The contact of the Molteno and Elliot formations through the main Karoo Basin, South Africa: a...

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

SOUTH AFRICAN JOURNAL OF GEOLOGY, 2005, VOLUME 108 PAGE 351-364

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Introduction and geological backgroundThe Late Carboniferous to Middle Jurassic Karoo retroarcforeland system, which includes the main Karoo Basinand Cape Fold Belt (e.g., Johnson, 1991; Veevers et al.,1994; Catuneanu et al., 1998; Pysklywec and Mitrovica,1999; Catuneanu and Elango, 2001), originated throughflexural deflection of the Gondwanan lithosphere due tothe shallow-angle subduction of the palaeo-Pacific plateunder the Gondwana plate (Lock, 1980; Pysklywec andMitrovica, 1989). Catuneanu et al. (1998) analyzed thestratigraphic pattern of the Karoo basin fill, andsuggested that Triassic-Jurassic sequence generation inthe Karoo Supergroup was primarily controlled byorogenic loading and unloading events in the adjacentCape Fold Belt (CFB). According to this model thepreserved strata of the informal “Stormberg” group(Molteno, Elliot and Clarens formations) (Figure 1) areexclusively foresag basin fills, generated during the final,first order unloading event of the Karoo foreland system.These authors further suggested that apart from the firstorder sequence boundary at the base of the “Stormberg”strata, the succession is interrupted by at least onefurther unconformity (second-order sequenceboundary), which they placed at the boundary betweenthe Molteno and Elliot formations. It is thought that thisunconformity resulted from a minor loading event in theCFB, the end of which was dated at 215 ± 3 Ma (Halbich

et al., 1983). Bordy et al. (2004a) demonstrated that thereis an additional subaerial unconformity within the ElliotFormation (Figure 2), which was explained to have alsobeen generated by a subsequent minor loading event ofthe CFB, and which separates the lower and upper partsof the Elliot Formation.

Following on from this work, one of the keyformational boundaries that still required researchattention was that between the Molteno and Elliotformations. This boundary is an important although ill-defined one, having previously been proposed to beintertongued (Johnson, 1976; Christie, 1981), gradational(Eriksson, 1984) and both abrupt and intertongued(Turner, 1975; 1977; 1983). Accepting the inter-tonguednature of the contact proposed by Turner (1975; 1983),Cole (1992) and Anderson et al. (1998) considered theupper part of the Molteno and lower part of the Elliotformations to be coeval facies equivalents. They arguedthat the Molteno Formation represented a proximalfacies, closer to the Cape Fold Belt, whereas the ElliotFormation represented its distal equivalent. Within thismodel, Elliot Formation strata migrated southwards andback-stepped over the older units of the coarser MoltenoFormation due to the source terrain (CFB) diminishing,and thus progressively supplying finer grainedsediments. This reasoning implies that the lower part ofthe Elliot Formation in the north of the basin would be

The contact of the Molteno and Elliot formations through the main Karoo Basin, South Africa:

a second-order sequence boundary

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

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

Corresponding author: e-mail: [email protected]

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

Private Bag 3, Wits 2050, South [email protected]; [email protected];

© 2005 Geological Society of South Africa

ABSTRACT

The Late Triassic to Early Jurassic Molteno, Elliot and Clarens formations of the uppermost part of the Karoo Supergroup in the main

Karoo Basin of South Africa form a distinct tectono-sedimentary sequence. A major problem with the modeling of this part of the

Karoo basinal fill is that the nature of the Molteno-Elliot contact is poorly understood. Previously the contact has been defined on a

mix of lithological and palaeontological criteria, and has been considered to be gradational or transitional in nature, with the Elliot

considered the distal equivalent of the upper Molteno Formation. Recent field investigations demonstrate that the boundary can be

defined on lithologic changes, including the gross and internal geometries of the sandstone units and contained lithofacies

associations, the presence/absence of coal seams and paleosols, and on palaeocurrent patterns, sandstone composition and grain-size

variations. This boundary represents a regional unconformity throughout the basin and the recognition of this cryptic second order

sequence boundary invalidates previous interpretations of an interfingering stratigraphic relationship between the two formations.

Stacking patterns in the upper Karoo Supergroup, and the tectonic framework of their development, suggest that the southern

margin of the Kaapvaal Craton acted as a regional control on sequence development during Late Triassic - Early Jurassic times.

older than that in the south, and that the grain-sizes andsandstone/mudstone ratio should be constant or shoulddecrease form north to south.

This paper presents the results of our investigationinto the stratigraphic relationship of the uppermostMolteno and lowermost Elliot formations throughout themain Karoo Basin, and analyzes the postulate ofCatuneanu et al. (1998) that the Molteno-Elliot contact is a second-order sequence boundary. Detailedinvestigation of changes in sandstone geometry resultingfrom the fluvial style transition from braided rivers in theupper Molteno Formation (Turner, 1975; Hancox, 1998)

to meandering river systems of the Lower ElliotFormation (LEF) (Bordy et al., 2004a; b) has beenundertaken. The results of this investigation, coupledwith basin-wide mapping of the transition betweenthese units, have elucidated the dynamics of theforeland basin system during the Late Triassic. Finally, abasinal analysis of the boundary between the Moltenoand Elliot formations provides significant newlithostratigraphic data (e.g., basinal distribution andinternal architecture of presently informal litho-stratigraphic units below and above the contact), whichtogether with observations of previous authors (Turner,

SOUTH AFRICAN JOURNAL OF GEOLOGY

THE CONTACT OF THE MOLTENO AND ELLIOT FORMATIONS352

Figure 1. Geological map of the upper part of the Karoo Supergroup in the Republic of South Africa and Lesotho (modified after the

1:1000000 Geological map of RSA and Lesotho, 1984).



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1975; 1977; 1983; Christie, 1981; Eriksson, 1983; 1984;Macdonald, 1993; Hancox, 1998 for the MoltenoFormation, and Botha, 1968; Le Roux, 1974; Botha andVisser, 1980; Eriksson, 1983; 1985; Bordy et al., 2004a, bfor the Elliot Formation) can improve the stratigraphic

nomenclature of the “Stormberg” rocks of South Africa.This paper relies to a certain extent on observations ofprevious upper Karoo researchers (see above), howeverit provides a new and unique stratigraphic interpretationof the existing and newly acquired data.

Figure 2. Generalized “Stormberg” group stratigraphic relations in the main Karoo Basin. Note the four coarsening-upward, unconformity-

bound sequences consisting of progradational and aggradational units. See text for details (after Christie, 1981; Catuneanu et al., 1998;

Hancox, 1998; Catuneanu and Elango, 2001; Bordy et al., 2004a).

Table 1. Comparison of the proposed lithostratigraphic subdivisions of the strata above the Indwe Sandstone Member of the Molteno

Formation.

Depositional environment of the Molteno andElliot formationsPrevious work on the non-marine Molteno and Elliotformations has established that these two lithologicalunits developed in different fluvio-depositional andclimatic settings (e.g., Turner, 1975; 1977; 1983; Christie,1981; Eriksson, 1983; 1984; Macdonald, 1993; Hancox,1998 for the Molteno Formation, and Botha, 1968; Le Roux, 1974; Botha and Visser, 1980; Eriksson, 1983;1985; Bordy et al., 2004a; b for the Elliot Formation).Following these workers, it is considered that theMolteno Formation was deposited under seasonallywarm to humid climatic conditions, in a broad, perennialbraided system associated with braidplain areas oflimited extent. In contrast to this, the lower part of theElliot Formation developed in a perennial, moderatelymeandering fluvial system, associated with extensiveoverbank areas, under semi-arid climatic conditions,which became increasingly drier through time.

Stratigraphy of the Molteno and Elliot formationsThe Molteno Formation has previously been subdividedinto a lower Bamboesberg and Indwe Sandstonemembers, and an upper unit that has been variouslygrouped and referred to by previous authors (Table 1).Strata above the Indwe Sandstone Member weresubdivided into three members (Mayaputi Shale, GlenRosa and Transitional) by Turner (1975), four members(Mayaputi, Qiba, Tsomo and Loskop) by Christie (1981),three members (Mayaputi, Qiba and Tsomo) byMacdonald (1993), and grouped into one informal“Transitional” member by Hancox (1998) (Table 1).Although Christie’s (1981) work is stratigraphically themost reliable, Macdonald (1993) correctly points out thatthe term “Loskop” is pre-occupied, and thus groups the upper two members of Christie (1981) together asthe Tsomo Member. With the exception of the IndweSandstone Member, no other unit of the MoltenoFormation has been formally defined or accepted by theSouth African Committee for Stratigraphy (SACS, 1980).For practical reasons, the strata above the IndweSandstone Member are here grouped together andinformally referred to as the Tsqima succession - anacronym derived from the Tsomo, Qiba and Mayaputimembers of Macdonald (1993). It should be stressed,however, that formally designated members shouldreplace this informal nomenclature once sufficientstratigraphic work has been undertaken on the upperMolteno Formation.

Although Turner (1975; 1978; 1983) makes variousmention as to the nature of the Molteno-Elliot contact, todate, work undertaken by Christie (1981) in the areasouth of the town of Elliot in the Eastern Cape (Figure1) is the only detailed study focusing on the stratigraphyof the upper part of the Molteno Formation. Based onChristie’s (1981) localized survey the upper part of theMolteno Formation contains at least ten upward-findingunits, eight of which occur in his Loskop member.Turner (1977; 1978; 1983) is vague about the exact

number of cyclothems in this upper part of the MoltenoFormation, but his figures indicate that the lateral extentof the cyclothems in this upper part gradually decreasewith time. In the northern part of the basin only thecyclothem represented by the Indwe Sandstone Memberis considered to be present (Turner, 1975; 1977; 1978;1983), however in two instances the same author clearlydocuments more than one sandstone in the northernpart of the basin (Turner, 1975 in attached Folders 1, 9;1986).

The Elliot Formation has never been subdivided intoformal lithostratigraphic units, however recent studies byBordy et al. (2004b) have shown that a regionalsubdivision of the Formation is possible, since the lowerand upper parts show considerable differences in theirdiagnostic lithological features, and as such may infuture be formally designated.

Field aspects of the boundary between the Moltenoand Elliot formationsOur recent fieldwork, which has focused on the natureof the boundary between the Molteno and Elliotformations, has shown that throughout the basin thecontact is abrupt. Neither the fossil assemblages nor the sedimentological record of these two very distinctformations indicate interfingering. Instead, theformations appear to be separated by a sequenceboundary probably of second-order (see Catuneanu et al., 1998, p. 430-431 for the nomenclature of the orderof stratigraphic sequences in the main Karoo Basin).Distinction between the Molteno and Elliot formations,however, becomes problematic in the sense that thecontact is disconformable since both formations arecharacterized by flat lying strata (cf., Lucas and Hancox,2001). In addition to the lack of an explicit angulardiscordance, the contact is sometimes obscured byvegetation and/or scree. It is therefore not surprisingthat there are large discrepancies in the literatureregarding the thicknesses of these two formations (e.g., Botha, 1968; Johnson, 1976 and references therein;Le Roux, 1974; Turner, 1975; Christie, 1981 andreferences therein; Eriksson, 1984; Macdonald, 1993;Hancox, 1998). In most places where the outcrops arepoor or absent, the boundary can be recognised by achange in geomorphic expression, especially where it isfound at the base of the slopes which expose thesedimentary rocks of the lower part of the ElliotFormation. Nevertheless, it has to be emphasized thatwherever the contact is exposed, the Molteno and Elliotformations show no evidence of inter-tonguing or agradational nature.

Methodology of Boundary IdentificationIn the upper part of the Karoo Supergroup, formationboundaries have previously been based oncombinations of changes in physical properties (e.g., colour, grain-size, sandstone/mudstone ratio) aswell as fossil content of the rock units across theboundary. Turner (1975) and Christie (1981) suggested





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that various criteria (e.g., colour, grain-size,sandstone/mudstone ratio, absence of coal seams,carbonaceous or calcareous mudstones, horizontal andvertical persistence of red mudstones, absence ofDicroidium flora and the abundance of dinosaurianfossils) are effective in determining the Molteno-Elliotboundary. Some of these criteria contradict theguidelines of the International Stratigraphic Guide(Hedberg, 1976), which states that boundaries oflithostratigraphic units must be based on changes inlithic properties only and not on age stipulations,interpretations of depositional environments, orcontained fossil assemblages. Apart from the fact thatpalaeontological evidence may not be used indetermining lithologic identities, the presence ofdinosaur trackways in the Molteno Formation (Raath etal., 1990) indicates that the faunistic criteria of Turner(1975) is not practical, even though to date, vertebratebody fossils are not known in the Molteno strata. In addition, the relative abundance of quartzite pebblesin the Elliot Formation (Bordy et al., 2004c) refutes thelithological assumption that quartzite pebbles areexclusive to Molteno Formation strata only (Christie,

1981, p. 14). Furthermore, as Le Roux (1974) and Turner(1975, p. 27) pointed out the differences in grain sizeand colouring of the two units are not always relevantin establishing the contact because red colouredsedimentary rocks are found not only in the Elliot, butalso in the Molteno Formation, and coarse grained,glittering sandstones similar to those of the MoltenoFormation also occur in the lower part of the ElliotFormation.

Recent developments in alluvial facies architectureanalysis and nonmarine sequence stratigraphy haveestablished new methods in determining sequenceboundaries. Following Miall’s (2001, p. 972, Table 1)suggestions, cryptic sequence boundaries may bedetermined by the identification of the followingcharacteristics of the sequences above and below aboundary:

“Facies characteristicsLaterally amalgamated channel-sandstone/conglomerate bodiesProminent, laterally extensive erosion surfacesSteep cut-and-fill relief, indication early lithification

Table 2. Generalized lithofacies types in the Molteno (A) and

Elliot (B) formations in the main Karoo Basin based on own

observations only (e.g., proper coal seams were not observed).

Outsized quartzite pebbles occur in association with the *-marked

lithofacies type in both formations.

Quartz pebble conglomerates: massive, matrix-supported

(Gmm) and massive, clast-supported (Gcm) more common –

horizontally stratified (Gh); planar (Gp) and trough (Gt) cross-

bedded types very rare

Granule- to boulder-grade, subangular to well-rounded clasts:

quartz pebbles>> feldspar>> quartzite >>> mudstone

Common (Gmm >> Gcm)

Mainly lag accumulations or less frequently longitudinal gravel bars

Mud-pebble conglomerates: massive, matrix-supported

(Gmmm)

Granule- to pebble-grade clasts: mudstone >>> >> quartz pebbles,

cobbles >> plant fragments, sandstones

Rare

Gravel bars from eroded floodplain fines and/or in-channel fines

(abandoned watercourses)

Trough cross-bedded sandstone (St*)

Medium to very coarse-grained sand

Very common

Sinuous-crested (3D) or linguoid dunes

Planar cross-bedded sandstone (Sp*)


Very common

Straight-crested (2D) or linguoid dunes

Massive sandstone (Sm*)


Rare

Deposition from hyperconcentrated flows or destruction of

primary structures by bioturbation, fluidization, etc.

Horizontally laminated sandstone (Sh)

Often associated with either planar cross-bedded sandstones or

small-scale trough and ripple cross-laminated sandstones

Medium to fine-grained

Very rare

Plane-bed (either upper or lower flow regime)

Ripple cross-laminated sandstone (Sr)

Fine grained

Very rare

Ripples (lower flow regime)

Massive mudstones (Fsm)

Predominantly green, khaki, very rare red, dark grey

Silt and clay grain-size classes

Associated carbonaceous or coaly intercalations very rare

Common

Overbank and/or waning flow deposits in standing waterbodies or

abandoned watercourses

Horizontally laminated mudstones (Fl)

Predominantly green, khaki, very rare red, dark grey

Silt and clay grain-size classes

Associated carbonaceous or coaly intercalations very rare

Rare

Overbank and/or waning flow deposits in standing waterbodies or

abandoned watercourses

Unusual abundance of plant fragments, bonefragments or other lags on sequence boundaryPervasive oxidation.

Changes in sequence architecture at the sequenceboundary:

Changes in isopach patternsMajor changes in palaeocurrent dispersal directions.

Petrographic characteristics:Pedogenic indicatorsSharp changes in detrital compositionOpen-grain framework below the sequenceboundaryVadose cements below the sequence boundary”

Cryptic sequence boundaries may thus be identified bystratigraphic changes in fluvial architectural styles(including the geometries of the sandstone units andcontained lithofacies associations), the presence/absence of organic matter (e.g., coal seams) andpaleosols, changes in regional thickness, palaeocurrentpatterns, sandstone provenance and grain-size (Eberth,2002). In order to assess the above characteristics of theMolteno and Elliot formations both outcrop- andmicroscopic-scale investigations were undertaken.It should however be emphasized that the most effectivemethod in identifying the contact between the Moltenoand Elliot formations was outcrop-scale differences in



Figure 4. The boundary of the Molteno and Elliot formations is

clearly demarcated by the distinctive sandstone geometries in the

uppermost Molteno and lowermost Elliot formations. Note that

the tabular, sheet-like sandstones of the Molteno Formation are

directly overlain by laterally accreted Elliot sandstones.

The thickness of Elliot Formation is limited in this section (note

Clarens Formation cliffs at top of the picture) (Caledonspoort,

South Africa). Car for scale.

Figure 3. Sandstone geometries in the uppermost Molteno

(A) and lowermost Elliot (B) formations. Note the sharp, sub-

horizontal external bounding surfaces and rare channel margins in

the Molteno Formation sandstones (A - person for scale) (west of

Elliot). Steep cutbanks are well-developed in the lower part of the

Elliot Formation (B - person for scale) (east of Elliot).



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the architecture of the sandstones across the boundary.Palaeocurrent and petrographic differences proved to beof lesser importance.

Sedimentological differences between the upperMolteno and lower Elliot formationsSedimentological characteristics of the uppermost part ofthe Molteno and lower part of the Elliot formations wereexamined throughout the outcrop extent of the mainKaroo Basin.

Sandstone body geometriesThe lithofacies types of the upper Molteno Formation inthe Elliot area are described in detail in Christie (1981),whereas those present in the lower part of the ElliotFormation are presented in Bordy et al. (2004b). The lithofacies of the two formations are listed in Table 2. Examples of the different sandbody geometriespresent in the two formations are illustrated in theaccompanying photographs (Figures 3 to 6). Fining-upward sandstone units typical of fluvial depositsdominate both successions, and a distinctive change inthe channel sandstone geometry across their junction isapparent throughout the basin. The sandstones in theupper Molteno Formation are laterally extensive(average >150 m), sheet-like, multi-storey bodies with

sub-horizontal, sharp external erosion surfaces, and highwidth to depth ratios (Figure 3a). The channel margins(e.g., cutbacks) of the major sand bodies were notobserved, and the margins of smaller sandstones wererarely encountered (Figure 3a). Within the Moltenosandstones, the internal erosion surfaces are mostly sub-horizontal (Figures 3a; 4; 5a and 5b) and very rarelyconvex-up. In the latter case, the internal accretionsurfaces dip in the same direction as the cross-bedsfound within them. Within the major sandstones, andespecially in the upper parts of sandstone bodies, lens-shaped minor channel fills (maximum 5 m wide,maximum 1.5 m thick) occur locally (Figures 3a and 6).The upper bounding surfaces of the major sandstonebodies are well-defined (i.e., mostly abrupt transitions),gradational contacts with the overlying mudstone unitsbeing absent. Poor quality exposures in the fine-grainedassociations of the upper part of the Molteno Formationshow mostly massive and less commonly finelylaminated mudstones. These fine-grained rocks arelocally interbedded with highly carbonaceousmudstones, but coal seams were not observed duringthis study. Other lithofacies types (e.g., interbeddedsandstone layers, pedogenic alteration features) werealso not encountered.

In contrast the sandstones of the lower part of theElliot Formation (LEF) are laterally non-persistent (max.100 to 150 m), and the deeply incised basal boundingsurfaces, steep cutbanks, and generally inclined internalerosion surfaces result in asymmetrical channel-fills(Figures 3b; 4; 5d and 5e). The dip direction of thecontained accretion surfaces and major inclined internalerosion surfaces are usually perpendicular to the dipdirection of the cross-bedded strata found within them.The upper junction of the sandstone bodies is horizontaland fairly sharp, rarely showing gradational contactswith the overlying mudstone units. The generally 20-30 m thick, fine-grained units in the lower part of theElliot Formation are mainly massive and very rarelyhorizontally laminated mudstones, with limitedpedogenic overprint (e.g., irregular mottles, fewdesiccation cracks, rare calcareous glaebules).

Figure 5. Tabular sandstones (A, B) and laterally accreted

sandstone and mudstone packages (pointbars)(C, D) in the

uppermost Molteno and lowermost Elliot formations, respectively

(A, D -Kramberg, west of Aliwal North, hammer in white circle for

scale; B - Caledonspoort, car in white circle for scale; C - west of

Elliot, person for scale).

Figure 6. Small, channel-shaped sandstones are very rare features

in the uppermost Molteno Formation (Eestegeluk farm, south of

Bethlehem). Tree (ca. 2 m) on top of the cliff for scale.

Intercalated with the mudstones, a variety of small-scalesandstone units have also been described (Bordy et al.,2004b).

At most localities in the southern pat of the basin,there is a marked grain-size difference between thesandstones of the Molteno and Elliot formations, withthose of the Molteno generally being coarser. At somelocalities (e.g., the farm Rietfontein near Jamestown) thelowermost sandstones of the Elliot Formation are ascoarse, and manifest the same glittering nature (due tosecondary quartz overgrowths) as the underlyingMolteno sandstones. North of Lady Grey, grain-size

differences between the Molteno and Elliot formations(Molteno very coarse to coarse; lower Elliotpredominantly medium to fine) are consistent enough torely mainly on this difference to identify the boundarybetween the two formations.

Number of cyclothems in the upper MoltenoFormationAs documented by Christie (1981), the sandstones in theupper part of the Molteno Formation are commonlylithologically indistinguishable from those of the IndweSandstone Member. Throughout the basin these



Figure 7. The number of upward-fining sandstone units in the upper part of the Molteno Formation decreases from south to north in the

basin (A)- Loskop, west of Elliot; B) - Liebe, west of Molteno; (C) - Avilion, south of Molteno; (D) - Thaba Nkulu, west of Lady Grey;

(E) -Koesberg, north of Zastron). Numbers represent the upward-fining cycles above the distinctive Indwe Sandstone Member. See text for

details.



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sandstones are yellowish brown, very coarse to mediumgrained, and have a characteristic glittering appearance.Based on regional mapping of the upper part of theMolteno Formation, the number of the fining-upwardcyclothems above the Indwe Sandstone Member varies

throughout the study area. The highest number ofindividual cyclothems (ten) was encountered in theElliot area (Figure 7a), and this is in agreement with the work of Christie (1981). In the southwestern part ofthe study area, the number of individual cyclothems was

Figure 8. Throughout the northern part of the study area (i.e., from the regions north of Zastron) the total number of the sandstones in

the Molteno Formation is two (A) - Constantia, northeast of Clocolan; (B) - Juliana, northeast of Bethlehem; (C, D) - Langberg, Sophiasrust,

east of Bethlehem.

more difficult to determine, due to stratigraphiccomplexities caused by dolerite intrusions, however the minimum number was estimated to vary from six to nine (near Molteno, on the farms Liebe andNorwood respectively) (Figures 7b and c). In a northerly direction, the number of individual cyclothemsgradually decreases, and west of Lady Grey (ThabaNkulu) (Figure 7d) the number is reduced to six. Further north, in the vicinity of Zastron (Koesberg)(Figure 7e) only four cyclothems are present. North ofZastron, the Molteno Formation comprises only two sandstones (Figures 8a to d) at all localities, as was briefly documented by Turner (1975, in attached Folders 1, 9; 1986) and Hancox (1998 p. 207and 233).

The additional Molteno sandstone reported here hasvirtually identical lithologic properties (e.g., grain-size,colour, facies architecture) to the Indwe SandstoneMember, and thus it is difficult, if not impossible, toestablish beyond reasonable doubt whether they: (1) both form part of the same Indwe Sandstone

Member;(2) both form part of the Tsqima succession or(3) that the lower sandstone represents the Indwe

Sandstone Member, while the upper one representsthe Tsqima succession of the Molteno Formation.

Apart from being the most plausible basin model (Figure 2), the only indication that the last scenariomight be the most applicable is the slight difference inthe sandstone composition of the samples taken fromthe Indwe Sandstone member and Tsqima succession inthe south. The Indwe Sandstone samples have higherquartz content (Hancox, 1998) in comparison with thoseof the Tsqima, which tend to have a higher proportionof lithic fragments. None of the upward-finding unitsabove the Indwe Sandstone Member can be associatedwith any outstanding, outcrop-scale characteristics thatwould aid intra-basinal correlation; however there is apattern in the thickness distribution of the upper threeunits, which emerges on a regional scale. The thirdsandstone below the first Elliot Formation sandstone isunusually thick (up to 25 m) at the following localities:Elliot (Loskop), Molteno (Liebe, Norwood, Avilion),Lady Grey (Thaba Nkulu) and Zastron (Koesberg)(Figures 7a to e). Because of the lack of other identifyingoutcrop-scale features, it cannot presently bedetermined with absolute certainty which cyclothems ofthe upper Molteno Formation diminish in their lateralextent and which one are present in the northern part ofthe basin.

Palaeocurrent changesPalaeocurrent measurements in the strata below andabove the contact of the two formations have beenpresented by previous researchers (e.g., Turner, 1975;1977; Christie, 1981; Eriksson, 1984; Bordy et al., 2004c).Sediment supply in the upper Molteno Formation wasmainly from south-southeast to north-northwest (343o)in the southeast (Christie, 1981) as well as in the rest ofthe basin. In the northern part of the basin, an additionaleast to west trend has been documented (Turner 1975;1977; Eriksson, 1984), but it is not clear if thesedirections were obtained from the Indwe SandstoneMember or the overlying upper Molteno sandstone. In the lower part of the Elliot Formation, the palaeo-slopes in the southern part of the study area weredirected from south-southeast to north-northwest (350o),whereas in the northern part mainly from southwest tonortheast (37o) (Bordy et al., 2004c). In the northern partof the basin, where poor quality outcrops do not alwaysallow straightforward identification of the sandstonebody geometries, differences in palaeocurrent directionsand grain-sizes are the most valuable tools in thedetermination of the contact.

Petrographic characteristicsIn order to gain a better understanding of the lithologicchanges that occur between the Molteno and Elliotformations, petrographic studies were undertaken onsample pairs (n=38) collected across the boundarybetween the two formations. Thin-section descriptionswere based on quantitative observations (i.e., 300 pointcounts per thin-section for mineral percentages), and theapplied Gazzi-Dickinson point-counting method helpedto alleviate the grain-size difference problem (see



Figure 9. Ternary diagram of the mineral composition of arenites

in the uppermost Molteno, and lowermost Elliot formations.

Qm:FP:Lt (monocrystalline quartz : feldspar : total rock fragments

including all microcrystalline quartz aggregates). Classification

after Pettijohn et al. (1987) and point-counting methods based on

Ingersoll et al. (1984) and Dickinson (1985). All samples were

taken across the boundary of the Molteno and Elliot formations

from sandstones immediately adjacent, below and above the

contact, respectively. They are hardly visible. See text for details.



361

Ingersoll et al., 1984 and Dickinson, 1985) between themedium to coarse-grained samples of the MoltenoFormation, and the mostly medium-grained sandstonessamples of the Elliot Formation. The relative abundanceof the different polycrystalline quartz types wasrecorded separately because this information isimportant in reconstructing possible provenance types(e.g., Basu et al., 1975; Basu, 1985).

All the samples from the uppermost Molteno andlowermost Elliot formations are highly quartzose (grandmean Qm81.61 FP7.36 Lt10.91 and Qm83.13 FP8.02Lt8.73, respectively – see Figure 9 for abbreviations andplots) ranging from mainly sub-litharenites to quartzarenites and sub-arkoses (sensu Pettijohn et al., 1987). 9 shows that based on the abundance of the mainframework minerals (i.e., monocrystalline quartz,feldspar and all lithic fragments), the petrographicdifference between the uppermost Molteno andlowermost Elliot Formation sandstones is negligible, asthe two sample populations strongly overlap, and thegrand averages of both sample populations are sub-litharenites. In both sample populations, most of thepolycrystalline quartz grains comprise less than four,irregular, non-equigranular crystals. Polycrystallinequartz grains that contain more than four crystals arerare, and they are mainly grains with irregular crystals ofsemi-equal sizes and non-sutured crystal boundaries, orless frequently, irregular, subequant and rarely slightlyelongated crystals with mainly sutured contacts. In bothsample populations, the proportion of polycrystallinequartz grains is far less than the monocrystalline ones.

A notable difference between the two samplepopulations is the relatively higher feldspar content inthe Elliot Formation samples (Figure 10). A sightdeparture from this general compositional trend isevident in those samples from the uppermost MoltenoFormation, which are very coarse grained. In addition tohaving increased feldspar content, the Elliot Formation

samples analyzed also have a higher content of heavyminerals (e.g., zircon, rutile, opaques) and themonocrystalline quartz grains of the uppermost Moltenomore often enclose small vacuoles, zircon and rutilegrains, as well as worm-like mica inclusions. Lithicfragment types in both sample populations arepredominantly chert, and on a few occasionsmetamorphic rock fragments. In spite of the generallycoarser grain-sizes of the Molteno Formation, on averagethe samples from the Elliot Formation contained ahigher proportion of subrounded to roundedmonocrystalline quartz grains.

The very similar modal composition of sandstones ofthe uppermost Molteno and lowermost Elliot formations,and the fact that almost all the studied samples fallwithin the recycled-orogen provenance field (sensuDickinson, 1985) show that both sample populationswere derived from similar source rocks types. The presence in the source areas of a mix of all threemajor rocks types (i.e., acid igneous, metamorphic andsedimentary) is evidenced by the various polycrystallinequartz types (derived from both igneous andmetamorphic suites), inclusions in monocrystallinequartz grains (again suggesting both igneous andmetamorphic suites), and rock fragment types ofmetamorphic and sedimentary origin (Blatt et al., 1972;Basu et al., 1975; Basu, 1985). The relatively higherfeldspar content of the Elliot samples may imply morearid climatic conditions during Elliot times, whichinhibited the chemical weathering of these sensitiveminerals. This tendency is also supported by the higherauthigenic clay content of the Molteno sandstones(Hancox, 1998). The higher proportion of subrounded torounded monocrystalline quartz grains in the Elliotsamples suggest a higher degree of sediment recyclingduring Elliot sedimentation.

DiscussionStacking patterns in the “Stormberg” groupOur results, and that of others (e.g., Hancox, 1998, p.318-319), indicate that the “Stormberg” group ischaracterized by a repetitive progradational stackingpattern. For instance, the geometry of the Bamboesbergmember of the Molteno Formation is a northwardtapering wedge (with its northernmost occurrence is inthe Zastron region), while that of the Indwe SandstoneMember is extensive, sheet-like, and traceablethroughout the basin (Hancox, 1998) (Figure 2).Our regional mapping showed that in the northern partof the basin, in addition to the Indwe Sandstone, thereis another Molteno sandstone unit. Although we cannotcorrelate it to any particular sandstone unit in the southof the basin, we prefer to explain this second sandstonein the northern reaches of the basin as part of theuppermost Tsqima succession of the Molteno Formation,and account for its genesis in terms of foreland basindevelopment. As for the scenario postulated for theBamboesberg and Indwe Sandstone members (Hancox,1998), the Tsqima succession was deposited after a

Figure 10. Chart showing the slight differences in the feldspar

content of the Molteno and Elliot formations samples. All samples

were taken across the boundary of the Molteno and Elliot

formations from sandstones immediately adjacent, below and

above the contact, respectively.

minor orogenic loading stage, which generated a regional second-order unconformity at the top of theIndwe Sandstone Member. Following this minororogenic loading event, renewed subsidence occurred inthe foresag, and the palaeoslope, connecting thedepocentre with the sediment source (CFB), becameprogressively steeper with time, providing enoughenergy for the sediment transport processes to transfermaterials into the northern part of the basin as well.According to Bordy et al. (2004a), a similarprogradational-aggradational pattern emerges twice inthe Elliot and Clarens formations (one in the Lower ElliotFormation and another one in the Upper Elliot andClarens formations) (Figure 2).

In the light of the above, the following units of“Stormberg” strata can be regarded as individual,unconformity-bounded sequences (i.e., stratigraphicunits comprising a relatively conformable succession ofgenetically related rocks bounded by regionallysignificant unconformities): (1) the Bamboesberg andIndwe Sandstone members of the Molteno Formation,(2) the Tsqima succession (i.e., the informal Tsomo,Qiba and Mayaputi members of Macdonald, 1993) of theupper Molteno Formation, (3) the Lower ElliotFormation (LEF) and (4) the Upper (UEF) and Clarensformations (Figure 2). The presence of a first-ordersequence boundary (major unconformity) at the base ofthe Molteno Formation has been demonstrated byHancox (1998) and documented by Catuneanu et al.(1998). Moreover, the presence of the second-ordersequence boundaries at the base of the Tsqimasuccession and between the Lower and Upper Elliotformations, have been documented by Hancox (1998 p.319) and Bordy et al. (2004a), respectively. These foursequences reflect the pulsating tectonic activity of theCFB, and are interpreted as sedimentary packages thatformed during periods of tectonic quiescence of the CFB(Catuneanu et al., 1998; Bordy et al., 2004a).

Relative subsidence rates in the “Stormberg” groupThe coal zones and the carbonaceous mudstonespresent in parts of the Molteno Formation (e.g., Bamboesberg and lower Tsqima) are evidence ofhigh water tables and low-gradient conditions (cf. Bohacs and Suter, 1997), and in contrast, theoverlying basin-wide units of coarser grained,amalgamated sandstones with virtually non-existentchannel margins of the Indwe Sandstone member andupper Tsqima succession which show periods of highersubsidence rates (i.e., steeper slopes thus coarsermaterial) that were outpaced by even higher sedimentsupply rates (i.e., accommodation rapidly consumed bythe overwhelming clastic input). The isolated coal seamsof the Molteno Formation are the thickest (ca. 3 to 4 m- Ryan, 1963; Christie, 1981) in the Bamboesbergmember and become gradually thinner in the lowerTsqima succession (ca. 0.5 to less than 2 m, Ryan, 1963;Christie, 1981). These may be taken as evidence for anincreasing rate of accommodation space creation, which

most likely was governed by subsidence resulting fromthe combined effects of first and second order tectonicunloading. This gradually intensified clastic inputprobably eventuated in the dilution of organic matterand unfavorable conditions for mire preservation (cf. Bohacs and Suter, 1997: Table 3). The succeedingLower Elliot Formation (LEF) is characterized byasymmetrical lens-like, laterally non-persistent sandstoneunits that are enveloped in overbank fines with rarepedogenic alterations. Similar to all underlying“Stormberg” strata, the LEF sandstones show aprogressive northward fining trend from medium tocoarse sandstones in the south to fine to medium in thenorth, but vertical grain-size variations are not evident.In contrast to the LEF, the Upper Elliot Formation (UEF)sandstones are sheet-like, laterally persistent andinterfinger with overbank fines that show a highabundance of pedogenic features (Bordy et al., 2004b).In its upper parts, the UEF sandstones are moreamalgamated and coarser grained, thus the whole unittogether with the Clarens Formation are taken as onecoarsening-upward sequence (Bordy et al., 2004b). Thegeometrical differences of the Elliot Formation can beexplained by moderate to high subsidence rates in theLEF, and low to gradually accelerating subsidence ratesin the UEF (Bordy et al., 2004a).

The influence of the Kaapvaal Craton margin onthe “Stormberg” group sedimentationThe entire outcrop area of the “Stormberg” is situatednorth of the northern portion of the Southern CapeConductive Belt (part of the Natal-Namaqua Mobile Belt)and southern part of the Kaapvaal Craton. Based on thebasinal extent of the different lithostratigraphic units ofthe “Stormberg” strata, the southern margin of theKaapvaal Craton (Figure 1) seems to play an importantrole in the south-north distribution of the sedimentsderived from the Cape Fold Belt. The Bamboesbergmember (Hancox, 1998), and lower part of Tsqimasuccession (this study) do not extend north of thistectonic boundary, and the Lower and Upper Elliotformations (Bordy et al., 2004a) manifest a significantthinning in the same region. In addition to the thicknessdecrease, palaeocurrent and thickness measurements inthe Elliot Formation (Bordy et al., 2004a; c) in the regionimmediately south of this boundary are anomalous. To explain the geometry of the Bamboesberg Member,Hancox (1998, p. 318) suggested that lithosphericheterogeneity between the Kaapvaal Craton and thelithosphere underplating the foresag created atopographic high against which the Bamboesbergsystem onlapped, but did not overtop. To explain thethickness changes and anomalous palaeocurrents for theElliot Formation, Bordy et al. (2004a) also postulatedelevated and irregular basin floor topography, offset tothe south of, and paralleling, the margin of the KaapvaalCraton. It thus seems that the rheological differencesbetween the older (~2.7 Ga) Kaapvaal Craton andyounger (~1.1 Ga) (Schmitz and Rooyani, 1987), more





363

viscoelastic Natal-Namaqua Mobile Belt played animportant role in the spatial distribution of varioussequences in the “Stormberg” group of the main KarooBasin, with flexural tectonics creating basin floortopography in this structural zone.

SummaryBased on modern, continental sequence stratigraphicmethods, rigorous field investigation has resulted informulation of guidelines that identify the Molteno-Elliotcontact on a regional scale. Marked differences arepresent in both the internal and external geometries ofthe major sandstone units above and below the contactof the Molteno and Elliot formations. That the twoformations are separate lithostratigraphic units iscorroborated by changes in palaeocurrent directions andpetrographic characteristics at this point showing thatthe boundary between the Molteno and Elliot formationsis a regional unconformity. The recognition of thiscryptic second-order sequence boundary thusinvalidates previous interpretations of an interfingeringstratigraphic relationship between the two units.

The Molteno, Elliot and Clarens formations form adistinct tectono-sedimentary unit. Currently, the“Stormberg” strata may be defined as the succession ofthose formations in the upper part of the KarooSupergroup that occur above the regional unconformity(Hancox, 1998) at the base of the Molteno Formation (or its correlatives throughout the satellite Karoo basins)and below the base of laterally continuous continentalflood basalts of the Drakensberg Group. In the mainKaroo Basin, their sedimentation seems to have beengreatly influenced by the southern margin of theKaapvaal Craton. We suggest that with a view tocorrelation of the stratigraphic units in the upper part ofthe Karoo Supergroup, the usage of the stratigraphicterm “Stormberg Group” should be reinstated, as suchreintroduction would aid in the simplification of thestratigraphic classification in the main Karoo and itssatellite basins.

AcknowledgementsThe manuscript was prepared while EMB was a NationalResearch Fund (South Africa) post-doctoral researchfellow at the School of Geosciences, the University ofthe Witwatersrand and the National ResearchFoundation funded University of the Witwatersrand, andfieldwork. EMB would like to thank her husband,Mamadou Diop, for his enthusiastic field assistance andcompanionship. We thank the reviewers B.Turner and S.Flint for their useful comments that whichenhanced the paper.

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