EMESE M. BORDY, P. JOHN HANCOX AND BRUCE S. RUBIDGE

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Provenance study of the Late Triassic - Early Jurassic Elliot Formation, main Karoo Basin, South Africa

Emese M. BordyUniversity of the Witwatersrand, School of Geosciences, Johannesburg,

Private Bag 3, Wits 2050, South Africa. Corresponding address: Department of Geology, Rhodes University, Grahamstown, 6140, South Africa

e-mail: e.bordy@ru.ac.za

P. John Hancox and Bruce S. RubidgeUniversity of the Witwatersrand, School of Geosciences, Johannesburg,

Private Bag 3, Wits 2050, South Africa. e-mail: HancoxP@geosciences.wits.ac.za; rubidgeb@geosciences.wits.ac.za

© 2004 Geological Society of South Africa

ABSTRACT

Based on regional palaeocurrent pattern analysis and petrographic studies, the Late Triassic-Early Jurassic Elliot Formation (Karoo

Supergroup, South Africa) is divisible into a Lower and an Upper Elliot Formation. Laterally, along a north-south section, the lower

unit displays both changes in the sediment supply pattern and petrological composition, whereas the upper unit lacks significant

petrological variations. A break in sedimentation took place after the deposition of the Lower Elliot Formation, and a regional

subaerial unconformity is detected between the Lower and Upper Elliot formations. This unconformity is emphasized by marked

differences in the sedimentological and palaeontological composition of the deposits below and above the surface. Prevailing

northerly sediment supply patterns of the Lower Elliot Formation are replaced by mainly easterly-northeasterly transportation

directions in the Upper Elliot Formation, trends which were mentioned, but not accounted for by earlier researchers of the Late

Triassic-Early Jurassic Elliot Formation. Simultaneously with the reorganization of the sediment supply patterns, the origin of the

sandstones also shifted from recycled-orogen provenance, to a transitional continental source situated between craton interior and

basement uplift provenances. This paper presents the contrast in palaeocurrent trends and sandstone composition across

this surface, and reports, for the first time, the basin-wide presence of outsized quartzite pebbles and boulders, sourced from the

Cape Fold Belt.

IntroductionThe continental red beds of the Elliot Formation providekey information on the Late Triassic-Early Jurassicdevelopment of the Karoo foreland system which cosistof the main Karoo Basin and the Cape Fold Belt (CFB)(Figure 1A). The uppermost sedimentary fill, theinformal “Stormberg Group” comprising of the Molteno,Elliot and Clarens formations, is considered to lackdrastic lateral facies changes or thickness variations.According to Catuneanu et al. (1998) there are in factonly two major lateral changes along the south-northprofile: a gradual grain-size reduction and an overallthickness decrease from south to north. The basindevelopment model of Catuneanu et al. (1998)anticipates that the vertical grain-size distribution of LateTriassic-Early Middle Jurassic sedimentary fill showsthree second-order coarsening-upward sequences: twoof them are preserved in the Molteno Formation(Hancox and Rubidge, 1995; Catuneanu et al., 1998;Hancox, 1998a; b) and the third is the Elliot-Clarensformations (Catuneanu et al., 1998) (Figure 1B). TheUpper Karoo sedimentary rocks are believed tooriginate from reworking of older Karoo Supergrouprocks, and Cape Fold Belt lithologies (Johnson, 1991;Catuneanu et al., 1998). This paper investigates theprovenance of the Elliot Formation sedimentary strataby documenting the major sediment supply patterns,sediment provenance (sediment dispersal patterns and

petrology combined), and lateral and vertical grain-sizechanges.

Geological backgroundThe compressive Karoo foreland system (including theCape Fold Belt and main Karoo Basin) developed inresponse to at least eight tectonic events related to the Late Palaeozoic-Early Mesozoic subduction of thepalaeo-Pacific plate beneath the Gondwana plate(Catuneanu et al., 1998). The entire ”Stormberg Group”was deposited in a foresag setting formed in response tothe final, first-order orogenic unloading of the system(Catuneanu et al., 1998; Hancox, 1998b) (Figure 1B).Two well-documented, smaller tectonic events (P7 finalphase ~223 Ma; P8 final phase ~215±3 Ma) (Halbich et al., 1983; Gresse et al., 1992; Catuneanu et al., 1998)are known to have occurred during “Stormberg”deposition (Figure 1B). These two brief orogenic pulses,recorded by two subaerial unconformities (one withinMolteno Formation and the other at the base of theElliot Formation - Figure 1B) were generated as a resultof forebulge uplift (Catuneanu et al., 1998). In otherwords, gradual steepening of the foreslope during thedominant, first-order orogenic unloading wasinterrupted at least two times, and resulted in the above-mentioned coarsening-upward sequences (two in theMolteno and and one in the Elliot-Clarens formations -Figure 1B).

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Figure 1. (A) Geological map of the main Karoo Basin (after Catuneanu et al., 1998) showing the Kaapvaal Craton (southern parts are

overlain by Karoo cover) boundaries as proposed by Skinner et al. (1992). (B) Hypothetical cross-section of the Karoo foreleand system

during the first-order orogenic unloading stage (i.e., during the deposition of the “Stormberg Group”). Stratigraphic table shows the

anticipated grain-size distribution and unconformities within the second-order sequences of the “Stormberg Group” (after Catuneanu et al.,

1998; Catuneanu and Elango, 2001).

Although the geological literature commonly agreesthat the Elliot Formation comprises laterally continuousfloodplain mudstones and associated fluvial sandstones(e.g., Johnson, 1976), the overall palaeoenvironmentalreconstruction and general stratigraphic profile has beenextrapolated from data collected in only a few, relativelysmall study areas. To date, only three publicationspresent comprehensive palaeocurrent measurements.Botha (1968) investigated one locality in the south

(north of Elliot in the Barkly Pass - Figure 2), Le Roux(1974) undertook a more systematic study of the entireElliot Formation, however, despite several sedimentarylog descriptions his work lacks a detailed provenancestudy, and Eriksson (1983; 1985) provided a limited setof measurements from the north-eastern outcropregions. Published sandstone petrography studies havebeen undertaken exclusively in the north-easternoutcrop area (Eriksson 1985; 1987; Eriksson et al., 1994).

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Figure 2. Palaeocurrent map of the Elliot Formation. The numbers show Lower Elliot Formation (LEF) measurements; while the letters

indicate Upper Elliot Formation (UEF) measurements.

Accordingly the general northerly sediment supplypattern (e.g., Visser, 1984; Smith, 1990; Turner, 1990;Johnson, 1991; Smith et al., 1993) and other publishedassumptions regarding the basin development duringthe Late Triassic-Early Jurassic (e.g., Johnson et al., 1997;Catuneanu et al., 1998) are based on a statisticallyinsignificant number of measurements, and there arelarge areas which have never been investigatedproperly.

The sedimentological history of the Elliot Formation,including stratigraphic logs, depositional patterns,architectural element analysis, palaeoenvironmentalreconstruction, proposed biostratigraphic and lithostrati-graphic subdivisions are presented by Botha (1968),Visser and Botha (1980), Kitching and Raath (1984),Smith and Kitching (1997) and Bordy et al. (2004a and b).

It is important to emphasize that, to date, detailedanalyses of provenance rock types and heavy mineralsassemblages are not available for any formation in themain Karoo Basin. It is possible that the reason for thisis partly related to the fact that the Cape Fold Belt, whichacted as a source of sediments especially during the“Stromberg” times, is only a northern remnant of a oncemassive orogen which was subsequently rifted apart(i.e., opening of the South Atlantic). Petrographic workundertaken by Johnson (1991) showed that the presentCape Fold Belt consists of compositionally monotonouslithologies, dominated by ultraquartzose, quartzose toslightly felsdspathic metasediments. Lithic fragment-richrock types are virtually absent (Johnson, 1991). Rust(1962) and Ryan (1963) analysed and identified the onlydirect link between the Karoo sedimentary rocks and the

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Figure 3. Trough cross-stratified sandstones (A) are more dominant especially in the Lower Elliot Formation, but exposures revealing their

three-dimensional geometry are rare (Leliekloof farm, east of Jamestown). The palaeocurrent measurements were carried out on medium-

and large-scale planar cross-stratified sandstones (B) (Barkly Pass, north of Elliot). Hammer for scale (28cm).

Figure 4. Summary palaeocurrent rose diagrams for planar cross-stratified sandstones in the Elliot Formation.

(A) Lower Elliot Formation (south of Zastron); (B) Lower Elliot Formation (north of Zastron); (C) Upper Elliot Formation (south of Zastron);

(D) Upper Elliot Formation (north of Zastron); (E) Elliot Formation (all measurements). See Figure 2 for localities.

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Cape Fold Belt. These are the extraformational quartzitepebbles in the Molteno Formation that have beenextensively documented by Turner (1975). The lack ofinformation on the composition of the ancient sourceareas undoubtedly hampers the identification of majorprovenance rocks, thus the present study provides onlya general discussion of the possible source rock typessimilar to other studies of sandstone petrography in theupper part of the Karoo Basin fill (e.g., Johnson, 1991;Eriksson et al., 1994; Hancox, 1998b). It is hopedhowever that future heavy mineral analyis of the KarooSupergoup formations will prove to be vital in thedetermination of major provenance rocks (M. Mange,personal communication, 2002)

Database and methodsPalaeocurrentsPalaeocurrent measurements are one of the mostpowerful tools in palaeodrainage reconstructions,providing information on the direction of the local orregional palaeoslope, depositional environment,direction of the sediment supply and geometry of lithologic units (Miall, 1984). A great number ofpalaeocurrent features (e.g., current ripples, trough andchannel axis orientations; grooves; flutes; partinglineations) were documented during the field work, butmeasured palaeocurrent data however, resulted mainlyfrom the foresets of medium- and large-scale planarcross-stratified sandstones in major sandstone bodies,since good quality exposures revealing the three-dimensional geometry of the more dominant troughcross-stratified sandstones were rarely available (Figures 3A and B). This means that the palaeocurrentresults might be somewhat ambiguous, since foresetorientations of planar cross-stratified sandstones areknown to be more variable than trough axis orientations(Tucker, 1988). However, comparison of meanpalaeocurrents and trough axis orientations showed thatdirections indicated by foresets of planar cross-stratifiedsandstones coincide with those of trough and largerchannel axis orientations. Ripples and small-scale cross-stratification were disregarded as they are notconsidered to be representative of regional trends,having been generated by less unimodal, local currents(Miall, 1984). A total of 539 palaeocurrent measurementswas collected in the field in order to determine possiblesediment dispersal patters within the formation. The

data are presented in the form of 72 palaeocurrent rosediagrams and maps (Figures 2 and 4; Appendix A, B andC). Each individual rose diagram corresponds to dataderived from separate sandstone bodies interpreted assingle channels in the LEF and single channels or majorsandstone sheets in the UEF, with the exception of thefollowing roses: 1, 35, A and C, where data werecombined from successive sandstone channels.

PetrographyThe petrography of sedimentary rocks generally reflectsthe source of the detritus, even though the original detritalcomposition may have been modified by interactionsbetween physical/ chemical processes (e.g., weathering,rigor and duration of erosion/transportation, selectiveentrainment and transport, diagenesis), geologicalconditions (e.g., source lithology, basin tectonics, baselevel changes, sediment supply) and palaeoclimate (e.g., water discharge, local and regional temperatureand precipitation). Petrographic studies wereundertaken in order to gain a better understanding ofthe palaeoenvironment, by supplying information aboutthe dynamics of sedimentation and the probable sourcerocks. Sampling of the sandstones (n=50) was carriedout systematically in the whole study area, from thelower to the upper parts of the Elliot Formation, in orderto obtain a fully representative selection from theformation. Acknowledging that comparison ofpetrographical composition should be carried out onsamples of similar grain sizes (e.g., Dickinson, 1985),this criteria was hampered because the UEF isdominated by finer grain sizes (except for its uppermostpart) than the LEF.

Graphical and statistical evaluations of thepetrographic data were obtained by means of ternarydiagrams and bar charts. Thin-section descriptions werebased on quantitative observations (250 point counts perthin-section for mineral percentages and 50 point countsfor grain-size determination), and semi-quantitativevisual estimates (comparison charts for sorting,roundness, sphericity, etc.). The number of pointscounted for mineral percentages was determined by thestatistics provided by Galehouse (1971), which showsthat 250 point counts are satisfactory to obtain reliablepercentages of the components present. The appliedpoint-counting method was that of Gazzi-Dickinson(1966-70) as described by Ingersoll et al. (1984). Based

Table 1. Polycrystalline quartz types in the Elliot Formation. Interpretation based on Basu et al. (1975) and Basu (1985).

on this method, coarse-grained lithic fragments wereseparated from the fine-grained ones, and the formerwas counted according to its mineral beneath the cross-hair (e.g., a granite fragment consisting of feldspar andquartz would either be counted as feldspar or quartzdepending on which one of the two minerals falls underthe cross-hair). In this way, coarse polycrystalline quartzgrains (Table 1 - type 1) were counted as quartz, andmicrocrystalline quartz aggregates (Table 1 - type 2 and 3) were considered as lithic fragments (Ingersoll et al. 1984; Dickinson, 1985). However, the relativeabundance of the different polycrystalline quartz typeswas recorded, since this information was important inreconstructing possible source rock types (Basu et al.,1975; Basu, 1985). Intrabasinal grains (e.g., intraform-ational mudstone clasts) were ignored (Dickinson, 1985)in the determination of detrital modes.

Considering the homogeneous fabric of the deposits(all samples were taken from massive, well- tomoderately sorted sandstones), it was felt that anincrease in the number of the measurements for thegrain size determination was unnecessary (Johnson,1994). For visual estimates, comparison charts by Powers(1953) and Compton (1962) were used.

Complementing the palaeocurrent and sandstonepetrographic data, the basin-wide occurrence of erratic,outsized quartzite pebbles and boulders in lags at thebase of the sandstone bodies is also documented for the first time.

DescriptionA recent in-depth facies analysis of the Elliot Formationby the present authors (Bordy et al., 2004a and b) hasrevealed major geological differences between the lowerand upper parts of the Elliot Formation, accordingly inthis paper the two units are referred to as the LowerElliot Formation (LEF) and Upper Elliot Formation (UEF).

PalaeocurrentsIn conjunction with other observations, the analysis ofthe palaeocurrent measurements showed that there aresome distinct vertical and horizontal sediment supplypattern differences in the Elliot Formation (Figures 2 and 4; Appendix A, B and C). With respect to lateraldifferences, the study area seems to be subdivided intwo sectors in the region of Zastron (Figure 2), whichcoincides with the southern limit of the Kaapvaal Cratonboundary (also in Figure 1). The current indicators of theLEF show that the major transportation direction in the region south of Zastron was predominantly fromsouth-southeast to north-northwest and southwest tonortheast (Figure 4A). In the region north of Zastron, thesoutherly sediment supply pattern is suppressed byeasterly and east-northeasterly current patterns (Figure 4B). As for the UEF, the sediments were carriedinto the basin from the south and west, with a smallproportion from the north (Figures 2, 4C and D). In thesouthern regions of the UEF, there are variouspalaeocurrent trends resulting in the very low

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Figure 5. Ternary diagram of mineral composition of arenites in the Elliot Formation. (A) Qm:FP:Lt (monocrystalline quartz : feldspar :

total rock fragments including all microcrystalline quartz aggregates). (B) Qt:FP:L (total quartz : feldspar : rock fragments). Classification

after Pettijohn et al., 1972 (Pettijohn et al., 1987), and point counting methods based on Ingersoll et al. (1984) and Dickinson (1985).

See text for details.

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consistency ratio of the measurements (Figure 4C). Inthe northern region of the UEF, the easterly currentsbecome more dominant than in the same region of theLEF sandstones (compare Figures 2 and 4D).

The overall sediment supply pattern of the ElliotFormation (Figure 4E) shows a mean palaeocurrentoriented approximately to the north. This is fairlyconsistent with the general palaeodepositional directionsgiven by Botha (1968: the south-southeast); Le Roux(1974: from the south-southeast - Appendix VIII),Eriksson (1983; 1985: from the south-southeast) andTurner (1999: from the southeast and south-southwest).No dominant palaeocurrents were directed from thesoutheast, in contrary to Turner (1999: his Figure 15).The conspicuous easterly palaeocurrents of the northernregions were recognised, but their significance was notexplained for by Le Roux (1974), Eriksson (1983; 1985)and Le Roux (1985). Botha (1968, pages 107 to 109, 112)also reported a westerly and a northerly source rockarea for the upper part of the Elliot Formation in BarklyPass (north of Elliot - Figure 2).

PetrographyThe fifty sandstone samples of the Elliot Formation arearenites (sensu Pettijohn et al., 1972 in Pettijohn et al.,1987), and their detailed thin-section descriptions areavailable, upon request, from the first author. The abundance of the monocrystalline quartz, feldsparand lithic fragments (i.e., main framework minerals)(Figure 5A) shows that most of the samples are eithersublitharenites or subarkoses, and quartz arenites andarkosic arenites are less common. Besides several otherdifferences that are explained later, there is a distinctcontrast between the lithic fragments and feldsparcontents of the lower and upper parts of the ElliotFormation. The thirty-five samples of the LEF are mainlyquartzo-lithic sandstones (sublitharenites), whereas thefifteen samples of the UEF are largely quartzo-feldspathic sandstones (subarkoses) (Figures 5 and 6,sensu Pettijohn et al., 1972 in Pettijohn et al., 1987). Thefeldspar content difference allowed the slight separationof the samples from the lower and upper parts of theElliot Formation even when the total quartz contents

Figure 6. Petro-micrographs of two sandstone samples from the Elliot Formation. (A) Quartzo-lithic sandstones (sublitharenites) of the

Lower Elliot Formation: medium-grained, poorly sorted, silica cemented sandstones with low-sphericity, subangular grains. Generally, these

samples have lower feldspar and higher rock fragment content, and contain more polycrystalline quartz grains as well as heavy minerals

than the Upper Elliot Formation samples. (B) Quartzo-feldspathic sandstones (subarkoses) of the Upper Elliot Formation: very fine grained,

moderately sorted, iron-oxide cemented sandstones with high-sphericity, angular grains. Images were taken with a 4X objective (image

width = 1.6mm).

Figure 7. Grain-size distribution of the Lower Elliot Formation (A) and Upper Elliot Formation (B) sandstones.

were plotted against the feldspar and lithic fragmentpercentages (Figure 5B).

The macroscopic grain-size estimates wereconfirmed by the microscopic observations. The grain-size distribution (Figure 7) indicates a size decrease andsorting increase from the predominantly fine- to mediumgrained, and moderately sorted LEF sandstones, to themainly very fine- to fine-grained, and better-sorted UEFsandstones (Figures 6 and 7). There is a slight regionalgrain-size decrease from south to north in the LEFsandstones, but this trend is not detectable in the UEFlithologies. On the other hand, the slight upwardcoarsening trend observable in the UEF sandstones isnot present in the LEF arenaceous deposits.

The abundance of strongly undulose quartz grains(slightly higher in the LEF), the proportion of high-

sphericity (higher in the UEF) and better rounded grains(slightly higher in the UEF), the state of sandstonematurity (more mature in the UEF), and theconcentrations of heavy minerals (e.g., rutile percentageis higher in the LEF, but still <1%) are the othermeaningful petrofacies differences noted between thelower and upper part of the formation (Figure 6). In addition, almost all the samples of the LEF are silicacemented, and show authigenic overgrowths on thequartz grains, a feature that is virtually absent from the iron-oxide cemented UEF.

Occasionally, the larger monocrystalline quartzgrains of the lower part contain zircon, mica and needle-like rutile inclusions. Occasionally monocrystallinequartz grains are rounded to well rounded in both LEFand UEF samples. The samples of the UEF often contain

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Figure 8. Map of the quartzite pebble localities in the Elliot Formation. Insert shows approximate stratigraphic positions, farm locations

and maximum length in centimeters of the newly found and previously described (* Le Roux, 1974; ** Visser and Botha, 1980) quartzite

pebbles of the Elliot Formation.

very fine grained angular (even triangular) grains withconchoidal surfaces. Polycrystalline quartz grainscontribute to the coarsest grain-sizes of the LEF samples(fine- or medium-grained fraction), and as previouslymentioned, depending on their crystal sizes, they wereincluded into quartz or lithic fragment categories. The most abundant composite grains (Table 1) are thosemade of less than four, irregular, non-equigranularcrystals (type 1). Another frequent polycrystalline quartztype has more than four irregular crystals of semi-equalsizes (type 2). The crystal boundaries are mainly non-sutured, with sutured contacts being virtually absent. A few polycrystalline grains consist of more than fourmedium or coarse silt-sized crystals (type 3). The crystalshapes are mostly irregular, subequant and rarely slightlyelongated. The crystal boundaries are mainly sutured,but non-sutured contacts are also present. In thesamples of the UEF only polycrystalline quartz grains,mainly of type 1 and rarely type 3, were detected.

Throughout the Elliot Formation, the very fine- tomedium-grained detrital feldspars are mostlyplagioclase, and less frequently microcline. Commonlythe plagioclase is twinned on the albite law, while themicrocline displays tartan (cross-hatched) twins. Usuallythe feldspars are rather altered (to mica, clay), andsericitization seems to be advanced, although themicrocline crystals are often fresh. The feldspar contentof the Elliot Formation in its upper part is twice as highas that in its lower part (UEF average 11.08% versus LEFaverage 6.04%).

The most common rock fragment types in the lowerpart of the formation are mudstone varieties (mainlymuddy siltstones, silty mudstones of red colour), butfollowing Dickinson (1985) these were excluded fromthe detrital framework. Chert and metamorphic rockfragments (consisting only of quartz and muscovite, i.e. no rock fragment contained any feldspar at all) areregular, but minor components. A reworked bonefragment is present in one specimen. Rock fragments inthe samples of the UEF are predominantly mudstone

and chert. Bone fragments and reworked pedogeniccarbonate glaebules with bioturbation features are alsopresent. Bioturbation features are found only in thesamples of the upper part. These were described andtentatively identified as Planolites isg. and Ancorichnus(?Beaconites) isp. by Bordy et al. (2004a). Contributing to the finer grain-size groups, rounded or subroundedheavy minerals (zircon, rutile, opaques, garnet, greenhornblende) as well as muscovite and biotite flakes arepresent in various amounts in all the samples.

Gravel-size particlesIsolated, rounded, pebble- to boulder-sized, exclusivelyquartzite clasts were recovered from basal lags of several LEF and UEF sandstone bodies throughout thesequences. The largest pebbles were found in the lowerparts of both the LEF and UEF. The pebbles, especially the larger ones, are solitary features inmassive or cross-bedded sandstones. The regionaldistribution of these extra-formational quartzite clasts,their sizes and stratigraphic localities are shown inFigure 9. The quartzite pebbles mentioned by Le Roux(1974, pages 69, 70 and 94) are 8 and 15cm long in theLEF (Onwerwacht farm, southern outcrop area) and25cm long in the UEF (Kramberg, western outcrop area)(Figure 8). Visser and Botha (1980: their Figure 2)observed 7.5cm long quartzite pebbles in the LEF (northof Elliot in the Barkly Pass, southern outcrop area)(Figure 8). During this study no detectible differencebetween the abundance, or size, of the single pebbles ofthe LEF and UEF was observed. Other gravel-sizesediment fractions are represented by mud-pebbleconglomerates (Figure 9A) found in both the LEF andUEF. This lithofaces seems to have uniform appearanceand abundance throughout the Elliot Formation. It iscommonly found as a channel lag, at the base of majorsandstone layers. In contrast, pedogenic carbonateglaebule conglomerates (Figure 9B) are found only atthe base of the sandstone bodies in the UEF. Theseconglomerates are clast supported, moderately sorted

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Figure 9. (A) Mud-pebble conglomerates are found in uniform appearance and abundance throughout the Elliot Formation (Withoogte

farm, south of Lady Grey). Hammer for scale (28cm). (B) Pedogenic carbonate glaebule conglomerates are unique to Upper Elliot

Formation. They are clast supported, moderately sorted and usually massive or planar cross-bedded (Mabula, west of Maseru, in South

Africa). Lens cap for scale (5.8cm).

and usually massive or planar cross-bedded. The grainsize varies from granules to medium pebble, howeversmall pebble conglomerates are rather common. Boththe intaformational mud-pebble and pedogeniccarbonate glaebule conglomerates are oligomict, andsolitary smaller (~1 to 2cm) quartz pebbles are rarelyassociated with them.

Discussion Palaeocurrents, Petrography and GeodynamicsThe drainage patterns and petrofacies differencesbetween the LEF and UEF arenaceous deposits arediscussed together, as they are both subject to the samegeodynamic factors (i.e., inconsistency of theprovenance terrains).

Based on theoretical sandstone provenance fieldsdetermined by Dickinson (1985), the sublitharenites ofthe LEF, which have a southerly and southwesterlyorigin, point to a recycled-orogen source dominated byrecycled-orogen sandstones, and metamorphic andigneous rock fragments. In contrast, the subarkoses ofthe UEF, with west to east sediment supply patterns,indicate a transitional continental source situatedbetween craton interior and basement upliftprovenances, dominated by granite-gneiss basement andrecycled sedimentary rock types (Dickinson, 1985).Apart from the subarkoses, the UEF also contains somesublitharenites, showing that sediment was continuouslybeing shed from the orogen until the final depositionalphases of the Elliot Formation. The most likely sourcearea of the sublitharenites (throughout the ElliotFormation) may be identified as the southern andsouthwestern sectors of the Cape Fold Belt. To accountfor the transition from transverse to semi-axial transport,especially during the UEF, a scenario of differential basinsubsidence is introduced (Cant and Stockmal, 1993).This might have been caused by diachronous evolutionof the orogen. If differential unloading took place alongthe CFB with a larger off-load in the Western Branch ofthe CFB, this would have resulted in greater uplift andmore sediment supply from the southwest to west. It should be emphasised that this explanation on its owncannot account for the high feldspar content of the UEFarenites, since it implies the same orogenic source rockas for the other feldspar-poor arenites. To account forthe different composition (e.g., feldspar) of the UEFsandstones, derived from an inferred transitionalcontinental source, the palaeocurrent discrepancies ofthe LEF and UEF are explained in terms of tectonicregime changes. Accordingly it is proposed here that theeastward-orientated palaeocurrents; the non-recycledorogen provenance of the UEF arenites, and other UEFextensional tectonic features (e.g., large-scale pene-contemporaneous faulting, large-scale convolutebedding as documented by Bordy et al. 2004a) might beexplained by tectonic movement along pre-existingstructural weaknesses within the Kaapvaal Craton(?boundary of the Wits and Kimberly Blocks). Thesemajor, crustal-scale faults and/or intrusions in the

basement were (re-) activated and shed feldspar-richsediments from the west into the UEF depository. Theseexclusively UEF features may be taken collectively as thefirst signals of a changing tectonic regime (Gondwanabreak-up) detectable only from Early Jurassic timesonward in contrast to an Early Mid-Triassic signalproposed by Turner (1999). On the other hand, agradual increase in feldspar content was previouslyreported within the Elliot Formation (e.g., Visser andBotha, 1980; Visser, 1984; Eriksson et al., 1994), and itwas taken as an exclusive reflection to the increasedaridity of the depositional setting. Although such aninterpretation is plausible, in light of the newly obtainedregional palaeocurrent information, the climaticinfluence on sandstone composition is considered to beof lesser importance than previously thought.

The finer grain-size and better sorting of the UEFcompared to the LEF could be also due to decreasingrelief differences (reduced subsidence), and increasingaridity (and aeolian activity) which promoted thepreservation of finer grained sediments, and reduced the amount of fluvial input. Based on the palaeocurrentindicators, the lateral, northward fining trend of the LEFsandstones appear to develop in a down-currentdirection as a result of abrasion. The slight coarsening-upward trend of the UEF sandstones, coupled with theoccasional presence of incised channels in the upperpart of the UEF, and the coarser grain-sizes of theoverlying Clarens Formation sandstones are interpretedhere as one of the indicators of a gradual steepening ofthe foreslope (increased subsidence) driven by the lastphase of the orogenic unloading of the CFB.

An alternative explanation for the feldspar contentdifferences between the LEF and UEF sandstones is themodifying effect of post-depositional, diagenetic eventson original grain composition (e.g., Helmond, 1985).McBride (1985) and Milliken (1988) showed thatalteration and dissolution of certain minerals (e.g., feldspars) increase with depth, and thusinterpretations of source rock compositions based onfinal detrital mineral assemblages of sandstones shouldbe treated with caution. In our case, the higher feldspar percentages (average 11.08%) in the UEFcompared to that in the LEF (average 6.04%) may be aresult of deeper and longer burial of the LEF sequencewhich promoted the alteration, and ultimately, thedissolution of the feldspars in the lower part of the Elliot Formation. In fact, the alteration of feldspar tomica and clay minerals, strongly suggests that the original modal compositions were modified in thediagenetic processes.

Igneous and metamorphic sourcesStrained (undulatory) single quartz grains,monocrystalline grains with rutile inclusions, andpolycrystalline quartz grains with coarser crystals (type 1) (Table 1) are not diagnostic, but are suggestiveof plutonic rocks in the catchment area (Basu et al.,1975; Basu, 1985). Type 1 polycrystalline quartz could

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also indicate high grade metamorphic source rocks,however the other composite quartz grain typesencountered (types 2, but especially type 3), themonocrystalline grains with strong undulose extinction,and the metamorphic rock fragments are the mostreliable indicators of derivation from metamorphicterrains (Basu et al., 1975; Basu, 1985). As thepolycrystalline quartz grains, the strained (undulatory)quartz grains and metamorphic grains are mechanicallyunstable, their presence in the LEF shows that, to someextent at least, the igneous and metamorphic sourcecomponents were proximal to the site of deposition inthe LEF. The presence of such rock types in the CFB maysuggest that unroofing of deeper levels of the orogenoccurred during the uplift of the proximal sector of themain Karoo Basin. Other accessory minerals suggestiveof a mixed (igneous, metamorphic and sedimentary)provenance are the micas, rounded zircon, rutile, garnetand hornblende.

The virtual absence of polycrystalline quartz grainsand some other petrofacies characteristic of the upperpart may be influenced by the previously mentioned factthat the UEF lithologies are predominantly very finegrained sandstones. Nevertheless, the fact that the UEFsamples have a higher content of feldspar than the LEFsamples cannot be explained by the smaller grain-size ofthe UEF samples. This relatively high feldspar content, inconjunction with the type 1 composite quartz grains andthe heavy minerals present in the upper part, seem topoint to the unroofing of an igneous source. Similar,intrabasinal palaeohighs of granitic composition havebeen demonstrated for Beaufort Group sediments, westof the present study area, in the vicinity of Bloemfontein(Theron, 1975; Cole, 1992; 1998) (Figure 1).

Sedimentary sourcesThe rare, rounded to well rounded quartz grains andother sedimentary rock fragments (e.g., chert) in bothLEF and UEF samples are interpreted to signal detritussupply from a pre-existent sedimentary source whichmight have been the metasedimentary sequences of theCape Fold Belt and/or older Karoo formations. The redmudstone grains and bone fragments in an otherwisenon-rubified LEF sandstone sample imply that theseparticles were ripped up from a nearby (underlying orlateral) units that underwent reddening prior toreworking. All other mudstone fragments both in LEFand UEF are taken as indicators of more localsedimentary sources. The angular, very fine grainedparticles in the UEF samples might be indicative offluvially reworked aeolian detritus, because suchtriangular-shaped grains are typical products of aeoliansaltation collisions, representing the corners of largergrains broken off during transport (P.G. Eriksson, writtencommunication 2003). The virtual lack of unstable rockcomponents, the occasional, rounded monocrystallinequartz grains, and the trends of increasing sorting,roundness, sphericity and maturity, support the idea ofa secondary provenance for at least part of the deposits

of the UEF. In addition, the high proportions ofundulatory, monocrystalline, finer-grained quartz in theUEF, might also suggest derivation from disintegrated,finely-crystalline composite quartz, perhaps reworkedfrom older deposits.

Gravel-size particlesBecause of the strong similarities to the extraformationalquartzite pebbles of the Molteno Formation, which werecorrelated to lithologies found within the CFB (e.g., Rust,1962; Ryan, 1963; Johnson, 1991), the outsized quartzitepebbles of the Elliot Formation are tentativelyinterpreted here as having been sourced from the CFB.These pebbles may also suggest that reworking of older,extraformational pebble-bearing deposits (i.e., underlyingMolteno Formation) occurred, but given theaforementioned sandstone compositions andpalaeocurrents, the former possibility is favoured herewhich implies that a minor thrust sheet movements (cf. Rogers, 1998; Burbank et al., 1988) took place in theCFB prior to the deposition of the LEF as well as beforethe accumulation of the UEF strata. The large size (~20-25cm) of the extrabasinal pebbles of the UEFrenders their interpretation as reworked clasts fromunderlying deposits less likely. Although the exacttiming of the causative thrusting cannot be assessed, it isclear that one thrusting event occurred prior to thedeposition of the lower part of the LEF, and another tookplace prior to the accumulation of the lower strata of theUEF (Bordy et al., 2004a). Thus, it seems that in additionto the last tectonic event at 215±3 Ma (Halbich et al.,1983; Gresse et al., 1992; Catunenau et al., 1998), a minor tectonic even in the CFB occurred prior to thedeposition of the UEF, and in contrast to the previousinterpretations (Catuneanu et al., 1998), the long-termorogenic unloading was interrupted by not only two, butthree short orogenic pulses.

Although, the lag time that elapsed between theactive thrusting and the deposition of thrust-signallingpebbles is unknown, the ephemeral fluvial style in theupper part of the formation (Visser and Botha, 1980;Eriksson, 1983; 1985) does not necessarily evoke afurther delay in pebble progradation, because these pebbles could have been transported into the depositional setting by rivers sourced outside thedryland region (e.g., in the mountain chain of the CFB).It is possible that these rivers, whose sediments are notpreserved, formed in the uplifted proximal sector, andprogressed into ephemeral or intermittent streams withinthe study area (similar, modern day perennial toephemeral river transition is documented e.g., in theOkavango Delta by McCarthy et al., 2002).

The reworked, carbonate glaebules found inconglomerates are very early diagenetic in origin (Bownand Kraus, 1981) and are a clear demonstration of thefact that calcrete-bearing palaeosols formed during the deposition of the UEF, and that the unconfinedrivers/streams combed the adjacent floodplains tosource these glaebules. Although their occurrence

indicates local base-level fall, it is believed that suchbase level fluctuations, which resulted in cycles ofentrenchment and backfilling, were driven by short-termclimate changes rather than by tectonic controls (e.g., Waters and Haynes, 2001).

SummaryPalaeocurrent data and thin-section evidence attest tothe fact that the boundary between the LEF and UEF isa second-order sequence boundary, as proposed byBordy et al. (2004a). Across this boundary reorganizationof the fluvial sedimentary systems took place, frommeandering in the LEF to ephemeral in the UEF (Bordyet al., 2004b) and changes in palaeocurrent patterns andsandstone provenance also occur. This regionalsubaerial unconformity detectable between the LEF andUEF is related to the last orogenic pulse of the CFB,which was not previously recognized. Due to this lasttectonic event, outsized quartzite pebbles and boulderswere released into the UEF depository, and theultimately final orogenic quiescence of the systemgenerated a slightly upward-coarsening sequencerepresented by the Upper Elliot and Clarens Formations.

The differences between the major sediment supplypatterns of the LEF and UEF are explained inconjunction with the differences of the sedimentprovenance of these two units. Sandstones of both theLEF and UEF, with recycled-orogen provenance from asoutherly and south-westerly source, indicate longdistance transport across, and oblique to, the tectonicstrike of the foreland system. The UEF arenites(predominantly subarkoses) of the northern regions alsoshow longitudinal transport from west to east. Thereorganization in the palaeocurrents in the northernareas, which became more pronounced during the UEFtimes, may be explained in terms of foreland basintectonics and change in the local/regional tectonicregime. It is proposed that feldspar-rich sediments mighthave been sourced from the west, where extensionaltectonic movements (Bordy et al., 2004a) unroofedsuitable (igneous) source rocks during the deposition ofthe UEF strata. Concurrently, the previously reportedincreasing aridity of the depositional setting is evidencedby the increase of feldspar content within the Elliot Formation, presence of reworked aeolian depositsas well as intraformational, pedogenic carbonateglaebule conglomerates in the upper part of theformation.

AcknowledgementsThe manuscript was prepared while EMB was a NationalResearch Found (South Africa) post-doctoral researchfellow at the School of Geosciences, University of theWitwatersrand. EMB would like to thank her husband,Mamadou Diop, for his enthusiastic field assistance andcompanionship. We thank the reviewers Drs H. Dypvik,P.G. Eriksson and U. Zimmermann for their thoughtfulcomments on the original manuscript.

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Appendix A. Palaeocurrent rose diagrams for planar cross-stratified sandstones in the lower part of the Elliot Formation (south of Lady

Grey) (roses listed in alphabetic order).

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Appendix B. Palaeocurrent rose diagrams for planar cross-stratified sandstones in the lower part of the Elliot Formation (north of Lady

Grey) (roses listed in alphabetic order).

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Appendix C. Palaeocurrent rose diagrams for planar cross-stratified sandstones in the upper part of the Elliot Formation (roses listed in

alphabetic order).