Latest proterozoic microfossils from the nama group, namibia (south west Africa)

Precambrian Research, 32 (1986) 45--62 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

45

LATEST PROTEROZOIC MICROFOSSILS FROM THE NAMA GROUP, NAMIBIA (SOUTH WEST A F R I C A )

GERARD J.B. GERMS 1, ANDREW H. KNOLLs and GONZALO VIDAL~ 1j. C.I. Research Unit, P.O. Box 976, 1760 Randfontein (South Africa) ~Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138 (U.S.A.) 3Department of Geology, Micropaleontological Laboratory, Kemicentrum Box 740, Lund University, 5-220 07 Lund (Sweden) (Received July 23, 1985; revision accepted February 6, 1986)

ABSTRACT Germs, G.J.B., Knoll, A.H. and Vidal. G.. 1986. Latest Proterozoic microfossils from the Nama Group, Namibia (South West Africa). Precambrian Res., 32: 45--62. Gray to black shales from the Kuibis and Schwarzrand subgroups of the Nama Group, South West Africa/Namibia, contain a distinctive, if depauperate, assemblage of organicwalled microfossils. The assemblage is dominated by leiosphaerid acritarchs and fragments of the ribbon-like macrofossil Vendotaenia. Small unicells and non-septate filaments interpreted as the sheaths of oscillatorian cyanobacteria occur with variable frequency throughout the subgroups, while Chuaria circularis, Bavlinella faveolata, and a Comasphaeridium-like form (known from a single specimen) are found as rarer components of the biota. Assemblage composition shows no strong stratigraphic trends within the subgroups. The Nama assemblage compares closely with biotas previously described from uppermost Proterozoic (Valdaian) sequences of the Baltic region, Scandinavia, and Australia, and confirms a latest Proterozoic age for the Kuibis and Schwarzrand subgroups and their contained metazoan fossils. Although its significance is unclear, Nama microfossils tend to be strongly corroded. This diagenetic feature is widespread among Valdaian assemblages, but appears to be much less characteristic of older or younger biotas.

INTRODUCTION The N a m a G r o u p o f S o u t h West A f r i c a / N a m i b i a c o n t a i n s an e x c e p t i o n a l fossil r e c o r d o f early invertebrate evolution. S a n d s t o n e s o f the Kuibis and S c h w a r z r a n d s u b g r o u p s c o n t a i n casts, molds, a n d impressions o f softb o d i e d m e t a z o a n s (Giirich, 1 9 3 0 , 1 9 3 3 ; Richter, 1 9 5 5 ; Glaessner, 1 9 6 3 , 1 9 7 8 , 1 9 7 9 ; Pflug, 1 9 6 6 , 1 9 7 0 a , b, 1 9 7 2 , 1 9 7 3 ; Germs, 1 9 7 2 a , 1 9 7 3 a , b), m a n y o f w h i c h are k n o w n o n l y f r o m this g r o u p , a l t h o u g h t h e f a u n a as a w h o l e is c o m p a r a b l e in architectural grade t o t h e Ediacara f a u n a o f S o u t h Australia (Glaessner, 1 9 8 4 ) . C a r b o n a t e s i n t e r b e d d e d w i t h t h e fossiliferous

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Nama sandstones contain abundant calcareous remains assigned to the genus Cloudina Germs (1972b). Germs (1972b) and Glaessner (1976) placed Cloudina among the cribricyathids, a largely Cambrian group of fossils interpreted as polychaete worm tubes. Both carbonates and siliciclastic rocks of the Kuibis and Schwarzrand subgroups contain evidence of bioturbation, and at least a dozen ichnotaxa have been identified from these units and the overlying Fish River Subgroup (Germs, 1972c; Crimes and Germs, 1982). Proper paleobiological interpretation of these fossils requires that the stratigraphic position of the Nama Group be accurately determined. The invertebrate remains themselves provide what some have considered to be conflicting biostratigraphic signals. The soft-bodied metazoan impressions have been correlated with latest Proterozoic Ediacara-type biotas from other continents, but paleontological tradition linking the first appearance of shelly fossils to the base of the Cambrian would suggest a younger age

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47 estimate based on Cloudina. Several of the ichnofossil genera described by Crimes and Germs (1982) from the Nama Group are best known from Cambrian sequences [although Glaessner (1984, p. 182) has questioned some Nama determinations]. On the basis of available paleontological evidence, Germs (1972a) concluded that the Kuibis and Schwarzrand subgroups (excepting the uppermost part of the Schwarzrand) are probably Vendian, while the uppermost Schwarzrand and Fish River successions are possibly Cambrian. Paleomagnetic studies (KrSner et al., 1980) similarly suggest that the largely unfossiliferous Fish River Group is Cambrian, while the fossil bearing strata of the underlying Kuibis and Schwarzrand subgroups are probably late Proterozoic in age. In this paper, we present micropaleontological data which confirm a latest Proterozoic age for the Kuibis and Schwarzrand subgroups and add a photoautotrophic dimension to our knowledge of Nama paleontology. GEOLOGICAL SETTING The Nama Group covers an area of approximately 125 000 km 2 in south. ern and central South West Africa/Namibia (Fig. 1). Over much of its extent it lies unconformably on crystalline basement; however, in some localities it sits paraconformably or with local unconformity atop sedimentary strata of the Numees Formation (Martin, 1965; McMillan, 1968; KrSner and Germs, 1971). In the vicinity of Vanrhynsdorp, South Africa, Nama units are overlain unconformably by the Table Mountain Sandstone Group which, according to Cocks et al. (1970), is Upper Cambrian near its base. This stratigraphic relationship places an upper limit on the timing of Nama deposition.

Lithologies West of the Groot Karas Mountains (Fig. 1), the Nama sequence has been divided into three lithologically distinct units, the previously mentioned Kuibis, Schwarzrand, and Fish River subgroups (Fig. 2). Lithologies in the Kuibis Subgroup vary according to distance from the Osis ridge, a paleostructural feature that divided the developing Nama basin into two distinct parts. Away from the ridge, Kuibis strata comprise two cycles, each consisting of a basal quartzitic sandstone (with local conglomerates) overlain b y shales and limestones (Fig. 2B). In what are thought to have been the deeper parts of the two subbasins, the sandstone units grade laterally into shale and limestone, and in the far northwest of the Nama basin, shales predominate (Germs, 1983). Stratigraphic thicknesses in the Kuibis Subgroup vary from a few tens of meters in the southeast and near the Osis ridge to as much as 800 m in the northwest. The Schwarzrand Subgroup generally overlies the Kuibis conformably. In the south it consists predominantly of thick limestones with intercalations

48

of sandstone and shales that become increasingly immature upwards (Fig. 2B). The carbonates become progressively thinner and eventually grade into shales to the east and north (Fig. 2A). Several unconformities are present within the sequence (Germs, 1983). The first red bed~ in the Nama succession occur in northern outcrops of the Nomtsas Formation near the top o f the Schwarzrand Group. These beds resemble overlying Fish River sediments, but pass laterally into thick stromatolitic limestone and blue-tinted shale to the south and southwest. At many localities, Nomtsas beds sit unconformably atop lower Nama units. In the north, the Nomtsas Formation is itself overlain unconformably by sandstones of the Vergesig Formation, the youngest unit of the Schwarzrand Subgroup. Like those of the Kuibis, Schwarzrand strata are thinnest to the southeast and thickest in the west and northwest where up to 1400 m of section have been noted. The Fish River Subgroup consists of red, medium- to coarse-grained, feldspathic sandstones and intercalated shales that rest unconformably on Stratigraphy

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Depositional environments Siliciclastic sediments "of the Kuibis and Schwarzrand subgroups were transported primarily from easterly source areas o n the Kaapvaal Craton.

50 The Fish River Subgroup, in contrast, is a distal molasse derived from northerly and westerly sources during the later stages of deformation in the Damara and Gariep Orogens. The Kuibis and Schwarzrand subgroups d o c u m e n t depositional environments ranging from braided streams to quiet subtidal marine conditions (Fig. 2). Intertidal to shallow subtidal conditions seem to have prevailed for much of the period during which the lower two subgroups were deposited, b u t in the upper Schwarzrand Subgroup fluvial deposits become increasingly significant, and in the Fish River Subgroup they constitute the dominant facies. There is little evidence for deposition in deep water; for example, no turbidites have been identified to date. The shale-rich units of the lower subgroups (i.e., the Urikos and Vingerbreek members, and the bluish-green shale unit of the Nomtsas Formation) indicate quiet shelf conditions below wave base and constitute the deepest depositional environments of any Nama unit. The depositional environments (and fossil contents) of Nama units to the north and south of the Osis ridge are summarised in the schematic stratigraphic sections shown in Fig. 2. For a detailed description of facies relationships within the Nama Group, see Germs (1983).

Location o f drill cores (boreholes) Because of pervasive surficial weathering, few samples collected from outcrop contain organically preserved fossils. The microfossils described in this paper were isolated from core samples obtained from the Tses (TS) and Nutupsdrift (ND) boreholes, drilled by Aquitain SWA and DeBeers Oil Holdings and the Tsumeb Corporation, respectively. Borehole locations and logs are shown in Figs. 1 and 3. The Tses borehole is situated just to the northeast of the area whose stratigraphy is represented by the schematic section shown in Fig. 2A, and its stratigraphy is similar to that of the schematic section (Fig. 3). The borehole section differs from that shown in Fig. 2A principally in the absence of the Mooifontein Member of the Zaris Formation (Kuibis Subgroup) and Huns Limestone Member of the Urusis Formation (Schwarzrand Subgroup); these carbonates thin to feather edge south and west of the borehole site. In the Tses borehole the stratigraphy of the Schwarzrand Subgroup above the Nasep Member is not clear. The grey limestone drilled at a depth of 1299 m (4245 ft) is a correlate of either the Spitskop Member or the Nomtsas Formation, and it is not certain to which member the shale just below the base of the Fish River Subgroup belongs. It seems quite possible that this shale is part of the Nomtsas Formation, in that Nomtsas beds usually underlie the Fish River Subgroup in the area near the borehole; however, this has not been demonstrated. The biostratigraphic significance of these uppermost Schwarzrand shales is discussed below. The Nutupsdrift borehole was drilled approximately 40 km from the localities where the trace fossils ?Chondrites, Diplocraterion, Planolites

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and Nereites were found in the Nudaus Formation of the Schwarzrand Subgroup (Crimes and Germs, 1982). The stratigraphy of this borehole corresponds in general to the schematic stratigraphic section of Fig. 2B. Much as is shown in that figure, the limestones of the upper Mooifontein Member, which are well developed elsewhere, grade into the green shales of the Urikos Member penetrated by the borehole (Fig. 3). Also, in the borehole section, the sandstone units of the Niederhagen Member are thinner and the Vingerbreek Member more shale-rich than shown in the schematic stratigraphical section of Fig. 2B. MICROFOSSIL PRESERVATION In general, dispersed organic matter in samples from the Nama Group is poorly preserved. Microfossils and disseminated organic residues display colors ranging from light to very dark-brown, and in some instances black, and the material shows no visible traces of induced fluorescence. These observations suggest an index of alteration close to AMC 5--6, which corresponds to R°% 1.5--1.55 -- approximately equivalent to a mesokatagenic stage of organic diagenesis (Mk4; cf. Rovnina, 1981). From this, one may infer conditions of thermal alteration corresponding to about 170--200°C. Most processed samples are rich in sapropel-like amorphous organic matter. This material has a granular or flaky appearance and is associated with pyrite pseudomorphs and extremely small (often <10 pro) pyrite framboids which often have left clear imprints on the surfaces of organic flakes and sheets. Microfossils in most examined samples are poorly preserved and display clear traces of extensive post-mortem degradation. Most microfossils, particularly spheroidal acritarchs attributable to Leiosphaeridia spp., exhibit irregularly subgranular to nearly psilate surface textures which appear to be the result of vesicle wall hydrolysis under alkaline, reducing conditions. Empty filamentous cyanobacterial sheaths often display a comparable degree of degradation, although a few samples (e.g. TS 3973 and 4533 from the upper Schwarzrand Subgroup) yielded relatively well preserved filamentous sheaths. It may be significant that the state of organic preservation noted for the Nama assemblage appears comparable to that characteristic of most other Valdaian (Ediacarian, late Vendian) organic-walled microfossils. This circumstance is particularly puzzling because older (e.g., Riphean and early Vendian) microfossils known from geographically widespread localities generally exhibit rather good preservation (e.g., Timofeev, 1959, 1966, 1969; Vidal, 1976, 1981a, b; Timofeev et al., 1976; Jankauskas, 1982; Knoll and Swett, 1985), as do Early Cambrian microfossil assemblages from northern Europe and Greenland (e.g., Vidal, 1979; Volkova et al., 1979; Downie, 1982). Valdai-age rocks appear in general to yield abundant sapropel-like, acidresistant residues closely comparable to the granular, flaky, amorphous

53

organic residues in the Nama Group. Because they have undergone a minim u m of burial diagenesis, Valdai and Valdai age-equivalent rocks in the East European Platform are insignificantly thermally altered (Keller and Rozanov, 1979). The organic residues from these rocks range in color from light greenish yellow to light yellow and, additionally, display considerable green, blue, violet, and UV induced fluorescence. Valdai correlatives on the Fennoscandian Shield (cf. Vidal, 1981a, b, 1985) are usually restricted to the eastern autochthonous margin of the Scandinavian Caledonian fold belt where they have been affected b y tectonic deformation to a variable extent. In most instances, these organic residues exhibit thermal alteration comparable to or even higher than those recorded in the Nama material under consideration. Sapropel A is generally thought to form in low energy aqueous environments where copious organic material can accumulate on a quiet sea or lake floor under conditions of restricted water circulation and low oxygen (Staplin et al., 1973; in Venkatachala, 1981). Sedimentological data for the Nama and some other Valdaian units do suggest deposition in quiet marine basins; however, it is not clear how, if at all, these latest Proterozoic deposits differ environmentally from older and younger successions whose fossils show no comparable degree of corrosion. As demonstrated b y East European and Fennoscandian assemblages, Valdaian microfossils showing comparable corrosion patterns can have very different burial and tectonic histories. Indeed, the only feature that appears to unite the Valdaian assemblages and separate them from other microfossils that do not exhibit similar degradation patterns is their age. What unique features of the Valdaian biosphere could have promoted such widespread formation of sapropellike organic accumulations and microfossil corrosion remains obscure. COMPOSITION OF THE MICROFOSSIL ASSEMBLAGE

Microfossils recovered from the Kuibis and Schwarzrand subgroups include the following forms: Leiosphaeridia spp. Chuaria circularis Walcott emend. Vidal and Ford 1985 Bavlinella faveolata (Shepeleva) Vidal 1976 Vendotaenia sp. Filamentous sheaths Comasphaeridium-like microfossil The microfossil assemblage is remarkably simple and invariant in composition. It is dominated by poorly preserved sphaeromorphic acritarchs attributed to Leiosphaeridia spp. Leiosphaerid individuals vary widely in diameter, although most specimens fall in the range of 30--70 ~m (Fig. 4a--d). Smaller uniceUs in the 10--16 p m size range also occur (Fig. 4e, h). Surface textures within these populations vary from smooth to markedly granular, but as discussed above, these variations appear to reflect diagenetic

54

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Fig. 4. Photographs of microfossils freed by maceration of Kuibis and Schwarzrand shales. For each specimen, sample number, slide number and England finder coordinates, and Harvard University Paleobotanical Collection number are given. Bar in b=40 g m for a and b, =20 um for c---e and g--j, and =250 um for f. a, Leiosphaeridia sp., TS-4533, K-1 at R]16-0, H.U. #62057. b, Leiosphaeridia sp. ND-93, V-1 at 0/31-0, H.U. #62058. c, Leiosphaeridia sp., TS-4533, K-2 at J/38-0, H.U. #62059. d, Leiosphaeridia sp., ND-93, V-1 at D/49-2, H.U. #62060. e, small unicell shown in two focal planes, TS-4533, V-1 at V/30-2, H.U. #62061. f, Chuaria circularis, T~-4533, V-1 at V/37-1, H.U. #62062. g, Bavlinella faveolata, ND-290, V-1 at C/23-4, H.U. #62063. h, small unicell, TS-4533, K-1 at R/48-0, H.U. #62064. i, B. faveolata, ND-87, V-1 at Z/29-3, H.U. #62065. j, B. faveolata, TS-5150, V-1 at H/28-0, H.U. #62066.

55 corrosion and, hence, have limited taxonomic value (see also Damassa and Knoll, 1986). In addition to leiosphaerids, the assemblage includes a number of less common microfossils. Large (114--319 um diameter), thick-walled, often strongly corroded sphaeromorphs occasionally occur in samples from the uppermost Kuibis and Schwarzrand subgroups (Fig. 4f). Their simple morphology, wall thickness and large dimensions permit taxonomic attribution to Chuaria circularis Walcott emend. Vidal and Ford (1985). The assemblage also includes filamentous microfossils which we interpret as empty sheaths of oscillatorian cyanobacteria. For the most part these filaments are 60-200 ~m long and 3--35 ~m wide (~ = 14.1 ~m; Fig. 5d); however, we have observed scattered specimens of generally larger (20--40 pm cross-sectional diameter) compressed filaments which display pseudosepta produced by imprints of trichome cells on the inner surface of the sheaths (Fig. 5e--h). Cyanobacterial filaments vary widely in abundance from one Nama sample to the next. Bavlinella faveolata (Shepeleva) Vidal (1976), a probable pleurocapsalean (Moorman, 1974; Knoll and Swett, 1985) or chroococcalean (Mansuy and Vidal, 1983) cyanobacterium, occurs sporadically in Kuibis and Schwarzrand samples (Figs. 4g-j, 6). Specimens consist of tiny (2 = 1.7 pm) unicells packed tightly into spherical aggregates 5--35 a m in diameter (2 = 20.4 ~m), much like previously described populations from uppermost Proterozoic successions of the Northern Hemisphere (e.g., Moorman, 1974; Vidal, 1976; Knoll et al., 1981; Mansuy, 1983). A single microfossil specimen 14 ~m in diameter and displaying a distinctive fringed surface ornamentation of short, densely packed processes was observed in sample TS-5150 from the upper Kuibis Subgroup. Among previously described acritarch genera, Comasphaeridium and, to a lesser extent, Micrhystridium most closely resemble this fossil. Morphologically similar microfossils are common in samples from the Platysolenites antiquissimus zone in cores from the Lublin slope of Poland (M. Moczyd~owska, personal communication to G.V., 1985). The final component of the Nama assemblage is Vendotaenia sp. (Gnilovskaya, 1971, 1975, 1976). Fragments of these distinctive ribbon-like fossils are common in most fossiliferous Nama samples. Like other fossils in this assemblage, Nama vendotaenids display variable degrees of postmortem alteration. When relatively well-preserved, they appear to be completely smooth except for longitudinal and transverse folds and wrinkles (Fig. 5a, c). Poorly preserved specimens, on the other hand, display granular to fibrous structures which undoubtedly reflect advanced degrees of corrosion and disintegration. Gnilovskaya (1971, 1975, 1976) interpreted vendotaenids as the remains of a more or less homogeneous group of metaphytic algae; however, a re-examination by one of us (G.V.) of type material and additional specimens from the Fennoscandian Shield and Poland suggests the need to rethink the systematic interpretation of these fossils. Few if any vendotaenids exhibit surficial patterns indicative of cellular organization

56

within the purported thallus; most in fact have a distinctly sheath-like texture. No unequivocal remains of differentiated reproductive bodies have been observed -- structures described by Gnilovskaya as sporangia or oogonia may more reasonably be interpreted as fortuitously superimposed,

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degraded filaments and spherical patches of sapropel and/or sphaeromorphic acritarchs. In a limited number of instances, highly degraded vendotaenids from Valdaian deposits contain abundant 20 ~m wide cylindrical sheaths (possibly cyanobacterial) aligned along their longitudinal axes. The systematic position of Vendotaenia thus remains problematic, and its attribution to any group of metaphytes or even to the algae at all remains in need of definitive supporting evidence. STRATIGRAPHIC DISCUSSION

Organic-walled microfossils are abundant in the upper Vendian Valdai 'Series' of the Baltic region (Volkova, 1973; Vokova et al., 1979; Korkutis, Fig. 5. Photographs of microfossils freed by maceration from Kuibis and Schwarzrand shales. For each specimen, sample number, slide number and England finder coordinates, and Harvard University Paleobotanical Collection number are given. Bar in b =200 #m for a, =20 ~m for b and i, =175 ~m for c and d, =25 ~m for e, f, and h, and =35 um for g. a, Vendotaenia sp. fragment, ND-290, V-1 at T/26-0, H.U. #62067. b, Spheroidal fossil, possibly Bavlinella, but showing the effects of diagenetic pyrite growth on surface morphology, TS-5150, V-1 at R/39-4, H.U. #62068. e, Vendotaenia sp. fragment, TS-4533, V-1 at H/29-3, H.U. #62069. d, mass of filamentous sheaths, TS-3973, V-1 at F/41-3, H.U. #62070. e, filamentous sheath containing partially degraded trichome, TS-4533, K-1 at M/58-1, H.U. #62071. f--h, large filamentous sheaths displaying pseudosepta --f, TS-4533, K-2 at B/53-3, H.U. #62072; g, ND-565, V-1 at M/47-0, H.U. #62073; h, TS-4533, K-2 at A/51-3, H.U. #62074. i, Comasphaeridium-like fossil, TS-5150, V-1 at L/25-2, H.U. #62075.

58 1981), but despite their abundance, morphologic diversity is low. Assemblages consist predominantly of undiagnostic leiosphaerid acritarchs, cyanobacterial filaments, and vendotaenids, along with occasional specimens of Chuaria and Bavlinella. Micrhystridium tornatum has also been reported from Valdaian rocks (Volkova et al., 1979), but detailed examination of core material from the Lublin Slope, Poland, by one of us (G.V.) and M. Modzyd~owska (unpublished observations) has failed to reveal any specimens of M. tornatum below beds of the P. antiquissimus zone (lowermost Cambrian). We suggest that if M. tornatum does occur in Valdaian assemblages, it must be extremely rare and limited in paleogeographic distribution. This assemblage can easily be distinguished from latest Riphean and earlier Vendian assemblages in the Northern Hemisphere, as well as from Early Cambrian biotas of the East European Platform and elsewhere. Valdai equivalents in Poland (the Lublin 'Series') contain assemblages that are essentially identical to those found in the Soviet Union (M. Moczyd~owska, personal communication to G.V., 1985), and sequences of post-Varangerian (Valdai) age in Finnmark and along the thin autochthonous rim of the Scandinavian Caledonides yield acid-resistant remains of comparable aspect (Vidal, 1981a, b, 1985). Other peri-North Atlantic assemblages that may be compared to the Valdai biota with less confidence include a rarely cited assemblage from the Brioverian of Normandy described by Roblot (1964) and a poorly preserved biota from the late Proterozoic Fermeuse Formation of eastern Newfoundland (Hofmann et al., 1979). It should be emphasized that in neither of the two last mentioned cases have vendotaenids been noted. Additionally, an assemblage described from the Muhos Formation of Finland (Tynni and Donner, 1980; Tynni and Uutela, 1984) may be of Valdai age. Leiosphaerid/vendotaenid assemblages have recently been discovered in latest Proterozoic successions of Australia (Damassa and Knoll, 1986). The Tent Hill Formation of the Stuart Shelf contains a biota similar to that of the Valdai. On lithological grounds, fossiliferous Tent Hill units have been correlated with the ABC Range Quartzite of the adjacent Adelaide Geosyncline (Preiss et al., 1981), a unit that lies just beneath the metazoan bearing beds of the Pound Quartzite and within the Ediacarian System stratotype proposed by Cloud and Glaessner (1982). Beds of the Central Mt. Stuart and Grant Bluff formations, Georgina Basin, Northern Territories, also contain Valdaian microfossil assemblages. Scattered Ediacaran-type metazoan fossils have been reported from sandstones of the Central Mt. Stuart sequence (Wade, 1969). The microfossil assemblage found in the Kuibis and Schwarzrand subgroups thus appears to be both geographically widespread and stratigraphically restricted, allowing the unequivocal assignment of a latest Proterozoic (Valdaian) age to the metazoan bearing subgroups of the Nama Group. The presence of the Comasphaeridium-like fossil in the Kuibis Subgroup might be used to corroborate earlier suggestions that Kuibis and Schwarzrand sediments were deposited relatively late m the Valdai interval (e.g.,

59

Cloud and Glaessner, 1982); however, insofar as only one specimen has been observed and its taxonomic identification remains uncertain, little confidence can be placed in stratigraphic inferences based solely on this fossil. There is no indication that any of the Nama microfossil assemblages recovered come from Cambrian rocks. In this context, the unresolved age of the Nomtsas Formation must be noted. In the Tses borehole, latest Proterozoic fossils occur in the uppermost beds of the Schwarzrand succession; however, as noted above, the relationship of these shales to ichnofossil bearing Nomtsas units is unclear. The uppermost Schwarzrand shales o f the borehole may belong in part or in t o t o to the Nomtsas Formation, b u t it is also possible that Nomtsas equivalents are locally absent at the borehole site. Nomtsas siltstones contain the trace fossil Phycodes pedum, known elsewhere only from Early Cambrian and younger rocks (Crimes and Germs, 1982), as well as two species of Neonereites, N. uniserialis and N. biserialis, known mostly from Phanerozoic rocks b u t described by Fedonkin (1976, 1977) from the Vendian of the Onega Peninsula. Microfossil assemblages thus confirm the latest Proterozoic age suggested for the Kuibis and Schwarzrand subgroups on the basis of paleomagnetic data (KrSner et al., 1980). Thus far, it has not proved possible to determine relative ages for early metazoan fossils using microfossil data. Whether such discrimination will be possible in the future remains to be seen. FOSSIL REPOSITORIES

Microfossils illustrated in this paper have been deposited in the Paleobotanical Collections of Harvard University and assigned collection numbers 62057 to 62074. Additional materials are housed in the collections of the Micropaleontological Laboratory, Lund University. ACKNOWLEDGMENTS

We thank Drs. R. McG. Miller and W. Hegenberger for supplying us with samples from the Tses borehole and E. Burkhardt for preparing the figures used in this paper. This research was supported in part b y NSF Grant BSR 82-13682 (to A.H.K.) and grants from the Natural Science Research Council (NFR) and the Knut and Alice Wallensbergs Stiftelsen (to G.V.). We also acknowledge N F R grants which made possible research collaboration with Drs. M.B. Gnilovskaya and M. Modzyd~owska during their tenures as visiting scientists at the Micropaleontological Laboratory, Lund University.

REFERENCES Cloud, P. and Glaessner, M.Fo, 1982. The Ediacarian period and system: Metazoa inherit the Earth. Science, 217 : 783--792.

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