Basin development during the deposition of the Elliot Formation (Late Triassic - Early Jurassic),...
16
EMESE M. BORDY, P. JOHN HANCOX AND BRUCE S. RUBIDGE SOUTH AFRICAN JOURNAL OF GEOLOGY,2004,VOLUME 107 PAGE 397-412 397 Basin development during the deposition of the Elliot Formation (Late Triassic - Early Jurassic), Karoo Supergroup, South Africa Emese M. Bordy, P. John Hancox and Bruce S. Rubidge University of the Witwatersrand, School of Geosciences, Johannesburg, Private Bag 3, Wits 2050, South Africa. e-mail: [email protected]; [email protected]; [email protected] © 2004 Geological Society of South Africa ABSTRACT The integrated results of a facies analysis and provenance study of the Late Triassic to Early Jurassic Elliot Formation (Karoo Supergroup) provide some new insights into the development of the main Karoo foreland system of South Africa. Based on changes in the fluvial style, palaeocurrent pattern, provenance, isopach trends and fossil content, a regional lithostratigraphic subdivision of the Elliot Formation is proposed. In addition, the boundary between the Lower and Upper Elliot formations appears to be a second order sequence boundary. This unconformity was probably generated by the last stage of orogenic loading of the Cape Fold Belt, which interrupted the overall, first order orogenic unloading of the system, suggesting that tectonically controlled flexural subsidence existed in the main Karoo Basin until at least the end of Triassic. The magnitude of this pre-Upper Elliot tectonic event is signified by the presence of outsized quartzite pebbles and boulders, believed to have originated in the Cape Fold Belt. A number of tectonic structures, e.g. pene-contemporaneous normal faults and large-scale convolute bedding, coupled to sandstones with basement uplift/craton interior provenance, and easterly palaeocurrent direction for the Upper Elliot Formation suggest that the first stages of inversion from a compressional to extensional tectonic regime began only in the Early Jurassic. Introduction Late Carboniferous to Middle Jurassic sedimentary and igneous rocks occur in the main Karoo Basin of South Africa (Figure 1) as well as in several separate outcrop areas in central and southern Africa (Johnson et al., 1996; Bordy, 2000; Bordy and Catuneanu, 2001). Undoubtedly the best studied and most complete Karoo succession is in the main Karoo Basin, and it serves as a firm base for comparison with other southern African outcrop areas (e.g., Bordy, 2000; Bordy and Catuneanu, 2001). The Basin is currently regarded by most authors as a retro-arc foreland basin developed in front of the Cape Fold Belt (e.g., Johnson, 1991; Veevers et al., 1994; Catuneanu et al., 1998; Pysklywec and Mitrovica, 1999; Catuneanu and Elango, 2001). The structural evolution of the Basin from Triassic to Jurassic has recently become the centre of some debate for this period of time (e.g., Turner, 1999), and for this reason it is essential to re-examine the basin development model. Although the Late Triassic to Early Jurassic Elliot Formation of the main Karoo Basin covers an important time period both palaeontologically and tectonically (for the timing of pre-break-up rifting), it is one of the most understudied successions. We propose that a key to understanding the development of the Elliot Formation lies in alluvial architectural and provenance studies. This paper therefore aims to interpret the controls on progradational patterns during the deposition of the Elliot Formation, by documenting the temporal and spatial changes in facies architecture, as well as isopach, palaeocurrent and petrographic patterns. Geological background The Elliot Formation (Figure 2), together with the underlying Molteno and overlying Clarens formations, comprise the “Stormberg” Group - a commonly used, but informal stratigraphic term for the uppermost sedimentary strata of the Karoo Supergroup (SACS, 1980; Johnson, 1994). This Late Triassic to Middle Jurassic basin fill (Smith and Kitching, 1997) is void of drastic lateral facies changes or thickness variations, and there are only two major lateral changes along the south-north profile: a gradual grain-size reduction, and an overall thickness decrease from south to north (Catuneanu et al., 1998). The southern limit of the present-day preservational area of the upper Karoo strata more or less demarcates the southern limit of the “Stormberg” Basin (i.e. north of Queenstown) (Catuneanu et al., 1998, p. 430). The northern limits of the Basin are believed to have been removed by post- Karoo erosion. According to Catuneanu et al. (1998), the compressive foreland system that existed north of the Cape Fold Belt developed in response to the late Palaeozoic-early Mesozoic subduction of the palaeo- Pacific plate below the Gondwanan plate. The flexural tectonism of the foreland system was drawn to a close during the first order unloading event which persisted from the Late Triassic until the Middle Jurassic (Catuneanu et al., 1998; Hancox, 1998). During the time of unloading, the proximal sector of the system was uplifted and eroded, and the reworked foredeep sediments were transported into the distal sector which acted as a depositional foresag preserving - among others - the Elliot Formation. In addition to the first order unloading, there are two well-documented, smaller tectonic events (P7 final phase ~223 Ma; P8 final phase 215±3 Ma) in the Cape Fold Belt (Halbich et al.,
Transcript of Basin development during the deposition of the Elliot Formation (Late Triassic - Early Jurassic),...
EMESE M. BORDY, P. JOHN HANCOX AND BRUCE S. RUBIDGE
SOUTH AFRICAN JOURNAL OF GEOLOGY, 2004, VOLUME 107 PAGE 397-412
397
Basin development during the deposition of the Elliot Formation (Late Triassic - Early Jurassic),
Karoo Supergroup, South Africa
Emese M. Bordy, P. John Hancox and Bruce S. RubidgeUniversity of the Witwatersrand, School of Geosciences, Johannesburg,
Private Bag 3, Wits 2050, South Africa. e-mail: [email protected]; [email protected]; [email protected]
© 2004 Geological Society of South Africa
ABSTRACT
The integrated results of a facies analysis and provenance study of the Late Triassic to Early Jurassic Elliot Formation (Karoo
Supergroup) provide some new insights into the development of the main Karoo foreland system of South Africa. Based on changes
in the fluvial style, palaeocurrent pattern, provenance, isopach trends and fossil content, a regional lithostratigraphic subdivision of
the Elliot Formation is proposed. In addition, the boundary between the Lower and Upper Elliot formations appears to be a second
order sequence boundary. This unconformity was probably generated by the last stage of orogenic loading of the Cape Fold Belt,
which interrupted the overall, first order orogenic unloading of the system, suggesting that tectonically controlled flexural
subsidence existed in the main Karoo Basin until at least the end of Triassic. The magnitude of this pre-Upper Elliot tectonic event
is signified by the presence of outsized quartzite pebbles and boulders, believed to have originated in the Cape Fold Belt. A number
of tectonic structures, e.g. pene-contemporaneous normal faults and large-scale convolute bedding, coupled to sandstones with
basement uplift/craton interior provenance, and easterly palaeocurrent direction for the Upper Elliot Formation suggest that the first
stages of inversion from a compressional to extensional tectonic regime began only in the Early Jurassic.
IntroductionLate Carboniferous to Middle Jurassic sedimentary andigneous rocks occur in the main Karoo Basin of SouthAfrica (Figure 1) as well as in several separate outcropareas in central and southern Africa (Johnson et al.,1996; Bordy, 2000; Bordy and Catuneanu, 2001).Undoubtedly the best studied and most complete Karoosuccession is in the main Karoo Basin, and it serves asa firm base for comparison with other southern Africanoutcrop areas (e.g., Bordy, 2000; Bordy and Catuneanu,2001). The Basin is currently regarded by most authorsas a retro-arc foreland basin developed in front of theCape Fold Belt (e.g., Johnson, 1991; Veevers et al., 1994;Catuneanu et al., 1998; Pysklywec and Mitrovica, 1999; Catuneanu and Elango, 2001). The structuralevolution of the Basin from Triassic to Jurassic hasrecently become the centre of some debate for thisperiod of time (e.g., Turner, 1999), and for this reason itis essential to re-examine the basin development model.
Although the Late Triassic to Early Jurassic ElliotFormation of the main Karoo Basin covers an importanttime period both palaeontologically and tectonically (forthe timing of pre-break-up rifting), it is one of the mostunderstudied successions. We propose that a key tounderstanding the development of the Elliot Formationlies in alluvial architectural and provenance studies. Thispaper therefore aims to interpret the controls onprogradational patterns during the deposition of theElliot Formation, by documenting the temporal andspatial changes in facies architecture, as well as isopach,palaeocurrent and petrographic patterns.
Geological backgroundThe Elliot Formation (Figure 2), together with the
underlying Molteno and overlying Clarens formations,comprise the “Stormberg” Group - a commonly used,but informal stratigraphic term for the uppermostsedimentary strata of the Karoo Supergroup (SACS,1980; Johnson, 1994). This Late Triassic to MiddleJurassic basin fill (Smith and Kitching, 1997) is void ofdrastic lateral facies changes or thickness variations, andthere are only two major lateral changes along thesouth-north profile: a gradual grain-size reduction, andan overall thickness decrease from south to north(Catuneanu et al., 1998). The southern limit of thepresent-day preservational area of the upper Karoostrata more or less demarcates the southern limit of the“Stormberg” Basin (i.e. north of Queenstown)(Catuneanu et al., 1998, p. 430). The northern limits ofthe Basin are believed to have been removed by post-Karoo erosion.
According to Catuneanu et al. (1998), thecompressive foreland system that existed north of the Cape Fold Belt developed in response to the latePalaeozoic-early Mesozoic subduction of the palaeo-Pacific plate below the Gondwanan plate. The flexuraltectonism of the foreland system was drawn to a closeduring the first order unloading event which persistedfrom the Late Triassic until the Middle Jurassic(Catuneanu et al., 1998; Hancox, 1998). During the timeof unloading, the proximal sector of the system wasuplifted and eroded, and the reworked foredeepsediments were transported into the distal sector whichacted as a depositional foresag preserving - amongothers - the Elliot Formation. In addition to the firstorder unloading, there are two well-documented,smaller tectonic events (P7 final phase ~223 Ma; P8 finalphase 215±3 Ma) in the Cape Fold Belt (Halbich et al.,
1983; Gresse et al., 1992; Catuneanu et al., 1998). Theconsequence of these smaller tectonic events wasforebulge uplift and two subaerial unconformities onewithin the Molteno Formation, and the other at the baseof Elliot Formation (Catuneanu et al., 1998; Catuneanuand Elango, 2001: Figure 2). In addition to theunloading-driven subsidence of the foresag region, the generation of accommodation space was alsocontrolled by the cessation of the subduction of thepalaeo-Pacific plate below the Gondwana plate(Pysklywec and Mitrovica, 1999). The geodynamicmodelling of Pysklywec and Mitrovica (1999) shows thatthe entire Karoo foreland system might have begun tobe regionally uplifted shortly (~0 to 20 million years)after subduction terminated.
Previous research and enigmasThe vertical profile of the “Stormberg” succession ischaracterized by three second-order coarsening-upwardsequences: two of them in the Molteno Formation(Hancox and Rubidge, 1995; Hancox, 1998; Catuneanuet al., 1998), and the third one represented by the Elliot-
Clarens formations (Catuneanu et al., 1998). Theseupward-coarsening sequences were formed when thelengthy first-order orogenic unloading of Late Triassic toEarly Middle Jurassic was interrupted by two brieforogenic pulses. Thus the coarsening-upward sequencesare explained by the northward progradation of the fluvial systems due to the gradual steepening of theforeslope during stages of orogenic unloading(Catuneanu et al., 1998). This model has beenchallenged by Turner (1999), who argues that fieldevidence supports upward-fining tendencies in the“Stormberg” basin fill. It must be emphasized thatTurner’s (1999: 228) Figure 11 depicts a coarsening-upward, prograding trend for the Bamboesberg Member, and his descriptions of fining-upward trends apply exclusively to the Molteno Formation,while the Elliot-Clarens sequence has not beeninvestigated in any detail. However, the general vertical profile of the Elliot and Clarens formationsshows an unambiguous coarsening-upward trend (i.e.there are more mudrocks in the Elliot Formation than in the arenaceous Clarens Formation). A basin-wide
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BASIN DEVELOPMENT DURING THE DEPOSITION OF THE ELLIOT FORMATION398
Figure 1. Geological map of the main Karoo Basin (after Catuneanu et al., 1998) showing the Kapvaal Craton (southern parts are overlain
by Karoo cover) boundaries as proposed by Skinner et al. (1992).
EMESE M. BORDY, P. JOHN HANCOX AND BRUCE S. RUBIDGE
SOUTH AFRICAN JOURNAL OF GEOLOGY
399
facies relationship survey as well as a detailedprovenance study of the Elliot Formation wasundertaken by the present authors to clarify thisprogradational trend suggested by the coarsening-upward sequence and predicted by the foreland system model. The major findings of this study areintegrated here to elucidate basin development duringElliot-times.
DescriptionStudy of the vertical and horizontal facies relationships,sedimentary structures, palaeocurrents and provenances,coupled to local and regional relationships used tointerpret the depositional history of the Elliot Formation,
revealed that throughout the Basin, there are somemajor geological differences between the lower andupper parts of the Elliot Formation (Table 1). In thiswork the lower part is therefore separated from theupper part of the Formation, and they are referred to asLower Elliot Formation (LEF) and Upper Elliot Formation(UEF), respectively. Details of the differences will beelaborated upon in subsequent papers, however acomparison of the Lower and Upper Elliot Formation isoutlined below. The two newly defined informal unitscorrespond with the biostratigraphic units defined byKitching and Raath (1984) as Euskelosaurus andMassospondylus Range Zones, respectively. However,the tripartite lithological subdivision of the Formation by
Figure 2. Geological map of the Elliot Formation in the Republic of South Africa and Lesotho (modified after the 1:1000000 Geological
map of RSA and Lesotho, 1984).
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BASIN DEVELOPMENT DURING THE DEPOSITION OF THE ELLIOT FORMATION400
Tab
le 1
.Com
par
ison o
f th
e Lo
wer
and U
pper
Elli
ot Fo
rmat
ions.
Fea
ture
sLo
wer
Ell
iot
Fo
rmat
ion
Up
per
Ell
iot
Fo
rmat
ion
Sandst
one
body
ove
rall
shap
em
ost
ly a
sym
met
rica
l ch
annel
most
ly tab
ula
r sh
eet,
few
chan
nel
char
acte
rist
ics
thic
knes
s20
to 2
5m5
to 6
mla
tera
l ex
tent
few
10s
of
mse
vera
l 10
's o
f m
relie
f of
maj
or/
exte
rnal
ero
sional
surf
aces
~ 5
to 1
0m (
irre
gula
r)<2
to 3
m (
tabula
r)ge
om
etry
multi
-sto
rey,
len
ticula
rm
ulti
-sto
rey,
shee
tre
lief
of
min
or/
inte
rnal
ero
sional
surf
aces
>1
m<1
mla
tera
l ex
tent of
min
or/
inte
rnal
ero
sional
surf
aces
non-p
ersi
sten
tper
sist
ent
poin
t bar
spre
sent,
but not w
ell-dev
eloped
abse
nt
late
ral ac
cret
ion s
urf
aces
pre
sent
abse
nt
sedim
enta
ry s
truct
ure
s trough
and p
lanar
cro
ss-b
eddin
g, m
assi
ve b
eds
mas
sive
bed
s, h
orizo
nta
l la
min
atio
n, ripple
-cro
ss
and les
s co
mm
only
low
-angl
e cr
oss
-bed
din
gla
min
atio
n, an
d r
are
trough
cro
ss-b
eddin
gM
udst
one
units
th
ickn
ess
20 to 3
0m<10
mca
rboniz
ed a
nd c
alcr
etiz
ed r
oot trac
esra
re (
occ
ur
mai
nly
in the
norther
n o
utc
rops)
com
mon (
espec
ially
in the
norther
n o
utc
rops)
calc
areo
us
concr
etio
ns
rare
(occ
ur
mai
nly
in the
norther
n o
utc
rops)
com
mon (
espec
ially
in the
norther
n o
utc
rops)
larg
e-sc
ale
calc
retiz
ed a
nd c
lay-
lined
shrinka
ge c
rack
sra
re (
occ
ur
mai
nly
in the
norther
n o
utc
rops)
com
mon (
espec
ially
in the
norther
n o
utc
rops)
irre
gula
r m
ottle
sra
re (
occ
ur
mai
nly
in the
norther
n o
utc
rops)
com
mon (
espec
ially
in the
norther
n o
utc
rops)
des
sica
tion c
rack
s ra
re (
occ
ur
mai
nly
in
the
norther
n o
utc
rops)
com
mon (
espec
ially
in the
norther
n o
utc
rops)
falli
ng-
wat
er lev
el m
arks
rare
(occ
ur
in the
norther
n o
utc
rops)
com
mon (
espec
ially
in the
norther
n o
utc
rops)
Colo
ur
light re
dco
mm
on
rare
dee
p r
edra
reco
mm
on
Foss
ilsla
rge
din
osa
ur
foss
ilsco
mm
on
rare
smal
l din
osa
ur
foss
ilsra
reco
mm
on
foss
il w
ood
pre
sent
abse
nt?
ichnofo
ssils
rare
com
mon
Pal
aeo-c
urr
ent pat
tern
sSo
uth
to n
orth
com
mon (
espec
ially
in the
south
ern r
egio
ns)
com
mon (
espec
ially
in the
south
ern r
egio
ns)
South
wes
t to
northea
stco
mm
on (
espec
ially
in the
norther
n r
egio
ns)
com
mon (
espec
ially
in the
norther
n r
egio
ns)
Wes
t to
eas
tab
sent
com
mon (
espec
ially
in the
norther
n r
egio
ns)
North to s
outh
abse
nt
rare
(in
the
norther
n r
egio
ns
only
)Pro
venan
cere
cycl
ed o
roge
nco
mm
on
rare
contin
enta
l in
terior/
bas
emen
t u
plif
tab
sent
com
mon
Gra
in-s
ize
med
ium
to f
ine
sandst
ones
com
mon
rare
fine
to v
ery
fine
sandst
ones
rare
com
mon
south
to n
orth g
rain
-siz
e ch
ange
sdec
reas
eco
nst
ant
vertic
al g
rain
-siz
e ch
ange
sco
nst
ant
slig
htly
coar
senin
g-upw
ards
Sortin
gpoorly
sorted
moder
atel
y so
rted
Gra
vel-si
ze p
artic
les
Out-si
zed q
uar
tzite
peb
ble
spre
sent
pre
sent
intraf
orm
atio
nal
mudst
one-
clas
t co
ngl
om
erat
espre
sent
pre
sent
rew
ork
ed, ped
oge
nic
car
bonat
e gl
aebule
congl
om
erat
esab
sent
com
mon
South
north iso
pac
h c
han
ges
dec
reas
esl
ight dec
reas
eEvi
den
ce o
fsy
n-s
edim
enta
ry f
aults
abse
nt
pre
sent
exte
nsi
onal
tec
tonic
s sy
n-s
edim
enta
ry d
eform
atio
n (
larg
e sc
ale
convo
lute
bed
din
g)
abse
nt
pre
sent
septa
rian
nodule
s, s
mal
l-sc
ale
dew
ater
ing
stru
cture
s, load
cas
ts,
wat
er-e
scap
e st
ruct
ure
s (T
anner
, 19
98)
and lar
ge-s
cale
cra
cks
abse
nt
pre
sent
EMESE M. BORDY, P. JOHN HANCOX AND BRUCE S. RUBIDGE
SOUTH AFRICAN JOURNAL OF GEOLOGY
401
Kitching and Raath (1984) was found to be applicableonly for the northern outcrop areas. In the northern partof the Basin, the Middle and Upper Elliot Formation ofKitching and Raath (1984) correlates with the UpperElliot Formation defined in this study.
The marked textural contrast of the LEF and UEFlithologies has resulted in differential weathering,generating a clearly detectable plateau in the otherwisesteep Elliot Formation outcrops (Figure 3). This break ofslope delineates the boundary between the LEF andUEF, and is well-developed in several outcrop areas inthe southern region (particularly around Barkly Pass, Jamestown, Lady Grey and Zastron). It, however,becomes obscured in the areas north of Zastron
where the sandstone content of the LEF shows a sharpdecline.
Sandstone body characteristicsThe LEF is dominated by channel-shaped sandstonebodies a few tens of metres wide, and a few metres(maximum 20 to 25m) high. Channel-fills wedge outlaterally towards the erosive basal bounding surfaces,which are concave-up, with reliefs of ~5 to ~10m. Thechannel-fills are asymmetrical, with well-defined, steepcutbanks on the convex, and gently inclined strata onthe concave margins. The sandstone bodies are usuallymade of multiple sandstone beds separated bynumerous laterally non-persistent internal erosion
Figure 3. Outcrop scale differences between the Lower and Upper Elliot Formations strata are clearly visible due to the differential
weathering of the dissimilar lithologies. The plateau arrowed is regionally traceable in areas south of Zastron. (A) Barkly Pass (Eastern
Cape); (B) Dili-dili (Tele River Valley - Eastern Cape).
A
B
surfaces of ~1m relief which are commonly gentlyinclined towards the thickest part of the multi-storeysandstone bodies. These are interpreted as lateralaccretion surfaces. Within the multi-storey sandstoneunits, well-developed point bar successions are present.However, the upward-fining units are rarely capped byextensive mudstones, but rather by conspicuous mud-pebble conglomerate lags. The sedimentary structures
within these channel-shaped sandstones of the LEF arepredominantly trough and planar cross-stratification,massive beds and less commonly low-angle cross-stratification.
The UEF does not display the same sandstone bodycharacteristics. Although rare lenticular sandstone bodiesare present at places, especially in its uppermost part,most sandstone bodies have sheet form, being severaltens of metres wide, and a maximum of 5 to 6 metreshigh. The sharp, tabular, bounding surfaces are laterallypersistent, and lack basal irregularities larger than a fewtens of centimeters. The sheet sandstone bodies containflat internal erosion surfaces, which are similar ingeometry to the basal bounding surface, and give theunits a multi-storied appearance. Internally, sandstonebodies of the UEF are dominated by massive beds,horizontal lamination, ripple-cross lamination, and raretrough cross-stratification.
Mudstone unitsThe multi-storey sandstone bodies of the LEF areseparated by thicker (average 20-30m) and laterallymore persistent mudstone intervals than those betweenthe sandstone sheets of the succeeding UEF, which areonly 0.5 to 10m thick. Invertebrate trace fossils,
SOUTH AFRICAN JOURNAL OF GEOLOGY
BASIN DEVELOPMENT DURING THE DEPOSITION OF THE ELLIOT FORMATION402
Figure 4. Summary palaeocurrent rose diagrams for planar cross-
bedded sandstones in the Elliot Formation. (A) Lower Elliot
Formation; (B) Upper Elliot Formation.
Figure 5. Ternary diagram of mineral composition of arenites in the Elliot Formation Qm:FP:RF (monocrystalline quartz: feldspar : rock
fragments).
EMESE M. BORDY, P. JOHN HANCOX AND BRUCE S. RUBIDGE
SOUTH AFRICAN JOURNAL OF GEOLOGY
403
carbonized and calcretized root traces, calcareousconcretions, large-scale calcretized and clay-linedshrinkage cracks, irregular mottles, desiccation cracks,falling-water level marks, and mud drapes are moreabundant in the UEF fine-grained facies than the LEFmudstones (Table 1). In both units, the degree ofpedogenic alteration becomes more evident towards thenorth than it is in the south.
ColourThere is a distinct colour difference between the LEFand UEF lithologies. While the rocks of the LEF are lightred or pink, with mottling of green and grey, the UEFdeposits are characteristically deep red or maroon, withsporadic light grey mottles, especially at the level ofpedogenic concretionary accumulations. In particular,the LEF sandstones are characterized by pale yellow
Figure 6. Quartzite pebbles are found in basal lags of sandstones
bodies in both Lower and Upper Elliot Formation.
Figure 7. Thickness measurements of the Elliot Formation (Lesotho data based on Stockley, 1947).
(5Y 7/4), pale yellowish brown (10YR 6/4), greyishorange (10YR 7/4), moderate red (5R 5/4; 5R /8) colours;while the mudstones are light red (5R 6/6), moderatepink (5R 7/4), dusky red (5R 3/4), very dusky red purple (5RP 3/2) with bluish grey (5B 7/1), pale blue (10G 6/1)light greenish grey (7.5GY 7/2) mottles. The UEFsandstones are red (5R 4/10, 5R 5/8), pale red (10R 6/10), dusky red (5R 3/4), moderate red (5R 4/8),dark reddish brown (10R 3/4), and mudstones are red(5R 4/10), dark red (5R 3/6, 5R 3/4), very dark red (5R 3/6), dusky red (5R 3/4) coloured with mottles ofbluish grey (5B 7/1, 10GY 7/2), pale blue (10G 6/1),light greenish grey (7.5GY 7/2).
FossilsCracked, disarticulated and fragmented fossil boneswere found in the overbank deposits of both the LEFand UEF, with taxa representative of both theEuskelosaurus and Massospondylus Range Zones ofKitching and Raath (1984). Fossil remains were notfound in the body of the channel-fill sandstones,however their basal channel-lags commonly containfragmentary fossil bones. These are in association withcarbonate glaebule conglomerates in the UEF. The majordifference shown by the vertebrate fossil record is therelative size of the specimens, with the LEF being
dominated by large taxa (prosauropods, cynodonts andamphibians of the Euskelosaurus Range Zone), incontrast to the smaller, more gracile specimens whichdominate the UEF (small sauropodomorphs andcrocodilians of the Massospondylus Range Zone).
Fossil wood fragments were collected in severaloutcrops of the LEF, but only in its southern outcroparea where carbonized wood fragments were alsodetected in channel lags. Fossil wood was not found inthe UEF during this study, however the unit yields fossilwood (Araucarioxylon arficanum - cataloged underBp/16/1099-1104 at the Bernard Price Institute forPalaeontology, University of the Witwatersrand) in thesouthern outcrop area in Lesotho.
Most trace fossils of the Elliot Formation occur asrelatively rare, but strongly bioturbated, shallow beddingplane features. While trace fossils are virtually absentfrom the LEF, the UEF ichnofossils are of relativelyhigher diversity and abundance. The most commontrace fossils are epichnial ridges which are horizontal,subhorizontal, straight, simple, and unlined with massiveburrow-fills identical to the host rocks. Similar traces areusually interpreted as feeding traces (Fodinichina)probably produced by arthropods or vermiform animals(Buatois et al., 1998). These burrows are thereforetentatively referred to Planolites isg. (sensu Pemberton
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Figure 8. Thickness relationships across the basin in the Lower and Upper Elliot Formations. Note the sudden break in the thickness trend
in the Lady Grey - Quthing area roughly coinciding with the southern limit of the Kaapvaal Craton (southern boundary as proposed by
Skinner et al., 1992). General vertical log profiles from the southern (A) and northern outcrop areas (B) (based on own data and Botha,
1968).
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and Frey, 1982). Other common ichnofossil types, whichwere observed only in the Upper Elliot Formation are traces with meniscate backfilling. The burrows aremostly straight, but irregularly meandering or slightlylooped burrows are also present. Similar, actively filled,lined, meniscate burrows were described as Scoyenia isp.by Bromley and Asgaard (1979). However, the moststriking characteristics of the Scoyenia isp., the denseexternal ornaments of the fill (longitudinal striations),were not noted. In the absence of surface ornaments,these burrows may be referred as Ancorichnus (sensuFrey et al., 1984) or Beaconites isp. (sensu Keighley andPickerill, 1994). All three ichnogenera (Scoyenia,Ancorichnus and Beaconites) are interpreted as feedingburrows (Frey et al., 1984) and belong to the Scoyeniaichnofacies (Buatois et al., 1998), which is characteristicof a low-energy, shallow-water environment. Thisichnoguild, therefore, documents periodical exposure tosubaerial conditions (Bromley and Asgaard, 1979; Frey etal., 1984; Buatois and Mangano, 1998; Buatois et al.,1998).
Palaeocurrents and provenanceCurrent indicators show that major rivers flowed fromsouth to north and south-west to north-east in the LEF,and from the south, southwest and west, with a smallproportion from the north in the UEF. Differences in thedrainage patterns are not only traceable between the southern and northern regions, but also between thetwo units. Figure 4A shows that the mean current vectorwas approximately north in the LEF changing to east-northeast during UEF times (Figure 4B).
Although there are various differences in thepetrofacies of the lower and upper part of the ElliotFormation, their most important implication is that theyindicate dissimilar source rock-types (Figure 5). Themainly quartzo-lithic sandstones (sublitharenites) of the LEF have a recycled-orogen parentage, whereas inthe UEF, in addition to the orogen-derivedsublitharenites, quartzo-feldspathic sandstones(subarkoses) also occur, which were shed from atransitional continental source situated between thecraton interior and basement uplift provenances.
Figure 9. Pene-contemporaneous deformation features detected in the upper part of Upper Elliot Formation. (A) Large-scale fault with
strongly deformed sandstone bed in the hanging wall and a horizontal, white silty layer in the mudstones of the footwall (location east of
Jamestown - near Rossouw). (B) Large-scale convolute bedding with deformation amplitude of more than 1m (location west of Jamestown
- near LeileKloof).
The sandstones of the LEF are fine- to mediumgrained and poorly sorted, in contrast to the very fine-to fine-grained and better-sorted UEF sandstones. Withinthe units, the LEF sandstones appear to lack any verticaltrends, but display a northward decrease in grain size,while the UEF sandstones reveal a slight upward-coarsening trend, but no lateral grain-size variations.
Throughout the Basin, basal lags of both the LEF andUEF sandstone bodies yield some outsized, roundedquartzite pebbles and boulders (Figure 6), but there isno detectible difference between the abundance or sizeof the single clasts of the LEF and UEF.
Isopach changesThe well-documented regionally traceable decrease inthe isopach trends of the Elliot Formation (Botha, 1968;Le Roux, 1974; Johnson, 1976; Visser and Botha, 1980;Kitching and Raath, 1984; Eriksson, 1985; Smith andKitching, 1997; etc.) is confirmed by this study. Based onpalaeocurrent indicators, the isopach variations appearto develop in both a down-current direction (from southto north) and across the strike of the Basin (in an east-west direction; Figure 7). It has to be emphasisedhowever, that both the northward and lateral reductionof the overall thickness are overwhelmingly governed bythe laterally and northwardly tapering wedge of the LEFdeposits (Figure 8). The thickness of the LEF in the southis ca. 300m (Figure 8A), but in the north the thickness iscommonly below 50m (Figure 8B). In the south, asimilar thickness reduction is detectable across the widthof the Basin, and because it takes place over a shorterdistance, the rate of reduction is more pronounced as itwedges out along the lateral margins of the Basin.
The UEF shows a much reduced thickness decreasefrom ca. 150m in the south to ca. 50m in the north. Inan east-west direction, across the Basin, the UEF showsconstant isopach trends. It is important to note that thenorthward thickness decrease of the Elliot Formation isnot a gradual one, but that there is a break in thegradient in the zone between Lady Grey-Zastron andQuthing, which appears to coincide with the southernedge of the Kaapvaal Craton as depicted by Skinner et al. (1992). While north of this zone the thickness ofthe Elliot Formation is generally below 250m, south of the zone the Elliot Formation is over 350m thick,except on the eastern and western flanks of the outcropbelt (Figures 7 and 8). It is noteworthy that in the samezone, between Lady Grey-Zastron to Quthing (southernLesotho), there are some pronounced thicknessvariations from 163 to almost 400m (Figure 7).
Pene-contemporaneous deformation featuresPene-contemporaneous deformation features have beendetected only in the upper part of the UEF. To date onlytwo features that may be structurally related have beendiscovered. These are one large ‘epi-depositional’normal fault observed east of Jamestown (nearRossouw) (Figure 9A), and one large-scale convolutebedding structure recorded west of Jamestown (near
Leilekloof) (Figure 9B). Other pene-contemporaneousdeformation features in the UEF are small-scaledewatering structures, load casts and water-escapestructures.
DiscussionThe deformation history of a thrusted mountain belt (in this case the CFB) is preserved in the stratigraphicrecord of its foreland basin. Climate, thrust lithology,basin floor geology and base level change alsoindependently influence the facies distribution andsedimentary petrology of the basin fill (Jordan et al.,1988). While the contribution of climate, source andbasement rock geology to the sedimentation style of theElliot Formation was significant, eustatic base-levelchanges had no influence on the stratigraphy, since theElliot Formation was deposited in a fluvio-lacustrineenvironment within a long-established internal drainagebasin, remote from any marine shoreline (Johnson et al.,1996; Smith and Kitching, 1997).
In such an internally drained basin, where directeustatic control can be ruled out due to the lack of amarine connection, and where the generation ofaccommodation space is governed by tectonic andclimatic changes, the application of classic sequencestratigraphy is difficult. It is proposed that the boundaryof the Lower and Upper Elliot Formations represents aregionally significant surface generated by tectonicactivity. We consider this surface to be the result of thefinal orogenic loading of the CFB, where the distal sectorof the system became elevated, and erosion took placedue to the uplift and southward migration of theforebulge, creating a second order sequence boundarybetween the LEF and UEF. Following the interruption bythis previously undetected tectonic event, the systemreturned being dominated by the first-order unloading ofthe CFB, creating a depression in the distal sector, whichwas subsequently blanketed by the upward coarseningUpper Elliot and Clarens formations. This unloading-driven subsidence was then uninterrupted until theonset of extensional tectonism related to the break-up ofGondwana in the Early Jurassic.
The proposed subaerial unconformity between theLEF and UEF is not an obvious sequence boundary, buta rather cryptic surface indicated by a series ofdiscrepancies detectable between the units below andabove it. Contrasts between the LEF and UEF are foundin the fluvial styles (demonstrated by dissimilararchitectural characteristics of sedimentary units e.g.,sedimentary structures, colour, grain-size, sorting.) aswell as in the change in provenance, the orientation ofthe drainage slopes, and in their fossil content.
In the section that follows the relationship betweenthe characteristics of the basin fill and its implication forthe development of the main Karoo Basin are discussed.The framework for the principles of fluvial stratigraphyand depositional patterns in a tectonically controlledforeland system are adopted from the work ofCatuneanu et al. (1998), Catuneanu and Sweet (1999),
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Catuneanu et al. (1999) and Catuneanu and Elango(2001). It has to be emphasised that the differences ofthe LEF and UEF basin fill could be due to acombination of tectonics and climate, and unravellingtheir individual contribution is difficult. The fact that thefluvial style difference of the LEF and UEF isaccompanied by a major shift in detrital compositionand regionally consistent, vertical changes inpalaeocurrent trends, suggests that the underlying causeof the LEF-UEF boundary is mostly tectonic, rather thanclimatic.
The contrasting characteristics of the sandstonebodies of the LEF and UEF are used here to give insightinto the relative rates of subsidence that took placeduring the deposition of the Elliot Formation.Backstripping of the Elliot Formation strata was notattempted, due to the lack of precise chronological data,and because the present strata thicknesses do not reflectaccurately the amount of flexural subsidence, as theeffects of sediment loading during Elliot times, and thecompaction under burial load of the pre-Elliot and ElliotFormation sediments, are also important controls on thepresent thickness.
The relative rates of subsidence during the LEF andUEF are interpreted based mainly on the work of Krausand Middleton (1987), Behrensmeyer (1988) andMarriott (1999). According to these authors, isolated,channel-shaped sandstone bodies such as those of LEFthat form discrete ribbons in thick overbank deposits areindicators of higher subsidence rates than the sheetsandstone bodies characteristic of the UEF, which implyslow subsidence. In addition to this, the distinctmeandering fluvial style of the LEF also shows moderatesubsidence with local cycles of avulsion (abandonedchannels). In contrast, the presence of wide, shallowscours and the lack of significant down-cutting in theUEF, suggests that floodwaters flowed in unconfined,shallow sheets rather than channels, evoking a picture ofa lower rate of subsidence. The sporadic lenticularchannels in the upper part of the UEF might beindicative of a relative increase in subsidence due to theprogressive steepening of the topographical slopesleading to the deposition of the upward-coarsening UEF-Clarens Formation sequence.
According to Jordan (1995, p. 353), the erosionalrelief of the sequence boundaries in the standardforeland basin of Bermejo (Central Andes) is less than 2 to 3m, and thus it is difficult to discern between theboundaries of stratigraphic sequences, and autogenicunits. Following the same line of evidence, it is thoughtthat the erosional surfaces of a few metres in relief foundin the LEF are probably erosional boundaries ofautocyclic avulsion sequences, and the only sequenceboundary is identified at the contact between the LEFand UEF.
The lack of incision in the UEF might have resultednot only from slower tectonic subsidence, but also fromthe progressive aridification during the deposition of theElliot Formation, resulting in less stable river banks, due
to the lack of cohesive bank substrates and protectiveriparian vegetation. There is a real scarcity of preservedwood remains in the UEF, despite the fact that there aremore preserved calcareous paleosols in the UEF than inthe LEF.
The rare pedogenic alterations in the LEF, and theabundance of pedogenic features in the UEF (e.g., calcareous concretions, large-scale calcretized andclay-lined shrinkage cracks, small-scale desiccationcracks, irregular mottles, carbonized and calcretized roottraces, and animal trace fossils) may be explained as aresult of changing tectonic and/or climatic conditions.An envisaged higher tectonic subsidence rate in theforesag during the LEF would result in more rapidfloodplain aggradation rates and lower soil profilematurity. In the UEF, tectonic change, triggered bydecreasing rates of orogenic unloading and totallydiminished thrusting, would lead to decreasing rates oftectonic subsidence in the foresag, limited sedimentsupply, and intense pedogenic development on themore stable and abandoned floodplain surfaces. Theprogressive aridification (climatic change) wouldenhance the effects of orogenic unloading and lead to agradual decrease in the frequency of flooding (limitedsediment supply) and increased pedogenic maturity. Inother words, the rare LEF paleosol profiles suggestthatthe sedimentation rate out-paced the rate ofpedogenesis, a situation which changed in the UEF.
The fact that in both units the pedogenic overprintingis more abundant in the north than in south mightsuggest that differential subsidence occurred on a basinalscale during sedimentation. It is therefore likely that inboth the LEF and UEF, the southern regions with fewpedogenic features experienced higher rates ofsubsidence and were more proximal, than those in the north which feature a variety of pedogenicmodifications. Alternatively, a lowered water table fromsouth to north may also contribute to the differences inpedogenic characteristics of the southern and northernfacies.
The colour of the lithofacies in the LEF impliesconditions of seasonally waterlogged drainage incontrast to a drier, better drained setting in the UEF,attesting to a long-suspected (Du Toit, 1954; Haughton,1924; 1969; Smith and Kitching, 1997; etc.) trend ofclimate aridification during “Stormberg” time.
The faunistic differences of the LEF and UEF mightalso be explained by changing climatic conditions, andaridification might account for the predominance ofgenerally smaller size animals in the UEF, as previouslysuggested by various workers (Haughton, 1924; Du Toit, 1954; Kitching and Raath, 1984; Gaffney and Kitching, 1994; Smith and Kitching, 1997; Warrenand Damiani, 1999).
The relative abundance of petrified fossil wood inthe LEF in the south might be explained by differentenvironmental factors (e.g., changes in the elevation ofwater table proximal to distal away from the orogen),which supported tree-growth and preservation in the
south, but not in the north. The absence of petrifiedplant remains from the UEF deposits does not directlyimply the lack of plants during UEF time. Their absencemight be due to unsuitable preservation conditionsand/or the presence of a different type of plantassociation. The fact that vegetation was present during the UEF times is clearly demonstrated by theabundance of pedogenic features, trace- and herbivorebody fossils.
The occurrence of strongly bioturbated beddingplanes has been interpreted in deposits similar to theElliot Formation as an indicator of rare but intensivebiological activities in dryland alluvial settingsdominated by episodic rainfall (Hasiotis, 2001). Thereason for the relatively higher diversity and abundanceof trace fossils in the UEF may be explained by the factthat in a depositional environment characterized bypunctuated episodic sedimentation, the land-derivedorganic debris is supplied more sporadically, but moreintensively than in a more humid setting (e.g., LEF).Since bioturbation intensity reflects moisture availability,their absence from the floodplain deposits might meanthat the overbank areas were moist for only shortperiods of time during the UEF.
Palaeocurrents and provenanceThe regionally consistent vertical changes inpalaeocurrent patterns are supported by the major shiftin detrital composition of the LEF and UEF. Thosearenites that show a recycled-orogen parentage in boththe LEF and UEF were mainly derived across the tectonicstrike of the main Karoo foreland basin from the south,as indicated by the northerly palaeocurrents typical inboth of the units. The palaeocurrents to the northeast,which become more dominant in the northern regionsof the LEF and through the UEF, show that the WesternBranch (northwest-southeast trending segment) of theCFB also acted as major sediment source during Elliot-times. This implies that supply of the detritus from thesetectonic uplands into the Basin was not only transverse,but also semi-axial. To account for the quartzo-feldspathic UEF sandstones with a transitionalcontinental source, situated between a craton interiorand basement uplift provenance, unroofing of probablyigneous source rocks within the western areas of theBasin is proposed. The unroofing might have takenplace along pre-existing structural weaknesses of theKaapvaal Craton boundary (Wits/Kimberley Block) inthe form of major, crustal-scale faults and/or intrusions,shedding feldspar-rich sediments from the west into theUEF depository. It is possible that the Clocolan Dome(Ryan and Whitfield, 1979), an intrabasinal, Archeangranite source for the Beaufort Group sediments(Theron, 1975; Cole, 1992; 1998), was renewed in theBloemfontein area during the deposition of the UEFstrata. In addition, the higher feldspar content in the UEFhas a climatic significance also, implying a more aridclimate in which chemical weathering was morerestricted.
The rare southerly palaeocurrents of the UEF areexplained by an elevated forebulge that was generatedduring the last (pre-Upper Elliot) orogenic-loading,which also generated the sequence boundary betweenthe LEF and UEF. These southerly denudation patternsmight also indicate the southward migration of theforebulge compared to the forebulge position that waseffective during the pre-Lower Elliot orogenic loading.Along a south-north strike, the more dominant northerly,and rare southerly, denudations in the UEF may beexplained as a result of a semi-symmetrical basingeometry, which formed as a combined effect of theongoing first-order unloading of the CFB (steeper north-facing slopes) and recovery from the last, minororogenic replenishing of CFB (gentler south-facingslopes).
It is noteworthy to emphasize that no dominantpalaeocurrents were directed from the Southeast in theLEF or UEF in contrast to that suggested by Turner’splume model (1999: Figure 15). In addition, the Elliot-equivalent strata in the southern Lebombo ‘Monocline’were also deposited on the south-southeasterly dippingslope, as determined by Turner and Minter (1985: Figure 5). These regional palaeodrainage patterns alsosuggest that the regional palaeoslopes were not slopingfrom a south-eastern area where Turner (1999) positionsthe most elevated region above his hypothetical Triassic-Jurassic plume. Therefore, the regional palaeocurrentdirections of the Elliot-strata recorded here are differentto the predicted radial drainage pattern of Turner (1999).
The finer grain-size and better sorting of the UEFcompared to the LEF could also be due to decreasingrelief differences (reduced subsidence) and increasingaridity, which promotes the preservation of finer grainedsediments, and reduces the amount of fluvial input.Based on the palaeocurrent indicators, the northwardfining trend of the LEF sandstones appear to develop ina down-current direction from abrasion. The slightupward-coarsening of the UEF, coupled with theoccasional incised channels in its upper parts, and thecoarser grain-sizes of the overlying Clarens Formationsandstones are interpreted as a positive indicator of thegradual steepening of the foreslope (increasedsubsidence) driven by the final quiescence of thesystem.
Tectonic events and subsidenceThe quartzite pebbles and boulders are interpreted hereas signals of major thrust sheet movements in the CFB.Those in the LEF sandstones attest to the orogenic pulsewhich ended at 215 ± 3 million years (Halbich et al.,1983; Gresse et al., 1992; Catuneanu et al., 1998), andgenerated the second-order subaerial unconformityfound at the base of the Elliot Formation (Catuneanu et al., 1998), whereas the UEF quartzite gravels areevidence for a previously undetected, final stage, minororogenic loading of the CFB, responsible for theunconformity between the LEF and UEF. In this way, it is suggested that in contrast to the previous
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interpretations (Catuneanu et al., 1998), the long-termorogenic unloading was interrupted by three, and nottwo short tectonic events.
The proximal-distal changes detected in the LEF, andthe fact the boundaries of the unit depict a wedge-shaped accumulation, are explained by the fact that thepreserved sediments of the LEF were deposited in the central and distal parts of a deeper foresag basin thatexperienced higher rates of subsidence in the middlethan either distally in the northern, or laterally in theeastern/western regions. In addition, the northern part of the original LEF basin was probably erodedduring the thrusting event and subsequent forebulgeuplift, which occurred pre-UEF, but post-LEF. The lack oftrends in the slab-shaped sedimentary rockaccumulation of the UEF (semi-parallel lower and upperboundaries) are explained by the fact the preservedsediments of the UEF represent a more distal part of ashallower foresag, where the proximal-distal differencesare not as easily detectable. Considering the channel-shaped cross-section of an “ideal” foresag basin(Catuneanu et al., 1998; Catuneanu and Sweet, 1999,Catuneanu et al., 1999; Catuneanu and Elango, 2001), it might be speculated that the tapering, southern limitsof both LEF and UEF might have been eroded duringpost-Karoo time.
It is speculated that the pronounced break in theisopach trends in the Lady Grey-Zastron and Quthingzone is probably related to weakness zones in theunderlying lithosphere along the southern edge of the Kaapval Craton, and suggests lithosphericheterogeneity of the Elliot Basin floor. Although thismatter clearly needs further investigation, it is interestingto note that Hancox (1998) observed similar trends in hisbasinal investigation of the Burgersdorp and Moltenoformations. The anomalously thin Elliot Formation in the area between Lady Grey-Zastron and Quthing(southern Lesotho) may be the result of elevated and irregular basin floor topography, offset to the southof, and paralleling, the margin of the Kaapvaal Craton.
The large-scale pene-contemporaneous deformationfeatures (i.e. normal faulting and convolute bedding) areinterpreted as evidence of early extensional tectonicswithin the foreland setting. Similar large-scale, laterallycontinuous convolute bedding has been linked toseismic activity by Rossetti (1999). Other features thatmight support syn-depositional extensional tectonicactivity in the UEF are the septarian concretions, small-scale dewatering structures, load casts, water-escapestructures (Tanner, 1998; Pratt, 2001) and the large-scalecracks found in the mudstone successions, all typical ofthe UEF. A plausible explanation of the origin of thelarge-scale cracks was given previously by Smith andKitching (1997) as shrinkage cracks, the dewatering ofsuch structures might have been promoted byliquefaction due to seismic shocks. Large-scaleconvolute bedding might be explained by densityinversions (Ankatell et al., 1970), which may cause
folding of the interference between the layers, but sincein this case both under- and overlying lithologies areidentical (i.e. sandstone), a density inversion origin canbe ruled out.
The observed extensional tectonic features augmentthe results of the UEF provenance study, suggestingtectonic movement along pre-existing structuralweaknesses within the Kaapvaal Craton (boundary ofthe Wits and Kimberley Blocks?). These exclusivelyupper UEF features, may be collectively taken as the firstsignals of the changeover to an extensional tectonicregime (Gondwana break-up) during the Early Jurassic,and not in the Early to Middle Triassic as suggested byTurner (1999).
ConclusionsThe magnitude of the lateral facies differences in the LEFand UEF suggest that the study area remained in a distalsetting with respect to the CFB throughout thedeposition of the Elliot Formation (Figure 10). In addition, the northward tapering wedge-shaped LEFdeposits and the rather slab-shaped UEF sequence,imply that the southern limit of the ancient foresag hasbeen eroded in post-Karoo times. The southerlypalaeoflows in the UEF are interpreted as forebulge-sourced drainage, showing that the strata in the upperpart of the Elliot Formation were deposited closer to adiminishing forebulge than to the CFB, and that thedepocentre of the UEF foresag was situated closer to theCFB than that of the LEF foresag basin.
The reorganization of the fluvial sedimentaryprocesses from meandering (LEF) to ephemeral fluvial(UEF) style, coupled with other differences in regionalpalaeocurrent patterns and source rock-types, suggestthe following development of the main Karoo Basinduring Late Triassic to Early Jurassic times:
As a consequence of the P8 tectonic event (as defined in Catuneanu et al., 1998) in the CFB, asubaerial unconformity developed across the top of theMolteno Formation (Figure 10A). During this time, dueto the elevated relief in the thrusts, some large quartziteboulders were transported from the CFB.
With time, as a result of the continued first-orderorogenic unloading of the CFB, the main Karoo Basinregained its foresag setting (Figure 10B). Along animaginary profile between the CFB and distal part of thesystem, the following sedimentary environments couldhave been found: braided rivers existed along theelevated foreslope (sediments not preserved) and, withdecreasing down-slope gradient, these rivers becamemeandering streams with extensive floodplains. TheCFB-sourced quartzite clasts and other meandering riversediments were finally deposited (as LEF) in the foresagbasin that was subsiding at a moderate rate. The climatewas humid to semi-arid, the vegetation was dominatedby riparian forests and inhabited by large bodied animals.
During the last localized thrusting event (here termedP9) in the CFB (without thrust front propagation) (Figure 10C), the foreslope experienced subsidence,
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Figure 10. Reconstructed tectonic profiles showing the development of the area between the Cape Fold Belt in the south, and northern
limits of the study area during (A, C) and following (B, D) the last two tectonic events (P8 and P9) in the thrust fold belt (see Conclusion
for details). Figure modified from Catuneanu et al. (1999) and Catuneanu and Elango (2001).
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while the subsidence in the foresag basin diminished,especially in its distal part, which became elevated as aforebulge. As a result, an unconformity developedacross the previous foresag basin deposits (LEF). Due toincreased relief of the thrust sheets, the CFB experiencederosion and large quartzite boulders entered thesedimentation cycle. This suggests that tectonicallycontrolled flexural subsidence existed in the main KarooBasin until the end of LEF times, in other words, until atleast the end of the Triassic.
Following the recovery period, the imaginary profile(Figure 10D) between the CFB and distal part of thesystem yet again showed foreslope and foresag settings,but in a more proximal position relative to the CFB.Since the unloading of the CFB was in its final phase, the subsidence gradually accelerated, creating thecoarsening-upward sequence of the UEF-ClarensFormation. The climate became more arid supporting afauna that was dominated by smaller species. Ephemeralstream sediments containing rare, CFB-derived quartzitepebbles accumulated in the foresag setting. In spite ofthe lack of precise chronological data, and thus well-defined accommodation rates, the frequent paleosolhorizons of this time indicate numerous small hiatusesand a slower rate of sediment accumulation than in theLEF. In addition to the foreland basin setting, the firstsigns of the break-up of Gondwana are also manifestedas various extensional tectonic features (faults,convolute-bedding, etc.). In a westerly direction, a largebasement fault or intrusion was (re)activated, whichshed feldspar-rich sandstones into the Basin. Otherbasement tectonics and their influence on thesedimentary fill are unclear, but it may be significant thatthe southern boundary of the Kapvaal Craton trendsparallel to the zone of some major facies changes (e.g., sudden decrease in LEF thickness, major shift inpalaeocurrents, increase in pedogenic alteration.)detected in the Elliot Formation.
AcknowledgementsThe manuscript was prepared while EMB was in receiptof a NRF post-doctoral bursary at the School ofGeosciences, University of the Witwatersrand. EMBwould like to thank her husband, Mamadou Diop for hisenthusiastic field assistance and companionship. The authors wish to thank Prof J.W. Kitching and M.A.Raath for introducing them to numerous fossil sites, andfor the countless and stimulating discussions regardingthe palaeoenvironmental conditions of the ElliotFormation. J.S. Marsh (Rhodes University) is thanked forbringing to EMB’s attention the large-scale fault east ofJamestown (near Rossouw). We thank reviewers O.Catunenau, R.M.H. Smith and M. Johnson for thoughtfultheir comments on the original manuscript. Also, specialthanks to L.D. Ashwal for his exceptional editorialsupport.
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Editorial handling: L.D. Ashwal
SOUTH AFRICAN JOURNAL OF GEOLOGY
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