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The Otavi Group of the Northern Platform and the northern margin zone, in Miller, R.McG., ed., The Geology of Namibia

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... The Otavi Group is a 2–4 km thick platform carbonate succession (Hoffman and Halverson, 2008, and references therein) that was deposited at subtropical latitudes (Li et al., 2008) over a $200 Ma period (Hoffmann et al., 2004) during Cryogenian and early Ediacaran age (ca. 770 to ca. 580 Ma;Halverson et al., 2005). ...
... Periods of rapid transgression followed by gradual regression produced the upward shallowing cycles of the Auros Fm (Miller, 2008, and references therein). The Gauss and Auros Fm (KLB) correlate to the Ombaatjie Fm (FSF), characterized by plane-parallel laminated limestones and dolostones (Hoffman and Halverson, 2008). Slope breccias, reworked oolite layers and pronounced erosion surfaces characterize the foreslope section, where a low-stand wedge (Franni-aus Member, Mb) indicates sea level fall preceding the Ghaub Fm (Domack and Hoffman, 2011, and references therein). ...
... Glaciogenic diamictites of the Ghaub Fm (correlated with the end-Cryogenian 'Marinoan' glaciation; e.g.Hoffmann and Prave, 1996;Hoffman et al., 1998;Bechstädt et al., 2009) occur in both sections, predominantly consisting of unsorted marine ice grounding-line diamictites and derived gravity flows with carbonate clasts, within a fine grained, bluish grey to cream or yellow carbonate matrix with angular quartz grains. The glacially transported debris is predominantly derived from the upper Ombaatjie Fm and the falling-stand wedge of the Franni-aus Mb, consistent with the parallel conformity of the glacial erosion surface on the Northern Platform (Hoffman and Halverson, 2008). On the Fransfontein Ridge, the Ghaub Fm is interpreted to consisting of subaqueous, glacial proximal and subglacial facies (Domack and Hoffman, 2011). ...
Article
Chromium isotopes constitute a powerful paleoenvironmental tracer recording fluctuations of atmospheric oxygenation and continental weathering thus facilitating the reconstruction of the redox state of ancient seawater. We use the δ⁵³Cr signature coupled with REE+Y patterns and redox-sensitive trace elements to monitor environmental changes recorded by marine carbonates of the Otavi Group, Namibia. These carbonates were deposited in a platform and foreslope setting in subtropical latitudes during the Neoproterozoic and comprise the transition from a marine depositional setting through glaciation into a postglacial environment in four stages. Preglacial carbonates (Stage 1) yield positively fractionated δ⁵³Cr values, increased U and Mn concentrations, indicative of mobilization during oxidative terrestrial weathering and stabilization in oxic surface waters. Carbonates deposited just before the Ghaub diamictites (Stage 2) record δ⁵³Cr values (>+0.4 ‰) comparable to modern seawater and negative Ce anomalies (∼0.7) characteristic for oxygenated seawater. We interpret this as a pulse of intense oxidative weathering shortly before the advance of the glaciers. Marginal shale contamination persists in carbonates of both sections and is slightly elevated during the glacial aftermath; Cr is vulnerable towards detrital contamination. Early postglacial cap dolostones (Stage 3) were influenced by enhanced detrital contamination potentially supplied by freshwater particulate load, which was then drastically reduced in the overlying postglacial limestones in the upper Maieberg Fm (Stage 4) where near-preglacial δ⁵³Cr values are reached again. REE+Y patterns along with Eu and Ce anomalies record a transformation from a marine, slightly anoxic and stratified water column with distal hydrothermal influence to a freshwater-influenced depositional environment with decreased hydrothermal activity and fluctuating oxic surface water conditions after glacial retreat. Here, we demonstrate that carbonate δ⁵³Cr signatures are sensitive to changes in continental weathering balanced between detrital contamination and oxidative weathering on land and are capable of tracing fluctuating redox conditions prior and after one of the major syn-Marinoan glaciations.
... In contrast, the Ghaub Formation is dominated by carbonate-clast diamictite. Regionally, the consistent lithologic contrast in diamictites reflects the tectonic transition from crustal stretching during the Chuos glaciation to passivemargin thermal subsidence during the Ghaub glaciation (Hoffman & Halverson, 2008). Yet, the basement-dominated diamictite wedge in the south dome is directly overlain by the Karibib Formation, including a well-developed Keilberg Member, which is the basal Ediacaran cap dolostone regionally affiliated with the younger Ghaub glaciation (Hoffmann & Prave, 1996; Hoffman & Halverson, 2008; Hoffman, 2011). ...
... Regionally, the consistent lithologic contrast in diamictites reflects the tectonic transition from crustal stretching during the Chuos glaciation to passivemargin thermal subsidence during the Ghaub glaciation (Hoffman & Halverson, 2008). Yet, the basement-dominated diamictite wedge in the south dome is directly overlain by the Karibib Formation, including a well-developed Keilberg Member, which is the basal Ediacaran cap dolostone regionally affiliated with the younger Ghaub glaciation (Hoffmann & Prave, 1996; Hoffman & Halverson, 2008; Hoffman, 2011). Accordingly, the Chuos-like diamictite body in the south dome was provisionally assigned to the Ghaub Formation (Maloof, 2000; Schreiber, 2006; Hoffman & Halverson, 2008) despite its atypical derivation and composition. ...
... Yet, the basement-dominated diamictite wedge in the south dome is directly overlain by the Karibib Formation, including a well-developed Keilberg Member, which is the basal Ediacaran cap dolostone regionally affiliated with the younger Ghaub glaciation (Hoffmann & Prave, 1996; Hoffman & Halverson, 2008; Hoffman, 2011). Accordingly, the Chuos-like diamictite body in the south dome was provisionally assigned to the Ghaub Formation (Maloof, 2000; Schreiber, 2006; Hoffman & Halverson, 2008) despite its atypical derivation and composition. As the stratigraphy in the domes exhibits considerable local variability, we mapped and measured 20 sections over a strike-length of 10.5 km around the periphery of the domes (Fig. 3, 4). ...
... Neoproterozoic successions of equivalent age also occur in central (Witvlei Group) and southwest (Gariep Group) Namibia [49]. During Otavi Group deposition, the Otavi carbonate platform in the north of the depositional basin (Fig. 1) was progressively differentiated from the deep-water Outjo basin to the south [50] as a result of north-south crustal stretching [51]. The mixed siliclastic-carbonate Ombombo Subgroup occurs only on the northern platform, where it was tilted on a Paleoprotoerozoic basement high [52] to the south and beveled beneath the Sturtian glacials [18]. ...
... A pair of precise U-Pb zircon ages from ashes in the equivalent lower Doushantuo Formation of South China [20], indicate that the entire Maieberg Formation was deposited in <3 Ma. The Maieberg Formation, defined by Hoffman and Halverson [50] to include the entire cap carbonate sequence, bound above by a mappable sequence boundary , is divided here into three members: the basal (formally defined) Keilberg Member (cap dolostone), a middle member comprising limestone and dolomite rhythmite, and an upper member consisting of dolomite grainstone. ...
... In most sections, the upper Keilberg Member is accompanied by a negative shift in δ 13 C and δ 18 O compositions (Figs. 3, 4). δ 13 C reaches a low of -5.5 in the middle member at approximately the level of the maximum flooding surface, above which it gradually rises, approaching 0 near the top of the formation [6, 50]. The Karibib Formation preserves a similar but condensed δ 13 C anomaly that has been reproduced in multiple sections [6, 19, 50]. ...
Article
We have analyzed δ34S (sulfate and pyrite), δ18Ocarbonate and δ13Ccarbonate, and major and trace elemental concentrations, including extractable Fe and Mn phases, in four sections of the Maieberg Formation, the cap-carbonate sequence to the Marinoan glaciation in northern Namibia. δ34Ssulfate profiles and other geochemical characteristics in the basal, transgressive cap dolostone (Keilberg Member) are nearly identical in all sections and indicate deposition from a water mass with very low sulfate concentrations. In the overlying interval that consists of rhythmites deposited during the transgressive high-stand, large geochemical disparities occur between sections that we interpret to have been deposited in open-ocean versus restricted settings. In the former, a large negative shift in δ34Ssulfate of ∼ 20‰ above the cap dolostone accompanies a change in mineralogy from dolomite to limestone, a sharp decrease in δ18O and δ13C, and a positive spike in Fe and Mn concentrations. In the latter, dolomite persists above the cap dolostone, δ34Ssulfate increases abruptly, δ18O and δ13C are invariant, and a spike in Mn and Fe concentrations is present, but subtle. These contrasting geochemical signatures in coeval sections can be explained by strong lateral chemical gradients that developed as cold, euxinic deep water and a cap of warm, oxic, brackish water that flooded the continental shelf during the post-glacial transgression. The geochemical differences between open-ocean and restricted sections diminishes upsection, presumably recording the gradual mixing and homogenization of the waters along the Otavi platform.
... The margins were established in three separate rifting events: at ca. 920 Ma (Aracuaí and West Congolese; Tack et al., 2001), ca. 750 Ma (northern Damara and Lufilian;Hoffman and Halverson, 2008;Key et al., 2001), and a poorly dated but likely mid-Neoproterozoic event (Oubanguide; Poidevin, 2007). The early-to mid-Neoproterozoic rifting events, along with at least one late Mesoproterozoic bounding orogen (Irumide belt; de and the presence of Neoproterozoic mobile belts (Araçuaí, Brasília, Riacho do Pontal, Rio Preto, and Sergipano) that surround SF, suggest that C/SF may have been a component of Rodinia supercontinent (Evans et al., 2016b), contrary to earlier suggestions (Kröner and Cordani, 2003). ...
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The Congo/São Francisco (C/SF) craton, one of the largest cratons in Proterozoic paleogeography, has been lacking reliable paleomagnetic data for the supercontinent Nuna interval (ca. 1600-1300 Ma). Here we provide a new paleomagnetic key pole for this craton from recently dated mafic dykes in the Curaçá (1506.7±6.9 Ma) region of Brazil. The characteristic remanent magnetization (ChRM) direction D=070.6°, I=54.0° (k=22.1 and a95=13.1°) corresponds with a paleomagnetic pole at 10.1°N, 009.6°E (K=15.6, A95=15.8°), which places C/SF craton in moderate paleolatitudes at the time of remanence acquisition. Primary nature of the paleomagnetic remanence is supported by a baked-contact test. A similar ChRM direction was obtained for four Mesoproterozoic mafic intrusions in Chapada Diamantina region. The new pole, only from Curaçá, for C/SF allows us to reconstruct the extended core of the supercontinent Nuna at 1.5 Ga. Based on coeval 1.5 Ga and 1.38 Ga magmatism in Baltica, Siberia and C/SF, we favor the position where Southwest Congo is reconstructed against present South-Southeast (S-SE) Baltica. We explore two alternative 1.5 Ga reconstructions of Nuna’s core. In both of them Baltica and Laurentia are shown in the well-defined NENA (Northern Europe North America) fit, together with Siberia in a tight fit to northern Laurentia. In reconstruction option A, more traditional fit of Amazonia with Baltica is shown, modified from the geologically based SAMBA (South AMerica BAltica) model to accommodate paleomagnetic data. In this option, however, West Africa must be extricated from SAMBA because C/SF has taken its place. For reconstruction option B, Amazonia is shifted to lie adjacent to NE Laurentia and West Baltica. In both options SW Congo is reconstructed against S-SE Baltica, but in option B there is a tighter fit between them, and there is a better match with our new paleomagnetic data for C/SF. In either option, separation of C/SF from Baltica and Siberia probably occurred at 1.38 Ga, the age of pronounced mafic magmatism throughout this sector of Nuna.
... Given the association with rhythmites, we suggest that the Carrancas Formation formed by the shedding of cap dolostone debris from a paleohigh or escarpment in the basin into a deeper water environment otherwise characterized by limestone rhythmites. Analogous facies relationships occur near shelf margin sections of the basal Ediacaran Maieberg Formation in Namibia (Hoffman and Halverson, 2008). This interpretation is consistent with the lack of evidence for glacial deposition in the Carrancas Formation (Vieira et al., 2007; Tuller et al., 2009 ). ...
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Remnants of a Neoproterozoic glaciation in east central Brazil are represented by thin diamictite layers (Jequitaí Formation and correlative units), locally overlying striated pavements on the São Francisco craton. The diamictites are covered by the Sete Lagoas Formation of the basal Bambuí Group, which is generally accepted to be a typical cap carbonate sequence. Although most authors have preferred a mid-Cryogenian (post-Sturtian) age for it, based mainly on Pb-Pb whole rock data, the Sete Lagoas Formation bears lithostratigraphic and isotopic characteristics that are identical to early Ediacaran cap carbonates worldwide, including a basal thin (0–10 m) pale and flinty cap dolostone, preserving a drop in δ13C values from around −3.2‰ to −4.5‰ with associated δ18O around −5‰, and crystal-fan facies interpreted as aragonite pseudomorphs. Ediacaran zircons have been recovered from the middle of the Sete Lagoas Formation, constraining the deposition of its upper half to be younger than 610 Ma (Rodrigues, 2008). Although there is an unconformity below the point where the zircons were collected, it is short-lived, as suggested by the identical, typically Ediacaran 87Sr/86Sr values above and below (0.7074–0.7076). Carbonate clasts from the Jequitaí Formation and correlative diamictite-bearing units in the fold belts that surround the São Francisco craton (Canabravinha and Serra do Catuni formations) display similar ranges in δ13C (−6.7 to +2.6‰), suggesting the erosion of a pre-glacial carbonate platform with negative δ13C values (i.e. the Islay and/or Trezona anomalies). The cratonic Carrancas Formation, on the other hand, yielded pale dolostone clasts with δ13C in a small range between −4.2 and −3.4‰, and δ18O values around −6.5‰. These clasts could be derived from the cap dolostone unit itself, in which case the Carrancas Formation would represent resedimented basal Sete Lagoas Formation and imply that sections of the Sete Lagoas Formation sitting atop the Carrancas Formation are incomplete. The base-truncated sections have confused previous attempts to correlate the Sete Lagoas Formation with other cap carbonate successions. In light of the available lithostratigraphic, isotopic and U-Pb zircon data, we propose that the Sete Lagoas Formation represents a basal Ediacaran cap carbonate sequence (∼635–610 Ma) deposited after the Marinoan glaciation in east central Brazil.
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Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 437 (2016): 76-88, doi:10.1016/j.epsl.2015.12.040.
Our detailed examination of the Ghaub Formation (possibly 635 Ma) on the distal foreslope of the Otavi carbonate platform is part of a regional study of the Congo paleo continental margin in northwestern Namibia. Detrital carbonates of the Ghaub Formation disconformably overlie the Franni-aus Member of the Ombaatjie Formation, a coarsening-upward stack of carbonate turbidites and oolite-clast debris-flow breccias interpreted to be a glacioeustatic falling-stand wedge. Within the main Ghaub Formation, carbonate diamictites are interleaved with mesoscale, laminated to cross-laminated (climbing rippled) grainstones and mudstones, and conglomeratic carbonates. Amalgamation of diamictite units is observed where interleaved facies (grainstones/mudstones) are laterally discontinuous due to reactivation of erosion, followed by renewed deposition. The diamictite package is progradational overall and 80 m thick on average. It is overlain by the 5-15-m-thick Bethanis Member, which is unique in its lateral continuity, composite fining-upward trend, and distinctive interbedding of turbidite grainstones, argillaceous siltstones, climbing-rippled mudstones, and meter-scale stromatolite dropstones. Dropstones are ubiquitous within the finer-grained (Ghaub) lithofacies, and their presence, along with the facies context for subglacial and near grounding-line deposition, indicates a glacigenic origin for the Ghaub Formation, despite its subtropical paleolatitude and distal foreslope setting. We infer a glacial maximum represented by the sub-Ghaub disconformity, followed by the main Ghaub interval when an ice grounding line on the distal foreslope experienced abrupt step backs and readvances of limited magnitude, terminated by the Bethanis episode of unusually widespread iceberg calving and slope instability. The Bethanis Member is overlain conformably by the Keilberg Member of the Maieberg Formation. Reconstruction of the foreslope places the Ghaub grounding-line wedge >1.3 km vertically below the rim of the platform, implying an enormous base-level change upon deglaciation, when the platform was drowned below wave base for a period far exceeding the time scale for isostatic adjustment. The magnitude of base-level change supports the panglacial hypothesis that dynamic (thick) ice sheets existed simultaneously on virtually all continents. The snowball hypothesis that the oceans were also covered by glacial ice (seaglacier) provides a simple explanation for the main Ghaub-to-Bethanis transition-terminal deglaciation was triggered by collapse of the sea-glacier.
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Morphological interpretation of regional and detailed bathymetric data sets on the 2500-km-long Norwegian shelf from the North Sea (57°N) to Svalbard (80°N) has revealed a dynamic ice-flow pattern along the western margin of the Scandinavian and Barents/Svalbard ice sheets. About 20 cross-shelf troughs with megascale glacial lineations (MSGL; elongate ridges and grooves oriented parallel to trough long axes) are interpreted as former pathways for fast-flowing ice streams. Studies of large-scale margin morphology and seismic profiles have identified large submarine fans at the mouths of several major cross-shelf troughs. Less dynamic ice probably existed on shallower banks. The two largest paleo-ice streams were the Norwegian Channel Ice Stream and Bear Island Trough Ice Stream, each 150-200 km wide at the mouth. The lengths of individual MSGL vary from hundreds of meters to several tens of kilometers, and the distance between ridges varies from 0.1 to 3 km. MSGL amplitudes reach 15 m, but are commonly <10 m. The onset of MSGL and, hence, fast ice flow is generally close to the outer coast, at the border zone between crystalline rocks and softer sedimentary rocks. Transverse submarine ridges on various scales, commonly parallel to the shelf edge, reflect either the maximum ice-sheet position or the recessional pattern of the ice sheet. Lateral ice-stream moraines several tens of kilometers long have also been mapped along the sides of several cross-shelf troughs, identifying the border zone between fast ice flow and stagnant or slow-flowing ice on intervening banks.
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Landsat images, ASTER digital elevation models, aerial photographs, and field investigations in the western Murzuq Basin (Libya) and the adjacent Tassili N'Ajjers (Algeria) provide paleogeomorphological evidence for the existence of a Late Ordovician ice stream at least 200 km long and 80 km wide. This includes mega-scale glacial lineations, an associated subglacial meltwater drainage system, and ice-front features. This first comprehensive description of a pre-Cenozoic ice stream may help to identify other examples in the Proterozoic to Paleozoic glacial record. Reconstruction of the extent and behavior of former ice sheets, and reservoir prospect analysis in glacially related successions, have to take into account the potential occurrence of ice streams.
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The most complete Neoproterozoic successions in south China contain three diamictite intervals in the Changan, Tiesiao, and Nantuo Formations. The youngest and most widespread Nantuo glacial deposit overlies the Datangpo Formation and underlies the fossil- rich Doushantuo Formation. Previous authors have correlated the Nantuo diamictite to either Sturtian or Marinoan glacial deposits elsewhere. Here we report sedimentary and delta13C chemostratigraphic data of the Doushantuo cap dolostone, which overlies the Nantuo Formation. Facies-dependent variation in delta13C is interpreted as evidence for spatial heterogeneity in delta13C and/or temporal diachroneity in the initiation of cap carbonate sedimentation. Sedimentary and chemostratigraphic data are indicative of a Marinoan age for the Nantuo glaciation. This inference is supported by a new U-Pb zircon age of 663 ± 4 Ma from a tuffaceous bed in the Datangpo Formation. The new date and other isotopic ages from south China constrain the age of the Changan and Tiesiao glaciation(s) as between 761 ± 8 Ma and 663 ± 4 Ma, and the Nantuo glaciation as between 663 ± 4 Ma and 599 ± 4 Ma.
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The Neoproterozoic Damara orogen in Namibia records the Gondwanan assembly of the Congo Kalahari Rio de la Plata cratons. Sedimentological and stratigraphic analyses of the Otavi (mostly carbonate) and Mulden (siliciclastic molasse) Groups exposed along the southwestern margin of the Congo craton indicate that foreland-basin orogenesis began during middle Otavi (ca. 600 750 Ma); this time frame is earlier than that for the Nama foreland (ca. 550 Ma) on the Kalahari craton. Evidence for this interpretation includes (1) an intra-Otavi angular unconformity; (2) concomitant development of increasing accommodation space, basin segmentation, and backstepping depositional systems composed mostly of deep-basin and slope hemipelagic and sediment gravity-flow deposits; and (3) a near reversal in paleocurrents (initially northward off the Congo craton, then switching to southeastward from a west-northwestern orogen). Thus, Congo Rio de la Plata suturing predated Congo-Kalahari suturing during the assembly of Gondwana.
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Reconstructions of the inland branch of the Late Proterozoic Damara orogen in Namibia indicate rifting and show a marked asymmetry in both sedimentary facies and structural styles from north to south. Early synrift sequences comprising terrigenous siliciclastics, minor carbonates, and local volcanics vary markedly within two initial half-grabens. With later subsidence, postrift sequences became laterally more continuous when sedimentation coalesced; they vary from carbonate-dominated in the north to siliciclastic in the south. This indicates sedimentation on a broad epicontinental shelf. Gentle open folds in the north increase in complexity southward, culminating in a fold-thrust belt. These tectonic styles are believed to be a direct result of reactivation of early extensional faults. Although most did not address this point, previous models for the development of Damara rift structures assumed pure-shear homogeneous stretching of the crust to produce the graben systems. These models, however, fail to account satisfactorily for observed asymmetries. Early rifting in the inland arm of the Damara orogen is proposed to have been controlled by two down-to-the-northwest low-angle detachment faults. Asymmetry in basin configuration was a direct consequence of this crustal thinning mechanism, which we believe controlled the depositional environments. This model is more appropriate in explaining the asymmetrical relations observed.