm. Sandstone in the outer zone of the Witkop III hydrothermal vent complex.
Pen (circled) for scale. C) Small scale weathering structures adjacent to the pipes in
photo b, as seen in map view. Pen for scale.
pressured groundwater during the final stages of Jurassic sedimentation may have been injected along relatively narrow vertical joints
(pipes) toward the surface, fluidizing overlying near-surface sediment
and incorporating silica from the host rock, this is likely to have
enhanced precipitation of silica along the pipe margins, and
particularly so toward the pipe surface where desiccation would
have been accelerated (Fig. 13). Should such a hypothesis be
applicable to some of the observed rock doughnuts, then the casehardened and more weathering resistant doughnut rims could
represent the circular pipe margins containing microcrystalline silica.
The sandstone petrography suggests that feldspar weathering is a
26
S. Grab, H. Svensen / Geomorphology 131 (2011) 14–27
Fig. 13. Schematic cross-section of the Stormberg Group sediments with a sill intrusion giving rise to vertical gas and water flow and a hydrothermal vent complex. Seepage and
dewatering resulted in fluidization of the Clarens Fm. sediments on a basin scale, whereas small and large scale pipes formed close to the vent complex. The inserted figures show
conceptualized cross sections through a rock doughnut (A), a dewatering pipe (B), and (C) a sandstone pipe. We hypothesize that the rock doughnuts result from weathering and
erosion of pre-existing seep anomalies in the Clarens Fm. Sandstone.
possible source of silica, resulting in hard silica crusts overlaying
kaolinite-rich porous sandstone. Dissolution/precipitation cycles are
possibly controlled by strong seasonality in rainfall and/or the high
diurnal rock surface temperature cycles, which today may range by at
least 40 °C/24 h on Clarens Fm. Sandstone (Sumner and Nel, 2006).
This implies that the mineralogy and weathering does not give any
fluidization process information per se.
Acknowledgments
Funding was provided to S. Grab by the University of Witwatersrand
Research Incentive Scheme and University Research Committee. We
would also like to thank the Norwegian Research Council for providing
funds to H. Svensen (SFF grant to PGP). We appreciate valuable
discussions with Sverre Planke, Bjørn Jamtveit and Luc Chevallier,
related to the Witkop III hydrothermal vent complex, and with Terrence
McCarthy and Andrew Goudie, related to the GGHNP rock doughnut
structures. Comments from two anonymous referees are much
appreciated.
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