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Discussion of “Depositional ages and provenance of the Neoproterozoic Damara Supergroup (northwest Namibia): Implications for the Angola–Congo and Kalahari cratons connection” by Débora B. Nascimento, Renata S. Schmitt, André Ribeiro, Rudolph A. J. Trouw, Cees W. Passchier, and Miguel A. S. Basei

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... In the North Dome, U2 is dominated by coarse-grained conglomerate. A maximum age constraint of 743 ± 10 Ma for the upper part of U2 from U-Pb ages on detrital zircons (Nascimento et al., 2017;Hoffman and Halverson, 2018) is consistent with the 746 ± 2 Ma date from the Nauuwpoort Formation (Hoffman et al., 1996). The top of U2 is defined by a flooding surface and U3 marks a transition to carbonate-dominated deposition. ...
... Based on the stratigraphic relationship between the Ghaub Formation (widely known as a Marinoan glacial diamictite interval) and the position of the Hüttenberg Formation that is located stratigraphically higher, the latter has been regarded to be an Ediacaran interval (Kennedy et al., 1998;Halverson et al., 2005;Hoffman, 2011;Miller, 2013;Prave et al., 2016;Bechstädt et al., 2018). For more details of the overall geology around the studied area, the readers are suggested to refer to the well-published studies (Frets, 1969;Kaufman et al., 1991;Germs, 1995;Hoffmann and Prave, 1996;Hoffman et al., 1998a;Hoffman et al., 2007;Kaufman et al., 2009;Hoffman, 2011;Miller, 2013;Hoffman et al., 2017;Hoffman and Halverson, 2018;Nascimento et al., 2018). ...
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The Neoproterozoic Hüttenberg Formation in northeastern Namibia records a remarkable δ13Ccarb positive excursion with a sustained plateau of values up to +12‰ (i.e., the Hüttenberg anomaly). High-resolution chemostratigraphic analyses of drill core samples spanning the upper Elandshoek and Hüttenberg formations reveal multiple new observations: (1) overall high but oscillatory δ13Ccarb values; (2) δ18Ocarb values ranging from −8‰ to −2‰; (3) significant enrichment of 13C in organic carbon and a broad co-variation between δ13Ccarb and δ13Corg; (4) a profound negative excursion in δ34Spyrite from +30‰ to −10‰; (5) an overall inverse δ13C–δ34S relationship; and (6) 87Sr/86Sr values down to 0.7074 in limestone samples. The new data suggest that the Hüttenberg anomaly records dynamic fluctuations in marine redox conditions, which may include an oxygenation event during the height of the δ13Ccarb positive excursion and a deoxygenation event at its termination. The δ34Spyrite negative excursion suggests the buildup of the marine sulfate reservoir, likely due to enhanced pyrite oxidation during the oxygenation event. The δ34Spyrite increase at the end of the Hüttenberg anomaly may result from a seawater sulfate concentration drawdown towards pre-anomaly conditions. On one hand, the Hüttenberg anomaly may reflect restricted basin signals that are deviated from the Ediacaran open ocean; on the other hand, the Ediacaran Hüttenberg anomaly, together with the Cryogenian δ13Ccarb positive excursions, suggests a stepwise pattern of the Neoproterozoic Oxygenation Event. Both local and global environmental factors may have contributed to the Hüttenberg anomaly. The Hüttenberg anomaly therefore represents a local enhancement of global oxygenation signals. Our data support the emerging view that the Neoproterozoic Oxygenation Event may have facilitated the evolution of early life at that time.
Article
After tilt correction for Ediacaran thick-skinned folding, a pair of Cryogenian half grabens at the autochthonous southwest cape of Congo Craton (CC) in northwest Namibia restore to different orientations. Toekoms sub-basin trended east-northeast, parallel to Northern Zone (NZ) of Damara belt, and was bounded by a normal-sense growth fault (2 290 m throw) dipping 57° toward CC. Soutput sub-basin trended northwest, oblique to NZ and to north-northwest-trending Kaoko Belt. It was bounded by a growth fault (750 m down-dip throw) dipping steeply (~75°) toward CC. Soutput growth fault could be an oblique (splay) fault connecting a Cryogenian rift zone in NZ with a sinistral transform zone in Kaoko Belt. A transform origin for the Kaoko margin accords with its magma-poor abrupt shelf-to-basin change implying mechanical strength, unlike the magma-rich southern margin where a gradual shelf-to-basin change implies a mechanically weak extended margin. A rift−transform junction is kinematically compatible with observed north-northwest−south-southeast Cryogenian crustal stretching within CC. Post-rift subsidence of the CC carbonate platform varies strongly across the south-facing but not the west-facing shelf. A sheared western CC margin differs from existing Kaoko Belt models that posit orthogonal opening with hyper-extended continental crust. Carbonate-dominated sedimentation over southwest CC implies palaeolatitudes ≤35° between 770 and 600 Ma.
Article
Otavi Group is a 1.5−3.5-km-thick epicontinental marine carbonate succession of Neoproterozoic age, exposed in an 800-km-long Ediacaran−Cambrian fold belt that rims the SW cape of Congo craton in northern Namibia. Along its southern margin, a contiguous distally tapered foreslope carbonate wedge of the same age is called Swakop Group. Swakop Group also occurs on the western cratonic margin, where a crustal-scale thrust cuts out the facies transition to the platformal Otavi Group. Subsidence accommodating Otavi Group resulted from S−N crustal stretching (770−655 Ma), followed by post-rift thermal subsidence (655−600 Ma). Rifting under southern Swakop Group continued until 650−635 Ma, culminating with breakup and a S-facing continental margin. No hint of a western margin is evident in Otavi Group, suggesting a transform margin to the west, kinematically consistent with S−N plate divergence. Rift related peralkaline igneous activity in southern Swakop Group occurred around 760 and 746 Ma, with several rift-related igneous centres undated. By comparison, western Swakop Group is impoverished in rift-related igneous rocks. Despite low paleoelevation and paleolatitude, Otavi and Swakop groups are everywhere imprinted by early and late Cryogenian glaciations, enabling unequivocal stratigraphic division into five epochs (period divisions): (1) non-glacial late Tonian, 770−717 Ma; (2) glacial early Cryogenian/Sturtian, 717−661 Ma; (3) non-glacial middle Cryogenian, 661−646±5 Ma; (4) glacial late Cryogenian/Marinoan, 646±5−635 Ma; and (5) non-glacial early Ediacaran, 635−600±5 Ma. Odd numbered epochs lack evident glacioeustatic fluctuation; even numbered ones were the Sturtian and Marinoan snowball Earths. This study aimed to deconstruct the carbonate succession for insights on the nature of Cryogenian glaciations. It focuses on the well-exposed southwestern apex of the arcuate fold belt, incorporating 585 measured sections (totaling >190 km of strata) and >8,764 pairs of δ13C/δ18Ocarb analyses (tabulated in Supplementary On-line Information). Each glaciation began and ended abruptly, and each was followed by anomalously thick ‘catch-up’ depositional sequences that filled accommodation space created by synglacial tectonic subsidence accompanied by very low average rates of sediment accumulation. Net subsidence was 38% larger on average for the younger glaciation, despite its 3.5−9.3-times shorter duration. Average accumulation rates were subequal, 4.0 vs 3.3−8.8 m Myr−1, despite syn-rift tectonics and topography during Sturtian glaciation, versus passive-margin subsidence during Marinoan. Sturtian deposits everywhere overlie an erosional disconformity or unconformity, with depocenters ≤1.6 km thick localized in subglacial rift basins, glacially carved bedrock troughs and moraine-like buildups. Sturtian deposits are dominated by massive diamictite, and the associated fine-grained laminated sediments appear to be local subglacial meltwater deposits, including a deep subglacial rift basin. No marine ice-grounding line is required in the 110 Sturtian measured sections in our survey. In contrast, the newly-opened southern foreslope was occupied by a Marinoan marine ice grounding zone, which became the dominant repository for glacial debris eroded from the upper foreslope and broad shallow troughs on the Otavi Group platform, which was glaciated but left nearly devoid of glacial deposits. On the distal foreslope, a distinct glacioeustatic falling-stand carbonate wedge is truncated upslope by a glacial disconformity that underlies the main lowstand grounding-zone wedge, which includes a proximal 0.60-km-high grounding-line moraine. Marinoan deposits are recessional overall, since all but the most distal overlie a glacial disconformity. The Marinoan glacial record is that of an early ice maximum and subsequent slow recession and aggradation, due to tectonic subsidence. Terminal deglaciation is recorded by a ferruginous drape of stratified diamictite, choked with ice-rafted debris, abruptly followed by a syndeglacial-postglacial cap-carbonate depositional sequence. Unlike its Sturtian counterpart, the post-Marinoan sequence has a well-developed basal transgressive (i.e., deepening-upward) cap dolomite (16.9 m regional average thickness, n=140) with idiosyncratic sedimentary features including sheet-crack marine cements, tubestone stromatolites and giant wave ripples. The overlying deeper-water calci-rhythmite includes crystal-fans of former aragonite benthic cement ≤90 m thick, localized in areas of steep sea-floor topography. Marinoan sequence stratigraphy is laid out over ≥0.6 km of paleobathymetric relief. Late Tonian shallow-neritic δ13Ccarb records were obtained from the 0.4-km-thick Devede Fm (~770−760 Ma) in Otavi Group and the 0.7-km-thick Ugab Subgroup (~737−717 Ma) in Swakop Group. Devede Fm is isotopically heavy, +4−8‰ VPDB, and could be correlative with Backlundtoppen Fm (NE Svalbard). Ugab Subgroup post-dates 746 Ma volcanics and shows two negative excursions bridged by heavy δ13C values. The negative excursions could be correlative with Russøya and Garvellach CIEs (carbon isotope excursions) in NE Laurentia. Middle Cryogenian neritic δ13C records from Otavi Group inner platform feature two heavy plateaus bracketed by three negative excursions, correlated with Twitya (NW Canada), Taishir (Mongolia) and Trezona (South Australia) CIEs. The same pattern is observed in carbonate turbidites in distal Swakop Group, with the sub-Marinoan falling stand wedge hosting the Trezona CIE recovery. Proximal Swakop Group strata equivalent to Taishir CIE and its subsequent heavy plateau are shifted bidirectionally to uniform values of +3.0−3.5‰. Early Ediacaran neritic δ13C records from Otavi Group inner platform display a deep negative excursion associated with the post-Marinoan depositional sequence and heavy values (≤+11‰) with extreme point-to-point variability (≤10‰) in the youngest Otavi Group formation. Distal Swakop Group mimics older parts of the early Ediacaran inner platform δ13C records, but after the post-Marinoan negative excursion, proximal Swakop Group values are shifted bidirectionally to +0.9±1.5‰. Destruction of positive and negative CIEs in proximal Swakop Group is tentatively attributed to early seawater-buffered diagenesis (dolomitization), driven by geothermal porewater convection that sucks seawater into the proximal foreslope of the platform. This hypothesis provocatively implies that CIEs originating in epi-platform waters and shed far downslope as turbidites are decoupled from open-ocean DIC (dissolved inorganic carbon), which is recorded by the altered proximal Swakop Group values closer to DIC of modern seawater. Carbonate sedimentation ended when the cratonic margins collided with and were overridden by the Atlantic coast-normal Northern Damara and coast-parallel Kaoko orogens at 0.60−0.58 Ga. A forebulge disconformity separates Otavi/Swakop Group from overlying foredeep clastics. In the cratonic cusp, where the orogens meet at a right angle, the forebulge disconformity has an astounding ≥1.85 km of megakarstic relief, and kmthick mass slides were displaced gravitationally toward both trenches, prior to orogenic shortening responsible for the craton-rimming fold belt.
Article
Lithospheric cusps occur where arcs are joined end to end. Where a subducting plate moves directly into a cusp, the slab experiences lateral constriction due to the cusp geometry. Buckled slabs of Cenozoic age occur at cusps (also known as ‘syntaxes’) in the Arabian, Indian, Pacific, Juan de Fuca and other plates. Here I report an Ediacaran example from the cusp of the Congo Craton where Pan-African collision zones meet at a right angle in NW Namibia. The craton was blanketed by syn- and post-rift Neoproterozoic marine carbonate, disconformably overlain by collision-related foredeep clastics. The disconformity has little stratigraphic relief in a 900 km-long fold belt rimming the craton, except within 60 km of the cusp apex where foredeep deposits bury a megakarst landscape floored by exhumed crystalline basement. Forebulge uplift, estimated from palaeokarst relief, was ≥1.85 km. This far exceeds characteristic forebulge heights of c. 0.5 km and matches the deepest part of the Grand Canyon of Arizona (USA). Coeval with megakarst development, map-scale mass slides moved coherently westwards and southwards towards the advancing accretionary prisms. Rapid burial by foredeep clastics preserved the megakarst palaeosurface and associated mass slides; folding them brought protection from complete destructive resurfacing for eons.
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Age calibrated deformation histories established by detailed mapping and dating of key magmatic time markers, are correlated across all tectono-metamorphic provinces in the Damara Orogenic System. Correlations across structural belts result in an internally consistent deformation framework with evidence of stress field rotations with similar timing, and switches between different deformation events. Horizontal principle compressive stress rotated clockwise ∼180º in total during Kaoko Belt evolution, and ∼135º during Damara Belt evolution. At most stages, stress field variation is progressive and can be attributed to events within the Damara Orogenic System, caused by change in relative trajectories of the interacting Rio De La Plata, Congo and Kalahari Cratons. Kaokoan orogenesis occurred earliest and evolved from collision and obduction at ∼590 Ma, involving E-W directed shortening, progressing through different transpressional states with ∼45º rotation of the stress field to strike-slip shear under NW-SE shortening at ∼550–530 Ma. Damaran orogenesis evolved from collision at ∼555–550 Ma with NW-SE directed shortening in common with the Kaoko Belt, and subsequently evolved through ∼90º rotation of the stress field to NE-SW shortening at ∼512–508 Ma. Both Kaoko and Damara orogenic fronts were operating at the same time, with all three cratons being coaxially convergent during the 550–530 Ma period; Rio De La Plata directed SE against the Congo Craton margin, and both together over-riding the Kalahari Craton margin also towards the SE. Progressive stress field rotation was punctuated by rapid and significant switches at ∼530–525 Ma, ∼508 Ma and ∼505 Ma. These three events included: (1) Culmination of main phase orogenesis in the Damara Belt, coinciding with maximum burial and peak metamorphism at 530–525 Ma. This occurred at the same time as termination of transpression and initiation of transtensional reactivation of shear zones in the Kaoko Belt. Principle compressive stress switched from NW-SE to NNW-SSE shortening in both Kaoko and Damara Belts at this time. This marks the start of Congo-Kalahari stress field overwhelming the waning Rio De La Plata-Congo stress field, and from this time forward contraction across the Damara Belt generated the stress field governing subsequent low-strain events in the Kaoko Belt. (2) A sudden switch to E-W directed shortening at ∼508 Ma is interpreted as a far-field effect imposed on the Damara Orogenic System, most plausibly from arc obduction along the orogenic margin of Gondwana (Ross-Delamerian Orogen). (3) This imposed stress field established a N–S extension direction exploited by decompression melts, switch to vertical shortening, and triggered gravitational collapse and extension of the thermally weakened hot orogen core at ∼505 Ma, producing an extensional core complex across the Central Zone.
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Cryogenian synglacial deposits are regionally thin but locally thick, considering glacial duration, but the reasons for local thickening are poorly known. We studied three local thickenings of the Sturtian Chuos Formation in northern Namibia by measuring closely spaced columnar sections, not only of the synglacial deposits but also of the bounding pre- and post-glacial strata. This enabled incised paleovalleys filled by glacial debris to be distinguished from morainal buildups. In case 1, a U-shaped paleovalley, ~450 m deep by ~3.0 km wide, is incised into pre-glacial strata and 10% overfilled by ice-contact and subglacial meltwater deposits. In case 2, a wedge of glacial diamictite, ~220 m thick by 2.0 km wide, overlies a disconformity that is demonstrably not incised into underlying pre-glacial strata. The wedge, draped by a post-glacial cap carbonate and argillaceous strata, is erosionally truncated at its apex by Marinoan glacial deposits and their basal Ediacaran cap dolomite. The wedge was a positive topographic feature, either a terminal moraine or an erosional outlier of formerly more extensive glacial deposits. In case 3, a wedge of conglomerate, glacial diamictite, and subglacial lake deposits thickens to >2000 m where it abuts against granitoid basement rock uplifted along a border fault. Fault movement ceased before the Sturtian cap carbonate was deposited. The locus of maximum deposition shifted over time from proximal to distal with respect to the border fault, similar to Mesozoic half grabens developed above listric detachments imaged seismically on offshore North Atlantic margins.
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The Cryogenian Period was first established in 1988 along with other Precambrian eon, era and period-level subdivisions that were defined numerically by Global Standard Stratigraphic Ages (GSSAs). As absolute age constraints have improved, some of these time intervals no longer bracket adequately the geological event(s), for which they were named. For example, the age discrepancy between the basal Cryogenian GSSA at 850 Ma and the onset of widespread glaciation ca. 717 Ma has rendered the 850 Ma boundary obsolete. The International Commission on Stratigraphy has now formally approved the removal of the Cryogenian GSSA from its International Chronostratigraphic Chart and supports its replacement with a rock-based Global Stratotype Section and Point (GSSP). The new Cryogenian GSSP will be placed at a globally correlative level that lies stratigraphically beneath the first appearance of widespread glaciation and is assigned in the interim a 'calibrated age' of circa 720 Ma. This new definition for the Tonian/Cryogenian boundary should be used in future publications until a formal Cryogenian GSSP can be ratified. The change marks progress towards establishment of a 'natural' (rock-based) scale for Precambrian time.
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The snowball Earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion year time scale. Geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons. Here we present four new Re-Os geochronology age constraints on Sturtian (717-660 Ma) and Marinoan (635 Ma termination) glacial deposits from three different paleocontinents. A 752.7 ± 5.5 Ma age from the base of the Callison Lake Formation in Yukon, Canada, confirms nonglacial sedimentation on the western margin of Laurentia between ca. 753 and 717 Ma. Coupled with a new 727.3 ± 4.9 Ma age directly below the glacigenic deposits of the Grand Conglomerate on the Congo craton (Africa), these data refute the notion of a global ca. 740 Ma Kaigas glaciation. A 659.0 ± 4.5 Ma age directly above the Maikhan-Uul diamictite in Mongolia confirms previous constraints on a long duration for the 717-660 Ma Sturtian glacial epoch and a relatively short nonglacial interlude. In addition, we provide the first direct radiometric age constraint for the termination of the Marinoan glaciation in Laurentia with an age of 632.3 ± 5.9 Ma from the basal Sheepbed Formation of northwest Canada, which is identical, within uncertainty, to U-Pb zircon ages from China, Australia, and Namibia. Together, these data unite Re-Os and U-Pb geochronological constraints and provide a refined temporal framework for Cryogenian Earth history.
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The Ombonde detachment is a primary low-angle normal fault that developed in an undeformed Neoproterozoic carbonate shelf succession as it entered a west-dipping Pan-African subduction zone. The fault is mappable from the top of the shelf succession to the granitic basement surface at a paleodepth of 1.5 km, and the hanging wall has not been significantly deformed. The primary fault geometry is well constrained by stratigraphic cutoff relationships, irrespective of secondary rotations. The dip direction of the fault was ˜270°, and its horizontal separation was 15 18 km. The fault plane is composed of two ramps separated by a long flat segment at a paleodepth of 0.55 km. The ramps are inclined 8° 14° relative to the carbonate strata, which underwent little or no compaction, and the mean cutoff angle overall is 1.3°. Given constraints on the contemporaneous tectonic setting, the primary fault dips must equal the stratigraphic cutoff angles augmented by a taper angle for lithospheric flexure of not more than 4°. Primary mean dips of 200 m, which would have significantly reduced the water load and thereby the normal stress on a subhorizontal plane, possibly leading to excess pore-fluid pressures. This scenario is consistent with the virtual absence of macroscopic shear deformation adjacent to the fault plane.
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Glacial deposits of Sturtian and Marinoan age occur in the well-studied Neoproterozoic successions of northern Namibia, South Australia, and northwestern Canada. In all three regions, the Marinoan glaciation is presaged by a large negative δ13C anomaly, and the cap carbonates to both glacial units share a suite of unique sedimentological, stratigraphic, and geochemical features. These global chronostratigraphic markers are the bases of a new correlation scheme for the Neoproterozoic that corroborates radiometric data that indicate that there were three glacial epochs between ca. 750 and 580 Ma. Intraregional correlation of Neoproterozoic successions in the present-day North Atlantic region suggests that glacial diamictite pairs in the Polarisbreen Group in northeastern Svalbard and the Tillite Group in eastern Greenland were deposited during the Marinoan glaciation, whereas the younger of a pair of glacials (Mortensnes Formation) in the Vestertana Group of northern Norway was deposited during the third (Gaskiers) Neoproterozoic glaciation. Gaskiers-aged glacial deposits are neither globally distributed nor overlain by a widespread cap carbonate but are associated with an extremely negative δ13C anomaly. The chronology developed here provides the framework for a new, high-resolution model carbon-isotope record for the Neoproterozoic comprising new δ13C (carbonate) data from Svalbard (Akademikerbreen Group) and Namibia (Otavi Group) and data in the literature from Svalbard, Namibia, and Oman. A new U-Pb zircon age of 760 ± 1 Ma from an ash bed in the Ombombo Subgroup in Namibia provides the oldest direct time-calibration point in the compilation, but the time scale of this preliminary δ13C record remains poorly constrained.
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A new U–Pb SHRIMP age of 551±4Ma on a mylonitic porphyry that intruded into the Sierra Ballena Shear Zone (Southernmost Dom Feliciano Belt, Uruguay) and a review of relevant published data make possible a more refined correlation and reconstruction of Brasiliano/Pan-African transpressional events. Paleogeographic reconstruction, kinematics and timing of events indicate a connection between the shear systems of the Dom Feliciano and Kaoko Belts at 580–550Ma. Sinistral transpression recorded in shear zones accommodates deformation subsequent to collision between the Congo and Río de la Plata Cratons. The correlation is strengthened by the similarity of magmatic and metamorphic ages in the Coastal Terrane of the Kaoko Belt and the Punta del Este Terrane of the Dom Feliciano Belt. This post-collisional sinistral transpression brought these units near to their final position in Gondwana and explains the different evolution at 550–530Ma. While in the Kaoko Belt, an extensional episode resulted in exhumation as a consequence of collision in the Damara Belt, in the Dom Feliciano Belt, sinistral transpression occurred associated with the closure of the southern Adamastor Ocean due to Kalahari-Río de la Plata collision. KeywordsDom Feliciano Belt–Kaoko Belt–Brasiliano–Pan-African–Transpressional deformation–Shear Zones
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The Damara Orogen is composed of the Damara, Kaoko and Gariep belts developed during the Neoproterozoic Pan-African Orogeny. The Damara Belt contains Neoproterozoic siliciclastic and carbonate successions of the Damara Supergroup that record rift to proto-ocean depositional phases during the Rodinia supercontinent break up. There are two conflicting interpretations of the geotectonic framework of the Damara Supergroup basin: i) as one major basin, composed of the Outjo and Khomas basins, related to rifting in the Angola-Congo-Kalahari paleocontinent or, ii) as two independent passive margin basins, one related to the Angola-Congo and the other to the Kalahari proto-cratons. Detrital zircon provenance studies linked to field geology were used to solve this controversy. U-Pb zircon age data were analyzed in order to characterize depositional ages and provenance of the sediments and evolution of the succession in the northern part of the Outjo Basin. The basal Nabis Formation (Nosib Group) and the base of the Chuos Formation were deposited between ca. 870 Ma and 760 Ma. The upper Chuos, Berg Aukas, Gauss, Auros and lower Brak River formations formed between ca. 760 Ma and 635 Ma. It also includes the time span recorded by the unconformity between the Auros and lower Brak River formations. The Ghaub, upper Brak River, Karibib and Kuiseb formations were deposited between 663 Ma and 590 Ma. The geochronological data indicate that the main source areas are related to: i) the Angola-Congo Craton, ii) rift-related intrabasinal igneous rocks of the Naauwpoort Formation, iii) an intrabasinal basement structural high (Abbabis High), and iv) the Coastal Terrane of the Kaoko Belt. The Kalahari Craton units apparently did not constitute a main source area for the studied succession. This is possibly due to the position of the succession in the northern part of the Outjo Basin, at the southern margin of the Congo Craton. Comparison of the obtained geochronological data with those from the literature shows that the Abbabis High forms part of the Kalahari proto-craton and that Angola-Congo and Kalahari cratons were part of the same paleocontinent in Rodinia times.
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The end-Cryogenian glaciation (Marinoan) is portrayed commonly as the archetype of snowball Earth, yet its duration and character remain uncertain. Here we report U-Pb zircon ages for two ash beds from widely separated localities of the Marinoan-equivalent Ghaub Formation in Namibia: 639.29 ± 0.26 Ma and 635.21 ± 0.59 Ma. These findings verify, for the first time, the key prediction of the snowball Earth hypothesis for the Marinoan glaciation, i.e., longevity, with a duration of ≥4 m.y. They also show that the nonglacial interlude of Cryogenian time spanned 20 m.y. or less and that glacigenic erosion and sedimentation, and at least intermittent open-water conditions, occurred 4 m.y. prior to termination of the Marinoan glaciation.
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The match of geological units of the dissected Kaoko-Dom Feliciano-Gariep orogenic system exposed along the coasts of the South Atlantic is poorly understood. Two suites of intrusive rocks crop out in the Angra Fria Bay area, a part of the Coastal Terrane of the Kaoko Belt in Namibia. U–Pb zircon dating of three samples from the younger suite provided ages of 574 ± 6, 586 ± 3 and 584 ± 7 Ma, similar to the ages of the oldest syn-collisional granitoids in the area. Three samples of the older suite gave ages of 626 ± 5, 622 ± 5 and 620 ± 6 Ma respectively, which have not previously been recorded in intrusive rocks of the Kaoko Belt, but which coincide with those from the Florianópolis Batholith in the Dom Feliciano Belt in Brazil. Zircon ages, Sr–Nd isotopic composition and tectonic position of these granitoids suggest that this magmatic complex could be a continuation of the Florianópolis Batholith on the African side of the Atlantic Ocean. Consequently, the Angra Fria intrusions and the Florianópolis Batholith may represent suitable localities for spatial reconstruction of the Gondwana supercontinent in the area, where the robust pre-Mesozoic connecting points are scarce.
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The existence of coherent, large-scale, submarine landslides on modern continental margins implies that their apparent rarity in ancient orogenic belts is due to non-recognition. Two map-scale, coherent, pre-orogenic, normal-sense detachment structures of Ediacaran age are present in the Kaoko belt, a well-exposed arc–continent collision zone in northwestern Namibia. The structures occur within the Otavi Group, a Neoproterozoic carbonate shelf succession. They are brittle structures, evident only through stratigraphic omissions of 400mor more, that ramp down to the west with overall ramp angles of 1.1° and 1.3° with respect to stratigraphic horizons. The separations of matching footwall and hangingwall stratigraphic cut-offs require horizontal translations >20 km for each detachment. One of the detachments is remarkably narrow (5 km) in the up-dip direction, just one fourth of its translation. The other detachment is stratigraphically dated at the shelf–foredeep transition, when the passive margin was abortively subducted westward, in the direction of submarine sliding. Trenchward sliding on the foreslope occurred concurrently with deep karstification of the autochthonous carbonate succession to the east, presumably due to forebulge uplift and (or) conjectural basin-scale base-level fall. We expect that similar detachments exist in other orogenic belts, and failure to recognize them can lead to misinterpretations of stratigraphy, sedimentary facies, and paleogeography.
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.
Article
The proximity of the Congo and Kalahari cratons during the Neoproterozoic breakup of the supercontinent Rodinia and during subsequent assembly of Gondwana is unclear. Neoproterozoic metasedimentary rocks from the rifted margins of Congo and Kalahari in the Damara Orogen yield distinctive detrital zircon U-Pb age distributions that correspond to the ages of prominent crustal components within the respective cratons. The most abundant zircons from Neoproterozoic strata deposited on the Congo margin give ages of 1150-1000 and 800-600 Ma, whereas, the most abundant zircons from the Kalahari margin strata range from 1350 to 1100 Ma. A 1350-1200 Ma detrital zircon population in the Kalahari margin strata is absent in the Damara-Congo strata. A prominent c. 1050-1000 Ma detrital zircon age population from Damara-Congo strata is nearly absent from the Damara-Kalahari strata, even though orogenic events of this age are found on both cratons. Damara strata on the Kalahari margin also lack detrital zircons with U-Pb ages of 900-600 Ma. The differences in detrital zircon age distributions are robust when comparing strata of the same age on both cratons, and remains so, even when younger, deeper water facies are excluded, which could have been biased by other sediment sources. These data suggest that the Congo and Kalahari cratons were not proximal in Rodinia, and did not establish their current relative positions until the end of the Neoproterozoic when they were sutured together during the collisional orogenies that formed Gondwana.
Article
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 Ecliacaran 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.
Article
The Sturtian is the oldest (ca 716 Ma) of three pan‐global glaciations in the Cryogenian. At Omutirapo, in northern Namibia, a 2 km wide, 400 m deep palaeovalley is filled by glaciogenic strata of the Chuos Formation, which represents the Sturtian glacial record. Sedimentary logging of an exceptionally high‐quality exposure permits detailed stratigraphic descriptions and interpretations, allowing two glacial cycles to be identified. At the base of the exposed succession, strong evidence supporting glaciation includes diamictites, ice‐rafted dropstones and intensely sheared zones of interpreted subglacial origin. These facies collectively represent ice‐proximal to ice‐rafted deposits. Upsection, dropstone‐free mudstones in the middle of the succession, and the absence of diamictites, imply sedimentation free from glacial influence. However, the reappearance of glacial deposits above indicates a phase of Sturtian glacial re‐advance. Comparison with age‐equivalent strata in South Australia, where evidence for sea‐ice free sedimentation has been established previously, suggests that a Sturtian interglacial may have been extensive, implying global‐scale waxing and waning of ice sheets during a Cryogenian glacial event.
Article
The geology of the Owambo Basin is known from outcrops along its margins, from interpretation of seismic, aeromagnetic and gravity surveys and from a few widely spaced wells. The Owambo Basin is floored by mid-Proterozoic crustal rocks of the Congo Craton and contains possibly as much as 8000 m of sedimentary rocks of the Nosib, Otavi and Mulden Groups of the late-Proterozoic Damara Sequence, 360 m of Karoo rocks and a blanket of semi-consolidated to unconsolidated Cretaceous to Recent Kalahari Sequence sediments up to 600 m thick.
Article
The Neoproterozoic succession in the Outjo District of northern Namibia is a mainly sedimentary sequence that was deposited in the time-interval of approximately 800 to 600 Ma ago and was deformed and suffered mild greenschist facies regional metamorphism during Damaran times ca. 550 Ma ago. The map area occupies the lithofacies and tectonic transition from the highly deformed more eugeoclinal Swakop Group sequence to the south to the coeval, uniformly southward-dipping, miogeoclinal Otavi Group of the Fransfontein Ridge to the north. The oldest unit is a largely sandstone and biotite-muscovite phyllite succession tentatively equated with the volcanogenic Naauwpoort Formation (Nosib Group) that pre-dates the Swakop Group in the Summas Mountains to the southwest. The overlying Saturn Formation comprises two distinct but coeval lateral carbonate facies: a 300 m thick sequence of bedded dolomite with a 20 m thick median limestone; and massive biohermal dolomite of undetermined thickness. The conformably overlying Landeck Formation comprises a lower succession of interbedded sandstone, phyllite and local carbonate rocks, a distinctive middle iron formation marker horizon with rather rare overlying dolomite, followed by a siliciclastic-carbonate sequence. The combined succession is up to 450 m in thickness, and glacial diamictite is recorded at 30+ localities sited below, within and above the iron formation. However, in the more distal parts of the Landeck Formation in the Saturn Dome, glacial diamictictite has not been identified, the iron formation is overlain by a distinctive dolomite, and the uppermost 200 to 300 m of the Landeck Formation is characterized by four horizons of siliciclastic sediments with three intervening horizons of blue limestone that reflect the concordant transition to the overlying Bergfriede Formation. New chemical analyses are reported for graphitic dolomite, magnetite-calcite schist, iron formation, ferruginous limestone, dolomitic sandstone and biotite-muscovite phyllite of the Landeck Formation. The Bergfriede Formation consists almost entirely of limestone and dolomite and is characterized by major changes in facies and thickness, and five lateral facies end-members are recognized: I. Hankow Facies, a five-fold succession of limestone, dolomite and calcarenite up to 500 m thick; II. Tsuwandes-Zuwitsaub Facies, a three-fold sequence of limestone-dolomite-limestone >300 m thick; III. Uranus Facies, entirely dolomite generally 150 m thick; IV. Belina-Sophienhof Facies, a limestone-dolomite sequence, with a distinctive arenaceous limestone at the top, 100 to 400 m thick; and V. Mooilagte-Mooihoek Facies, largely dolomite-chert-limestone ≥1500 m thick, that is transitional to the dolomite-dominated upper Otavi Tsumeb Subgroup of the Fransfontein Ridge to the north. The overlying Okaua Formation is composed of a >3000 m thick sequence of muscovite-chlorite phyllite and calcareous sandstone that occupy, inter alia, the troughs of the regional Ugab and Uranus-Neuland Synclines; it is most likely correlative with the Mulden Group of the Otavi Mountainland. The Damaran regional structure of area is characterized by: D1 an early phase of southward-directed gravity nappes; D2 the regionally dominant northeast-southwest to east-west trending tight compressional upright or northward-verging folds, and local thrusts; and D3 late strike-slip faulting.
Article
Dropstone-bearing glaciomarine sedimentary rocks of the Ghaub Formation within metamorphosed Neoproterozoic basinal strata (Swakop Group) in central Namibia contain interbedded mafic lava flows and thin felsic ash beds. U-Pb zircon geochronology of an ash layer constrains the deposition of the glaciomarine sediments to 635.5 ± 1.2 Ma, providing an age for what has been described as a “Marinoan-type” glaciation. In addition, this age provides a maximum limit for the proposed lower boundary of the terminal Proterozoic (Ediacaran) system and period. Combined with reliable age constraints from other Neoproterozoic glacial units—the ca. 713 Ma Gubrah Member (Oman) and the 580 Ma Gaskiers Formation (Newfoundland)—these data provide unequivocal evidence for at least three, temporally discrete, glacial episodes during Neoproterozoic time with interglacial periods, characterized by prolonged positive δ13C excursions, lasting at most ˜50 80 m.y.
Article
Tectonic fold tests conducted in Namibia demonstrate that the inclination with respect to bedding of geoplumb (palaeovertical) tubular structures in the Marinoan (635 Ma) syndeglacial cap dolostone is mainly the result of tectonic strain. Therefore, tubestone inclination data cannot be used to estimate the gradient of the sea floor on the foreslope of the Otavi carbonate platform during the Marinoan glaciation. A gradient steeper than 0·1 (slope angle ca 5·7°), implying a glacial base-level fall ≥0·5 km, is nevertheless supported by boulder-size intraclast debrite in the falling-stand wedge directly beneath the glacigenic sequence. Cryogenian oceans lacked skeletal carbonate production, raising the carbonate saturation state and persistent deep water anoxia excluded acid-producing aerobic respiration, facilitating early diagenetic carbonate precipitation, lithification and steep submarine slopes.
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