Download Deposition of the Early to Late Permian Whitehill Formation during a

S.AfrJ.Geol.,1992,95(5/6),181-193
181
Deposition of the Early to Late Permian Whitehill Formation during a sea-level
highstand in a juvenile foreland basin
J.N.J. Visser
Department of Geology, University of the Orange Free State, P.O. Box 339, Bloemfontein 9300, Republic of South Africa
Accepted 11 November 1992
The black, laminated, carbonaceous shales of the Whitehill Formation were deposited in a very young, underfilled
foreland basin under anoxic bottom conditions. A sea-level highstand, basin tectonics, and climate were the controlling
factors - interplay of which resulted in bounding conditions for organic-rich mud deposition during a specific time
slot in the history of the basin. Coal-forming environments along the steep palaeo-eastern basin margin were the
source of mud and organic matter transported as fresh-water plumes in an offshore direction during episodic flooding
and erosion of the organic-rich deposits. Air-borne volcanic ash deposited together with the muds as well as in discrete
layers was derived from a tectonic arc in the palaeo-west. The high concentration of organic matter in the water body
and the restricted oceanic circulation in the morphologically complex basin created anoxia in the water column.
Preservation of organic matter in the absence of benthonic fauna was high. Less anoxic conditions prevailed in the
shallow marginal regions where deposition of siltstone and carbonate rocks interbedded with the black shales took
place. Continuous inflow of fresh-water plumes in the restricted basin progressively caused brackish conditions
suitable for the proliferation of aquatic fauna.
Die swart, gelamineerde, koolstofryke skalies van die Whitehill-formasie is onder anoksiese bodemtoestande in 'n baie
jong, ondergevulde voorlandkom afgeset. 'n Hoe seevlak, komtektoniek, en klimaat was die kontrolerende faktore
waarvan tussenwerking tot begrensingstoestande vir die afsetting van organiesryke modder gedurende 'n spesifieke
tydvak in die geskiedenis van die kom, aanleiding gegee het. Omgewings van steenkoolvorming langs die steil
oeroostelike grens van die kom was die bron van modder en organiese materiaal wat as varswaterpluime in 'n
wegstrandse rigting gedurende episodiese oorstroming en erosie van die organiesryke afsettings, gevoer is.
Lugstroomvervoerde vulkaniese as afkomstig van 'n tektoniese boog in die oerweste is tesame met modder asook in
onderskeie lagies afgeset. Die hoe konsentrasie van organiese materiaal in die watermassa en die beperkte oseaniese
sirkulasie in die morfologies-komplekse kom, het anoksia in die waterkolum tot gevolg gehad. Preservering van
organiese materiaal was hoog in die afwesigheid van bentoniese fauna. Minder anoksiese toestande het in die vlak
randsones waar afsetting van sliksteen en karbonaatgesteentes tussengelaagd met die swart skalies plaasgevind het,
geheers. Aanhoudende invloei van varswaterpluime in die beperkte kom het progressief braktoestande, geskik vir die
opbloei van akwatiese fauna, veroorsaak.
Introduction
The Whitehill Formation, formerly known as the 'White
Band', is a very conspicuous stratigraphic unit near the base
of the Ecca Group in the western half of the Karoo Basin
(Figure 1). This white-weathering unit is persistent throughout the area which makes it an excellent marker horizon
and, together with its characteristic fossil assemblage, a
time-stratigraphic marker. It was furthermore considered as
a potential hydrocarbon source rock on account of its
organic content.
The Whitehill Formation is probably the one formation in
the Karoo Supergroup 'we know little about despite a large
number of publications on its palaeontology, sedimentology,
and economic potential (e.g. McLachlan & Anderson,
1977a; Anderson, 1979; Oelofsen, 1981; 1987; Cole &
McLachlan, 1991). In most of these publications reference is
made to the origin of the formation. All authors reached a
consensus that the formation was deposited as black,
organic, highly sulphuretted muds in a large sea and which
were preserved as black shales in the Parana Basin of South
America and the Karoo Basin of southern Africa. This is,
however, a fairly simplistic view which leaves several
questions unanswered.
1. Why is the Whitehill Formation such a distinctive unit in
an overall mudrock succession?
2. Why does the organic content of the formation increase
with respect to the under- and overlying strata?
3. Why is there an abrupt change in style of mud
sedimentation with respect to the under- and overlying
strata?
The objective of this paper is to formulate answers to
these questions and, in the process, define a more acceptable
Figure 1 The Whitehill Formation in outcrop north of the
Orange River on the farm Aussenkjer. Note the presence of blackweathering shale interbedded in white-weathering shale above the
lower dolerite and a white streak below the base of the formation.
The top of the Whitehill Formation was taken at the whiteweathering shale above the black horizon (Aussenkjer section in
Figure 4). Do = dolerite.
S.-Afr.Tydskr.Geol.,1992,95(5/6)
182
\'\,
II
NAMIBIA
BOTSWf/~,A
'I
:,;
\
\
I
,,/
"---~SOUTH
/'-?
,)
k.---."
AFRICA
.,:
<:
,
Main Karoo ..
Basin \ :
Douglas
,
Auss enkJe~...........:
, '-----..,....-..--,,--,----t
..:' Kenh~';dt '" .. : Lateral cut-off ,
':'"
.
.
for Whitehill'
Deep-water
' . :~~esfon:~~n '--', Formation~
facies of
..
-I
lo,.
Shallow
Whitehill
. De Bos
i Prince Albert
\
facies of
Formation
'
Whitehill
Ceres
.... Mer~eviJIe ----, Willowmore
Formation
Grahamstown
PORT ELIZABETH
o
200
400km
Figure 2 Locality map showing the major facies distribution in
the Whitehill Formation.
model for Whitehill deposition. This means that in the
analysis aspects of the sedimentology of the underlying
Prince Albert and overlying Collingham Formations in the
southern Karoo need to be covered as well as the regional
reconstruction of southwestern Gondwana during the Early
to Late Permian transition. This is necessary to get a
perspective of the morphology and tectonic setting of the
Whitehill basin.
predominantly of dark-grey, carbonaceous, pyrite-bearing,
splintery shale, olive-grey micaceous shale, dark mudstone,
and subordinate bluish-grey chert nodules and lenses,
nodules as well as thin beds of phosphorite and carbonate
concretions. Viljoen (1990) recognized widely distributed
volcanigenic material in the formation, whereas discrete tuff
beds are present in the northwestern Karoo (McLachlan &
Jonker, 1990) and near the top of the formation between
Laingsburg and Klaarstroom.
Lithologies within the formation can be grouped into five
lithofacies (Figure 3). The mudstone-shale facies which
contains subordinate claystone breccia, fine-grained wacke,
chert, and dolomitic limestones, pinches out in the Prince
Albert area, but merges with the greenish-grey shale facies
to the north. The chert-shale facies is a very distinctive unit
and forms a useful marker in the formation. It consists of
reddish weathering chert, containing abundant pyrite, and
subordinate dark-coloured shale. Phosphorite and chert
bodies in the south and dolomitic limestone in the north are
the main constituents of the greenish-grey shale-chert
facies. The olive-grey micaceous shale facies containing
phosphorite bodies forms lenticular units in the Laingsburg
and Grahamstown areas (ef Wright, 1969). At the top of the
formation the dark-grey shale-phosphorite facies is developed in the Laingsburg-Prince Albert area and then again to
the east of Klaarstroom. It also contains dolomitic limestone
and pyrite.
Stratigraphy and lithology
Prince Albert Formation
Whitehill Formation
The Prince Albert Formation attains a maximum thickness
of just under 200 m in the Laingsburg area and east of
Klaarstroom (Figures 2 and 3). The contact with the Whitehill Formation is sharp, except in the north (Loeriesfontein Aussenkjer area) where thin white-weathering beds are
present near the top of the formation (Figure 1).
The outcrops of the Prince Albert Formation consist
The white-weathering Whitehill Formation which is a
synchronous marker horizon (Oelofsen, 1987), is present
only in the western and central parts of the Karoo Basin
(Figure 2). Mapping of the Whitehill Formation shows that
it loses its 'character' laterally and can no longer be
recognized as such (ef Cole & McLachlan, 1991). Along
strike in the Karoo Basin the lateral cut-off mostly
CD
Mudstone-shale facies: minor claystone breccia,
wacke and chert; chert pinches out southward;
dolomitic limestone content increases northward
@ Greenish-grey shale-chert facies: phosphorite in the
south; dolomitic limestone content increases
northwards; chert pinches out towards the north
MERWEVILLE
® Chert-shale facies: reddish marker;
abundant pyrite
Phosphorite
boundary
LAINGSBURG
@
Olive-grey micaceous shale facies: phosphorite in the south
m
@ Dark-grey shale-phosphorite facies:
100
carbonaceous and dolomitic limestone;
pyrite; tuffaceous beds
Chert/phosphorite
boundary
MF
= Marine fossils
o
Chert/phosphorite
boundary
WILLOWMORE
@
WEST
EAST
Figure 3 Fence diagram of facies distribution in the Prince Albert Formation along the southern basin margin. The heavy line
represents a distinctive break within the Prince Albert Formation.
en
>
::i'
o
¥~
(1)
"Tl~61~
C. ~
::s fD
a.~§:n ~
(1)
..... 0Cl
a:ci Jg (1) II en
II
II
>-j
.....
n
~
~
~O.,
'"1
"Tl::rN
~
3
3g
~
~~
'"1 q:;
CD t=.
AUSSENKJER
GENERAL KAROO
STRATIGRAPHY
~
o
a
'"1
rz..
;" (S.
0
I-:
o' g:;?Ro~
~ g.
en (") 0
(1)
::s
~s ____
~
o.."Tl
g
II
~<
.....
(1)
'"1
0Cl
en
~g:.....,
~ § So
(1)
,--,
~~~
(")
::r :>_::r
II ~~.
61~
3
en
0
~
~
~N[
::1. II
-
c. "Tl ~ ~ 61
o 0 (1) ~ '"1
P
:> '"1 3
t:C ~
3
a g.
P
II
'"1
0:
;?
-
0
t;r>
~
_
~.
0
?
t:C l ' o"Tl (1) en
~ '"1 ::s (1)
~
3
(1)
,=",>"Tl~ -"Tl
~
::>
rz..
I:t;
0
II
g.
~~.
a~P~~
a
s:::Jg..... ~VJN
~
':='
en
II
II
'="'>
g"~~~
_
(1)
'"1
~OCl2:~So
~
=::
61
0
0
I'zl
...>
~
•
~
g
tI
<
-
0
::to ~.....,
~g~s•
•
(1)
-----_.
I
--
\~.--
//;
\
\
Notocaria tapscotti
Palaeoniscoid fish
'\. "
'
o
\.
rz..
\.
\.
\.
"\.
\
I%:
",
10
0
\
\
,,
,
~
Dark-grey splintery
shale
~
~
Laminated black
carbonaceous shale
B:::..: -::1
Grey silty shale
(siliceous in south)
r';.::':.-:':'
:':<1
\
\\F".:.:':.":':'.':.:':
Fin~
to mediumgrained sandstone
2
~ Coal
PRINCE ALBERT
2 ~
KLAARSTROOM
.
~
Fe-rich carbonates
III
T
\
\
>
\
m
/
LAINGSBURG
>t
I:t;
\
20
\
A
....
I'zl
\.
\
::::::: 7.
Chert (siliceous mudrock)
Tuffaceous
T
\
\
,
\
o
.__---- to't~
/.--'-- ;\;e't\
\ ./ /'~~'"
\
A
---.__'-- --~\\o~
/'-0-(,
,,-
\
~
'
.--
./ "'\
Mesosaurus
>t
~
__ __
\
0
~
I:t;
\
I
-- --
<
\.
I
\
<
\.
I
Fossil Range Zones
I:t;
' ' \ \ "'
\
E-!
\
\
::>
'
f
~
~
"?
"?.
I
I
o
o
o
0
'
z
o....
\ \
I
...
VI
'cc" , " '
L_ __ .. _.. .c
.i
/
."CC'
\
I
I
0
I-:
0
3 ::s ::s
'"1
/
/'
//
/
'
//
/:
//
:,
rz..
~ ~.
::to
I
I
-
a -0
E? (")
·:
~
0
o ..... _.
"Tlg.
3
I
I
I
as
(1)
§.
·,
·
///
:
,
·
~
~
...-:-.::.:-: ....
I
I
I
0
<
··
I
I
I
I
~
KENHAR DT 2
I
1I
e::
/
:1"
il
rz..
l ' ::s
~
(")
C'I
f---
"Tl P
II
·,,
. . . ."-,,.--'"":1
f
o
o
o
I' >"
Jt::L / //~:"-Cc:~
,,
I
~. 61 ~ on~::r
'"1
~
o
P 3
a
:N:B
i.e
BOSHOF 1
LOERIESFONTEIN
'-"
DOUGLAS
B. GR.
-
WELKOM 3
,
\
00
VJ
S.-Afr.Tydskr.Geol.,1992,95(5/6)
184
+ JOHANNESBURG@
+
+
DUR,~,7'
/
...., /
Figure 5 Tentative isopach map of the Whitehill Formation.
Orientation of fossils in the formation (arrows) after Oelofsen
(1981).
comprizes a change in weathering characteristics of the
mudrock, whereas a lithofacies change takes place towards
the northern basin margin (Figure 4). It is also only along
the southern basin margin that the upper and lower contacts
are well defined, whereas northwards the formation becomes
less distinct (cf Figure 4). There organic-rich shales, which
weather black, occur interbedded, below or above the
Whitehill Formation (cf Figure 1). This illustrates that the
deposition of the Whitehill muds represents a change in
bounding conditions in a specific sector of the basin during
overall mud deposition.
The Whitehill Formation has a maximum thickness of
about 80 m in the Loeriesfontein area (Figure 4). Along the
Cape Fold Belt, complex folding occurs within the formation with the result that thickness measurements are
unreliable. Formation contacts are poorly defined along the
northern basin margin. Therefore only a tentative isopach
map which shows an apparent northwest - southeast
thickness trend with maximum thicknesses in the west,
could be constructed (Figure 5). The formation thicknesses
show a correlation with the palaeotopography along the
northern basin margin in that greater shale thicknesses occur
near palaeovalleys.
Lithology
Lithologically the formation can be subdivided into a 'deepwater' facies characterized by the presence of chert and
carbonate concretions, and a 'shallow-water' facies with
numerous silty horizons and subordinate carbonate beds, but
with chert noticeably absent (Figures 2 and 4). Along the
western outcrop belt the shallow-water facies consists of a
basal and an upper shale horizon separated by a carbonatebearing zone. Black, carbonaceous, pyrite-bearing shale
constitutes between 75 and 100 per cent of the formation.
The shale is very thinly laminated and the carbonaceous
material consists mostly of amorphous kerogen (Cole &
McLachlan, 1991). In the Boshof area the black shales
contain siltstones and very fine-grained sandstone beds,
laminae, and lenses (Figure 4). These become thicker and
more abundant northward. They exhibit wave ripples, horizontal laminations, and minor inverse grading and loadcasting (cf Cole & McLachlan, 1991).
Thin, yellowish-weathering rhyodacitic tuffs associated
with silty shale are interbedded in the formation in the
Loeriesfontein - Kenhardt area, Douglas - Boshof area, and
east of Laingsburg along the southern basin margin
(Rowsell & De Swardt, 1976; McLachlan & Anderson,
1977a; Viljoen, 1990; McLachlan & Jonker, 1990). The
carbonate-bearing zone consists of ferruginous, dolomitic
limestone containing organic-rich laminae (McLachlan &
Anderson, 1977a), whereas dolomitic concretions are found
throughout the formation. Halite imprints were found on
bedding planes along the northern basin margin (Van der
Westhuizen et al., 1981). Primary gypsum crystals within
the carbonate beds in the southwestern Cape were apparently replaced by dolomitic material (McLachlan &
Anderson, 1977a) and Cole & McLachlan (1991) reported
gypsum lenticles in shale from the Boshof area.
Palaeontology
The biostratigraphy of the Whitehill Formation suggests
synchronous deposition in the Karoo Basin, as the fossil
range zones are confined mostly to the upper part of the
formation (Figure 4). The remains of a primitive swimming
reptile (Mesosaurus tenuidens) (Oelofsen, 1981) and of
plants (Glossopteris leaves and fossil wood), sponge
spicules, palaeoniscoid fish (Palaeoniscus capensis), and
arthropods (No tocaris tapscotti) are common (Figure 6).
Rare fossil insect wings were also reported by McLachlan &
Anderson (1977b). Worm trails are present in grey silty
shale near the middle of the formation at Loeriesfontein.
Anderson (1975) reported the presence of limulid trackways
at the top of the formation in the Aussenkjer area.
Correlation
Van Eeden (1973), McLachlan & Anderson (1975), and
Anderson (1977) suggested a correlation between the Whitehill Formation and the Middle Ecca coal measures. Such a
correlation is also substantiated by borehole sections in the
Boshof area where the fine-grained sandstone facies may
represent the distal extent of the coal measures (Vryheid
Formation) deposited along the northern and northeastern
basin margins (cf Cole & McLachlan, 1991) (Figure 4).
Mapping of the Dwyka - Ecca transition in the southern
Karoo indicates a facies change as the ice retreated towards
the palaeo-south and palaeo-east. Therefore facies 1 and 2
and probably also facies 3 of the Prince Albert Formation
are the lateral equivalents of the upper diamictites in the
eastern and northern parts of the basin (Figure 3 and 7).
Van Vuuren (1983) also showed that the coal beds are overlain by transgressive units which enables a correlation with
the offshore shale and siltstone facies. The upper part of the
Prince Albert Formation is thus correlated with the lower
coal-bearing cycle of the Vryheid Formation in the Kroonstad area, whereas the upper two cycles correlate with the
Whitehall Formation (Figure 7).
The Whitehill Formation is also present in southern and
northern Namibia where Horsthemke et al. (1990) correlated
the beds lithologically with the Irati Formation in the Parana
Basin. Oelofsen (1987) and Oelofsen & Araujo (1987)
correlated the succession in the Karoo and Parana Basins
ecologically and palaeontologically.
S.Afr.J.Geol.,1992,95(5/6)
185
ELAND$VLEILOERIESFONTEIN
KROONSTAD
AREA
AGE
z
<
a:
;:)
<.?
Z
;:)
Basement
'"
z
<
::E --
a:
>...J
a:
<
z
<
~
•
Coal
~
Limestone
D
Sandstone
U
(/)
z
~
Siltstone
~ Shale
<
~
Diamictite
G
Glauconite
Tr
Transgression
• Highly carbonaceous shale, but not while-weathering,
present in Aussenkjer section
Figure 7 Generalized sections of the Prince Albert and
Whitehill Formations along the western basin margin and of the
coal-bearing Vryheid Formation along the northern basin margin
(after Stavrakis, 1986), showing correlations and sea-level
fluctuations during the Early to Late Permian. PMB =
Pietermaritzburg, Wh = Whitehill, Fm = Formation. Modified
after Visser (in press).
Figure 6 Fossils from the Whitehill Formation from near
Loeriesfontein. A. Primitive free-swimming reptile (Mesosaurus
tenuidens). B. Arthropod (Notocaris tapscotti). C. Palaeoniscoid
fish (Palaeoniscus capensis).
Age
The age of the Prince Albert and Whitehill Formations is
based on palynology, their correlation with the coal-bearing
beds of the Vryheid Formation in the Karoo Basin and
formations in the Parana Basin of Brazil, the presence of
volcaniclastics, and the Eurydesma fossils near the top of
the glacigenic Dwyka Group in Namibia. The Eurydesma
fossils within the Dwyka Group have a Sakmarian age
(Dickins, 1984) which led Visser (1990) to suggest an
Artinskian age limit for the deposition of the upper Dwyka
beds. A Late Permian age had been assigned to the Irati
Formation (Archangelsky & Toigo, 1978). Pollen analyses
from the coal-bearing strata which correlate with the Prince
Albert and Whitehill Formations, indicate an Artinskian to
Kungurian age ~ (Rio Bonita beds in the Parana Basin; Dos
Santos et al., 1988) and a Late Sakmarian to Late Artinskian
age (Waterberg coalfield; MacRae, 1987). MacRae (1992)
assigned a Late Permian age, based on palynology, to the
Whitehill Formation in the Douglas - Boshof area. Recent
work on the Glossopteris floras of the coal measures also
indicates a Late Permian age (Kovacs-Endr6dy, 1991).
Tuffaceous beds occur from near the top of the Prince
Albert Formation to the Collingham Formation and show
the same rhyodacitic composition of tuffaceous beds in
similar strata in the Parana, Paganzo, and Sauce Grande
Basins of South America. There the rhyodacitic volcanism
started at about the Early to Late Permian boundary and
continued until the Early Triassic (Coutinho et al., 1988;
Breitkreuz et al., 1989; Lopez-Gamundi et al., 1990). Based
on these data Visser (1990) suggested a Permian age;
possibly Artinskian to Kungurian for the Prince Albert
Formation and Late Kungurian to Early Ufimian for the
Whitehill Formation.
Collingham Formation
The formation consists of a rhythmically bedded succession
of dark-grey splintery shale, yellowish-weathering tuffaceous beds, dark-grey chert, and subordinate very finegrained sandstone. About 10 m above the base of the
Collingham Formation a white-weathering, dark-grey chert
(Matjiesfontein chert) forms a prominent marker bed. The
tuffaceous beds have a rhyodacitic composition (ef. Viljoen,
1990).
S.-Afr.Tydskr.Geol.,1992,95(5/6)
186
Table 1 Chemical data on mudrocks and cherts from
the southern Karoo
2
3
4
5
6
80.23
Si02
Ti02
64.86
88.59
58.31
81.05
61.48
0.61
0.20
0.62
0.12
0.61
0.12
A120 3
17.71
5.09
15.95
9.04
18.04
10.12
F~03
5.28
2.91
6.48
0.67
6.70
0.81
MnO
0.10
0.05
0.07
0.03
0.11
0.06
0.09
MgO
1.48
0.59
2.36
0.10
1.86
CaO
0.33
0.05
0.94
1.67
1.01
1.22
Na20
0.97
0.23
1.26
5.11
1.41
5.51
K20
3.10
0.69
3.23
0.07
3.64
0.12
P2 0 S
0.10
0.01
0.16
0.12
0.23
0.05
LOI
5.66
1.41
10.20
1.66
4.85
1.46
100.20
99.82
99.58
99.64
99.88
99.79
159
64
153
92
154
93
0.0055
0.0098
0.0046
0.0033
0.0055
0.0059
1.2
0.8
2.8
0.3
1.1
Total
Zr
MnO/
Al20 3
Org. C
Carbo C
0
0.4
0.6
0.4
<1
<1
32
51
58
67
43
38
0
5
3
20
105
45
*Organic
matter
4
2
*Oastic
detritus
*Biog.
silica
*Dolomitic
limestone
tSedimentation
rate
1. Prince Albert shale (3 analyses)
2. Prince Albert chert (red marker), Laingsburg
3. Whitehill shale (7 analyses)
4. Whitehill chert, Laingsburg
5. Tierberg/Collingham shale (6 analyses)
6. Collingham chert (3 analyses), Laingsburg - Matjiesfontein
*After Tada (1991)
tAfter Iijima et al. (1985)
Facies interpretation
Carbonaceous mudrocks
The presence of thinly-bedded shale, mudstone, claystone
breccia, silty rhythmite, and fine-grained wacke in the
Prince Albert Formation suggests deposition by sediment
gravity flow (mostly distal turbidites) and subordinate
suspension settling. The muds contain about 2 per cent
organic matter (Table 1) indicating accumulation under
anoxic conditions as is also shown by the rarity of
bioturbation and the presence of pyrite. The anoxic
conditions were probably caused by oxygen consumption in
the water during the degradation of the descending organic
matter through the water column (ef. Demaison & Moore,
1980). The geometry of the mudrock facies along the
southern basin margin is explained by the formation of
stacked basin floor mud fans - the distribution of which
was controlled by the sediment entry points.
The thinly laminated Whitehill shales formed by slow
suspension settling of mud, whereas the interbedded siltstone and sandstone were deposited by tractional bottom
currents in a shallow environment as shown by the crossbedding and ripple-lamination. However, some of the
coarser-grained units in the Loeriesfontein and Aussenkjer
sections may have formed by rapid suspension fall-out of
fine sand and silt from large inflows of sediment-laden
water as shown by the presence of slumping and parallel
lamination (cf. Domack, 1983). The black muds deposited
along the southern outcrop belt contain about 4 per cent
organic matter consisting of plant algae, bacterially altered
land plant cuticle and pollen, and possible benthic
cyanobacterial mats (Cole & McLachlan, 1991). This
material accumulated under anoxic conditions as indicated
by the presence of pyrite and lamination in the mudrocks
emphasizing the absence of bioturbation. The increased
preservation of organic matter is attributed, firstly, to a
higher input flux of organic matter whereby more organic
material was entrained during suspension settling of the
sediment with reduced dilution by mud (ef. Pedersen &
Calvert, 1990) and, secondly, to increased anoxic conditions
in the water column due to a greater demand on oxygen
leading to an almost complete absence of scavenging fauna
(ef. Demaison & Moore, 1980). Cole & McLachlan (1991)
attributed the anoxic bottom conditions to the formation of
benthic microbial mats, but up to now no evidence for the
presence of such mats has been found and their effectiveness on oxygen drain in the water column over such a large
basinal area is doubtful. The presence of trace fossils in the
middle and arthropods in the upper part of the formation
along the northern basin margin indicates episodic
oxygenated bottom conditions. These conditions can be
attributed to the shallower environment where oxygen drain
in the bottom waters was sporadically reduced.
The organic-rich mud formed a blanket deposit on top of
the Prince Albert Formation in the southern Karoo. Along
the northern basin margin increased formation thicknesses
can be attributed to mud and silt accumulations in the
vicinity of possible sediment entry points (ef. Figure 5). The
average chemical index of alteration for 6 Whitehill shales
is 67 (Visser & Young, 1991) which is below that of an
average shale (= 72). This lower value can be attributed to
the presence of unweathered volcaniclastic material in the
black shales (ef. Fritz & Vanko, 1992).
Strydom (1950) described worm trails on bedding planes
and Viljoen (1990) bioturbation of the Collingham mudrocks. The shales may represent distal mud turbidites which
accumulated
under
oxygenated bottom conditions.
Geochemically there is a consistent increase in CaO, Na20,
and K20 from the Prince Albert to the Collingham shales
(Table 1) suggesting a progressively larger input of
volcaniclastic material to the bottom muds.
Chert
Interpretation of the chert formation is important in that
chert occurs interbedded with the deep-water facies of the
Prince Albert, Whitehill, and Collingham Formations. The
Prince Albert chert has a higher Fe content than that of the
S.Afr.J.Geol., 1992,95(5/6)
Whitehill and Collingham Formations, whereas all the cherts
contain organic material and clastic detritus (el. Table 1).
The cherts can be regarded as a binary mixture of
biogenic silica and terrigenous detritus. Radiolarian spicules
were observed in the cherts (el. Haughton et al., 1953), but
most of the skeletal material was dissolved during silica
phase transformation. The Ti~ (>0.1 %), A12 0 3, and Zr
contents indicate a large input of clastics (el Adachi et al.,
1986).
The Prince Albert chert has been influenced for a longer
period by sea-water as indicated by its F~03 content of
about 3 per cent and its low sedimentation rate of about
20 m/Ma (Table 1). It consists of about 67% biogenic
silica, 32% clastic detritus and 1% organic matter (Table 1)
which, together with the presence of parallel lamination,
suggest slow suspension fall-out of largely siliceous skeletal
material. The chert formation can be attributed to a high
frequency of biogenic silica productivity in the sea (el
Tada, 1991).
The Whitehill chert consists of about 43% biogenic silica,
51 % clastic detritus, 5% dolomitic limestone, and 1%
organic matter. Sedimentation rate was about 105 m/Ma
which was about 5 times as high as the Prince Albert chert,
but still within the range of biostratigraphically calculated
rates (el. Tada, 1991). The higher sedimentation rate may
imply emplacement by turbidity currents, but because clastic
detritus exceeds the biogenic silica input, high-density
suspension fall-out is considered more likely for the origin
of the chert. Although a high frequency of biogenic silica
productivity may have still occurred in the Whitehill sea,
suspension fall-out of skeletal material was partly masked
by a higher terrigenous clastic input flux as evidenced by
the 24 per cent decrease in biogenic silica content in the
chert. The nodular nature of the chert may then in part be
attributed to density inversion and diagenesis. The carbonate
in the chert is of diagenetic origin. Anoxic bottom conditions during deposition are indicated by the presence of
pyrite.
The rhythmically bedded Collingham chert alternates with
shale and tuffaceous beds. It consists of about 57% clastic
detritus, 38% biogenic silica, 4% dolomitic limestone, and
1% organic matter. Theron (1967) ascribed the graded
bedding in the Matjiesfontein chert to turbidity current
deposition; thereby implying that the organic material had
been transported from a shelf to its present position.
According to Murray et al. (1992) grading in chert is not
necessarily an indication of turbiditic input, whereas Viljoen
(1990) suggested that most of the chert beds had a
volcaniclastic origin. The sedimentation rate of the cherts
was about 45 m/Ma (Table 1) which rather suggests fall-out
of predominantly terrigenous material (mostly ash?) together
with siliceous skeletal material. The carbonate in the chert is
of diagenetic origin.
187
chemistry of the Collingham shales suggests also the
presence of volcaniclastic material. The tuffaceous material
represents air-borne volcanic ash (Viljoen, 1990). The
presence of mostly well-defined tuffaceous beds, as well as
disseminated volcaniclastic particles in the host-rocks,
suggests different levels of ash concentration in the
atmosphere over the Karoo Basin. The deposition of a bed
occurred as a sudden episodic event and required an initial
introduction of a large volume of volcaniclastic material in
an upwind dIrection (el. Moore, 1991). Depletion of the ash
source would have resulted in lower levels of ash fall-out
and minor contamination of bottom deposits by the ash. The
geochemistry of the mudrocks and the clastic detritus
content of the cherts suggest an increase in ash contents
from the Prince Albert Formation to the Collingham
Formation which may imply increased volcanism in the
source areas or a change to drier and windier climatic
conditions during the Permian.
Carbonate bodies.
The carbonate nodules, which are mostly confined to the
deep-water facies of the Prince Albert and Whitehill
Formations, are early diagenetic in origin. Those from the
Whitehill Formation are almost black in colour due to their
organic material content (Strydom, 1950). They formed
below the water-sediment interface during suspension
settling of mud and organic matter with high levels of
organic preservation. Bottom conditions were anoxic (el.
Scotchman, 1991). The absence of carbonates in the
Collingham Formation may reflect the change to more
oxygenated bottom conditions.
Along the northern basin margin, the dolomitic limestones
of the Whitehill Formation are associated with silty horizons
suggesting a shallower depositional environment (Oelofsen,
1987). The limestone beds contain carbon-rich microlaminations and casts of gypsum crystals, but show a lack of
shallow-water sedimentary structures (el McLachlan &
Anderson, 1977a). These features suggest deposition below
wave-base from bottom waters with greatly increased
salinities (brines?), which could be attributed to stratification
in the water column in a possible silled basin, or as a result
of high aridity or to the influx of brines formed by
evaporation in shoreward areas during a sea-level lowstand.
Oelofsen (1987) reported the presence of traces made by
benthic fauna in the associated silty shales, but normally, the
presence of bottom brines would have prevented sessile
fauna from entering the depository and establishing themselves (el. Eugster, 1985). However, during deposition of
the shallow-water facies episodic oxygenated bottom conditions must have occurred, ruling out stratification as a
possible cause for the high salinities. The carbonate horizon
correlates with a temporary lowstand (Figure 7) which
would have favoured the influx of brines from shoreward
regions.
Volcaniclastic deposits
Thin, yellowish-weathering tuffaceous beds occur towards
the top of the Prince Albert Formation, throughout the
Whitehill Formation, and increase in number in the Collingham Formation where they constitute between 30 and 40 per
cent of the rock volume (Lock & Wilson, 1975). The geo-
Phosphorites
The phosphorites, which also contain pyrite and up to 4 per
cent argillaceous material, are notably absent from the
Whitehill and Collingham Formations. The black colour of
the rock is due to the presence of carbonaceous material.
S. -Afr.Tydskr.Geol., 1992,95(5/6)
188
Deposition of phosphatic material occurred in the muds
below the water-sediment interface in a restricted anoxic
environment from interstitial waters enriched in phosphorous (ef Cook, 1984).
According to Cook (1984), a marine environment with the
minimal input of terrigenous sediment and a sea-level rise
are prerequisites for the formation of phosphorites. During
the early stages of the sea-level rise oxygen-depleted and
phosphorus-enriched waters were brought into the basin
leading to phosphorite deposition (ef Brasier, 1992). This
was followed by deeper anoxic waters which enhanced the
deposition of the organic-rich black shales of the Whitehill
Formation. The increased concentration of suspended terrigenous material in the water column and the reduced rate of
oceanic overturn due to the closing of sea ways also contributed to the absence of phosphorites in the Whitehill shales.
The high input of volcaniclastic material into the basin
during deposition of the Collingham Formation and the
more oxygenated bottom conditions were also unfavourable
for phosphorite formation.
AGE
(Ma)
248
SEDIMENTATION
RATE(m/Ma)
0
TECTONIC EVOLUTION
200
100
z
.~ : Compressional deformation.
w~
I-~
~
w
a.
'tl
<CO:
..J
258
.r.
.~
.:.:
()
~o
(1)
0:
I
(1)
.5 .5
~ ~
w
a.
>-
..J
0:
<C
E
E
c:
'tl
(1)
(1)
a 0 Of
w
.!l!
286
t
Basin tectonics and eustacy
The Karoo Basin fonned part of a much larger depository in
southwestern Gondwana of which remnants today can be
found in South America, Falkland Islands, and Antarctica. It
started as a back-arc basin probably during the MidCarboniferous, then became partly segmented and ended as
a foreland basin during the Late Pennian and Triassic
Substantial change in basin
configuration. Start of submerged fold-thrust belt.
Rhyodacitic volcanism
~
~
B
u.
~
Z
<C
: Uplift of fold-thrust belt.
• Subsidence of foreland ramp
Evidence for marine conditions
The remains of a shark, sponge spicules, foraminifera,
radiolaria, and acritarchs in the basal shales and the presence
of marine invertebrate fossils at higher stratigraphic levels in
the Prince Albert Formation are strong evidence for marine
conditions (ef McLachlan & Anderson, 1973; Oelofsen,
1986). The phosphorites in the formation, and the Rb/K
ratio of the shales suggest marine conditions during
deposition of the entire Prince Albert Formation (ef. Cook,
1984; Visser & Young, 1990). The Rb/K ratio of the
Whitehill shales suggests marine conditions for at least the
lower part of the formation, whereas the Rb/K ratio of the
upper Whitehill shales and the palaeontology suggest
brackish conditions during deposition (Visser & Young,
1990; Oelofsen, 1981). Glauconitic sandstone overlying coal
in the Vryheid Formation indicates a marine transgression
(Van Vuuren, 1983). The coal-bearing cycles are correlated
with the Whitehill Formation (Figure 7) which implies the
presence of marine conditions in an offshore direction. It is
concluded that overall marine conditions prevailed in the
basin up to at least the end of the Early Permian. Thereafter
conditions may have changed to brackish due to changes in
basin configuration restricting connections with the ocean.
A
t
PR
rn
()
(1)
~
0:
C)
.5
rn
tll
~
()
en
::;)
~
I
..:.:
0
0:
()
u.
III
w
tll
Decrease in plate movement.
Thermal subsidence phase.
Passive back-arc basin
in extensional setting
Z
0
III
0:
<C
(.)
w
,,
t
\
I<C
..J
315
en
::;)
0
0:
IW
Qu.
~z
0
... Windblown
volcanic ash
..-----Sasin axis
~. =
Whitehill
Formation
High rate of plate movement.
Subduction of Proto-Pacific
oceanic crust beneath
Gondwana. Partial destruction
of shelf
III
0:
<C
(.)
Figure 8 Sedimentation rates and tectonic evolution of the
Karoo Basin from the Late Carboniferous to Late Permian. A and
B represent compressional events in the pre-Karoo strata along the
southern basin margin (cf. Halbich et ai., 1983). Quiescent periods
shown by dotted lines. Modified after Visser (in press).
::. Fluviodeltaic
. :. deposits
-::-::;: Deep-water facies
+ + Highland
..-/ Palaeocurrent direction
.....,... -y- Fold-thrust
belt
~ ~~I~~~~~m
N~
1000 km
Figure 9 Palaeogeographic reconstruction of southwestern
Gondwana at about the end of the Early Permian (~ 260 Ma).
Distribution of South American basins after Lopez-Gamundi et ai.
(1990). Ch = Chaco Basin, Kr = Karoo Basin, Pr = Parana Basin,
pz = Paganzo Basin, Sg = Sauce Grande/Colorado Basin,
Sr = San Rafael Basin, * = early Late Permian tuff in Sauce
Grande Basin (Lopez-Gamundi, pers. comm.). Palaeo-southern
limit of Whitehill deposition shown by dotted line.
189
S.AfrJ.Geol.,1992,95(5/6)
(Figure 8). Towards the end of the Early Permian the basin
configuration consisted of an incipient fold-thrust belt in the
palaeo-west, a foredeep (depocentre of the deep-water facies
of the Prince Albert and Whitehill Formations), a gently
sloping foreland ramp (shallow-water facies of the Whitehill
Formation), and a forebulge forming part of a dissected
continental highland in the palaeo-east (Figure 9).
During the Early Permian the transition from a back-arc
to a foreland basin involved the formation of an incipient
retro-arc fold-thrust belt, the first evidence of which along
the southern margin of the Karoo Basin was a compressional event dated at about 278 Ma in pre-Karoo strata
(Hiilbich et ai., 1983; Figure 8). According to Hiilbich et ai.
(1983) the main compressional event occurred about 20 Ma
later at 258 Ma, but even at that stage sedimentation of the
glacial diamictites and post-glacial mudrocks took place
uninterruptedly in the region without any evidence of major
sediment input from the growing thrust belt. This is
supported also by the low sedimentation rates (on average
<20 m/Ma - Figure 8) in the basin. Although glacial
debris was derived from the palaeo-west, the clast
composition suggests that the material was distantly derived.
It is concluded that the palaeo-western margin of the
Early Permian basin consisted of a broad tectonic belt
slowly encroaching into the basin by means of a series of
fold-thrust belts (Figures 9 and 10; cf Dalziel, 1986). It
may have been comparable to the present North American
Cordillera bordering the Pacific margin which has a width
of about 1200 km. At the time of the first two compressional events the sea extended at least 200 km beyond the
southern margin of the Karoo Basin which implies that the
palaeo-eastern margin of the tectonic belt (i.e. the precursor
of the Cape Fold Belt) remained submerged until the end of
the Early Permian. However, its influence on basin floor
morphology was manifested in the formation of the foredeep
due to thrust-induced subsidence and oscilladon in the
position of the forebulge during periodic crustal deformation
®
..
and lithospheric relaxation (cf Flemings & Jordan, 1990).
According to these authors rejuvenated thrusting created, by
subsidence, more space that can be filled by sediment.
Because the thrust belt did not yet feature as a sediment
source, rapid subsidence trapped the available sediment
during an underfilled stage causing sediment starvation in
the basin (c/. Allen et ai., 1986). However, during
deposition of the basin floor mud fans of the upper Prince
Albert Formation, the growing fold-thrust belt temporarily
influenced depositional slope, thereby initiating sediment
transport from the palaeo-west.
Emergence of the rising fold-thrust belt and its erosion
are indicated by the sharp rise in sedimentation rates after
deposition of the Whitehill Formation (Figure 8). Sediment
was derived from the palaeo-west and deposition occurred
as deltas and basin floor turbidite fans. During development
of the foreland basin in the interior of southwestern
Gondwana, faulted basins formed and acidic volcanism
occurred in an extensional regime along the palaeo-western
margin of the tectonic belt (cf Breitkreuz, 1991)
(Figure 10). The plinian type volcanism was the source of
the air-borne ash deposited during the latest Early Permian
to Early Triassic in the Karoo Basin.
A relative sea-level rise inundating parts of the palaeoeastern highland (Pietermaritzburg transgression) followed
on the collapse of the Gondwana Ice Sheet (Figure 7).
Dark-coloured marine muds were deposited over the entire
basin as part of a transgressive system tract (c/. Haq, 1991).
This transgression can be attributed to lithospheric relaxation which followed on the 278 Ma compressional event,
resulting in craton-ward migration of the forebulge (c/.
Flemings & Jordan, 1990). Additional support for the
marine transgression is the deposition of phosphorites in the
Prince Albert Formation (cf Follmi et ai., 1992; Brasier,
1992).
The distinctive Whitehill Formation was deposited during
a relative highstand on which two third-order sea-level falls
Proto - Precordillera
Late Carboniferous
to Permian volcanosedimentary strata
..
Permian continental
basins
~~
®
Whitehill depository
Permian acidic volcanism
(Plinian)
lJ/, __
Proto Pacific
Karoo Basin
~
-- -- -- __
~17>
Transport of windblown
- . ash
.
Foredeep
Early Palaeozoic
to early Carboniferous
strata
f
+
+
+
+
Basement
--Exten,'onal ,eglme _ _
/
Fold-thrust belt
Cape Supergroup
. . . . - - Compressional
regime
300 km
Figure 10 Schematic NW - SE section across southwestern Gondwana at about the end of the Early Permian (± 260 Ma).
Northwestern part of the section after Breitkreuz (1991). See Figure 9 for the position of the section.
190
were superimposed (Figure 7). These minor regressions are
attributed to basinward migration of the forebulge during the
258 Ma thrusting episode (cf. Flemings & Jordan, 1990).
Additional evidence for the overall highstand in the Karoo
Basin at the end of the Early Permian is the deposition of
the coal-bearing strata in valleys and embayments along the
basin margin forming part of a highstand system tract (cf.
Haq, 1991). Lithospheric relaxation after the 258 Ma period
of crustal deformation resulted in basinward migration of
facies derived from the fold-thrust belt. A prograding
sequence of distal mud turbidites followed by proximal sand
turbidites (Collingham, Vischkuil, and Laingsburg Formations in Figure 4) was deposited during a regression in the
southern part of the Karoo Basin.
Discussion
The study of the characteristics and lithological interpretation of the Whitehill Formation and associated strata raise
the following points which require in depth discussion to
arrive at a sensible depositional model for the Whitehill
Formation.
1. At the time of deposition of the Whitehill Formation the
foreland basin was in a juvenile stage. Rapid subsidence
during the 258 Ma compressional event resulted in a
well-defined foredeep, but part of the fold-thrust belt was
still submerged, thereby delaying the input of medium- to
coarse-grained clastics (c/. Allen 1986). Mud was
therefore the main clastic component fed into the basin.
Alternating crustal deformation and lithospheric
relax~tion in the foreland basin controlled retrogradation
of facies along the palaeo-eastern basin margin creating
suitable conditions for the formation of peatlands
(Figure 10). Periodic flooding of these peatlands
enhanced the supply of suspended terrigenous organic
material in the basin.
2. The palaeogeography of the region had an overriding
control on sedimentation in that maximum sediment
accumulation was adjacent to the rugged topography of
the eastern basin margin where major rivers entered the
basin via deep valley systems (Figures 5 and 9). In the
Orange Free State coalfield which was approximately
opposite the palaeo-southern limit of Whitehill sedimentation, deposition of the coal-bearing strata occurred in
valleys and small inlets. The coal-bearing strata there
form aggradational units with down-valley shale-out
occurring over relatively short distances (Stavrakis,
1986). Thus no delta progradation into the basin took
place with the result that predominantly clay and organic
matter were washed basinwards during shoreline
sedimentation.
Basin morphology was also rapidly changing during
this period and was such that basin floor sinks for the
accumulation of organic-rich sediment formed during the
Whitehill highstand.
3. The Prince Albert Formation had a bidirectional sediment
input from the palaeo-east and the palaeo-west. The isopach map of the Whitehill Formation suggests sediment
input from the palaeo-east, thereby supporting the
directional data obtained from the fossils (Figure 5). The
sediment input volume from the tectonic belt in the
S.-Afr.Tydskr.Geol.,I992,95(5/6)
palaeo-west, except for volcaniclastics, was probably
small. It can thus be concluded that rivers draining the
supercontinent in the palaeo-east were the main mud
source during Whitehill sedimentation (c/. Figure 9).
4. The concentration of suspended organic matter was
exceptionally high in the Whitehill sea and during fallout up to 4 per cent of organic material was preserved in
muds of the deep-water facies. Most of the organic
matter was transported into the depository by river
plumes or washed in during flooding of peatlands during
transgressions (c/. Demaison & Moore, 1980; RossignolStrick, 1982; Garzanti, 1991). The subsequent regressions also eroded the coal-forming strata, the extent of
which is manifested in the form of up-valley in-seam
channelling (Stavrakis, 1986) and the amount of fossil
wood reported from the Whitehill Formation. Schlanger
& Jenkyns (1976) also reported an abundance of
terrestrial plant remains derived from the inundation of
vegetated lowlands in deep sea floor muds of the
northern Pacific Ocean. The high concentration of
terrigenous sediment in the water column may also have
contributed to the absence of phosphorites in the
Whitehill muds (cf. Cook, 1984).
5. A consistent volume of rhyodacitic volcaniclastic
material was supplied to the basin from the palaeo-west
either as air-borne ash or as devitrified ash by surface
run-off. McLachlan & Jonker (1990) suggested a source
for the volcaniclastic material within 100 km from the
northwestern margin of the Karoo Basin, but no evidence
for Late Permian volcanism has yet been found near the
basin. The major centre of volcanic eruption was located
about 1000 kIn from the depocentre (Figures 9 and 10),
which makes the formation of centimetre-thick tuffaceous
layers in the Karoo Basin at the time of a volcanic
eruption unlikely (cf. Lowe, 1988). Limarino & Spaletti
(1986) and Lopez-Gamundi et al. (1990 and 1992)
described Early to Late Permian aeolian deposits
containing interbedded acidic volcaniclastics in the
central Argentine basins. Recently early Late Permian
tuffaceous beds were also reported from the nearby Sauce
Grande Basin (Lopez-Gamundi, pers. comm.; Figure 9).
The airfall ash was derived from the palaeo-west during
cool arid conditions (Lopez-Gamundi et al., 1992).
Volcanic ash is chemically stable under dry conditions so
that it could have been transported intermittently by
regional westerly winds over the supercontinent and
blown out for hundreds of kilometres over the Whitehill
sea. Windblown ash from the 1991 volcanic eruptions in
southern Chile was widely distributed over Patagonia
(300 to 400 km from the source) and was consistently
blown by westerly winds into the Atlantic Ocean
(personal observation). The palaeolatitudinal setting of
the region (500 to 700 S after Smith et al., 1981) was
most favourable for the presence of a strong westerly
wind system. Therefore, the actual distance of the
volcanoes from the Karoo Basin becomes less critical.
6. The sharp lower contact of the Whitehill Formation, and
thus the change from basin floor mud fan deposition of
the upper Prince Albert to suspension settling sedimentation, is attributed to the abrupt termination of mud
S.Afr.J.Oeo!., 1992,95(5/6)
turbidites entering the depository from the palaeo-west.
This change is attributed to the sea-level highstand with
landward displacement of shorelines (cf Figure 10).
Also, if the upper part of a muddy slope lacked any
conspicuous feeder channels through which sediment
could be transferred to deeper water, a situation could
have developed whereby turbidity flows no longer
operated. The diversion or blocking of major drainage
systems feeding sediment to the basin may have been the
reason for such a change. The late Palaeozoic volcanism
in the tectonic belt reached a peak during the latest Early
Permian to Early Triassic (245 - 265 Ma) (Kay et ai.,
1989; Lopez-Gamundi et ai., 1990; Gonzalez-Bonorino,
1991). Volcanic events occurred on a short time scale
and could have severely affected drainage patterns in the
region.
7. Bottom conditions were more anoxic during deposition
of the Whitehill Formation than during either Prince
Albert or Collingham times. Anoxia in a water body is
caused by excessive oxygen demand resulting from the
degradation of organic matter and a deficient oxygen
supply by circulation within the water body (Demaison &
Moore, 1980). Increased anoxic conditions resulted in a
higher preservation of organic matter primarily due to a
lack of benthonic scavenging and absence of bioturbation
(Demaison & Moore, 1980). During deposition of the
Prince Albert Formation, oceanic circulation probably
compensated in part for the oxygen drain caused by the
suspension fall-out of organic matter, so that only 2 per
cent organic matter was entrained in the muds (cf
Table 1). However, during Whitehill deposition vertical
mixing and oxygen renewal in the water column were
greatly exceeded by the oxygen demand of the high
organic matter concentration in the water with the result
that extreme anoxic conditions prevailed in the depository. There was a lower input flux of organic matter in
the shallow-water environment during regressions and
episodic oxygenated conditions thus occurred at the
water-sediment interface. During the later Volksrust
transgression (Figure 7), large tracts of the highlands
were completely inundated thereby reducing the input of
organic matter. Bottom circulation in the basin could also
have been improved by the presence of turbidity currents
with the result that oxygenated bottom conditions
prevailed.
8. The presence of marine or brackish conditions in the
basin had apparently no effect on sedimentation. The
basin initially had marine connections to the palaeo-north
and palaeo-west, although the tectonic belt is shown as a
continuous landmass on the map (Figure 9). Episodic
brackish conditions probably caused by the inflow of
large fresh-water plumes into the restricted basin, may
have been present during deposition of the upper part of
the formation when aquatic fauna (a primitive swimming
reptile, fish, and arthropods) were introduced to the basin
(Oelofsen, 1987). The vertical salinity gradient stratified
the upper part of the sea and may have contributed to
overall anoxic bottom conditions (cf Rossignol-Strick,
1982).
9. The sea-level highstand was the major driving force in
191
producing a high organic matter concentration in the
basin by flooding the peatlands and indirectly controlled
anoxia in the water column (cf Demaison & Moore,
1980). Minor regressions superimposed on the overall
sea-level highstand caused a lowering in water depth
which led to the deposition of interbedded siltstones and
carbonate rocks in the shallow-water facies. The shallow
facies was also more susceptible to changes in the anoxic
bottom conditions and the concentration of bottom brines
in sea-floor lows.
To understand the distinctive Whitehill depositional
event, the above conclusions must be integrated to give a
coherent working model applicable on the scale of the
Whitehill and Irati seas. Sedimentation of coarse- and
medium-grained clastics and organic material (coal) was
confined to valleys and inlets along the steep palaeo-eastern
basin margin during the late Early Permian. Deposition of
black carbonaceous mud started when the peatlands were
flooded during a highstand, thereby increasing the input of
terrigenous organic matter in the basin as well as the anoxic
conditions in the water body. Mud and terrigenous organic
matter were transported into the basin as large river plumes,
the dispersion of which was inhibited by restricted oceanic
circulation caused by the uneven basin morphology. High
preservation of organic matter in the bottom muds occurred
in those areas of the basin within the influence of the river
plumes. Minor regressions contributed to the flux of organic
matter to the basin as well as the deposition of carbonate
sediment and silts in the shallow-water facies under
oxygenated bottom conditions. Settling of predominantly
air-borne ash from volcanism in the palaeo-west episodically masked the sedimentation of mud and organic matter.
The seas became progressively more brackish due to the
inflow of fresh water and restricted oceanic conditions, and
the environment became favourable for the introduction of
aquatic fauna to the basin. Black mud deposition was
terminated when lithospheric relaxation occurred after the
258 Ma thrusting event whereby basinward migration of
facies derived from the fold-thrust belt took place. This
caused a progressive return to oxygenated bottom conditions
in the basin.
The abrupt appearance and disappearance of carbonaceous shales in the stratigraphic record are a function of
changing bounding conditions in the depositional environment. The changes in bounding conditions during deposition
of the Whitehill muds in the distal areas (deep-water facies)
were more limited than closer to the source areas where
input flux and external factors in the shallow environment
greatly influenced deposition. The formation of the carbonaceous shales within the Lower Ecca mudrock succession
results from the concurrence of three related factors.
Sedimentation was primarily controlled by basin tectonics
which involved sediment supply and a possible silled basin,
a sea-level highstand which involved the input of terrigenous organic matter, anoxia in the water column and preservation of organic matter, and climate which involved the
proliferation of plants and input of air-borne volcaniclastics.
Sea-level was the first-order control on the accumulation of
the organic-rich facies (cf Pedersen & Calvert, 1990). The
interaction of these controls must be seen in a retro-arc fore-
192
land basin setting when predominantly mud was supplied
during the juvenile stage of foreland basin development.
The depositional model outlined for the Whitehill
Formation can also be applied to the Irati Formation
between the Parana and Chaco Basins (Figure 9). The
eastern, central, and northern parts of the Irati Formation
contain more carbonate rocks deposited in an offshore
shallow environment or on carbonate mud flats (Oelofsen,
1987; Horsthemke et al., 1990) than the southwestern facies.
The organic carbon content of the shales also increases
towards the more proximal and shallower facies and, in the
case of the Parana Basin, forms a source of hydrocarbons. In
areas away from the entry points of terrigenous organic
matter or where prograding deltas enhanced offshore mud
and silt deposition (e.g. eastern part of the Karoo Basin) a
subtle lateral transition from black, organic-rich mud to
dark-grey marine muds occurred. Therefore, the deposition
of the distinctive Whitehill- Irati muds was a localized
event in part of southwestern Gondwana during a specific
time slot in the Permian.
Acknowledgements
Financial assistance for the study of the Whitehill Formation
by the Foundation for Research Development (FRD) and the
University of the Orange Free State is gratefully acknowledged. Mr J.C. Loack and Dr W.A. van der Westhuizen are
thanked for the fossil samples from the Whitehill Formation
and the chemical analyses respectively. Ken Eriksson and
Ian McLachlan are thanked for constructive comments on an
earlier draft of the paper.
References
Adachi, M., Yamamoto, K. & Sugisaki, R (1986). Hydrothennal chert
and associated siliceous rocks from the northern Pacific: their
geological significance as indication of oceanic ridge activity. Sedim.
Geol., 47, 125-148.
Allen, P.A., Homewood, P. & Williams, G.D. (1986). Foreland basins: an
introduction. In: Allen, P.A. & Homewood, P. (Eds.), Foreland
Basins. Spec. Publ. Internat. Assoc. Sedim., 8, 3-12.
Anderson, A.M. (1975). Limulid trackways in the late Palaeozoic Ecca
sediments and their palaeoenvironmental significance. S. Afr. J. Sci.,
71,249-251.
---- (1979). The oil-shale potential of the Early Pennian White Band
Fonnation in southern Africa. In: Anderson, A.M. & Van Biljon, W.J.
(Eds.), Some Sedimentary Basins and Associated Ore Deposits of
South Africa. Spec. Publ. geol. Soc. S. Afr., 6, 83-89.
Anderson, J.M. (1977). The biostratigraphy of the Pennian and the
Triassic. Part 3: A review of Gondwana Pennian palynology with
particular reference to the northern Karoo Basin, South Africa. Mem.
Bot. Surv. S. Afr., 41, 188 pp.
Archangelsky, S. & Toigo, M.M. (1978). La palinologia y el problema del
limite Carbonico - Pennico en el Gondwana Sudamericano. Actas 2nd
Congr. Paleont. Biostrat., Argentina, Buenos Aires, 257-269.
Brasier, M. D. (1992). Nutrient-enriched waters and the early skeletal
fossil record. J. geol. Soc. Lond., 149, 621-629.
Breitkreuz, C. (1991). Fluvio-Iacustrine sedimentation and volcanism in a
Late Carboniferous tensional intra-arc basin, northern Chile. Sedim.
Geol.,74, 173-187.
----, Bahlburg, H., Delakowitz, B. & Pichowiak, S. (1989). Paleozoic
volcanic events in the Central Andes. J. S. Amer. Earth Sci., 2,
171-189.
Cole, OJ. & McLachlan, I.R (1991). Oil potential of the Pennian
Whitehill Shale Fonnation in the main Karoo Basin, South Africa. In:
Ulbrich, H. & Rocha Campos, A.C. (Eds.), Gondwana Seven
Proceedings. Inst. Geocien., Univ. Sao Paulo, 379-390.
Cook, P.J. (1984). Spatial and temporal controls on the fonnation of
S.-Afr.Tydskr.Geol.,l992,95(5/6)
phosphate deposits - a review. In: Nriagu, 1.0. & Moore, P.B.
(Eds.), Phosphate Minerals. Springer-Verlag, New York, 242-274.
Coutinho, J.M.V., Dos Santos, P.R & Coimbra, A.M. (1988). Ash-fall
derived vitroclastic tuffaceous sediments in the Pennian of the Parana
Basin and its provenance. Abstr. 7th Gondwana Symp., Sao Paulo,
Brazil, 78.
Dalziel, I.W.D. (1986). Collision and Cordilleran orogenesis: an Andean
perspective. In: Coward, M.P. & Ries, A.C. (Eds.), Collision
Tectonics. Spec. Publ. geol. Soc. Lond., 19, 389-404.
Demaison, G.J. & Moore, G.T. (1980). Anoxic environments and oil
source bed genesis. Bull. Amer. Assoc. Petrol. Geol., 64, 1179-1209.
Dickins, J.M. (1984). Late Palaeozoic glaciation. J. Austr. Geol.
Geophys., 9, 163-169.
Domack, E.W. (1983). Facies of late Pleistocene glacial-marine sediments
on Whidbey Island, Washington. In: Molnia, B.P. (Ed.), Glacialmarine Sedimentation. Plenum Press, New York, 535-570.
Dos Santos, P.R., Rocha Campos, A.C. & Canuto, J.R (1988).
Paleogeographic evolution of the late Paleozoic glaciation in the
Parana Basin, Brazil. Abstr. 7th Gondwana Symp., Sao Paulo, Brazil,
131.
Eugster, H.P. (1985). Oil shales, evaporites and ore deposits. Geochim.
Cosmochim. Acta, 49,619-635.
Flemings, P.B. & Jordan, T.E. (1990). Stratigraphic modelling of foreland
basins: interpreting thrust defonnation and lithosphere rheology.
Geology, 18, 43Q....434.
Follrni, K.B., Garrison, RE., Ramirez, P.c., Zambrano-Ortiz, F.,
Kennedy, W.J. & Lehner, B.L. (1992). Cyclic phosphate-rich
successions in the upper Cretaceous of Colombia. Palaeogeogr.,
Palaeoclimatol., Palaeoecol., 93, 151-182.
Fritz, W.J. & Vanko, D.A. (1992). Geochemistry and origin of a black
mudstone in a volcaniclastic environment, Ordovician Lower Rhyolitic
Tuff Fonnation, North Wales, UK. Sedimentology, 39,663-674.
Garzanti, E. (1991). Non-carbonate intrabasinal grains in arenites: their
recognition, significance, and relationship to eustatic cycles and
tectonic setting. J. Sedim. Petrol., 61, 959-975.
Gilligan, R.N. (1986). OFS-Vierfontein coalfield. In: Anhaeusser, C.R &
Maske, S. (Eds.), Mineral Deposits of Southern Africa, II. Geol. Soc.
S. Afr., Johannesburg, 1929-1937.
GonzaIez-Bonorino, G. (1991). Late Paleozoic orogeny in the
northwestern Gondwana continental margin, western Argentina and
Chile. J. S. Amer. Earth Sci., 4, 131-144.
Hiilbich, I.W., Fitch, F.J. & Miller, J.A. (1983). Dating the Cape orogeny.
In: SOhnge, A.P.G. & Hiilbich, I.W. (Eds.), Geodynamics of the Cape
Fold Belt. Spec. Pub!. geol. Soc. S. Afr., 12, 149-164.
Haq, B.U. (1991). Sequence stratigraphy, sea-level change, and
significance for the deep sea. In: Sedimentation, Tectonics and
Eustasy: Sea-level Changes at Active Margins. Spec. Publ. Internal.
Assoc. Sedim., 12,3-39.
Haughton, S.H., Blignaut, J.J.G., Rossouw, PJ., Spies, 1.1. & Zagt, S.
(1953). Results of an investigation into the possible presence of oil in
Karroo rocks in parts of the Union of South Africa. Mem. geol. Surv.
S. Afr., 45, 130 pp.
Horsthemke, E., Ledendecker, S. & Porada, H. (1990). Depositional
environments and stratigraphic correlation of the Karoo sequence in
northwestern Damaraland. Comm. geol. Surv. Namibia, 6,63-73.
lijima, A., Matsumoto, R & Tada, R (1985). Mechanism of
sedimentation of rhythmically bedded chert. Sedim. Geol., 41,
221-233.
Kay, S.M., Ramos, V.A., Mpodozis, C. & Sruoga, P. (1989). Late
Paleozoic to Jurassic silicic magmatism at the Gondwana margin:
analogy to the middle Proterozoic in North America? Geology, 17,
324-328.
Kovacs-Endrody, E. (1991). On the late Pennian age of Ecca Glossopteris
floras in the Transvaal province with a key to and description of
twenty five Glossopteris species. Mem. geol. Surv. S. Afr., 77, III pp.
Limarino, C.O. & Spaletti, L.A. (1986). Eolian Pennian deposits in west
and northwest Argentina. Sedim. Geol., 49, 109-127.
Lock, B.E. & Wilson, J.D. (1975). Discussion on a paper 'The nature and
origin of volcaniclastic material in some Karroo and Beacon rocks'.
Trans. geol. Soc. S. Afr., 78, 171.
Lopez-Gamundi, 0., Espejo, I.S. & Alonso, M.S. (1990). Sandstone
composition changes and paleocurrent reversal in Upper Paleozoic and
Triassic deposits of the Huaco area, western Paganzo Basin, west-
S.AfrJ.Oeo!., 1992,95(5/6)
central Argentina. Sedim. Geol., 66, 99-111.
----, Limarino, e.o. & Cesari, S.N. (1992). Late Paleozoic
paleoclimatology of central west Argentina. Palaeogeogr.,
Palaeoclimatol., Palaeoecol., 91, 305-329.
Lowe, D.1. (1988). Stratigraphy, age, composition, and correlation of late
Quaternary tephras interbedded with organic sediments in Waikato
lakes, North Island, New Zealand. N. Z. J. Geol. Geophys., 31,
125-165.
MacRae, e. (1987). Palynostratigraphic correlation between the lower
Karoo sequence of the Waterberg and Pafuri coal-bearing basins and
the Hammanskraal plant macrofossil locality, Republic of South
Africa. Ph.D. Thesis (unpub!.), Univ. Orange Free State,
Bloemfontein, 421 pp.
---- (1992). Age of the Whitehill Formation in the Hopetown area,
northeastern Cape Province. Abstr. 7th Conf. Palaeont. Soc. s. Afr.,
Johannesburg, 29.
McLachlan, I.R. & Anderson, A. (1973). A review for the evidence of
marine conditions in southern Africa during Dwyka times. Palaeont.
Afr., 15, 37-<;4.
---- & ---- (1975). The age and stratigraphic relationship of the glacial
sediments in southern Africa. In: Campbell, R.S.W. (Eds.), Gondwana
Geology. Australian Nat. Univ. Press, Canberra, 415-422.
---- & ---- (1977a). Carbonates, 'stromatolites' and tuffs in the lower
Permian White Band Formation. S. Afr. J. Sci., 73, 92-94.
---- & ---- (1977b). Fossil insect wings from the early Permian White
Band Formation, South Africa. Palaeont. Afr., 20, 83-86.
---- & Jonker, J.P. (1990). Tuff beds in the northwestern part of the Karoo
Basin. S. Afr. J. Geol., 93, 329-338.
Moore, C.L. (1991). The distal terrestrial record of explosive rhyolitic
volcanism: an example from Auckland, New Zealand. Sedim. Geol.,
74,25-38.
Murray, R.W., Jones, D.L. & Buchholtz ten Brink, M.R. (1992).
Diagenetic formation of bedded chert: evidence from chemistry of the
chert-shale couplet. Geology, 20, 271-274.
Oelofsen, B.W. (1981). An anatomical and systematic study of the Family
Mesosauridae (Reptilia; Proganosauria) with special reference to its
associated fauna and palaeoecological environment in the Whitehill
Sea. Ph.D. Thesis (unpub!.), Univ. Stellenbosch, 163 pp.
---- (1986). A fossil shark Neurocranium from the Permo-Carboniferous
(lowermost Ecca Formation) of South Africa. In: Uteno, T., Arai, R,
Taniuchi, T. & Matsuura, K. (Eds.), Indo-Pacific Fish Biology: Proc.
Second Internat. Conf. on Indo-Pacific Fishes. Ichthyological Society
of Japan, Tokyo, 107-124.
---- (1987). The biostratigraphy and fossils of the Whitehill and Irati Shale
Formations of the Karoo and Parana Basins. In: McKenzie, G.D.
(Ed.), Gondwana Six: Stratigraphy, Sedimentology, and Paleontology.
Geophys. Monogr. Amer. Geophys. Union, 41, 131-138,
---- & Araujo, D.e. (1987). Mesosaurus tenuidens and stereosternum
tumidum from the Permian Gondwana of both southern Africa and
South America. S. Afr. J. Sci., 83,370-371.
193
Pedersen, T.F. & Calvert, S.E. (1990). Anoxia vs. productivity: What
controls the formation of organic-carbon-rich sediments and
sedimentary rocks? Bull. Amer. Assoc. Petrol. Geol., 74, 454-466.
Rossignol-Strick, M. (1982). Petroleum origin: heavy rains, river plume,
ocean stratification. Bull. Amer. Assoc. Petrol. Geol., 66, 625-<;26.
Rowsell, D.M. & De Swardt, A.M.J. (1976). Diagenesis in Cape and
Karroo sediments, South Africa, and its bearing on their hydrocarbon
potential. Trans. geol. Soc. S. Afr., 79, 81-145.
Schlanger, S.O. & Jenkyns, H.C. (1976). Cretaceous oceanic anoxic
events: causes and consequences. Geol. Mijnb., 55, 179-184.
Scotchman, I.C. (1991). The geochemistry of concretions from the
Kimmeridge Clay Formation of southern and eastern England.
Sedimentology, 38, 79-106.
Smith, A.G., Hurley, A.M. & Briden, J.e. (1981). Phanerozoic
Paleocontinental World Maps. Cambridge Univ. Press, Cambridge,
102 pp.
Stavrakis, N. (1986). Sedimentary environments and facies of the Orange
Free State coalfield. In: Anhaeusser, e.R & Maske, S. (Eds.), Mineral
Deposits of Southern Africa, II. Geo!. Soc. S. Afr., Johannesburg,
1939-1952.
Strydom, H.C. (1950). The geology and chemistry of the Laingsburg
phosphorites. Ann. Univ. Stellenbosch, 26A, 267-285.
Tada, R (1991). Origin of rhythmical bedding in Middle Miocene
siliceous rocks of the Onnagawa Formation, northern Japan. J. Sedim.
Petrol., 61, 1123-1145.
Theron, A.C. (1967). The sedimentology of the Koup Subgroup near
Laingsburg. M.Sc. thesis (unpub!.), Univ. Stellenbosch, 22 pp.
Van der Westhuizen, W.A., Loock, J.e. & Strydom, D. (1981). Halite
imprints in the Whitehill Formation, Ecca Group, Carnarvon District.
Ann. geol. Surv. S. Afr., 15, 43-46.
Van Eeden, O.R (1973). The correlation of the subdivisions of the
Karroo System. Trans. geol. Soc. S. Afr., 76, 201-206.
Van Vuuren, C.J. (1983). A basin analysis of the northern facies of the
Ecca Group. Ph.D. Thesis (unpub!.), Univ. Orange Free State,
Bloemfontein, 249 pp.
Viljoen, J.H.A. (1990). K-bentonites in the Ecca Group of the south and
central Karoo Basin. Abstr. 23rd Congr. geol. Soc. S. Afr., Cape
Town, 576-579.
Visser, J.NJ. (1990). The age of the late Palaeozoic glacigene deposits in
southern Africa. S. Afr. J. Geol., 93, 366-375.
---- (in press). Sea-level changes in a back-arc - foreland transition: the
Late Carboniferous - Permian Karoo Basin of South Africa. Sedim.
Geol., 00, 000--000.
---- & Young, G.M. (1990). Major element geochemistry and
paleoclimatology of the Permo-Carboniferous glacigene Dwyka
Formation and post-glacial mudrocks in southern Africa.
Palaeogeogr., Palaeoclimatol., Palaeoecol., 81, 49-57.
Wright, A.B. (1969). The geology of a portion of north-western Albany.
M.Sc. Thesis (unpub!.), Rhodes Univ., Grahamstown, 114 pp.