- © 2005 Cambridge University Press
The occurrence of organic-walled microfossils is reported for the first time from the Neoproterozoic Port Nolloth Group, Gariep Belt (southern Namibia). Acritarchs assigned to Bavlinella faveolata occur in the Hilda Subgroup below the younger of two glaciogenic diamictite units (Numees Formation) within the Port Nolloth Group. The microfossil assemblage in the overlying upper Holgat Formation, above the Numees Formation, is characterized by low diversity (six species), dominance of Soldadophycus bossii, and absence of acanthomorphic or large sphaeromorphic acritarchs. The agglutinated foraminifer Titanotheca also occurs in the Holgat Formation. Combined with available chemostratigraphic data and Pb–Pb ages, this microfossil assemblage indicates an upper Ediacaran age 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 (Witvlei Group, central Namibia), the Kombuis Member (Cango Caves Group, southern South Africa) and the uppermost 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 loosely constrained so far and subject to considerable debate, can now be assigned to the c. 580 Ma Gaskiers or the possibly younger (< 570 Ma) Moelv glacial event. The Numees glacial event may be represented in the uppermost Nooitgedagt Member (Cango Caves Group, South Africa) and the lower Barriga Negra formations (Arroyo del Soldado Group, Uruguay), characterized by a negative δ13C excursion and a strong sea-level drop. If this correlation is confirmed, lack of glacial deposits there might have implications for the palaeogeographic extent of upper Ediacaran glaciations. Our preliminary studies show that acritarch biostratigraphy can make a significant contribution to unravelling the stratigraphy of Neoproterozoic glacial deposits, especially when combined with C and Sr isotopic data.
The Gariep Supergroup, exposed in the Pan-African Gariep Belt in southwestern Namibia and westernmost South Africa, represents the fill of the Neoproterozoic Gariep Basin in SW Gondwana (Fig. 1⇓). It serves as a prime example of one of the major conundrums of Neoproterozoic stratigraphy worldwide, namely the number, timing and intercontinental correlation of Neoproterozoic glacial events.
Similar to many other Neoproterozoic successions (Fig. 2⇓) in southern Africa (e.g. Damara Belt, the Lufilian Arc and the West Congolian Belt) and elsewhere (e.g. the Dalradian in Scotland and the Huqf Supergroup in Oman: Brasier & Shields, 2000; Brasier et al. 2000), the Gariep Supergroup contains two diamictite units for which a glaciogenic origin is indicated, namely the Kaigas and Numees formations (Frimmel, Fölling & Eriksson, 2002). Both of these are overlain by distinct cap carbonate sequences that illustrate the apparent change from global icehouse to greenhouse conditions (Frimmel, Fölling & Eriksson, 2002). The older cap carbonate is typically rich in organic carbon and medium to dark grey, whereas the younger cap carbonate is light grey, cream or pale pink in colour and poor in organic matter. This intercalation of glaciogenic and warm water deposits, some of which have been laid down at low latitudes, forms the basis of extreme climate models, with the ‘snowball Earth’ model (Hoffman, Kaufman & Halverson, 1998b) probably the most popular one at present.
As the assessment of the applicability of any of these global palaeoclimate models (or the establishment of alternative new models) is critically dependent on the interbasinal correlation of the climate-sensitive lithostratigraphic units, a considerable effort by a number of research groups has gone into the regional and intercontinental correlation of Neoproterozoic glaciogenic units. Most of that correlation has been carried out based on the isotopic composition of marine carbonate, which is taken as a contemporaneous seawater proxy. In particular, C isotopes play an important role in these chemostratigraphic studies. Stratigraphic correlation across depositional basins remains hampered, however, mainly for two reasons: (1) the use of C isotopes for chemostratigraphic correlation is limited, because the relative short residence time of C in seawater determines a strong dependence on local environmental conditions (e.g. palaeobathymetry: Frimmel & Fölling, 2004); and (2) a reliable geochronological control is lacking. There is a difference of opinion as to how many glaciations occurred during the Neoproterozoic, which is illustrated by the various attempts to correlate globally up to four negative δ13C excursions (Jacobsen & Kaufman, 1999). Some authors believe that there were at least four glaciations (Saylor et al. 1998), and some prefer three (Halverson et al. 2003), whereas others suggested only two glaciations (Kennedy et al. 1998). The former group ascribes the two older glaciations to an older (c. 750 Ma) and a younger (c. 720 Ma) Sturtian ice age (Fig. 2⇑), whereas the two younger glacial events are considered to reflect the c. 635 Ma Marinoan and the c. 560 Ma Moelv glaciations (Brasier & Shields, 2000). Even for type sections, such as those through the Varangerian deposits in northern Norway, no agreement exists regarding the correlation of a given diamictite bed with the Sturtian, Marinoan or Moelv glaciation (compare Brasier & Shields, 2000; Kennedy et 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 of the Adelaide Rift Complex and the Elatina Formation of Australia, respectively (Fig. 2⇑).
A number of Neoproterozoic glacigenic units have been unambiguously dated by different methods as post-Marinoan, suggesting the occurrence of at least three glacial epochs in the Neoproterozoic (Fig. 2⇑). The Gaskiers Formation of the Conception Group (southeastern Newfoundland) has been dated at c. 580 Ma by U–Pb geochronology of zircons separated from ash-beds interlayered with the glaciomarine diamictites (Bowring et al. 2003). The Moelv Formation of the Hedmark Group (southern Norway) overlies the Biskopåsen and Biri formations, which yielded a complex 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 the complex acantomorph assemblage (Grey, Walter & Calver, 2003). Finally, the glaciogenic Egan Formation of the Louisa Downs Group (Western Australia, Kimberley region) is regarded as post-Marinoan (Grey & Corkeron, 1998), on the basis of biostratigraphic correlation 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 Ma is assumed for the Egan glaciation (K. Grey, pers. comm. 2005), roughly coincident with the Moelv event (Fig. 2⇑).
Ultimately, chemostratigraphic correlation should be tested by chronostratigraphic means. The age of the Kaigas Formation is reasonably well constrained between 741 and 751 Ma by U–Pb and Pb–Pb single zircon data from associated felsic volcanic rocks (Borg et al. 2003; Frimmel, Klötzli & Siegfried, 1996). Based on these age data (Fig. 2⇑), as well as on chemostratigraphic data, a correlation of the Kaigas Formation with the Sturtian glaciation has been established with a fair degree of confidence (Frimmel, Fölling & Eriksson, 2002). In contrast, the age of the younger Numees Formation remains problematic due to a lack of easily dateable units therein. In the Damara Belt (central and northern Namibia), the younger of two glaciogenic diamictite horizons has been precisely dated at 636 ± 1 Ma with a U–Pb single zircon age for 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, in the Gariep Belt, the age of the younger glaciogenic unit (Numees Formation), a regionally extensive sheet of massive diamictite that is overlain by typical post-glacial cap carbonates (Holgat Formation), remains problematic. A syn-Marinoan age has been suggested from imprecise Pb–Pb carbonate data, Ar–Ar hornblende ages and theoretical considerations on the tectonic evolution (Frimmel & Fölling, 2004). However, in the light of the recent revised age constraint on the Marinoan glaciation, such a correlation has become doubtful, and correlation with the younger Gaskiers or Moelv glaciations cannot be ruled out. Clarification of this issue is pivotal for any future climate and geodynamic model that aims to explain the ambiguous Neoproterozoic rock record.
In order to improve the correlation of the post-Kaigas lithostratigraphic units in the Gariep Belt, we systematically sampled carbonate, banded iron formation (BIF) and shale of the upper Port Nolloth Group for the study of organic-walled microfossils. On the basis of our new micropalaeontological data and equivalent studies in the Saldania Belt (Gaucher & Germs, 2003), a significant improvement in the regional correlation of the younger diamictite units in the Gariep Belt is achieved. The implications of these results for the understanding of global Neoproterozoic glaciations and the distribution of microfossils will be discussed.
2. Geological setting
The Gariep Belt, which continues towards the north into the Damara and Kaoko belts of central and northwestern Namibia and to the south into the Saldania Belt on the southern tip of South Africa, is exposed in a number of outcrops in the southern Namib desert from the coast between Lüderitz and Port Nolloth to up to 100 km inland (Fig. 3⇓). South of Port Nolloth, most of the belt strikes out to sea with only a few small outcrops along the coastline, and then re-emerges in an erosional outlier around 31°S (Vredendal Outlier). It represents a Pan-African fold-thrust belt in which two major tectonic units are distinguished (Frimmel, 2004). In the internal, western part of the belt, largely oceanic rocks that lack any continental basement occur (Marmora Terrane). The external part further east (Port Nolloth Zone) consists of continental sedimentary successions with subordinate volcanic rocks (Port Nolloth Group), which, though internally intensely deformed, still rest on their Palaeo- to Mesoproterozoic basement. A major thrust fault (Schakalsberge Thrust) separates the allochthonous Marmora Terrane from the para-autochthonous Port Nolloth Zone. Only in the latter is a relatively complete Neoproterozoic succession exposed (Port Nolloth Group), including the two diamictite and associated cap carbonate units, and it therefore is the main focus of this study.
A low-grade metasedimentary succession occurs in the area around Vanrhynsdorp (Gifberg Group), which has been correlated with large parts of the Port Nolloth Group. The Gifberg Group is included in the Gariep Supergroup, whereby the Vredendal Outlier represents the southernmost exposures of the Gariep Belt (Fig. 3⇑). Based on recent mapping and chemostratigraphic studies (H. E. Frimmel, unpub. data), a correlation at formation level (Fig. 4⇓) is proposed.
Three megasequences (M1–M3) are distinguished within the Port Nolloth Group (Frimmel, Fölling & Eriksson, 2002). Sequence M1 starts with alluvial fan deposits (Fig. 4⇑) in an emerging continental rift graben and evolves to alluvial plain and fan delta deposits in the widening rift (Stinkfontein Subgroup). A maximum age of 771 ± 6 Ma is indicated for the onset of sedimentation from the youngest age obtained from intrusive rocks in the pre-Gariep basement (Frimmel, Zartman & Späth, 2001). Locally, at the flanks of a major growth fault along the eastern boundary of the Gariep Basin, laterally discontinuous diamictite, intercalated with upward-fining arkose and greywacke beds and dolomitic olistostromes are developed (Kaigas Formation). They are interpreted as representing, respectively, debris flow sediments, proximal to medial turbidity fan deposits, and large slump masses, laid down adjacent to drowned rift shoulders. A glacio-marine or fluvio-glacial origin of parts of this formation is indicated. Bimodal, predominantly felsic, continental, 741 to 751 Ma rift magmatism accompanied sedimentation near the eastern rift graben margin (Rosh Pinah Formation). Based on low δ13C and low 87Sr/86Sr ratios and chemostratigraphic similarities with Neoproterozoic sequences elsewhere, this glacial influence is ascribed to the global Sturtian glaciation (Fölling & Frimmel, 2002).
A succession of post-glacial carbonates with intercalated argillite, marl and minor arenite (Pickelhaube Formation, M2) reflects a change from icehouse to greenhouse conditions as indicated by a shift from distinct negative to strongly positive δ13C ratios (Fölling & Frimmel, 2002). This was followed by a dramatic fall in sea-level, which led to re-working of the older stratigraphy in the clastic Wallekraal Formation that is characterized by immature arenite, quartz pebble conglomerate channels that cut across carbonate platforms, and chaotic dolomite breccias and olisthostromes. In areas that escaped erosion, carbonate deposition continued in the form of stromatolitic bioherms and oolites (Dabie River Formation). This major regression towards the top of M2 has been explained by eustatic sea-level drop in advance of a major glacial event (Frimmel, Fölling & Eriksson, 2002), which is recorded in the form of a massive, up to 500 m thick, blanket of glaciogenic diamictite of the Numees Formation. The lower part of this formation contains a thin, but distinct, intercalation of banded 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 occasionally the dominant iron oxide. Dropstones of different composition occur in the Jakkalsberg Member. These facts enable classification of the Jakkalsberg BIF into the Rapitan type (Maynard, 1991).
The Numees Formation diamictite is overlain by a typical, up to 100 m thick, carbonate succession (Bloeddrif Member, Holgat Formation, M3; see Frimmel & Fölling, 2004). It starts with a flat, thinly laminated, coarsely recrystallized limestone that releases a distinct smell of H2S upon breaking. This limestone is characterized by very high Sr concentrations, resulting in very low Mn/Sr, Fe/Sr and Ca/Sr, and is devoid of terrigenous material. In sections from a proximal position relative to the palaeo-shoreline, the lower parts of this limestone are dolomitized and contain thin intercalations of quartz arenite. Deposition of aragonite mud in a generally euxinic, marine environment below wave base is inferred for the Bloeddrif Member based on its structure and chemistry. Only in proximal sections are found internal, vertical, tube-like structures of infilled micritic sediment and cement. They are typically a few centimetres across and several decimetres high. Similar structures have been described from many other Neoproterozoic post-glacial cap carbonates (Cloud, 1974; Hegenberger, 1993; Kennedy, 1996; Hoffman, Kaufman & Halverson, 1998a). While some workers interpret them as being of microbial origin (Hegenberger, 1993; Hoffman, Kaufman & Halverson, 1998a), others explain them by gas escape following the destabilization of gas hydrate during warming of terrestrial permafrost (Kennedy, 2001). Columnar, current-elongated, closely spaced stromatolites 2.5 to 5 cm in diameter occur in the Bloeddrif Member at the Witputs section (Figs 3⇑, 4⇑). The cap carbonates typically start with a negative δ13C excursion only to recover to values around 0 ‰ (PDB) in most sections, except for the most proximal domains where a rise to distinctly positive δ13C ratios has been noted (Frimmel & Fölling, 2004).
The remainder of the Holgat Formation comprises upward-fining cycles of medium-bedded sandstone, greywacke and arkose with minor siltstone, mudstone and intraformational conglomerate. At Farm Witputs (Figs 3⇑, 4⇑), the siltstone is mainly composed of detrital micas (muscovite, biotite, chlorite), quartz and detrital carbonates. The apparent thickness of this flysch-like succession is about 7 km. Due to tectonic thickening, the true thickness is considerably less, however, and is probably some several hundred metres. In contrast to M1 and M2, the Holgat Formation, representing M3, has equivalents across the Marmora Terrane. From this regional distribution, which probably also extended further to the east onto the Kalahari craton, it has been concluded that M3 was deposited during the closure of the Gariep Basin in a foredeep position on top and in front of the advancing thrust sheets of the Marmora oceanic crustal fragments (Frimmel & Fölling, 2004).
A first dynamic metamorphic stage is recognized only in the mafic rocks of the Marmora Terrane and is probably related to their accretion that began about 575 Ma (Frimmel & Frank, 1998). The peak of metamorphism, attaining lowermost amphibolite facies conditions in the Port Nolloth Zone, was reached as a consequence of the emplacement of the Marmora Terrane on top of the Port Nolloth Zone, which has been dated at 545 ± 2 Ma (Frimmel & Frank, 1998). Syn-orogenic sedimentation in the corresponding foreland began around 550 Ma with shallow marine deposits of the lower Nama Group followed by c. 540 Ma siliciclastic molasse sediments of the upper Nama 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 the adjacent Mesoproterozoic basement, with the most reliable age so far for this magmatic pulse being 507 ± 6 Ma (Frimmel, 2000).
3. Materials and methods
Fifteen palynological macerations and ten thin-sections of carbonates, pelites and BIF from the Holgat, Numees (Jakkalsberg Member), upper Wallekraal and Pickelhaube formations were prepared at the Micro-palaeontology Laboratory of the Facultad de Ciencias (Montevideo). Following crushing and digestion of samples (c. 150 g) with concentrated HCl, 72 % HF was applied for 24 hours to the silicate/organic residues. After neutralization, boiling concentrated HCl was used to dissolve flourides. Remaining organic residues were recovered by means of a 5 μm sieve, stored in glass flasks and mounted with glycerin-gelatine on standard glass slides. Throughout the preparation, gravity settling was used instead of centrifugation, to avoid destruction of fragile, large acritarchs. Microfossils were determined and counted under a Leica DM LP polarizing microscope, using transmitted, reflected and combined reflected–transmitted light (in the latter cases with oil immersion objectives). This allowed observation of opaque (carbonized) microfossils, and also assessment of the fossil nature of the isolated microstructures. This method, developed by Pflug & Reitz (1992 and references therein), makes possible the comparison of the reflectivity and transparency of microfossils. Modern contaminants are non-reflective, regardless of their transparency and colour, because they have not undergone carbonization. Moreover, the epi-illumination method allows discarding of opaque mineral structures that resemble microfossils (e.g. pyrite framboids). All the structures described here are clearly reflective and organic in nature. Finally, a few specimens were observed in thin-sections (Fig. 7⇓), which clearly represent true fossils indigenous to the host rock.
Five from a total of fifteen samples analysed were found to contain identifiable organic remains. Three shale samples belonging to the upper Holgat Formation (Farm Witputs section, Figs 3⇑, 5⇓) yielded well-preserved organic-walled microfossils. Thermal alteration index (TAI: Staplin in Hunt, 1996) according to the schemes of Hunt (1996) and Teichmüller, 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 preservation of oncolites 3 to10 mm in diameter from the Dabie River Formation shows that shearing stress was low in the Dreigratberg section (Fig. 3⇑). Therefore, it is possible that a large part of the degradation exhibited by organic matter in the pre-Numees samples is bacterial in origin. One sample from the upper Wallekraal Formation and one from the Pickelhaube Formation contain scattered and poorly preserved Bavlinella faveolata vesicles (Fig. 6⇓). Carbonization corresponds to TAI 5 for these samples, indicating palaeotemperatures higher than 250 °C. It is worth noting that, as observed in thin-sections of siltstones (Fig. 7⇓), Soldadophycus bossii is often hematized or embedded in calcite and less flattened, suggesting early cementation, as also observed in the Arroyo del Soldado Group (Gaucher, 2000). An alternative explanation for hematized fossils would be that they were originally pyritized and the pyrite later altered to hematite.
4.b. Systematic palaeontology
All palynological slides, containing specimens described here, are kept in the Precambrian collection of the Departamento de Paleontología, Facultad de Ciencias (Montevideo, Uruguay). Positions of specimens in the slides are clearly marked on corresponding duplicates.
Kingdom EUBACTERIA Woese & Fox, 1977
Phyllum CYANOBACTERIA Stanier et al. 1978
Class, Order and Family indet.
Genus Bavlinella (Schepeleva) Vidal, 1976
Bavlinella faveolata (Schepeleva) Vidal, 1976.
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.
Vidal (1976) adopted the diagnosis given by Moorman (1974) for Sphaerocongregus variabilis as the valid diagnosis for Bavlinella faveolata. Vidal & Nystuen (1990, p. 194) stated that the type specimen illustrated by Shepeleva (1962, in Vidal, 1976) ‘is in fact the organic residue after maceration of framboidal pyrite’, and recommended the use of the junior synonym instead of Bavlinella faveolata for this species. Nevertheless, German, Mikhajlova & Yankauskas (1989) had already designated a lectotype for the species from the Kotlin Formation of the former USSR. This lectotype has been also illustrated by Schopf (1992, pl. 54J). Therefore, the valid designation of a lectotype supersedes any previous restriction of the application of the name of the genus and species Bavlinella faveolata. Sphaerocongregus variabilis Moorman, 1974 is thus to be considered as a junior synonym (Gaucher et al. 2003).
Five specimens occur in organic-rich marls and shales of the Pickelhaube and upper Wallekraal formations.
The observed specimens are relatively large, single spheroidal vescicles made up of hundreds of tightly packed, micron-sized microspheres (Fig. 6⇑), thus corresponding to the endosporangia morphotype of Moorman (1974). One specimen is composed of two attached vesicles (Fig. 6c⇑). Preservation is rather poor due to complete carbonization and advanced degradation of fossils, which can only be observed in reflected light. Despite their opacity (due to carbonization), the specimens do not represent framboidal pyrite, because (1) they are bluish-white under epi-illumination; (2) they show considerable flattening and folding of the 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 to that reported from bona fide Bavlinella and distinctly different from pyrite framboids (Fig. 6b, f, i⇑). The diameter of the vesicles ranges between 20 and 30 μm (mean = 25 μm, sd = 3.5 μm, N = 5).
Bavlinella faveolata reached its acme in the late Vendian, when it was a dominating component of 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 of the organic structures isolated in acid macerations, in contrast with other occurrences associated with organic-rich sediments in a number of Vendian basins worldwide (Germs, Knoll & Vidal, 1986; Palacios, 1989; Vidal & Nystuen, 1990; Gaucher, 2000; Gaucher et al. 2003; Gaucher & Germs, 2003). Since an advanced degradation prevents recognition of other organic remains co-occurring with B. faveolata in the studied samples, it is so far difficult to make detailed biostratigraphic inferences for the Pickelhaube and Wallekraal formations (see below).
Group acritarcha Evitt, 1963
Genus Leiosphaeridia Eisenack, 1958, emend
Leiosphaeridia baltica Eisenack, 1958.
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 & Montaña, fig. 4.6.
2000 Leiosphaeridia tenuissima Gaucher, pl. 11.5.
2003 Leiosphaeridia tenuissima Gaucher & Germs.
Two well-preserved specimens in macerations of siltstones of the upper Holgat Formation at Witputs.
Thin-walled, compressed, psilate spheroidal vesicles with common folds. Observed specimens measure 90 and 105 μm in diameter. The individuals show some bacterial (?) degradation of the vesicle wall.
Genus Synsphaeridium Eisenack, 1965
Synsphaeridium gotlandicum Eisenack, 1965.
Synsphaeridium sp. Figure 9h⇓
1997 Stictosphaeridium sp. Samuelsson, fig. 12C–E.
Two vesicle-aggregates in a maceration of shales of the upper Holgat Formation, containing 30 to 50 vesicles each.
Loose aggregates of thin-walled, compressed spheroidal vesicles. Vesicle walls psilate or slightly degraded, with common folds. Diameter of vesicles ranges between 6.2 and 14 μm (mean = 10.5 μm; sd = 2.6 μm; N = 8).
Morphological simplicity of these fossils and the lack of diagnostic features prevent a specific assignation of the material.
Genus Coniunctiophycus Zhang, 1981
Coniunctiophycus gaoyuzhuangense Zhang, 1981.
1981 Coniunctiophycus conglobatum Zhang, pl. 4.
2000 Coniunctiophycus conglobatum Gaucher, pls 12.8–9, 13.5.
Holotype: specimen figured by Zhang (1981, pl. 4, fig. 11), with catalogue number BGP 7804.
Two well-preserved colonies occurring in shales of the upper Holgat Formation.
Colonial, originally spherical to subpolyhedral cells, showing distortion by mutual compression. Colonies irregular to ellipsoidal, exceeding 60 μm in 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).
These are the smallest colonial spheroids of the Holgat Formation. The epiphytic association to Soldadophycus and Myxococcoides reported by Gaucher (2000) for C. conglobatum has not been observed so far in the material described here.
Genus Myxococcoides Schopf, 1968
Myxococcoides minor Schopf, 1968.
2000 Myxococcoides siderophila Gaucher, pl. 13.1–5.
Ten well-preserved colonies with dozens of cells, recovered from shale samples of the Holgat Formation.
Colonial, mostly spherical cells with psilate to slightly granular, robust walls, 0.5 to 1.5 μm in thickness, compressed to different degrees. Approximately half of the vesicles occur in dyads, which show the greatest wall thickness. The spheroids are arranged in loose colonies of up to some dozens of individuals. Diameter of spheroids ranges between 5.0 and 10.0 μm (mean = 7.2 μm; sd = 1.3 μm; N = 21).
While these fossils fit the generic diagnosis of Myxococcoides, they are somewhat problematic in their specific assignment. Common occurrence of dyads points toward M. distola Knoll, Sweet & Mark, 1991, but these fossils are considerably larger and show thinner cell-walls. Size-frequency distribution of the Holgat Formation material closely resembles that of M. siderophila Gaucher, 2000. The original material of the latter species from the Arroyo del Soldado Group does not show, however, the abundance of dyads observed in the specimens from the Holgat Formation. Nevertheless, dyads representing binary fission of cells do occur in the Uruguayan fossils, as illustrated by Gaucher (2000, pl. 13.4–5). The higher abundance of dyads in the material described here might reflect environmental factors (e.g. greater abundance of nutrients, higher temperature) that locally promoted reproduction by binary fission. Therefore, we tentatively assign the Holgat microfossils to M. siderophila.
Genus Soldadophycus Gaucher, Sprechmann & Schipilov, 1996
Soldadophycus bossii Gaucher, Sprechmann & Schipilov, 1996.
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 & Montaña, figs 4.7, 4.8
1998 Soldadophycus bossii Gaucher & Sprechmann, p. 184.
2000 Soldadophycus bossii Gaucher, pls 14–15, 17.4.
2003 Soldadophycus bossii Gaucher et al., figs 5B, 6A, B.
2003 Soldadophycus bossii Gaucher & Germs.
Holotype: specimen FCDP 3188, figured by Gaucher Sprechmann & Schipilov (1996, fig. 6.1).
Eighteen colonies and colony fragments in palynological macerations and thin-sections of marl, siltstone and limestone of the upper Holgat Formation (Witputs section).
The diameter of the spheroidal cells ranges between 3.1 and 6.2 μm (mean = 5.0 μm, sd = 1.0 μm, N = 40). Maximum width of one filamentous cell observed is 3.7 μm. The diameter of spherical colonies ranges between 30 and 34 μm, and of saucer-shaped colonies from 40 to 43 μm. These values are well within the cell and colony sizes typical of the species (Gaucher, Sprechmann & Schipilov, 1996; Gaucher, 2000, p. 78).
Soldadophycus bossii is characterized by the co-occurrence of psilate, originally spheroidal cells and septate, branched filaments (Gaucher Sprechmann & Schipilov, 1996). Nevertheless, some colony types are made up only of spheroidal cells, which are spherical, saucer-shaped or parenchymatous (Gaucher, 2000). In the upper Holgat Formation, the spherical and saucer-shaped colonies dominate, but larger, irregular colony fragments also occur. The typical transition from spheroids to filaments and vice versa is observed in two specimens (Figs 7a⇑, 9a,b⇑).
This species dominates the Holgat Formation assemblage, along with Soldadophycus major.
1998 Soldadophycus sp. A Gaucher & Sprechmann, p. 184.
2000 Soldadophycus major Gaucher, pls 16.1–6, 17.6.
2003 Soldadophycus major Gaucher & Germs.
Colony fragment FCDP 3216 figured by Gaucher (2000, pl. 16.2).
Ten well-preserved colonies and colony fragments in palynological macerations of shale of the upper Holgat Formation.
Compressed colonial spheroids characterized by hyaline, psilate, flexible walls 0.5–1 μm thick. Spheroids range between 4.7 and 10.2 μm in diameter (mean = 6.7 μm; sd = 1.3 μm; N = 68). Filaments are absent in the material examined. Colony types occurring in the Holgat Formation include saucer-shaped, ellipsoidal, spheroidal and irregular colonies.
The material described under this species clearly belongs to Soldadophycus major Gaucher, 2000, because of the occurrence of similar colony habits (saucer-shaped, ellipsoidal and spheroidal) and the same size-frequency distribution of spheroids as the type material from the Arroyo del Soldado Group. Colony-types present in the Holgat Formation suggest that they were planktonic, and might explain the absence of filaments in the observed population.
Order FORAMINIFERIDA Eichwald, 1830
Suborder TEXTULARIINA Delage & Hérouard, 1896
Family saccamminidae Brady, 1884
Subfamily sacammininae Brady, 1884
Genus Titanotheca Gaucher & Sprechmann, 1999
Titanotheca coimbrae Gaucher & Sprechmann, 1999.
Titanotheca sp. Figure 7c⇑
Dozens of specimens and fragments in thin-sections and acid macerations of coarse siltstone, Holgat Formation, Witputs Farm.
Spheroidal to vase-shaped fossils with walls composed of a single layer of agglutinated rutile grains. Maximum diameter of specimens reaches 50 μm. Largest specimen observed is 80 μm long (Fig. 7c⇑).
On the basis of the agglutinated nature of the wall, which is composed exclusively by tiny rutile grains, and the shape of the fossils, we assign them to Titanotheca Gaucher & Sprechmann, 1999. The size of the Holgat specimens, however, is smaller than the type material from the Arroyo del Soldado Group (Uruguay). Therefore, we leave it in open nomenclature until a more detailed study of the Holgat Formation population can be undertaken.
Titanotheca has been described from the Yerbal Formation of the Arroyo del Soldado Group (Gaucher & Sprechmann, 1999; Gaucher, 2000), the Corumbá Group of southwestern Brazil (Gaucher et al. 2003) and the Pico de Itapeva, Eleutério and Cajamar basins, 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 the pre-Numees units studied here do not permit a reliable biostratigraphic age assignment. The occurrence of Bavlinella faveolata in the upper Wallekraal and Pickelhaube formations, however, indicates a maximum late Riphean age for these units (Vidal, 1976; Samuelsson, 1997), which according to Vidal & Moczydlowska (1997) corresponds to c. 700 Ma. Vidal & Moczydlowska (1995) interpret a Rb–Sr age for a shale of 707 ± 37 Ma (Bonhomme & Welin in Vidal & Moczydlowska, 1995) as representing the maximum age of the upper Visingsö Group, where the oldest known Bavlinella faveolata occurrences have been reported (Vidal, 1976). Such an age is in perfect agreement with existing geochronological data for the Pickelhaube and equivalent formations in the Port Nolloth Zone (e.g. Fölling, Zartman & Frimmel, 2000).
The microfossil assemblage recovered from the upper Holgat Formation (Fig. 10⇓) is characterized by: (1) low diversity (6 species); (2) dominance of the genus Soldadophycus, especially S. bossii; and (3) absence of acanthomorphic and large (> 500 μm) sphaeromorphic acritarchs. Similar assemblages have been reported from the Cango Caves Group of South Africa (Gaucher & Germs, 2003), Arroyo del Soldado Group of Uruguay (Gaucher, 2000), Corumbá Group and Pouso Alegre Basin of Brazil (Gaucher et al. 2003; Teixeira & Gaucher, 2004) and also the Nama Group in Namibia (Germs, Knoll & Vidal, 1986). Although most of the taxa occurring there are long-ranging, all the mentioned units are demonstrably late Ediacaran (Vendian) in age, as shown by radiometric dating (Grotzinger et al. 1995; Barnett, Armstrong & de Wit, 1997; Fölling, Zartman & Frimmel, 2000), skeletal fossils (mainly Cloudina, Namacalathus and Titanotheca: Germs, 1972; Zaine & Fairchild, 1985; 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; Fölling & Frimmel, 2002; Boggiani et al. 2003; Gaucher et al. 2003; Gaucher et al. 2004a). Furthermore, the Holgat Formation microflora matches the Kotlin-Rovno assemblage of Vidal & Moczydlowska (1997), characterized by low diversity and absence of acanthomorphs and large sphaeromorphs.
A more refined biostratigraphic scheme for the Ediacaran has recently been proposed (Fig. 11⇓), which recognizes three informal acritarch biozones between the Marinoan glacials and the base of the Cambrian (Knoll, 2000; Grey, Walter & Calver, 2003). The Marinoan glacials and strata immediately above are characterized by a simple leiosphere palynoflora (SLP, Fig. 11⇓), followed by an Ediacaran complex acanthomorph palynoflora (ECAP: Grey, Walter & Calver, 2003; Fig. 11⇓) between 580 and 570 Ma, described from Australia, Siberia and China. In the uppermost Ediacaran, plankton diversity decreased dramatically, leading again to a depauperate assemblage dominated by small sphaeromorphs (Kotlin-Rovno assemblage of Vidal & Moczydlowska, 1997; Knoll, 2000; Grey, Walter & Calver, 2003). Based on available 87Sr/86Sr ratios and Pb–Pb carbonate age data (Fölling, Zartman & Frimmel, 2000; Fölling & Frimmel, 2002), correlation of the Holgat assemblage with the younger of the two Ediacaran low-diversity palynofloras (SLP and Kotlin-Rovno assemblage, Fig. 11⇓) of Grey, Walter & Calver (2003) is suggested. Whereas the older SLP occurs in successions characterized by carbonates with 87Sr/86Sr ratios between 0.7065 and 0.7078, carbonates deposited during the younger Kotlin-Rovno interval yielded ratios between 0.7075 and 0.7085 (Walter et al. 2000; Fig. 11⇓). The Holgat Formation carbonates have 87Sr/86Sr ratios in the narrow range of 0.7083 to 0.7085 (Fölling & Frimmel, 2002) and yielded a Pb–Pb age of 555 ± 28 Ma (Fölling, Zartman & Frimmel, 2000), strongly supporting our biostratigraphic inferences. Finally, the genus Titanotheca has been found only in upper Ediacaran successions of South America, co-occurring with Cloudina and low-diversity acritarch assemblages. These successions include the Arroyo del Soldado Group of Uruguay (Gaucher & Sprechmann, 1999; Gaucher, 2000), the Corumbá Group (Gaucher et al. 2003) and the Pico de Itapeva, Eleutério and Cajamar basins of Brazil (Teixeira & Gaucher, 2004).
Whereas a late Ediacaran age is indicated by the above mentioned data, it is still difficult to determine into which part of this c. 30 million year period the Holgat Formation fits. This is due to the still poor resolution of Ediacaran palynostratigraphy. Within the framework of SW Gondwana, two units show palynomorph assemblages strikingly similar to the Holgat Formation, namely the lower Kombuis Member of the Cango Caves Group (South Africa) and the uppermost Polanco–lowermost Cerro Espuelitas formations of the Arroyo del Soldado Group (Uruguay).
The Kombuis Member limestone bears all the geochemical and isotopic characteristics of the Bloeddrif Member in the Port Nolloth Group (Fölling & Frimmel, 2002). The lower Kombuis Member shares four species with the upper Holgat Formation (Fig. 10⇑), the assemblage being also dominated by Soldadophycus bossii (Gaucher & Germs, 2003). Moreover, the Kombuis Member yielded an identical Pb–Pb carbonate age of 553 ± 30 Ma (Fölling, Zartman & Frimmel, 2000), confirming the biostratigraphic data (Fig. 10⇑). Finally, the underlying Nooitgedagt Member of 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 have been reported (Gaucher, 2000) from the transition Polanco-Cerro Espuelitas/Barriga Negra Formation of the Arroyo del Soldado Group (in order of decreasing abundance, Fig. 10⇑): Soldadophycus bossii, S. major, Myxococcoides siderophila, Coniunctiophycus conglobatum, Siphonophycus kestron and unnamed microfilaments (Fig. 10⇑). Note that diversity is comparable to the upper Holgat Formation assemblage, with four species in common (67 %). The microflora is dominated by S. bossii, as in the Holgat Formation. Therefore, we conclude that the palynomorph assemblage of the Holgat Formation is best compared to that 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, and eustatic sea-level drop at the Polanco-Barriga Negra transition (Gaucher, 2000; Gaucher et al. 2004a,b; Fig. 10⇑) possibly hint at the Numees glacial event being recorded there. Such a lower Barriga Negra–Numees correlation would imply that the Arroyo del Soldado shelf was ice-free during the Numees event, because no glacial rocks occur there. This fact militates against a ‘snowball Earth’ scenario for this glaciation (Gaucher et al. 2004b).
The combination of the Soldadophycus–Myxococcoides–Coniunctiophycus–Leiosphaeridia assemblage with negative-to-positive δ13C and 87Sr/86Sr ratios between 0.7080 and 0.7085 seems characteristic of post-Numees/Gaskiers units. This integrated approach might help solve the large uncertainties in the stratigraphy of Neoproterozoic glacial deposits (Kennedy et al. 1998; Saylor et al. 1998).
The impressive similarity of the palynomorph assemblages preserved in the Holgat Formation and Kombuis Member, combined with their identical Pb–Pb carbonate ages of 555 ± 28 Ma and 553 ± 30 Ma, respectively (Fölling, Zartman & Frimmel, 2000), strongly suggests that the cap carbonate (Bloeddrif Member of the Holgat Formation) above the Numees Formation is considerably younger than typical Marinoan 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 the age of the Marinoan glacation comes from the U–Pb zircon age of 636 ± 1 Ma for an ash bed intercalated in the Marinoan-correlative Ghaub Formation of Namibia (Hoffmann et al. 2004). As cap carbonates are believed to have been deposited immediately after the respective glacial event (Kennedy, 1996; Hoffman & Schrag, 2002), a late Ediacaran age (as shown by organic-walled microfossils) around 555 Ma for the Holgat Formation implies that the Numees Formation represents 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 Numees formations. The different degree of thermal alteration of organic matter beneath and above the Numees Formation diamictites could be interpreted as evidence supporting a hiatus between these units. However, the temperature gap of c. 150 °C could also be due to locally variable metamorphic grades, because the pre- and post-Numees units were sampled at different localities (Dreigratberg and Witputs Farm, respectively, Fig. 3⇑). In the Gobabis area an unconformity occurs at the base of the Bildah Member (Buschmannsklippe Formation, Witvlei Group) (Hegenberger, 1993), which according to the authors is a correlative to the Bloeddrif Member of the Holgat Formation. This unconformity truncates the underlying Blässkrans tillite (Hoffmann, 1989) which is equivalent to the Numees tillite. Chemostratigraphic data (Kennedy et al. 1998) seem to indicate that the hiatus at the base of the Bildah Member is less than 50 Myr. Similarly, the C, O and Sr isotope data from the lowermost parts of the Bloeddrif Member (Fölling & Frimmel, 2002) militate against a major hiatus but agree with typical cap carbonate development immediately above a glacial deposit.
More biostratigraphic data, especially from the lower Port Nolloth Group, are needed, however, to solve these uncertainties.
Organic-walled microfossils are reported for the first time from the Gariep Belt (Port Nolloth Group), and represent a highly promising tool to unravel the stratigraphy and age of the different glacial events recorded there. Poorly preserved, highly carbonized acritarchs characterize units that underlie glaciogenic diamictite of the Numees Formation. Based on the occurrence of Bavlinella faveolata in the Pickelhaube and upper Wallekraal formations, an upper Neoproterozoic age younger than c. 700 Ma is inferred for these units. Acritarchs of the upper Holgat Formation, on the other hand, are comparable to many upper Ediacaran assemblages occurring in SW Gondwana and elsewhere. The assemblage is characterized by low diversity (six species), dominance of Soldadophycus bossii, and absence of acanthomorphs and large sphaeromorphs. The agglutinated foraminifer Titanotheca sp. co-occurs with this acritarch assemblage. Biostratigraphic correlation of the Holgat Formation with the lower Kombuis Member (Cango Caves Group) corroborates previously reported Pb–Pb ages for these units of around 555 Ma. This implies that either the Numees Formation was deposited during the post-Marinoan Gaskiers or Moelv glacial events (Halverson et al. 2003; Bowring et al. 2003), or that a large hiatus of c. 50 Myr exists between the Holgat and Numees formations, with the latter reflecting the Marinoan glaciation. The former interpretation is preferred here, and differs from previous inferences (Frimmel, Fölling & Eriksson, 2002). Post-Numees deposits in SW Gondwana are characterized by a Soldadophycus–Myxococcoides–Coniunctiophycus–Leiosphaeridia assemblage, Titanotheca, negative-to-positive δ13C excursions and 87Sr/86Sr values around 0.7080–0.7085. These include, apart from the Holgat Formation, the Buschmannsklippe Formation (Witvlei Group), the Kombuis Member (Cango Caves Group) and the uppermost Polanco–lowermost Barriga Negra Formation (Arroyo del Soldado Group). In the latter unit, a negative δ13C excursion and strong sea-level drop recorded at the Polanco–Barriga Negra transition (Gaucher et al. 2004a,b) may be linked with the Numees glacial event. Finally, a hiatus of less than 5 Myr is represented by the erosive surface at the base of the Nama Group, on the basis of available U–Pb age data of 548.8 ± 1 Ma for the lower Nama Group (Grotzinger et al. 1995) and the age suggested in this study for the Holgat Formation.
We thank Leticia Chiglino (Montevideo) for valuable help during field work in Namibia and preparation of palynological samples. Part of the field expenses and analyses were financed by a research grant from Rand Afrikaans University to G. J. B. Germs. Financial support from the Comisión Sectorial de Investigación Científica (CSIC, Uruguay) to C. Gaucher, and the South African National Research Foundation (gun No. 2053697) to H. Frimmel are gratefully acknowledged. This is a contribution to IGCP project 478 (‘Neoproterozoic–Early Palaeozoic Events in SW-Gondwana’).
- Received January 13, 2004.
- Revision received April 27, 2005.
- Accepted June 22, 2005.