Research ArticleARTICLES

Stratigraphy of the Port Nolloth Group of Namibia and South Africa and implications for the age of Neoproterozoic iron formations

Francis A. Macdonald, Justin V. Strauss, Catherine V. Rose, Francis Ő. Dudás and Daniel P. Schrag
American Journal of Science November 2010, 310 (9) 862-888; DOI: https://doi.org/10.2475/09.2010.05

Abstract

Uncertainties in the number and age of glacial deposits within the Port Nolloth Group have hindered both structural and stratigraphic studies in the Neoproterozoic Gariep Belt of Namibia and South Africa. These uncertainties are compounded by major lateral facies changes that complicate correlations locally. Herein, we report the results of integrated geological mapping, chemo- and litho-stratigraphic, and sedimentological studies that shed light on the age and stratigraphic architecture of the Port Nolloth Group. Particularly, we have distinguished an additional glacial deposit, herein referred to as the Namaskluft diamictite, which is succeeded by a ca. 635 Ma basal Ediacaran cap carbonate. This interpretation indicates that the stratigraphically lower, iron-bearing Numees diamictite is not Marinoan or Gaskiers in age, as previously suggested, but is instead a ca. 716.5 Ma Sturtian glacial deposit. A Sturtian age for the Numees Formation is further supported by the discovery of microbial roll-up structures in the dark limestone of the Bloeddrif Member that caps the diamictite. A re-evaluation of the age constraints indicates that all Neoproterozoic iron formations may be of Sturtian age, and thus indicative of secular evolution of the redox state of the ocean.

INTRODUCTION

The Port Nolloth Group (PNG) of South Africa and Namibia hosts glacial deposits, iron formations, mixed carbonate-siliciclastic rocks, enigmatic microbialites, economically significant sedimentary exhalative Pb-Zn deposits, and datable volcanic rocks (Rogers, 1915; Kröner, 1974; Frimmel, 2008). However, due to structural complexities and large lateral facies changes, stratigraphic correlations within the PNG have remained unclear. Particularly, much debate has centered around the number and age of the glacial deposits (Jasper and others, 2000; Frimmel, 2008). The most recent review of the stratigraphy of the PNG concludes that the Kaigas Formation (table 1) is a pre-Sturtian ca. 750 Ma glacial deposit, the Numees Formation and associated iron formation are ca. 580 Ma Gaskiers-age glacial deposits, and that both the ca. 716.5 Ma Sturtian and ca. 635 Ma Marinoan glaciations are missing (Frimmel, 2008). This interpretation has significant implications not only for the tectonic evolution of the Kalahari Craton, but also for the evolution of ocean chemistry. The presence of extensive iron formation is an important indicator of the redox state of the ocean, and thus constraining the temporal range of major iron deposits in Earth history is critical to a better understanding of the co-evolution of oxygen, climate, and life.

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Table 1

Generalized stratigraphy of the Port Nolloth Group, contrasting the stratigraphic scheme presented herein with that of Frimmel (2008)

Previous studies have relied on regional lithostratigraphic correlations of the diamictites (for example Von Veh, 1993). However, such correlations can be compromised by complexities in the stratigraphy of glacial deposits: lateral facies changes are abundant, and glaciers access different sedimentary sources over time. In contrast, carbonate rocks that bound Neoproterozoic diamictites are likely to reflect conditions in a well-mixed ocean reservoir and have distinct geochemical and sedimentological features that can be used for both local and global correlation (Hoffman and Schrag, 2002; Halverson and others, 2005). In this paper, we use carbon and strontium isotope chemostratigraphy linked to measured stratigraphic sections and geological mapping to test regional correlations and map relationships. We extend these regional correlations and construct a new age model for deposition of the PNG.

STRATIGRAPHIC SETTING

The Gariep Belt is a Pan-African/Brasiliano orogenic belt exposed on the western margin of the Kalahari Craton in southwestern Namibia and northwestern South Africa (Stowe and others, 1984). Folded strata in this Ediacaran to Cambrian transpressional orogen (Davies and Coward, 1982) include the PNG, which formed on the Kalahari margin of the Adamastor Ocean after early Neoproterozoic to Cryogenian rifting (Jasper and others, 2000; Frimmel and others 2001), and the Nama Group, which was deposited in a foreland basin in response to the collision between the Congo, Kalahari, and Rio de la Plata cratons (Germs and Gresse, 1991). The PNG refers specifically to the Neoproterozoic stratigraphy in the Port Nolloth Zone, which is exposed on the autochthon and para-autochthon of the Gariep Belt (fig. 1), as distinguished from the allochthonous Mamora Terrane west of the Schakalsberge Thrust (Frimmel, 2008).

Fig. 1.
Fig. 1.

Geological map of the Gariep Belt autochthon with inset of location map. Mapping southwest of Rosh Pinah modified from Von Veh (1993). Boxes mark the extent of the small-scale maps in figure 3. Stars mark locations discussed in the text.

The PNG formed in NNW-SSE trending grabens, which post-date 771 ± 6 Ma granites of the Richtersveld suite (single grain Pb/Pb zircon evaporation age, Frimmel and others, 2001). A minimum age is provided by overlying foreland sedimentation of the Nama Group, which began by ∼548 Ma (Grotzinger and others, 1995). The PNG in the South African portion of the Gariep Belt, including the Kaigas Series and the Numees Series, was first described by Rogers (1915), however diamictite was only identified in the Numees Series. The Kaigas diamictite was later distinguished from the Numees diamictite by De Villiers and Sohnge (1959). More recent mapping of the Gariep Belt, both on the South African side of the Orange River (Kröner, 1974; Von Veh, 1993) and the Namibian side (Martin, 1965b; McMillan, 1968; Jasper and others, 2000; Frimmel, 2008), has resulted in conflicting interpretations of the stratigraphy. Particularly, Von Veh (1993) and Frimmel (2008) introduced an array of thrust faults to accommodate their stratigraphic correlations. Our correlations suggest simpler structure (fig. 1), more akin to the earlier work of Rogers (1915), McMillan (1968), and Kröner (1974); however, the identification of the Namaskluft diamictite, which rests above the Numees Formation (table 1), allows many exposures previously correlated with the Kaigas Formation to be reassigned to the Numees Formation (fig. 1).

In contrast to previous studies of the PNG that focused on para-autochthonous and allochthonous exposures (for example Frimmel and others, 2002), we centered our litho- and chemo-stratigraphic studies on the least-deformed, most-autochthonous sections (fig. 1). Where possible we have retained the current stratigraphic nomenclature as reviewed by Frimmel (2008), but separated the Wallekraal and Dabie River Formations from the Hilda Subgroup and added informal members to distinguish the Namaskluft diamictite and Dreigratberg cap carbonate within the Holgat Formation (table 1). A critical difference in this stratigraphic scheme from previous work is that we place the Wallekraal and Dabie River Formations above the Numees diamictite.

STRATIGRAPHY OF THE PORT NOLLOTH GROUP

Stinkfontein and Hilda Subgroups

Deposition of the PNG commenced with the accumulation of approximately 800 m of coarse siliciclastic and bimodal volcanic rocks of the Stinkfontein Subgroup (Von Veh, 1993). These strata are succeeded by the Hilda Subgroup, which is subdivided into the Kaigas, Rosh Pinah, and Picklehaube Formations (table 1). The Kaigas Formation is up to 100 m thick and consists predominantly of subrounded, gravel- to boulder-sized basement clasts suspended in a matrix that ranges from argillite to feldspathic sandstone with complex lateral facies changes. The glacigenic origin of this diamictite remains questionable and depends on correlations with rocks that could alternatively be assigned to the Numees Formation. The Kaigas Formation is succeeded by the Rosh Pinah Formation, which consists of up to 850 m of arkosic sandstone, organic-rich shale, carbonate, and felsic volcanic rocks that were deposited in an actively rifting graben (Alchin and others, 2005). The volcanic rocks are thickest ∼15 km north of Rosh Pinah near the Skorpion Mine (fig. 1), where they have also been referred to as the Spitzkop Formation. Rhyolite flows within the Rosh Pinah Formation contain zircons that have been dated at 752 ± 6 Ma (U/Pb zircon evaporation age, Borg and others, 2003) and at 741 ± 6 Ma (Pb/Pb age, Frimmel and others, 1996).

The Picklehaube Formation is present predominantly west of Rosh Pinah, and is composed of greater than 200 meters of carbonate. A stratigraphic thickness is difficult to determine due to the strong deformation in these western exposures. On paleo-highs these carbonates consist of upward-shallowing parasequences capped by microbialaminates, including stromatolites and giant ooids (>0.5 cm diameter) that are lithologically very similar to those in the Dabie River Formation. In deeper-water settings, the Picklehaube Formation is composed of hundreds of meters of allodapic limestone.

Numees Formation

The Hilda Subgroup is overlain by the Numees Formation, which contains both massive and stratified glacial diamictites with dropstones (Rogers, 1915; Martin, 1965b) and the iron formation of the Jakkalsberg Member (Mb) (fig. 2A) (Frimmel and Von Veh, 2003). The Numees Formation is thickest in the syncline west of the Numees Mine (fig. 1) where it is estimated to be as much as 500 m thick (Frimmel and Von Veh, 2003). Clasts within the diamictite are derived from all of the underlying stratigraphy and basement; outsized clasts reach several meters in diameter.

Fig. 2.
Fig. 2.

Field photographs: (A) iron formation with dropstone in Numees Formation, northwest of Dreigratberg syncline, coin for scale is 2.5 cm in diameter; (B) microbialaminite of Bloeddrif member from Namaskluft Camp, ballpoint pen for scale; (C) carbonate dropstone in the Namaskluft diamictite at Dreigratberg, coin for scale is 2.0 cm in diameter; (D) tubestone stromatolites in plan view in the Dreigratberg cap carbonate on the top of the escarpment near Namaskluft Farm, lighter for scale; (E) giant wave ripples in the Dreigratberg cap carbonate on the top of the escarpment near Namaskluft Farm, 33 cm hammer for scale; (F) sheet-crack cements in the Dreigratberg cap carbonate at Dreigratberg, 33 cm hammer for scale.

Holgat, Wallekraal, and Dabie River Formations

The Numees diamictite is capped by the Bloeddrif Member of the lower Holgat Formation, a dark gray laminated limestone that ranges in thickness from 1 to 120 m. This limestone unit contains microbial roll-up structures and abundant crinkly lamination that we interpret as sublittoral microbialaminite (fig. 2B), and it is commonly interbedded with thin grainstone and sandstone turbidite beds. The lower Holgat Formation consists of as much as 250 m of allodapic carbonate and argillite, including olistoliths of the Dabie River Formation.

The Wallekraal and Dabie River Formations have previously been included with the Hilda Subgroup (Frimmel, 2008). However, our mapping shows that these formations rest between the Numees and Namaskluft diamictites (figs. 1 and 3). On the autochthon, the Wallekraal Formation interfingers with the lower Holgat Formation (see description below in “Key Localities”), and consists predominantly of sandstone turbidites that were deposited in submarine channels. The Wallekraal Formation is distinguished from the Rosh Pinah Formation by the presence of boulder-sized olistoliths and coarse feldspathic grit. The Dabie River Formation is the shallow water equivalent of the lower Holgat Formation and consists of as much as 160 m of carbonate, including conophyton stromatolites, giant ooids, and intraclast breccias.

Fig. 3.
Fig. 3.

Geological maps of (A) Namaskluft Camp, (B) Namaskluft Farm, (C) Dreigratberg, and the (D) Kaigas River region. Legend is the same as figure 1.

Throughout the Gariep Belt, the Namaskluft diamictite has either been overlooked or mis-mapped as the Numees diamictite. This has been a major source of stratigraphic confusion. The Namaskluft diamictite ranges from 5 to 240 m thick and consists of both massive and stratified diamictite units with clasts from all of the underlying stratigraphy and the basement. Outsized clasts and bed-penetrating dropstones suggest a glacial origin for the Namaskluft diamictite (fig. 2C). This diamictite is capped by the informally named Dreigratberg member of the upper Holgat Formation (table 1). In more proximal settings, the Dreigratberg member is up to 40 m thick and composed predominantly of buff-colored, fine-laminated, micropeloidal dolomite with stromatolite bioherms and giant wave ripples (figs. 2D and 2E). In deeper water settings, the Dreigratberg member is less than 5 m thick, and hosts sheet-crack cements (fig. 2F). These dolostones are overlain by as much as 400 m of allodapic carbonate, shale, and sandstone turbidites of the upper Holgat Formation. On the autochthon, the upper Holgat Formation is commonly cut out under the sub-Nama Group unconformity.

KEY LOCALITIES

Namaskluft Camp

At Namaskluft Camp (fig. 3A), the Stinkfontein Subgroup is missing and a diamictite lies directly on basement with a sharp erosional contact. This diamictite ranges from 0 to 35 m thick, and is composed predominantly of stratified diamictite. Clasts range in size from pebbles to boulders, and consist predominantly of granitic and gneissic basement with rare limestone and sandstone. The diamictite fills an ∼8 km wide and ∼1 km deep E-W oriented paleo-canyon (fig. 3A) and contains ripple cross-lamination with flow towards the west. A glacigenic origin for the diamictite is suggested by the presence of bed penetrating dropstones. This diamictite has previously been mapped as the Kaigas Formation (McMillan, 1968). However, we assign this diamictite to the Numees Formation because it is overlain by blue fine-laminated limestone that is lithologically and isotopically similar to the Bloeddrif Formation. These carbonate beds comformably grade into quartz sandstone turbidites (Bouma Ta-b) of the Wallekraal Formation in fining-upwards cycles ranging from <1 m to over 20 m. Scours and deep-water trough cross-beds are present that indicate flow to the west. Map relationships and measured sections indicate that the Wallekraal Formation thins dramatically both to the west and the east, consistent with the filling of a submarine channel that dissects a rift shoulder. The sandstone turbidites are up to 600 m thick and are succeeded by as much as 200 m of green argillite, allodapic limestone, and channelized debris flows dominated by very coarse subangular feldspathic grit with rafted angular limestone cobbles (fig. 4). The Wallekraal Formation grades upwards into micrite and marl of the lower Holgat Formation with carbonate olistostromes derived from the Dabie River Formation.

Fig. 4.
Fig. 4.

Chemo-stratigraphic correlations of the PNG in Namibia. Trekpoort Farm is located near Skorpion mine. Locations of sections are in figures 1 and 3.

The overlying Namaskluft diamictite has an erosional base and rests on all of the underlying stratigraphy and basement. The diamictite is channelized and up to 240 m thick. Within the channel, the base of the diamictite consists of 20 to 70 m of green to purple laminated siltstone with granite boulder dropstones, limestone pebbles and gravel, and lenses of pebble conglomerate. This lower interval includes evidence of ice-grounding in the form of truncations, plowed clasts, and abundant soft-sedimentary deformation. The Namaskluft diamictite is succeeded by an additional ∼90 m of sandstone matrix diamictite with lenses of carbonate pebble clasts, and intervals of green siltstone to marl matrix diamictite with granite boulder lonestones. These stratified diamictite units are overlain, across a sharp, erosional contact, by a massive diamictite that is up to 80 m thick and consists of clasts of limestone, dolomite, and sandstone in a chocolate brown calc-arenite matrix. The upper meter of the Namaskluft diamictite has a laminated matrix with bed-penetrating dropstones, flame structures, and festoon cross-lamination. The Namaskluft diamictite is overlain by a fine-laminated, buff-colored micropeloidal dolomite with low angle cross-stratification that we assign to the Dreigratberg Member. Outsized clasts occur in the lower 0.5 m of the Dreigratberg Member. The cap dolostone is up to 30 m thick and also contains giant wave ripples and tubestone stromatolites (Corsetti and Grotzinger, 2005). It is overlain by >300 m of upper Holgat strata consisting of pink to light gray, allodapic limestone and siltstone with occasional hummocky cross stratification that are preserved under the sub-Nama unconformity.

Namaskluft Farm

The carbonates exposed on Namaskluft Farm (fig. 3B) are stratigraphically above the Numees diamictite and below the Namaskluft diamictite, and thus should be included with the lower Holgat Formation. On the easternmost exposures, limestone and a thin, laterally discontinuous diamictite rest with a tectonic contact against crystalline basement. We interpret this contact as a syn-sedimentary normal fault. Above the diamictite are ∼120 m of limestone rhythmite, ∼80 m of argillite with carbonate olistostromes, and an additional ∼30 m of massive carbonate grainstone with stromatolite bioherms (fig. 4). The stromatolites are sharply overlain by the Namaskluft diamictite, which consists of ∼30 m of carbonate matrix diamictite with predominantly sub-rounded carbonate cobbles and boulders with rare sandstone clasts. The Namaskluft diamictite is capped by ∼15 m of buff-colored dolomite with bioherms that are filled with irregular cements, herein assigned to the Dreigratberg Member. This is overlain by an ∼50 m thick transgressive sequence of folded pink limestone rhythmite and an additional ∼50 m of mixed allodapic carbonate and siliciclastic rocks of the upper Holgat Formation. To the west, the Holgat Formation is truncated by a thrust fault that places the Fe-rich Numees Formation structurally above the Holgat Formation (fig. 3B). To the south, along the Orange River, this thrust cuts out the Holgat Formation, and the Hilda Subgroup is present in the hanging wall in the core of an anticline, which is thrust onto a sliver of diamictite that is most likely the Numees Formation (fig. 3C). In contrast to previous mapping, this new unit assignment leads to kinematically feasible structures. Moreover, the northern extensions of these structures comprise an exposure of limestone and diamictite in the footwall of a thrust on the east side of Rosh Pinah Mountain that have previously been assigned to the Pickelhaube and Kaigas Formations (Von Veh, 1993; Alchin and others, 2005; Frimmel, 2008). A simpler interpretation is that these units belong to the Wallekraal, lower Holgat, and Numees Formations.

Dreigratberg

At Dreigratberg, the Holgat Formation is exposed at the center of a simple syncline surrounded by the Numees diamictite and its associated iron formation (fig. 3C). This correlation is supported by the presence of a fine-laminated, dark gray cap limestone with sub-littoral microbialaminite and roll-up structures that rests on the diamictite. The cap is ∼10 m thick and is succeeded by ∼50 m of argillite and marl (fig. 4). These marls grade into a stratified diamictite with rare, subrounded carbonate and sandstone cobble lonestones. We assign this stratified diamictite to the Namaskluft diamictite because it is capped by 5 m of white dolomite with bed-parallel cements and an additional ∼50 m of pink limestone, characteristic of the upper portion of the Dreigratberg cap carbonate. In the core of the syncline is an additional ∼250 m of allodapic carbonate, shale and wackestone of the upper Holgat Formation.

Numees Mine to Bloeddrif

At Numees Mine, the Stinkfontein Subgroup is unconformably overlain by up to 100 m of diamictite. This diamictite is capped by more than 20 m of blue-gray limestone and ∼300 m of arkosic turbidites. The limestone and arkose units are indistinguishable from those exposed at Namaskluft Camp and are assigned to the Bloeddrif Member and Wallekraal Formation respectively. The upper ∼100 m of strata exposed on the escarpment above the Numees Mine consist of a poorly sorted, carbonate-clast conglomerate (fig. 5) with boulder olistoliths of giant ooids assigned to the Dabie River Formation (fig. 5).

Fig. 5.
Fig. 5.

Chemo-stratigraphic correlations of the PNG in South Africa. Locations of sections are in figures 1 and 3.

The type locality of the Numees Formation is located in South Africa in the broad syncline ∼10 km due west of Numees Mine (fig. 1), where it is >500 m thick and consists predominantly of a massive diamictite with subrounded clasts of granite, quartzite, schist, and dolomite in a dark colored, ferruginous, fine- to medium-grained quartz-mica schist matrix (Frimmel and Von Veh, 2003). The diamictite is interbedded with a ferruginous feldspathic arenite and banded iron formation of the Jakkalsberg Member. In the core of the syncline the Numees Formation is capped with a dark gray, fine laminated limestone that grades upwards into limestone marl and metapelite. This exposure can be correlated with the type section of the Bloeddrif Member located ∼10 km to the west.

Kaigas River

On the autochthon, near the Kaigas River (fig. 3D), the Numees Formation is >1 km thick and composed of a sandstone matrix diamictite with boulder-sized clasts of crystalline basement rocks and sandstone. This diamictite is interbedded with thick-bedded sandstone turbidites of the Wallekraal Formation. It has previously been mapped as the Kaigas Formation with several thrust repetitions (Von Veh, 1993). However, our mapping has demonstrated that the sandstone bodies are channelized and interbedded with the diamictite, and like Kröner (1974), we conclude that there are no such thrust repetitions. Moreover, the diamictite is capped by the Bloeddrif Member, which consists of 12 m of dark gray limestone rhythmite and microbialaminite that is succeeded by ∼200 m of argillite and marl.

On the autochthon, the Namaskluft diamictite is typically massive and consists of cobble-sized carbonate and sandstone clasts in a sandstone to argillite matrix. It is ∼100 m thick and poorly exposed. Stratified units with bed-penetrating dropstones are also present. The Namaskluft diamictite is capped by the Dreigratberg Member, which consists of up to 200 m of light gray to buff-colored dolomite. The Dreigratberg Member is formed predominantly of recrystallized grainstone, but also contains giant wave ripples and massive stromatolite bioherms. The transgressive sequence above the Dreigratberg Member consists of pink marl and rhythmite with occasional hummocky cross-stratification, as well as mixed carbonate and sandstone in well-developed parasequences. We interpret these beds as a proximal facies of the upper Holgat Formation.

On the para-autochthon, the Stinkfontein Subgroup rests on basement and is beveled by a diamictite (fig. 3D). We suggest this diamictite is the Numees Formation because it is capped by a limestone that is indistinguishable from the Bloeddrif Member. To the west, in the Dolomite Peaks, the Jakkalsberg iron formation is present in the Numees Formation and the overlying Bloeddrif Member contains microbial roll-up structures, reminiscent of Sturtian-age cap carbonates elsewhere (Hoffman and Schrag, 2002). The Bloeddrif Member is less than 3 m thick and is succeeded by over 30 m of argillite and marl (fig. 5). In the Dolomite Peaks, the lower Holgat Formation grades into a stratified diamictite with rare carbonate lonestones. This unit, assigned to the Namaskluft diamictite, is capped by ∼5 m of dolomite with sheet-crack cements. An additional ∼100 m of allodapic carbonate and argillite are present in the Dolomite Peaks above the Dreigratberg Member, but these units are highly folded and difficult to measure with confidence.

CHEMOSTRATIGRAPHY

Previous Studies

Carbon, oxygen, and strontium isotopes were previously reported from Dreigratberg and Namaskluft Farm in Namibia, and from near Numees Mine and Bloeddrif in South Africa (Foëlling and Frimmel, 2002). Our mapping suggests that the data previously attributed to the Picklehaube Formation is actually from strata between the Numees and Namaskluft diamictites (Frimmel, 2007), and should thus be assigned to the lower Holgat Formation.

Frimmel and Foëlling (2004) report two additional sections of the Bloeddrif Member from the Kaigas River and from the Bloeddrif type locality along the Kuboos River (fig. 1), but the section from the Kaigas area is actually from the Dreigratberg cap carbonate.

Carbon Isotopes

Above the lower diamictite at Namaskluft Camp, δ13C values in the Bloedriff Member start near −6 permil and increase to −2 permil over ∼30 m with considerable scatter (fig. 4). A similar pattern is present in the Bloedriff Member at Numees Mine (fig. 5). In more distal, condensed sections, carbon isotope values in the Bloeddrif Member rise from −6 permil to 0 permil in ∼10 m of strata. The succeeding δ13C values in the lower Holgat Formation describe an arc with values increasing up to +8 permil before plummeting into a negative anomaly (fig. 4). At Namaskluft Camp, Namaskluft Farm and Dreigratberg, this pronounced negative δ13C anomaly is developed under the Namaskluft diamictite with values as low as −9 permil.

The Dreigratberg Member at Namaskluft Camp, Namaskluft Farm, and Dreigratberg all display a sigmoidal δ13C profile with values decreasing from −2 permil to below −5 permil. In the Kaigas River region, δ13C values in the Dreigratberg cap carbonate vary between −2 permil and 0 permil (fig. 5).

Strontium Isotopes

We report 23 new 87Sr/86Sr measurements (table 2). Unlike Halverson and others (2007), we are unable to define a reliable alteration cutoff based on Sr concentration. Mn/Sr, Mg/Sr, and δ18O values provide a rough guide to the extent of alteration, but we again cannot define a meaningful cutoff value in any of these proxies. We distinguish “most reliable” data based on the absolute 87Sr/86Sr value, because diagenetic overprinting usually increases 87Sr/86Sr (Banner and Hanson, 1990), and consequently, unradiogenic values (here less than 0.7080) are likely near primary values.

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Table 2

Strontium isotope and trace element data from the Port Nolloth Group

DISCUSSION

Regional and Global Correlations

Previous age constraints on the Kaigas and Rosh Pinah Formations of 752 ± 6 Ma (Borg and others, 2003) and 741 ± 6 Ma (Frimmel and others, 1996), respectively, come from exposures on Trekpoort Farm, near Skorpion Mine (fig. 1), where the Kaigas Formation is tectonically dismembered. At this locality, the Hilda Subgroup is overlain, in the core of the syncline, by a thick, Fe-rich diamictite that we have assigned to the Numees Formation. That is, the ca. 750 Ma ages are from volcanic rocks that are stratigraphically below the Numees diamictite. This interpretation is consistent with maximum age constraints on the Sturtian glaciation of 726 ± 1 Ma in Oman (Bowring and others, 2007), 725 ± 10 Ma on the Tarim Block (Xu, 2009), and 717.43 ± 0.14 in the Yukon (Macdonald and others, 2010). Therefore, the Kaigas Formation was deposited prior to the ca. 716.5 Ma Sturtian glaciation (Macdonald and others, 2010). Moreover, a glacial origin of the Kaigas Formation is highly questionable and is based largely on miscorrelations with the glacigenic Numees Formation.

A Gaskiers age has been proposed for the Numees diamictite (Frimmel and Foëlling, 2004; Frimmel, 2008) on the basis of a 555 ± 28 Ma Pb/Pb carbonate age on the Bloeddrif Member (Foëlling and others, 2000), and radiogenic Sr isotope values (87Sr/86Sr > 0.7082; Foëlling and Frimmel, 2002). However, this age and the Pb/Pb carbonate dating technique are unreliable because Pb is mobile in carbonate (Sumner and Bowring, 1996). Radiogenic Sr isotope compositions may, in part, be due to sample preparation procedures that do not attempt to remove clay- and surface-bound Sr (compare methods of Derry and others, 1989 and Asmerom and others, 1991, with Gao and others, 1996, and Bailey and others, 2000), and to disturbance of the Sr system during Pan-African orogenesis. Our 87Sr/86Sr data for the Bloeddrif Member are from the least deformed sections on the autochthon, whereas Foëlling and Frimmel (2002) reported results from the highly tectonized exposures on the para-autochthon at the Bloeddrif type section. Our data for the Bloeddrif Member indicate 87Sr/86Sr as low as 0.7071, a value that is most consistent with marine Sr isotopic compositions before the ca. 635 Ma Marinoan glaciation (Sawaki and others, 2010), and similar to 87Sr/86Sr values from Sturtian cap carbonates in the Rasthof Formation of northern Namibia (Halverson and others, 2007) and the Tsaagan Oloom Formation of Mongolia (Brasier and others, 1996).

This new geochemical dataset is supported by sedimentological and chemostratigrahic data from the Bloeddrif Member, which are characteristic of Sturtian-age cap carbonates globally. This unit consists of dark colored, fine-laminated limestone with distinct microbial roll-up structures, and a sharp negative carbon isotope anomaly (Hoffman and Schrag, 2002). Moreover, an additional diamictite, herein referred to as the Namaskluft diamictite, is stratigraphically above the Numees diamictite and the Bloeddrif Member in multiple sections. This uppermost diamictite is capped by the Dreigratberg member, a micro-peloidal dolostone with low-angle cross-lamination, tubestone stromatolites, and giant wave ripples. Carbon isotope analyses through the Dreigratberg member display a sigmoidal profile with a nadir at ∼ −5 permil. These sedimentological and geochemical features are characteristic of basal Ediacaran cap carbonates globally (Hoffman and others, 2007). Our Sr isotopic data for the Dreigratberg member show a minimum 87Sr/86Sr near 0.7077, which is consistent with basal Ediacaran marine Sr compositions (Halverson and others, 2007; Sawaki and others, 2010). Thus, the Numees diamictite is stratigraphically between the ca. 750 Ma Kaigas Formation and the ca. 635 Ma Namaskluft diamictite. Given this relationship, we suggest that the Numees Formation and associated iron formation are correlative with the ca. 716.5 Ma Sturtian glaciation. As such, the ca. 750 Ma ages from the Hilda Subgroup are maximum age constraints on the Sturtian glaciation.

In this new stratigraphic framework, the composite δ13C curve from the PNG can be used to construct an age model (fig. 6) in which the negative δ13C anomaly beneath the Numees diamictite is correlative with the pre-Sturtian Islay anomaly (Calver, 1998; Halverson, 2006; Prave and others, 2009), the negative δ13C anomaly in the Bloeddrif Member is correlative with the Rasthof anomaly that is present in Sturtian cap carbonates around the world (Yoshioka and others, 2003; Halverson and others, 2005), the negative δ13C anomaly below the Namaskluft diamictite is correlative with the Trezona anomaly that is below the Marinoan glaciation at several localities globally (McKirdy and others, 2001; Hoffman and Schrag, 2002; Halverson and others, 2005), and the negative δ13C anomaly in the Dreigratberg cap carbonate is equivalent to the Maieberg anomaly in basal Ediacaran cap carbonates (Kennedy, 1996; Halverson and others, 2005) (fig. 6). These new correlations demonstrate that the carbon isotope chemostratigraphy of the Port Nolloth Group is consistent with global composite curves. Claims to the contrary (Frimmel, 2010) are a product of regional stratigraphic miscorrelations.

Fig. 6.
Fig. 6.

Composite carbon isotope curve of the PNG plotted against time. Boxes representing the glacial events are faded to lighter shades to represent the uncertainty in the maximum age constraint for the Sturtian glaciation and for the minimum age constraint for the Marinoan glaciation. Pre-Numees data are from the Skorpion mine area and post-Numees data are from Namaskluft Farm and Dreigratberg. Carbon isotope anomalies are discussed in text.

Implications for the Micropaleontology Record

Microfossils have been described previously in the PNG (Gaucher and others, 2005) and have been cited to support a Gaskiers age of the Numees Formation (Frimmel, 2008). Bavlinella was identified in samples from organic rich marl and shale in the Dreigratberg section that were assigned to the Picklehaube and Wallekraal Formations (Gaucher and others, 2005). In our new stratigraphic framework, the entire Holgat Formation is exposed in the Dreigratberg section, and it is unclear if these samples were collected above or below the Namaskluft diamictite. Thus, these Bavlinella specimens are constrained to post-date the Sturtian-age (ca. 716.5 Ma) Numees Formation. The range of Bavlinella is not well constrained as an index fossil (Knoll and others, 2006), and its presence does not provide any further constraints on the age of the PNG stratigraphy.

Other microfossils identified in the PNG consist of large colonies of cells extracted from the upper Holgat Formation at Witputs Farm (north of map area in fig. 1) (Gaucher and others, 2005). These samples are from marl and shale above a diamictite and buff-colored cap dolomite with stromatolite bioherms that is indistinguishable from the Dreigratberg cap carbonate near Namaskluft Camp. Thus, the underlying diamictite unit should be assigned to the Namaskluft diamictite rather than the Numees Formation, and the microfossils are in post-635 Ma strata. This age assignment is consistent with the micropaleontology; the clusters of cells described are similar to forms in the post-635 Ma Doushantuo Formation in South China (Xiao and others, 2004), but again the ranges of such forms are not well constrained as index fossils.

Structural and Tectonic Implications

The new mapping and correlations presented herein suggest a much simpler structure in the Gariep Belt than envisioned by some previous geologists who invoked thrust repetitions to reconcile stratigraphic inconsistencies (for example, Von Veh, 1993). Broadly, the Numees Formation is thickest on the hanging-wall of a west dipping, syn-sedimentary normal fault that defines the eastern margin of the Rosh Pinah graben and the para-autochthon. To the west, the para-autochthon is folded into an approximately 20 km wide anticline-syncline pair, with the Hilda Subgroup well-developed in the axis of the Rosh Pinah graben.

Recently, it has been suggested that the upper portion of the PNG includes both a successor back-arc basin and the beginnings of foreland deposition (Frimmel and Foëlling, 2004; Basei and others, 2005; Frimmel, 2008). This interpretation stems from the proposed Gaskiers-age of the Numees Formation which implies a ∼150 Myr hiatus in the PNG and an Ediacaran tectonic reactivation of the margin to accommodate the Holgat Formation. Our new age model (fig. 6) makes these hypothesized tectonic events unnecessary and points to a much simpler evolution of the margin. Particularly, we suggest that rifting began between ∼770 and ∼750 Ma forming the Rosh Pinah graben and the margin remained tectonically active through the deposition of the Numees diamictite at ca. 716.5 Ma. In this model, the Holgat Formation was deposited on a thermally-subsiding passive margin, which was only reactivated in the latest Ediacaran with the foredeep deposition of the Nama Group at the onset of the Gariep orogeny.

Neoproterozoic Iron Formations and Oceanic Redox

Neoproterozoic iron formations are present on nine separate paleocontinents marking a return to the stratigraphic record after an absence of over one billion years (Klein and Beukes, 1993) (table 3). The new age assignment of the Numees Formation is of particular interest because it hosts the iron formation of the Jakkalsberg Member (fig. 5) (Frimmel and Von Veh, 2003). In addition to the iron formations of the Jacadigo Group in the Urucum District of Brazil and Bolivia (Dorr, 1945; Urban and others, 1992; Trompette and others, 1998; Klein and Ladeira, 2004), and the Rizu Formation of Iran (Kianian and Khakzad, 2008), the Jakkalsberg Member of the Numees Formation was previously considered one of three Neoproterozoic iron formations that do not belong to the Sturtian glaciation (Frimmel, 2008; Hoffman and Li, 2009). As discussed above, a Gaskiers age of the Numees diamictite is espoused by several authors, whereas Hoffman and Li (2009) mis-assigned the Jakkalsberg Member to the Kaigas Formation and correlated the cratonic Jacadigo Group with the Puga Formation in the adjacent Paraguay fold belt (Alvarenga and Trompette, 1992; Nogueira and others, 2003). The Puga Formation is overlain with a typical Marinoan-type cap dolostone (Font and others, 2006; Nogueira and others, 2007; Alvarenga and others, 2008); however, no cap carbonate is preserved on the Jacadigo Group, and the contact with the overlying Ediacaran-age Corumbá Group is not exposed and possibly unconformable. Moreover, the age constraints on the Rizu Formation are so poor that any assignment is arbitrary (Kianian and Khakzad, 2008). Thus, it is possible that all Neoproterozoic iron formations are of a ca. 716.5 Ma Sturtian age.

View this table:
Table 3

Neoproterozoic iron formations and their respective age constraints

View this table:
Table A1

Carbon and oxygen isotope data

Iron formation requires anoxic deep waters and either S:Fe flux ratios < 2 to enable reduced Fe to travel freely in solution without being titrated out as pyrite during bacterial sulfate reduction (BSR) (Canfield, 2004), or insufficient organic substrate for BSR (Mikucki and others, 2009). A low S:Fe flux into the ocean may have been accomplished by diminished sulfate input from the continents during glaciation (Canfield, 2004), and by lowered S:Fe flux in hydrothermal vent fluids due to the decrease in hydrostatic pressure resulting from glacioeustatic sea level fall (Kump and Seyfried, 2005). Moreover, ice cover may have caused primary productivity to crash, thereby limiting BSR.

Interestingly, while Neoproterozoic iron formations are predominately hosted by Sturtian glacial deposits, sedimentary barite is present in several Marinoan cap carbonates (Deynoux and Trompette, 1981; Kennedy, 1996; Hoffman and Schrag, 2002; Jiang and others, 2006; Shields and others, 2007). Barium is also soluble in anoxic water and precipitates as barite in the presence of sulfate. An atmospheric sulfur isotope signal in the Marinoan barites suggests the sulfur was derived from shallow water as sulfate rather than deep waters as sulfide (Bao and others, 2008). Moreover, FeP:FeHR < 0.8 in Ediacaran shales indicates that ferruginous, and not euxinic conditions, prevailed after the Marinoan glaciation (Canfield and others, 2008; Shen and others, 2008). Thus, like the ca. 716.5 Ma Sturtian ocean, the ca. 635 Ma Marinoan ocean appears to have been anoxic but not euxinic, yet one produced iron formation and the other produced barite. This difference could be due to: 1) different S:Fe flux into the ocean at the onset of the two glaciations; 2) an increase in atmospheric oxygen and sulfate availability between the two glaciations; 3) different degrees of ice cover, primary productivity, and organic carbon availability for BSR during the two glaciations; and 4) different durations of the two glaciations with a longer Sturtian glaciation driving decreased S:Fe ratios in the ocean. Future geochemical and geochronological studies will help distinguish between these models and lead towards an understanding of the origin of Cryogenian chemical sediments and the secular evolution of the environments that produced them.

CONCLUSIONS

Integrated geological mapping with chemo- and litho-stratigraphic studies in the PNG of Namibia and South Africa suggest that the Numees Formation is a Sturtian-age diamictite. With existing age constraints, this scenario is consistent with Neoproterozoic glacial records globally. Particularly, 1) the ca. 750 Ma age constraint on the Kaigas Formation is too old for a Sturtian glacial deposit; 2) the presence of microbial roll-up structures, a characteristic δ13C profile, and relatively low Sr isotopic ratios in the Bloeddrif cap limestone is consistent with its assignment as a Sturtian-age cap carbonate; and 3) the δ13C profile, elevated Sr isotopic composition, and sedimentary structures of the Dreigratberg cap carbonate are characteristic of basal Ediacaran cap carbonates. The correlations presented herein suggest that recent tectonic models that claim the upper portion of the Hilda Subgroup and Numees Formation were deposited in a back-arc basin and the overlying Holgat Formation formed in a foredeep basin, are untenable. We suggest that the entire PNG was deposited along a rifted passive margin and that foredeep sedimentation began at the base of the Nama Group. The new correlations also suggest that the Jakkalsberg Member iron formation of the Numees diamictite is Sturtian in age. This leaves few, if any, possible Marinoan-aged iron formations.

APPENDIX

Geochemical Methods

Over 1000 samples were collected for carbon and oxygen isotopic analyses. All samples were cut perpendicular to lamination, revealing internal textures. Between 5 and 20 mg of powder were micro-drilled from the individual laminations (where visible), avoiding veining, cleavage, and siliciclastic components. Subsequent isotopic analyses were performed on aliquots of this powder. Carbonate δ13C and δ18O isotopic data were acquired simultaneously on a VG Optima dual inlet mass spectrometer attached to a VG Isocarb preparation device (Micromass, Milford, MA) in the Harvard University Laboratory for Geochemical Oceanography. Approximately, 1-mg micro-drilled samples were reacted in a common, purified H3PO4 bath at 90°C. Evolved CO2 was collected cryogenically and analyzed using an in-house reference gas. External error (1σ) from standards was better than ± 0.1 permil for both δ13C and δ18O. Samples were calibrated to VPDB (Vienna Pee-Dee Belemnite) using the Cararra marble standard. The memory effect potentially resulting from the common acid-bath system was minimized by increasing the reaction time for dolomite samples. Memory effect is estimated at <0.1 permil based on variability of standards run after dolomite samples. Carbon (δ13C) and oxygen (δ18O) isotopic results are reported in permil notation of 13C/12C and 18O/16O, respectively, relative to the standard VPDB.

All 87Sr/86Sr data were acquired at the MIT Radiogenic Isotope Laboratory. Sample preparation methods are based on Gao and others (1996) and Bailey and others (2000). Approximately 10 mg of each powdered carbonate sample was first leached sequentially 3 to 5 times for 15 to 45 minutes in an ultrasonic bath, in 1.0 mL of 0.2 M ammonium acetate, to remove loosely bound Sr cations. The remaining solid was then washed 3 times in an ultrasonic bath with 1.0 mL of ultrapure water, to remove excess ammonium and suspended clays. Carbonate was reacted for 5 min. with 1.0 mL 1.4 M acetic acid and insoluble residue was removed by centrifuging. Sr was isolated via standard chromatographic techniques using 50 μL columns of EIChroM SR-spec resin. Samples were analyzed by thermal ionization mass spectrometry (TIMS) on a GV IsoProbe T in dynamic mode, with target intensity of 3V 88Sr. All data were corrected to 86Sr/88Sr = 0.1194 for internal mass bias. Each analysis represents a minimum of 60 ratio measurements, with internal precision of better than 0.001 percent (1-σ). Analyses were referenced against NBS SRM 987 (0.710250), with a long-term average of 0.710240 and 2-σ external precision of 0.000014 (n > 100). Data were not corrected for the slight low bias of measured values compared with the expected value of NBS 987.

Strontium concentrations from section F539 were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) on a Jobin-Yvon 46P in the Harvard University Laboratory for Geochemical Oceanography. Samples were prepared by dissolving ∼6 mg of carbonate powder in ∼6 ml of 2 percent nitric acid. SCP single element standards were used for element-specific instrumental calibration. External error (1-σ) was determined by repeat analyses and was less than 5 percent. Additional elemental analyses were performed commercially at Actlabs in Ancaster, Ontario, where 0.5 g samples were digested in Aqua Regia at 90°C in a microprocessor controlled digestion block for 2 hours. The solution was diluted and analyzed using a Perkin Elmer SCIEX ELAN ICP/MS.

Acknowledgments

We thank Sarah Bahan, Alexandra Breus, Tom Benson, and Hess Yntema for help in the field. We thank Greg Eischeid and John Higgins for assistance with elemental and isotopic analyses. We thank Sam Bowring for use of MIT's Radiogenic Isotope Lab. We thank Tanja Bosak, Sara Pruss and the NSF for support in 2009 (EAR-0843358). We thank Paul Hoffman for suggesting beginning study of the Gariep belt at the Namaskluft Camp section, for providing support (pre-2009) from NSF grant EAR-0417422, and for comments on the manuscript. We thank the Geological Survey of Namibia for assistance with logistics. We also thank Nick Beukes, Simon Wilde, and an anonymous reviewer for their comments. We acknowledge Alfred Kroner for his thesis mapping in the Gariep Belt, which has held up with time and paved the way for our study.

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Stratigraphy of the Port Nolloth Group of Namibia and South Africa and implications for the age of Neoproterozoic iron formations
Francis A. Macdonald, Justin V. Strauss, Catherine V. Rose, Francis Ő. Dudás, Daniel P. Schrag
American Journal of Science Nov 2010, 310 (9) 862-888; DOI: 10.2475/09.2010.05
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