Organic-walled microfossils and biostratigraphy of the upper Port Nolloth Group (Namibia): implications for latest Neoproterozoic glaciations

Geol. Mag. 142 (5 ), 2005, pp. 539–559. c© 2005 Cambridge University Press 539doi:10.1017/S0016756805001123 Printed in the United Kingdom

Organic-walled microfossils and biostratigraphy of the upperPort Nolloth Group (Namibia): implications for latest

Neoproterozoic glaciations

CLAUDIO GAUCHER*, HARTWIG E. FRIMMEL† & GERARD J. B. GERMS‡*Departamento de Geologıa, Facultad de Ciencias, Igua 4225, 11400 Montevideo, Uruguay

†Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa‡Department of Geology, Rand Afrikaans University, Auckland Park 2092, South Africa

(Received 13 January 2004; revised version received 27 April 2005; accepted 22 June 2005)

Abstract – The occurrence of organic-walled microfossils is reported for the first time from theNeoproterozoic Port Nolloth Group, Gariep Belt (southern Namibia). Acritarchs assigned to Bavlinellafaveolata occur in the Hilda Subgroup below the younger of two glaciogenic diamictite units (NumeesFormation) within the Port Nolloth Group. The microfossil assemblage in the overlying upper HolgatFormation, above the Numees Formation, is characterized by low diversity (six species), dominanceof Soldadophycus bossii, and absence of acanthomorphic or large sphaeromorphic acritarchs. Theagglutinated foraminifer Titanotheca also occurs in the Holgat Formation. Combined with availablechemostratigraphic data and Pb–Pb ages, this microfossil assemblage indicates an upper Ediacaranage of around 555 Ma for the Holgat Formation. Virtually identical microfossil assemblages, negative-to-positive δ13C trends, 87Sr/86Sr values between 0.7080 and 0.7085, as well as Pb–Pb carbonate ages,make it possible to correlate the Holgat Formation with the Buschmannsklippe Formation (WitvleiGroup, central Namibia), the Kombuis Member (Cango Caves Group, southern South Africa) and theuppermost Polanco to lowermost Cerro Espuelitas Formation (Arroyo del Soldado Group, Uruguay).Based on these data, the underlying Numees Formation, the age of which has been only looselyconstrained so far and subject to considerable debate, can now be assigned to the c. 580 Ma Gaskiers orthe possibly younger (< 570 Ma) Moelv glacial event. The Numees glacial event may be representedin the uppermost Nooitgedagt Member (Cango Caves Group, South Africa) and the lower BarrigaNegra formations (Arroyo del Soldado Group, Uruguay), characterized by a negative δ13C excursionand a strong sea-level drop. If this correlation is confirmed, lack of glacial deposits there might haveimplications for the palaeogeographic extent of upper Ediacaran glaciations. Our preliminary studiesshow that acritarch biostratigraphy can make a significant contribution to unravelling the stratigraphyof Neoproterozoic glacial deposits, especially when combined with C and Sr isotopic data.

Keywords: Neoproterozoic, Ediacaran, glaciation, acritarchs, Namibia.

1. Introduction

The Gariep Supergroup, exposed in the Pan-AfricanGariep Belt in southwestern Namibia and westernmostSouth Africa, represents the fill of the NeoproterozoicGariep Basin in SW Gondwana (Fig. 1). It serves as aprime example of one of the major conundrums ofNeoproterozoic stratigraphy worldwide, namely thenumber, timing and intercontinental correlation ofNeoproterozoic glacial events.

Similar to many other Neoproterozoic successions(Fig. 2) in southern Africa (e.g. Damara Belt, theLufilian Arc and the West Congolian Belt) andelsewhere (e.g. the Dalradian in Scotland and theHuqf Supergroup in Oman: Brasier & Shields, 2000;Brasier et al. 2000), the Gariep Supergroup contains

* Author for correspondence: [email protected]

two diamictite units for which a glaciogenic origin isindicated, namely the Kaigas and Numees formations(Frimmel, Folling & Eriksson, 2002). Both of theseare overlain by distinct cap carbonate sequences thatillustrate the apparent change from global icehouse togreenhouse conditions (Frimmel, Folling & Eriksson,2002). The older cap carbonate is typically rich inorganic carbon and medium to dark grey, whereas theyounger cap carbonate is light grey, cream or pale pinkin colour and poor in organic matter. This intercalationof glaciogenic and warm water deposits, some of whichhave been laid down at low latitudes, forms the basisof extreme climate models, with the ‘snowball Earth’model (Hoffman, Kaufman & Halverson, 1998b) pro-bably the most popular one at present.

As the assessment of the applicability of any of theseglobal palaeoclimate models (or the establishment ofalternative new models) is critically dependent onthe interbasinal correlation of the climate-sensitive

540 C. GAUCHER, H. E. FRIMMEL & G. J. B. GERMS

Figure 1. Pan-African/Brasiliano orogenic belts and Vendian–Early Cambrian basins in southwestern Gondwana (modified afterGresse et al. 1996 and Gaucher & Germs, 2003).

lithostratigraphic units, a considerable effort by a num-ber of research groups has gone into the regional andintercontinental correlation of Neoproterozoic glacio-genic units. Most of that correlation has been carriedout based on the isotopic composition of marine car-bonate, which is taken as a contemporaneous seawaterproxy. In particular, C isotopes play an important role inthese chemostratigraphic studies. Stratigraphic correla-tion across depositional basins remains hampered, how-ever, mainly for two reasons: (1) the use of C isotopesfor chemostratigraphic correlation is limited, becausethe relative short residence time of C in seawater

determines a strong dependence on local environmentalconditions (e.g. palaeobathymetry: Frimmel & Folling,2004); and (2) a reliable geochronological control islacking. There is a difference of opinion as to how manyglaciations occurred during the Neoproterozoic, whichis illustrated by the various attempts to correlate glob-ally up to four negative δ13C excursions (Jacobsen &Kaufman, 1999). Some authors believe that there wereat least four glaciations (Saylor et al. 1998), and someprefer three (Halverson et al. 2003), whereas otherssuggested only two glaciations (Kennedy et al. 1998).The former group ascribes the two older glaciations to

Biostratigraphy of Port Nolloth Group 541

Figure 2. Stratigraphic subdivision of the late Neoproterozoic: (A) ratified by IUGS (Knoll et al. 2004) and (B) equivalence withRussian intervals. Glacial units within selected late Neoproterozoic successions and their radiometric age constraints are shown to scale.Note that apart from the Ghubrah, Ghaub and Gaskiers formations, which have been directly dated (Brasier et al. 2000; Bowring et al.2003; Hoffmann et al. 2004), correlation of all other glacial units remains interpretative. ECAP: Ediacaran Complex AcanthomorphPalynoflora (Grey, Walter & Calver, 2003). Sources of data: (1) Frimmel, Klotzli & Siegfried (1996), Folling, Zartman & Frimmel(2000) and Grotzinger et al. (1995); (2) Hoffman et al. (1996), Hoffmann et al. (2004); (3) Ireland et al. (1998), Preiss (2000); (4) Grey &Corkeron (1998); (5) Zhou et al. (2004); (6) Brasier et al. (2000); (7) Myrow & Kaufman (1999 and references therein), Bowringet al. (2003); and (8) Vidal & Nystuen (1990).

an older (c. 750 Ma) and a younger (c. 720 Ma) Sturtianice age (Fig. 2), whereas the two younger glacialevents are considered to reflect the c. 635 Ma Marinoanand the c. 560 Ma Moelv glaciations (Brasier &Shields, 2000). Even for type sections, such as thosethrough the Varangerian deposits in northern Norway,no agreement exists regarding the correlation of a givendiamictite bed with the Sturtian, Marinoan or Moelvglaciation (compare Brasier & Shields, 2000; Kennedyet al. 1998; Prave, 1999; Halverson et al. 2003; Fig. 2).Given these uncertainties, the terms ‘Sturtian’ and‘Marinoan’ are here used to mean the Sturt Tillite ofthe Adelaide Rift Complex and the Elatina Formationof Australia, respectively (Fig. 2).

A number of Neoproterozoic glacigenic units havebeen unambiguously dated by different methods aspost-Marinoan, suggesting the occurrence of at leastthree glacial epochs in the Neoproterozoic (Fig. 2). TheGaskiers Formation of the Conception Group (south-

eastern Newfoundland) has been dated at c. 580 Ma byU–Pb geochronology of zircons separated from ash-beds interlayered with the glaciomarine diamictites(Bowring et al. 2003). The Moelv Formation ofthe Hedmark Group (southern Norway) overlies theBiskopasen and Biri formations, which yielded acomplex acantomorph acritarch assemblage (Vidal &Nystuen, 1990) currently assigned to the Ediacaran(Knoll, 2000; Grey, Walter & Calver, 2003). Therefore,the Moelv glacial event must be younger than 570 Ma(Fig. 2), the estimated age of disappearance of thecomplex acantomorph assemblage (Grey, Walter &Calver, 2003). Finally, the glaciogenic Egan Formationof the Louisa Downs Group (Western Australia, Kim-berley region) is regarded as post-Marinoan (Grey &Corkeron, 1998), on the basis of biostratigraphiccorrelation with the Julie Formation (Amadeus Basin)and the upper Wonoka to lower Bonney Formation(Adelaide Geosyncline). Based on a U–Pb zircon date


of 556 Ma for the latter unit, an age of around 560 Mais assumed for the Egan glaciation (K. Grey, pers.comm. 2005), roughly coincident with the Moelv event(Fig. 2).

Ultimately, chemostratigraphic correlation shouldbe tested by chronostratigraphic means. The age ofthe Kaigas Formation is reasonably well constrainedbetween 741 and 751 Ma by U–Pb and Pb–Pb singlezircon data from associated felsic volcanic rocks (Borget al. 2003; Frimmel, Klotzli & Siegfried, 1996).Based on these age data (Fig. 2), as well as on chemo-stratigraphic data, a correlation of the Kaigas Forma-tion with the Sturtian glaciation has been establishedwith a fair degree of confidence (Frimmel, Folling &Eriksson, 2002). In contrast, the age of the youngerNumees Formation remains problematic due to a lackof easily dateable units therein. In the Damara Belt(central and northern Namibia), the younger of twoglaciogenic diamictite horizons has been preciselydated at 636±1 Ma with a U–Pb single zircon agefor an intercalated ash bed (Hoffmann et al. 2004),the best constraint thus far on the age of a Marinoan-correlative glaciogenic unit. However, further south, inthe Gariep Belt, the age of the younger glaciogenicunit (Numees Formation), a regionally extensive sheetof massive diamictite that is overlain by typical post-glacial cap carbonates (Holgat Formation), remainsproblematic. A syn-Marinoan age has been suggestedfrom imprecise Pb–Pb carbonate data, Ar–Ar horn-blende ages and theoretical considerations on the tec-tonic evolution (Frimmel & Folling, 2004). However,in the light of the recent revised age constraint on theMarinoan glaciation, such a correlation has becomedoubtful, and correlation with the younger Gaskiers orMoelv glaciations cannot be ruled out. Clarificationof this issue is pivotal for any future climate andgeodynamic model that aims to explain the ambiguousNeoproterozoic rock record.

In order to improve the correlation of the post-Kaigas lithostratigraphic units in the Gariep Belt,we systematically sampled carbonate, banded ironformation (BIF) and shale of the upper Port NollothGroup for the study of organic-walled microfossils. Onthe basis of our new micropalaeontological data andequivalent studies in the Saldania Belt (Gaucher &Germs, 2003), a significant improvement in theregional correlation of the younger diamictite units inthe Gariep Belt is achieved. The implications of theseresults for the understanding of global Neoproterozoicglaciations and the distribution of microfossils will bediscussed.

2. Geological setting

The Gariep Belt, which continues towards the northinto the Damara and Kaoko belts of central andnorthwestern Namibia and to the south into the SaldaniaBelt on the southern tip of South Africa, is exposed in a

number of outcrops in the southern Namib desert fromthe coast between Luderitz and Port Nolloth to up to100 km inland (Fig. 3). South of Port Nolloth, most ofthe belt strikes out to sea with only a few small outcropsalong the coastline, and then re-emerges in an erosionaloutlier around 31◦ S (Vredendal Outlier). It representsa Pan-African fold-thrust belt in which two majortectonic units are distinguished (Frimmel, 2004). In theinternal, western part of the belt, largely oceanic rocksthat lack any continental basement occur (MarmoraTerrane). The external part further east (Port NollothZone) consists of continental sedimentary successionswith subordinate volcanic rocks (Port Nolloth Group),which, though internally intensely deformed, still reston their Palaeo- to Mesoproterozoic basement. Amajor thrust fault (Schakalsberge Thrust) separatesthe allochthonous Marmora Terrane from the para-autochthonous Port Nolloth Zone. Only in the latteris a relatively complete Neoproterozoic successionexposed (Port Nolloth Group), including the twodiamictite and associated cap carbonate units, and ittherefore is the main focus of this study.

A low-grade metasedimentary succession occurs inthe area around Vanrhynsdorp (Gifberg Group), whichhas been correlated with large parts of the Port NollothGroup. The Gifberg Group is included in the GariepSupergroup, whereby the Vredendal Outlier representsthe southernmost exposures of the Gariep Belt (Fig. 3).Based on recent mapping and chemostratigraphicstudies (H. E. Frimmel, unpub. data), a correlation atformation level (Fig. 4) is proposed.

Three megasequences (M1–M3) are distinguishedwithin the Port Nolloth Group (Frimmel, Folling &Eriksson, 2002). Sequence M1 starts with alluvial fandeposits (Fig. 4) in an emerging continental rift grabenand evolves to alluvial plain and fan delta depositsin the widening rift (Stinkfontein Subgroup). A maxi-mum age of 771±6 Ma is indicated for the onset ofsedimentation from the youngest age obtained fromintrusive rocks in the pre-Gariep basement (Frimmel,Zartman & Spath, 2001). Locally, at the flanks of amajor growth fault along the eastern boundary of theGariep Basin, laterally discontinuous diamictite, inter-calated with upward-fining arkose and greywacke bedsand dolomitic olistostromes are developed (KaigasFormation). They are interpreted as representing, res-pectively, debris flow sediments, proximal to medialturbidity fan deposits, and large slump masses, laiddown adjacent to drowned rift shoulders. A glacio-marine or fluvio-glacial origin of parts of this formationis indicated. Bimodal, predominantly felsic, contin-ental, 741 to 751 Ma rift magmatism accompaniedsedimentation near the eastern rift graben margin(Rosh Pinah Formation). Based on low δ13C andlow 87Sr/86Sr ratios and chemostratigraphic similaritieswith Neoproterozoic sequences elsewhere, this glacialinfluence is ascribed to the global Sturtian glaciation(Folling & Frimmel, 2002).


Figure 3. Main tectonic units of the Gariep Belt (after Frimmel & Hartnady, 1992). D – Dreigratberg section. W – Farm Witputs Westsection.

A succession of post-glacial carbonates with inter-calated argillite, marl and minor arenite (PickelhaubeFormation, M2) reflects a change from icehouseto greenhouse conditions as indicated by a shiftfrom distinct negative to strongly positive δ13C ratios(Folling & Frimmel, 2002). This was followed by adramatic fall in sea-level, which led to re-workingof the older stratigraphy in the clastic WallekraalFormation that is characterized by immature arenite,quartz pebble conglomerate channels that cut acrosscarbonate platforms, and chaotic dolomite breccias andolisthostromes. In areas that escaped erosion, carbonatedeposition continued in the form of stromatoliticbioherms and oolites (Dabie River Formation). Thismajor regression towards the top of M2 has beenexplained by eustatic sea-level drop in advance ofa major glacial event (Frimmel, Folling & Eriksson,2002), which is recorded in the form of a massive,up to 500 m thick, blanket of glaciogenic diamictiteof the Numees Formation. The lower part of thisformation contains a thin, but distinct, intercalation ofbanded iron formation (Jakkalsberg Member, Fig. 4).This BIF is made up of alternating hematite/magnetite-rich and cherty, hematite/magnetite-poor laminae.Hematite crystals are subhedral and fairly constant in

size (diameter 20–80 µm). Magnetite is occasionallythe dominant iron oxide. Dropstones of differentcomposition occur in the Jakkalsberg Member. Thesefacts enable classification of the Jakkalsberg BIF intothe Rapitan type (Maynard, 1991).

The Numees Formation diamictite is overlain bya typical, up to 100 m thick, carbonate succession(Bloeddrif Member, Holgat Formation, M3; seeFrimmel & Folling, 2004). It starts with a flat, thinlylaminated, coarsely recrystallized limestone that re-leases a distinct smell of H2S upon breaking. Thislimestone is characterized by very high Sr concentra-tions, resulting in very low Mn/Sr, Fe/Sr and Ca/Sr,and is devoid of terrigenous material. In sections froma proximal position relative to the palaeo-shoreline,the lower parts of this limestone are dolomitized andcontain thin intercalations of quartz arenite. Depositionof aragonite mud in a generally euxinic, marine envir-onment below wave base is inferred for the BloeddrifMember based on its structure and chemistry. Only inproximal sections are found internal, vertical, tube-likestructures of infilled micritic sediment and cement.They are typically a few centimetres across and severaldecimetres high. Similar structures have been describedfrom many other Neoproterozoic post-glacial cap


Figure 4. (a) Stratigraphy of the Port Nolloth Group in the Port Nolloth Zone and correlation with the Gifberg Group in the VredendalOutlier. Note stratigraphic location of samples in the Port Nolloth Group. (b) Stratigraphic subdivision of the Port Nolloth Group inthe Port Nolloth Zone; numbers in brackets refer to maximum thickness (after Frimmel, Folling & Eriksson, 2002).

carbonates (Cloud, 1974; Hegenberger, 1993; Kennedy,1996; Hoffman, Kaufman & Halverson, 1998a). Whilesome workers interpret them as being of microbialorigin (Hegenberger, 1993; Hoffman, Kaufman &Halverson, 1998a), others explain them by gas escapefollowing the destabilization of gas hydrate duringwarming of terrestrial permafrost (Kennedy, 2001).Columnar, current-elongated, closely spaced stro-matolites 2.5 to 5 cm in diameter occur in the BloeddrifMember at the Witputs section (Figs 3, 4). The capcarbonates typically start with a negative δ13C excur-sion only to recover to values around 0 ‰ (PDB) inmost sections, except for the most proximal domainswhere a rise to distinctly positive δ13C ratios has beennoted (Frimmel & Folling, 2004).

The remainder of the Holgat Formation comprisesupward-fining cycles of medium-bedded sandstone,greywacke and arkose with minor siltstone, mudstoneand intraformational conglomerate. At Farm Witputs(Figs 3, 4), the siltstone is mainly composed of detritalmicas (muscovite, biotite, chlorite), quartz and detritalcarbonates. The apparent thickness of this flysch-likesuccession is about 7 km. Due to tectonic thickening,the true thickness is considerably less, however, andis probably some several hundred metres. In contrastto M1 and M2, the Holgat Formation, representingM3, has equivalents across the Marmora Terrane. Fromthis regional distribution, which probably also extendedfurther to the east onto the Kalahari craton, it has beenconcluded that M3 was deposited during the closure


of the Gariep Basin in a foredeep position on top andin front of the advancing thrust sheets of the Marmoraoceanic crustal fragments (Frimmel & Folling, 2004).

A first dynamic metamorphic stage is recognizedonly in the mafic rocks of the Marmora Terraneand is probably related to their accretion that beganabout 575 Ma (Frimmel & Frank, 1998). The peak ofmetamorphism, attaining lowermost amphibolite faciesconditions in the Port Nolloth Zone, was reached asa consequence of the emplacement of the MarmoraTerrane on top of the Port Nolloth Zone, which hasbeen dated at 545±2 Ma (Frimmel & Frank, 1998).Syn-orogenic sedimentation in the correspondingforeland began around 550 Ma with shallow marinedeposits of the lower Nama Group followed by c.540 Ma siliciclastic molasse sediments of the upperNama Group (Germs, 1983; Germs & Gresse, 1991).Subsequently, post-orogenic alkaline intrusive bodies,ranging from alkali granite to syenite and carbonatite,were emplaced along a NE-trending line (Kuboos–Bremen line) into the southern Gariep Belt and theadjacent Mesoproterozoic basement, with the mostreliable age so far for this magmatic pulse being507±6 Ma (Frimmel, 2000).

3. Materials and methods

Fifteen palynological macerations and ten thin-sectionsof carbonates, pelites and BIF from the Holgat,Numees (Jakkalsberg Member), upper Wallekraal andPickelhaube formations were prepared at the Micro-palaeontology Laboratory of the Facultad de Ciencias(Montevideo). Following crushing and digestion ofsamples (c. 150 g) with concentrated HCl, 72 % HFwas applied for 24 hours to the silicate/organic residues.After neutralization, boiling concentrated HCl wasused to dissolve flourides. Remaining organic residueswere recovered by means of a 5 µm sieve, stored in glassflasks and mounted with glycerin-gelatine on standardglass slides. Throughout the preparation, gravitysettling was used instead of centrifugation, to avoiddestruction of fragile, large acritarchs. Microfossilswere determined and counted under a Leica DMLP polarizing microscope, using transmitted, reflectedand combined reflected–transmitted light (in the lattercases with oil immersion objectives). This allowedobservation of opaque (carbonized) microfossils, andalso assessment of the fossil nature of the isolatedmicrostructures. This method, developed by Pflug &Reitz (1992 and references therein), makes possiblethe comparison of the reflectivity and transparency ofmicrofossils. Modern contaminants are non-reflective,regardless of their transparency and colour, becausethey have not undergone carbonization. Moreover, theepi-illumination method allows discarding of opaquemineral structures that resemble microfossils (e.g.pyrite framboids). All the structures described hereare clearly reflective and organic in nature. Finally, a

Figure 5. Stratigraphic column of the sampled section at FarmWitputs West (Fig. 3, point W). Note location of samples.

few specimens were observed in thin-sections (Fig. 7),which clearly represent true fossils indigenous to thehost rock.


Figure 6. Bavlinella faveolata (Schepeleva) Vidal, 1976 recovered from palynological macerations of the Pickelhaube and upperWallekraal formations, Dreigratberg section (Fig. 3, point D). (a, b) Specimen 20 µm in diameter in transmitted (a) and reflected-transmitted (b) light, showing typical microspheres; Upper Wallekraal Formation. (c) Specimen composed of two attached vesicles(colony fission?), upper Wallekraal Formation. Note semi-translucent microspheres near the edge of the vesicle (arrowed). (d–f)Flattened specimen in transmitted (d) and reflected-transmitted (e, f) light, Pickelhaube Formation. Photograph (f) was taken ata slightly higher focus level than (e). (g–i) Specimen extracted from marly limestones, showing considerably less flattening, intransmitted (g) and reflected-transmitted light (h, low focus; i, high focus level); Pickelhaube Formation. For all specimens figured,colour of fossils in reflected light is bluish-white and equal to other organic remains in the same samples. Scale bar in (a) represents10 µm for (a) and (b), and scale bar in (d) represents 10 µm for the rest of the photographs.

4. Palaeontology

4.a. Preservation

Five from a total of fifteen samples analysed werefound to contain identifiable organic remains. Threeshale samples belonging to the upper Holgat Formation(Farm Witputs section, Figs 3, 5) yielded well-preserved organic-walled microfossils. Thermal alter-ation index (TAI: Staplin in Hunt, 1996) according tothe schemes of Hunt (1996) and Teichmuller, Littke &Robert (1998) is TAI 3 for the Holgat microfossils,indicating palaeotemperatures between 100 and 170 ◦C.On the other hand, all the pre-Numees samples(Dreigratberg section, Figs 3, 4) contain abundant,

completely carbonized and degraded organic remains,which are mostly unidentifiable. Fairly good preser-vation of oncolites 3 to10 mm in diameter from theDabie River Formation shows that shearing stress waslow in the Dreigratberg section (Fig. 3). Therefore, it ispossible that a large part of the degradation exhibited byorganic matter in the pre-Numees samples is bacterialin origin. One sample from the upper WallekraalFormation and one from the Pickelhaube Formationcontain scattered and poorly preserved Bavlinellafaveolata vesicles (Fig. 6). Carbonization correspondsto TAI 5 for these samples, indicating palaeotempera-tures higher than 250 ◦C. It is worth noting that,as observed in thin-sections of siltstones (Fig. 7),


Figure 7. Microfossils in thin-sections of pelites and marls, Holgat Formation, Farm Witputs (Fig. 3, point W). (a) Section of colonyassigned to Soldadophycus bossii Gaucher, Sprechmann & Schipilov, 1996, embedded in calcite. Note (1) chain of spheroids, (2) twospheroids passing into a filamentous (cylindrical) cell, and (3) group of larger spheroids up to 5.5 µm in diameter. (b) Soldadophycusbossii, ellipsoidal colony composed of spheroids 3–4 µm in diameter. The fossil is slightly hematized. (c) Buteliform structure composedof tiny, agglutinated rutile crystals, here assigned to Titanotheca Gaucher & Sprechmann, 1999. All scale bars represent 10 µm.

Soldadophycus bossii is often hematized or embeddedin calcite and less flattened, suggesting early cement-ation, as also observed in the Arroyo del SoldadoGroup (Gaucher, 2000). An alternative explanation forhematized fossils would be that they were originallypyritized and the pyrite later altered to hematite.

4.b. Systematic palaeontology

Repository. All palynological slides, containing spe-cimens described here, are kept in the Precambriancollection of the Departamento de Paleontologıa,Facultad de Ciencias (Montevideo, Uruguay). Positionsof specimens in the slides are clearly marked oncorresponding duplicates.

Kingdom EUBACTERIA Woese & Fox, 1977Phyllum CYANOBACTERIA Stanier et al. 1978

Class, Order and Family indet.Genus Bavlinella (Schepeleva) Vidal, 1976

Type species. Bavlinella faveolata (Schepeleva) Vidal,1976.

Bavlinella faveolata (Schepeleva) Vidal, 1976Figure 6

1974 Sphaerocongregus variabilis Moorman,pls 1–3.

1976 Bavlinella faveolata Vidal, fig. 7A–C.1990 Sphaerocongregus variabilis Vidal & Nystuen,

fig. 9A, B, D, E, G–L.1992 Bavlinella faveolata Schopf, pl. 54J1–J3.1996 Bavlinella faveolata Gaucher, Sprechmann &

Schipilov, figs 7.1–7.2.2000 Bavlinella faveolata Gaucher, pl. 9, pls 18.1–

18.2.2003 Bavlinella faveolata Gaucher et al., figs 5C–H,

6F.

Type specimen. Vidal (1976) adopted the diagnosisgiven by Moorman (1974) for Sphaerocongregus vari-abilis as the valid diagnosis for Bavlinella faveolata.Vidal & Nystuen (1990, p. 194) stated that the typespecimen illustrated by Shepeleva (1962, in Vidal,1976) ‘is in fact the organic residue after macerationof framboidal pyrite’, and recommended the use ofthe junior synonym instead of Bavlinella faveolatafor this species. Nevertheless, German, Mikhajlova &Yankauskas (1989) had already designated a lectotypefor the species from the Kotlin Formation of the formerUSSR. This lectotype has been also illustrated bySchopf (1992, pl. 54J). Therefore, the valid designa-tion of a lectotype supersedes any previous restrictionof the application of the name of the genus and spe-cies Bavlinella faveolata. Sphaerocongregus variabilisMoorman, 1974 is thus to be considered as a juniorsynonym (Gaucher et al. 2003).

Material. Five specimens occur in organic-rich marlsand shales of the Pickelhaube and upper Wallekraalformations.

Description. The observed specimens are relativelylarge, single spheroidal vescicles made up of hundredsof tightly packed, micron-sized microspheres (Fig. 6),thus corresponding to the endosporangia morphotypeof Moorman (1974). One specimen is composed of twoattached vesicles (Fig. 6c). Preservation is rather poordue to complete carbonization and advanced degrada-tion of fossils, which can only be observed in reflectedlight. Despite their opacity (due to carbonization), thespecimens do not represent framboidal pyrite, because(1) they are bluish-white under epi-illumination;(2) they show considerable flattening and folding ofthe vesicle wall (Fig. 6a–f), (3) at least one specimen(Fig. 6c) is translucent near the vesicle edge, and(4) the morphology of the microspheres is identical tothat reported from bona fide Bavlinella and distinctly


Figure 8. For caption see facing page.


different from pyrite framboids (Fig. 6b, f, i). Thediameter of the vesicles ranges between 20 and 30 µm(mean = 25 µm, sd = 3.5 µm, N = 5).

Remarks. Bavlinella faveolata reached its acme in thelate Vendian, when it was a dominating componentof the biota worldwide (Moorman, 1974; Mansuy &Vidal, 1983; Knoll & Sweet, 1985; Germs, Knoll &Vidal, 1986; Palacios, 1989; Vidal & Nystuen, 1990;Gaucher, 2000). However, in the studied material,B. faveolata only represents a small fraction ofthe organic structures isolated in acid macerations,in contrast with other occurrences associated withorganic-rich sediments in a number of Vendian basinsworldwide (Germs, Knoll & Vidal, 1986; Palacios,1989; Vidal & Nystuen, 1990; Gaucher, 2000; Gaucheret al. 2003; Gaucher & Germs, 2003). Since anadvanced degradation prevents recognition of otherorganic remains co-occurring with B. faveolata in thestudied samples, it is so far difficult to make detailedbiostratigraphic inferences for the Pickelhaube andWallekraal formations (see below).

Incertae sedisGroup ACRITARCHA Evitt, 1963

Genus Leiosphaeridia Eisenack, 1958, emendDownie & Sarjeant, 1963

Type species. Leiosphaeridia baltica Eisenack, 1958.

Leiosphaeridia tenuissima Eisenack, 1958Figure 8g, h

1958 Leiosphaeridia tenuissima Eisenack, pl. 1.2–1.3.

1994 Leiosphaeridia tenuissima Butterfield,Knoll & Sweet, fig. 16I.

1994 Leiosphaeridia tenuissima Hofmann &Jackson, fig. 12E.

1998 Leiosphaeridia tenuissima Gaucher,Sprechmann & Montana, fig. 4.6.

2000 Leiosphaeridia tenuissima Gaucher, pl. 11.5.2003 Leiosphaeridia tenuissima Gaucher & Germs.

Material. Two well-preserved specimens in macera-tions of siltstones of the upper Holgat Formation atWitputs.

Description. Thin-walled, compressed, psilate spher-oidal vesicles with common folds. Observed specimensmeasure 90 and 105 µm in diameter. The individualsshow some bacterial (?) degradation of the vesicle wall.

Genus Synsphaeridium Eisenack, 1965

Type species. Synsphaeridium gotlandicum Eisenack,1965.

Figure 8. Soldadophycus major and Leiosphaeridia tenuissima from the upper Holgat Formation, farm Witputs West (Fig. 3, pointW). (a–f) Soldadophycus major Gaucher, 2000. (a) Saucer-shaped colony showing concentric arrangement of cells; (b) flat, ellipsoidalcolony; (c, f) spheroidal colonies; (d) bilobate colony; (e) irregular colony-fragment. (g, h) Leiosphaeridia tenuissima Eisenack, 1958.Note thin, somewhat corroded vesicle walls and concentric folds. Scale bars represent 20 µm for all figures.

Synsphaeridium sp.Figure 9h

1997 Stictosphaeridium sp. Samuelsson, fig. 12C–E.

Material. Two vesicle-aggregates in a maceration ofshales of the upper Holgat Formation, containing 30 to50 vesicles each.

Description. Loose aggregates of thin-walled, com-pressed spheroidal vesicles. Vesicle walls psilate orslightly degraded, with common folds. Diameter of ves-icles ranges between 6.2 and 14 µm (mean = 10.5 µm;sd = 2.6 µm; N = 8).

Remarks. Morphological simplicity of these fossilsand the lack of diagnostic features prevent a specificassignation of the material.

Incertae sedisGenus Coniunctiophycus Zhang, 1981

Type species. Coniunctiophycus gaoyuzhuangenseZhang, 1981.

Coniunctiophycus conglobatum Zhang, 1981Figure 9c

1981 Coniunctiophycus conglobatum Zhang, pl. 4.2000 Coniunctiophycus conglobatum Gaucher,

pls 12.8–9, 13.5.

Type specimen. Holotype: specimen figured by Zhang(1981, pl. 4, fig. 11), with catalogue number BGP 7804.

Material. Two well-preserved colonies occurring inshales of the upper Holgat Formation.

Description. Colonial, originally spherical to subpoly-hedral cells, showing distortion by mutual compres-sion. Colonies irregular to ellipsoidal, exceeding 60 µmin length and composed of hundreds of cells. Cell-diameter varies between 0.8 and 2.5 µm (mean =1.8 µm; sd = 0.45 µm; N = 15).

Remarks. These are the smallest colonial spheroidsof the Holgat Formation. The epiphytic associationto Soldadophycus and Myxococcoides reported byGaucher (2000) for C. conglobatum has not beenobserved so far in the material described here.

Genus Myxococcoides Schopf, 1968

Type species. Myxococcoides minor Schopf, 1968.

Myxococcoides cf. M. siderophila Gaucher, 2000Figure 9g

2000 Myxococcoides siderophila Gaucher, pl. 13.1–5.


Figure 9. For caption see facing page.


Material. Ten well-preserved colonies with dozensof cells, recovered from shale samples of the HolgatFormation.

Description. Colonial, mostly spherical cells withpsilate to slightly granular, robust walls, 0.5 to1.5 µm in thickness, compressed to different degrees.Approximately half of the vesicles occur in dyads,which show the greatest wall thickness. The spheroidsare arranged in loose colonies of up to some dozens ofindividuals. Diameter of spheroids ranges between 5.0and 10.0 µm (mean = 7.2 µm; sd = 1.3 µm; N = 21).

Remarks. While these fossils fit the generic diagnosisof Myxococcoides, they are somewhat problematicin their specific assignment. Common occurrence ofdyads points toward M. distola Knoll, Sweet & Mark,1991, but these fossils are considerably larger andshow thinner cell-walls. Size-frequency distributionof the Holgat Formation material closely resemblesthat of M. siderophila Gaucher, 2000. The originalmaterial of the latter species from the Arroyo delSoldado Group does not show, however, the abundanceof dyads observed in the specimens from the HolgatFormation. Nevertheless, dyads representing binaryfission of cells do occur in the Uruguayan fossils, asillustrated by Gaucher (2000, pl. 13.4–5). The higherabundance of dyads in the material described heremight reflect environmental factors (e.g. greater abund-ance of nutrients, higher temperature) that locallypromoted reproduction by binary fission. Therefore,we tentatively assign the Holgat microfossils to M.siderophila.

Genus Soldadophycus Gaucher, Sprechmann &Schipilov, 1996

Type species. Soldadophycus bossii Gaucher,Sprechmann & Schipilov, 1996.

Soldadophycus bossii Gaucher, Sprechmann &Schipilov, 1996

Figures 7a, b, 9a, b, d–f

1989 Tipo B Palacios, pl. V; figs 1–4.1996 Soldadophycus bossii Gaucher, Sprechmann &

Schipilov, figs 6.1–6.5, 6.7.1998 Soldadophycus bossii Gaucher, Sprechmann &

Montana, figs 4.7, 4.81998 Soldadophycus bossii Gaucher & Sprechmann,

p. 184.2000 Soldadophycus bossii Gaucher, pls 14–15,

17.4.

Figure 9. Organic-walled microfossils from the upper Holgat Formation, Farm Witputs West (Fig. 3, point W). (a, b) Soldadophycusbossii Gaucher, Sprechmann & Schipilov, 1996, irregular colony fragment and enlargement of one sector (square) showingtransition from spheroidal to filamentous cells (arrowed). (c) Coniunctiophycus conglobatum Zhang, 1981; large, elongated colony.(d) Soldadophycus bossii, irregular colony fragment. (e) Soldadophycus bossii, saucer-shaped colony with concentric arrangementof spheroids. (f) Soldadophycus bossii, small, irregular colony fragment. (g) Myxococcoides cf. M. siderophila Gaucher, 2000. Notecommon occurrence of dyads. (h) Synsphaeridium sp. Scale bars represent 20 µm for all figures.

2003 Soldadophycus bossii Gaucher et al., figs 5B,6A, B.

2003 Soldadophycus bossii Gaucher & Germs.

Type specimen. Holotype: specimen FCDP 3188,figured by Gaucher Sprechmann & Schipilov (1996,fig. 6.1).

Material. Eighteen colonies and colony fragments inpalynological macerations and thin-sections of marl,siltstone and limestone of the upper Holgat Formation(Witputs section).

Dimensions. The diameter of the spheroidal cellsranges between 3.1 and 6.2 µm (mean = 5.0 µm,sd = 1.0 µm, N = 40). Maximum width of one fila-mentous cell observed is 3.7 µm. The diameter ofspherical colonies ranges between 30 and 34 µm, and ofsaucer-shaped colonies from 40 to 43 µm. These valuesare well within the cell and colony sizes typical ofthe species (Gaucher, Sprechmann & Schipilov, 1996;Gaucher, 2000, p. 78).

Description. Soldadophycus bossii is characterizedby the co-occurrence of psilate, originally spher-oidal cells and septate, branched filaments (GaucherSprechmann & Schipilov, 1996). Nevertheless, somecolony types are made up only of spheroidal cells,which are spherical, saucer-shaped or parenchymatous(Gaucher, 2000). In the upper Holgat Formation, thespherical and saucer-shaped colonies dominate, butlarger, irregular colony fragments also occur. Thetypical transition from spheroids to filaments and viceversa is observed in two specimens (Figs 7a, 9a,b).

Remarks. This species dominates the Holgat Formationassemblage, along with Soldadophycus major.

Soldadophycus major Gaucher, 2000Figure 8a–f

1998 Soldadophycus sp. A Gaucher & Sprechmann,p. 184.

2000 Soldadophycus major Gaucher, pls 16.1–6,17.6.

2003 Soldadophycus major Gaucher & Germs.

Type specimen. Colony fragment FCDP 3216 figuredby Gaucher (2000, pl. 16.2).

Material. Ten well-preserved colonies and colonyfragments in palynological macerations of shale of theupper Holgat Formation.

Description. Compressed colonial spheroids charac-terized by hyaline, psilate, flexible walls 0.5–1 µm


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thick. Spheroids range between 4.7 and 10.2 µmin diameter (mean = 6.7 µm; sd = 1.3 µm; N = 68).Filaments are absent in the material examined. Colonytypes occurring in the Holgat Formation include saucer-shaped, ellipsoidal, spheroidal and irregular colonies.

Discussion. The material described under this speciesclearly belongs to Soldadophycus major Gaucher,2000, because of the occurrence of similar colonyhabits (saucer-shaped, ellipsoidal and spheroidal) andthe same size-frequency distribution of spheroids asthe type material from the Arroyo del Soldado Group.Colony-types present in the Holgat Formation suggestthat they were planktonic, and might explain theabsence of filaments in the observed population.

Order FORAMINIFERIDA Eichwald, 1830Suborder TEXTULARIINA Delage & Herouard, 1896

Family SACCAMMINIDAE Brady, 1884Subfamily SACAMMININAE Brady, 1884

Genus Titanotheca Gaucher & Sprechmann, 1999

Type species. Titanotheca coimbrae Gaucher & Sprech-mann, 1999.

Titanotheca sp.Figure 7c

Material. Dozens of specimens and fragments in thin-sections and acid macerations of coarse siltstone,Holgat Formation, Witputs Farm.

Description. Spheroidal to vase-shaped fossils withwalls composed of a single layer of agglutinatedrutile grains. Maximum diameter of specimens reaches50 µm. Largest specimen observed is 80 µm long(Fig. 7c).

Discussion. On the basis of the agglutinated nature ofthe wall, which is composed exclusively by tiny rutilegrains, and the shape of the fossils, we assign themto Titanotheca Gaucher & Sprechmann, 1999. Thesize of the Holgat specimens, however, is smaller thanthe type material from the Arroyo del Soldado Group(Uruguay). Therefore, we leave it in open nomenclatureuntil a more detailed study of the Holgat Formationpopulation can be undertaken.

Remarks. Titanotheca has been described from theYerbal Formation of the Arroyo del Soldado Group

Figure 10. Correlation chart between different late Neoproterozoic basins in SW Gondwana, showing available biostratigraphic,isotopic and geochronologic data. (a) Cango Caves Group (Republic of South Africa). Huis Riv. – Huis Rivier Formation.Lithostratigraphy after Le Roux & Gresse (1983), biostratigraphic data according to Gaucher & Germs (2003), chemostratigraphyafter Folling & Frimmel (2002) and Pb–Pb age from Folling, Zartman & Frimmel (2000). (b) Middle–upper Port Nolloth Group andlower Nama Group (southern Namibia). Pick. Fm – Pickelhaube Formation; W – Wallekraal Formation, DR – Dabie River Formation.Sources of data: litho- and chemostratigraphy: Folling & Frimmel (2002, Port Nolloth Group), Germs (1995) and Grotzinger et al.(1995, Nama Group); biostratigraphy: this work (Port Nolloth Group), Germs, Knoll & Vidal (1986) and Germs (1995, Nama Group).Pb–Pb ages (Port Nolloth Group) according to Folling, Zartman & Frimmel (2000), and U–Pb single zircon ages for the Nama Groupfrom Grotzinger et al. (1995). (c) Arroyo del Soldado Group (Uruguay). Litho- and biostratigraphy after Gaucher (2000), C isotopicdata after Gaucher et al. (2003, 2004a,b) and Sr isotopic data according to Kawashita et al. (1999) and Gaucher et al. (2004b). CSF –Cerros San Francisco Formation, CV – Cerro Victoria Formation.

(Gaucher & Sprechmann, 1999; Gaucher, 2000), theCorumba Group of southwestern Brazil (Gaucher et al.2003) and the Pico de Itapeva, Eleuterio and Cajamarbasins, southeastern Brazil (Teixeira & Gaucher, 2004).These occurrences are all upper Ediacaran (Vendian)in age, as demonstrated by chemostratigraphy (C, Sr)and radiometric dating (see below).

5. Biostratigraphy and correlations

The scarce and poorly preserved microfossils of thepre-Numees units studied here do not permit a reliablebiostratigraphic age assignment. The occurrence ofBavlinella faveolata in the upper Wallekraal andPickelhaube formations, however, indicates a max-imum late Riphean age for these units (Vidal, 1976;Samuelsson, 1997), which according to Vidal &Moczydłowska (1997) corresponds to c. 700 Ma. Vidal& Moczydłowska (1995) interpret a Rb–Sr age for ashale of 707±37 Ma (Bonhomme & Welin in Vidal &Moczydłowska, 1995) as representing the maximumage of the upper Visingso Group, where the oldestknown Bavlinella faveolata occurrences have beenreported (Vidal, 1976). Such an age is in perfectagreement with existing geochronological data for thePickelhaube and equivalent formations in the PortNolloth Zone (e.g. Folling, Zartman & Frimmel, 2000).

The microfossil assemblage recovered from theupper Holgat Formation (Fig. 10) is characterizedby: (1) low diversity (6 species); (2) dominance ofthe genus Soldadophycus, especially S. bossii; and(3) absence of acanthomorphic and large (>500 µm)sphaeromorphic acritarchs. Similar assemblages havebeen reported from the Cango Caves Group of SouthAfrica (Gaucher & Germs, 2003), Arroyo del SoldadoGroup of Uruguay (Gaucher, 2000), Corumba Groupand Pouso Alegre Basin of Brazil (Gaucher et al.2003; Teixeira & Gaucher, 2004) and also the NamaGroup in Namibia (Germs, Knoll & Vidal, 1986).Although most of the taxa occurring there are long-ranging, all the mentioned units are demonstrably lateEdiacaran (Vendian) in age, as shown by radiometricdating (Grotzinger et al. 1995; Barnett, Armstrong &de Wit, 1997; Folling, Zartman & Frimmel, 2000),skeletal fossils (mainly Cloudina, Namacalathus andTitanotheca: Germs, 1972; Zaine & Fairchild, 1985;


Figure 11. Neoproterozoic to Cambrian Sr and C isotopic composition of marine carbonates and acritarch biostratigraphic zones(informal). Shaded area represents the interval recorded in the Holgat Formation. Cambrian isotopic record according to Brasier &Sukhov (1998) and Montanez et al. (2000). SLP – Simple Leiosphere Palynoflora (shaded), ECAP – Ediacaran Complex AcritarchPalynoflora (Grey, Walter & Calver, 2003). Note negative-to-positive δ13C trend, 87Sr/86Sr ratios between 0.7082 and 0.7085, and adepauperate acritarch assemblage (Kotlin-Rovno assemblage, shaded) for post-Moelv strata (560–555 Ma), in agreement with datapresented here for the Holgat Formation.

Gaucher & Sprechmann, 1999; Grotzinger, Watters &Knoll, 2000; Gaucher et al. 2003; Teixeira & Gaucher,2004), vendotaenids (Germs, Knoll & Vidal, 1986;Gaucher et al. 2003), C and Sr chemostratigraphy(Saylor et al. 1998; Folling & Frimmel, 2002; Boggianiet al. 2003; Gaucher et al. 2003; Gaucher et al.2004a). Furthermore, the Holgat Formation microfloramatches the Kotlin-Rovno assemblage of Vidal &Moczydłowska (1997), characterized by low diversityand absence of acanthomorphs and large sphaero-morphs.

A more refined biostratigraphic scheme for the Edi-acaran has recently been proposed (Fig. 11), which re-cognizes three informal acritarch biozones between theMarinoan glacials and the base of the Cambrian (Knoll,2000; Grey, Walter & Calver, 2003). The Marinoan

glacials and strata immediately above are characterizedby a simple leiosphere palynoflora (SLP, Fig. 11),followed by an Ediacaran complex acanthomorphpalynoflora (ECAP: Grey, Walter & Calver, 2003;Fig. 11) between 580 and 570 Ma, described fromAustralia, Siberia and China. In the uppermostEdiacaran, plankton diversity decreased dramatically,leading again to a depauperate assemblage dominatedby small sphaeromorphs (Kotlin-Rovno assemblage ofVidal & Moczydłowska, 1997; Knoll, 2000; Grey, Wal-ter & Calver, 2003). Based on available 87Sr/86Sr ratiosand Pb–Pb carbonate age data (Folling, Zartman &Frimmel, 2000; Folling & Frimmel, 2002), correlationof the Holgat assemblage with the younger of thetwo Ediacaran low-diversity palynofloras (SLP andKotlin-Rovno assemblage, Fig. 11) of Grey, Walter &


Calver (2003) is suggested. Whereas the older SLPoccurs in successions characterized by carbonates with87Sr/86Sr ratios between 0.7065 and 0.7078, carbonatesdeposited during the younger Kotlin-Rovno intervalyielded ratios between 0.7075 and 0.7085 (Walter et al.2000; Fig. 11). The Holgat Formation carbonateshave 87Sr/86Sr ratios in the narrow range of 0.7083to 0.7085 (Folling & Frimmel, 2002) and yieldeda Pb–Pb age of 555±28 Ma (Folling, Zartman &Frimmel, 2000), strongly supporting our biostrati-graphic inferences. Finally, the genus Titanothecahas been found only in upper Ediacaran successionsof South America, co-occurring with Cloudina andlow-diversity acritarch assemblages. These successionsinclude the Arroyo del Soldado Group of Uruguay(Gaucher & Sprechmann, 1999; Gaucher, 2000), theCorumba Group (Gaucher et al. 2003) and the Picode Itapeva, Eleuterio and Cajamar basins of Brazil(Teixeira & Gaucher, 2004).

Whereas a late Ediacaran age is indicated by theabove mentioned data, it is still difficult to determineinto which part of this c. 30 million year period theHolgat Formation fits. This is due to the still poorresolution of Ediacaran palynostratigraphy. Within theframework of SW Gondwana, two units show palyno-morph assemblages strikingly similar to the HolgatFormation, namely the lower Kombuis Member of theCango Caves Group (South Africa) and the uppermostPolanco–lowermost Cerro Espuelitas formations of theArroyo del Soldado Group (Uruguay).

The Kombuis Member limestone bears all thegeochemical and isotopic characteristics of the Bloed-drif Member in the Port Nolloth Group (Folling &Frimmel, 2002). The lower Kombuis Member sharesfour species with the upper Holgat Formation (Fig. 10),the assemblage being also dominated by Soldado-phycus bossii (Gaucher & Germs, 2003). Moreover,the Kombuis Member yielded an identical Pb–Pbcarbonate age of 553±30 Ma (Folling, Zartman &Frimmel, 2000), confirming the biostratigraphic data(Fig. 10). Finally, the underlying Nooitgedagt Memberof the Cango Caves Group yielded a Bavlinella-dominated assemblage (Gaucher & Germs, 2003),which may be correlative to the Pickelhaube Formation.

The following organic-walled microfossils havebeen reported (Gaucher, 2000) from the transitionPolanco-Cerro Espuelitas/Barriga Negra Formationof the Arroyo del Soldado Group (in order ofdecreasing abundance, Fig. 10): Soldadophycus bossii,S. major, Myxococcoides siderophila, Coniunctiophy-cus conglobatum, Siphonophycus kestron and unnamedmicrofilaments (Fig. 10). Note that diversity is com-parable to the upper Holgat Formation assemblage,with four species in common (67 %). The microflorais dominated by S. bossii, as in the Holgat Forma-tion. Therefore, we conclude that the palynomorphassemblage of the Holgat Formation is best compared tothat occurring at the transition between the Polanco and

Barriga Negra formations. A negative δ13C excursion,rising 87Sr/86Sr ratios between 0.7073 and 0.7085, andeustatic sea-level drop at the Polanco-Barriga Negratransition (Gaucher, 2000; Gaucher et al. 2004a,b;Fig. 10) possibly hint at the Numees glacial event beingrecorded there. Such a lower Barriga Negra–Numeescorrelation would imply that the Arroyo del Soldadoshelf was ice-free during the Numees event, becauseno glacial rocks occur there. This fact militates againsta ‘snowball Earth’ scenario for this glaciation (Gaucheret al. 2004b).

The combination of the Soldadophycus–Myxococ-coides–Coniunctiophycus–Leiosphaeridia assemblagewith negative-to-positive δ13C and 87Sr/86Sr ratiosbetween 0.7080 and 0.7085 seems characteristic ofpost-Numees/Gaskiers units. This integrated approachmight help solve the large uncertainties in the strati-graphy of Neoproterozoic glacial deposits (Kennedyet al. 1998; Saylor et al. 1998).

6. Discussion

The impressive similarity of the palynomorph as-semblages preserved in the Holgat Formation andKombuis Member, combined with their identical Pb–Pb carbonate ages of 555±28 Ma and 553±30 Ma,respectively (Folling, Zartman & Frimmel, 2000),strongly suggests that the cap carbonate (BloeddrifMember of the Holgat Formation) above the Nu-mees Formation is considerably younger than typicalMarinoan cap carbonates (Hoffman & Schrag, 2002;Halverson et al. 2003), which are older than 601±4 Ma(Dempster et al. 2002) and 604+4

−3 Ma (Myrow &Kaufman, 1999). The best constraint so far on theage of the Marinoan glacation comes from the U–Pbzircon age of 636±1 Ma for an ash bed intercalated inthe Marinoan-correlative Ghaub Formation of Namibia(Hoffmann et al. 2004). As cap carbonates are believedto have been deposited immediately after the respectiveglacial event (Kennedy, 1996; Hoffman & Schrag,2002), a late Ediacaran age (as shown by organic-walled microfossils) around 555 Ma for the HolgatFormation implies that the Numees Formation repres-ents the c. 580 Ma Gaskiers Glaciation (Hoffman &Schrag, 2002; Halverson et al. 2003; Bowring et al.2003; Fig. 2) or the probably younger (<570 Ma)Moelv or Egan events (Vidal & Nystuen, 1990; Knoll,2000; Fig. 2).

An alternative explanation is to assume a hiatus of c.50 Myr between deposition of the Holgat and Numeesformations. The different degree of thermal alterationof organic matter beneath and above the NumeesFormation diamictites could be interpreted as evidencesupporting a hiatus between these units. However,the temperature gap of c. 150 ◦C could also be dueto locally variable metamorphic grades, because thepre- and post-Numees units were sampled at differentlocalities (Dreigratberg and Witputs Farm, respectively,


Fig. 3). In the Gobabis area an unconformity occursat the base of the Bildah Member (BuschmannsklippeFormation, Witvlei Group) (Hegenberger, 1993), whichaccording to the authors is a correlative to the BloeddrifMember of the Holgat Formation. This unconformitytruncates the underlying Blasskrans tillite (Hoffmann,1989) which is equivalent to the Numees tillite.Chemostratigraphic data (Kennedy et al. 1998) seemto indicate that the hiatus at the base of the BildahMember is less than 50 Myr. Similarly, the C, O and Srisotope data from the lowermost parts of the BloeddrifMember (Folling & Frimmel, 2002) militate againsta major hiatus but agree with typical cap carbonatedevelopment immediately above a glacial deposit.

More biostratigraphic data, especially from the lowerPort Nolloth Group, are needed, however, to solve theseuncertainties.

7. Conclusions

Organic-walled microfossils are reported for the firsttime from the Gariep Belt (Port Nolloth Group),and represent a highly promising tool to unravel thestratigraphy and age of the different glacial eventsrecorded there. Poorly preserved, highly carbonizedacritarchs characterize units that underlie glaciogenicdiamictite of the Numees Formation. Based on theoccurrence of Bavlinella faveolata in the Pickelhaubeand upper Wallekraal formations, an upper Neoprotero-zoic age younger than c. 700 Ma is inferred forthese units. Acritarchs of the upper Holgat Formation,on the other hand, are comparable to many upperEdiacaran assemblages occurring in SW Gondwanaand elsewhere. The assemblage is characterized by lowdiversity (six species), dominance of Soldadophycusbossii, and absence of acanthomorphs and largesphaeromorphs. The agglutinated foraminifer Titan-otheca sp. co-occurs with this acritarch assemblage.Biostratigraphic correlation of the Holgat Formationwith the lower Kombuis Member (Cango Caves Group)corroborates previously reported Pb–Pb ages for theseunits of around 555 Ma. This implies that either theNumees Formation was deposited during the post-Marinoan Gaskiers or Moelv glacial events (Halversonet al. 2003; Bowring et al. 2003), or that a largehiatus of c. 50 Myr exists between the Holgat andNumees formations, with the latter reflecting theMarinoan glaciation. The former interpretation ispreferred here, and differs from previous inferences(Frimmel, Folling & Eriksson, 2002). Post-Numeesdeposits in SW Gondwana are characterized bya Soldadophycus–Myxococcoides–Coniunctiophycus–Leiosphaeridia assemblage, Titanotheca, negative-to-positive δ13C excursions and 87Sr/86Sr values around0.7080–0.7085. These include, apart from the HolgatFormation, the Buschmannsklippe Formation (WitvleiGroup), the Kombuis Member (Cango Caves Group)and the uppermost Polanco–lowermost Barriga Negra

Formation (Arroyo del Soldado Group). In the latterunit, a negative δ13C excursion and strong sea-leveldrop recorded at the Polanco–Barriga Negra transition(Gaucher et al. 2004a,b) may be linked with theNumees glacial event. Finally, a hiatus of less than5 Myr is represented by the erosive surface at the baseof the Nama Group, on the basis of available U–Pbage data of 548.8±1 Ma for the lower Nama Group(Grotzinger et al. 1995) and the age suggested in thisstudy for the Holgat Formation.

Acknowledgements. We thank Leticia Chiglino (Mon-tevideo) for valuable help during field work in Namibiaand preparation of palynological samples. Part of the fieldexpenses and analyses were financed by a research grantfrom Rand Afrikaans University to G. J. B. Germs. Financialsupport from the Comision Sectorial de InvestigacionCientıfica (CSIC, Uruguay) to C. Gaucher, and the SouthAfrican National Research Foundation (gun No. 2053697)to H. Frimmel are gratefully acknowledged. This is acontribution to IGCP project 478 (‘Neoproterozoic–EarlyPalaeozoic Events in SW-Gondwana’).

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Organic-walled microfossils and biostratigraphy of the upper Port Nolloth Group (Namibia): implications for latest Neoproterozoic glaciations