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Evidence for participation of microbial mats in the deposition of the siliciclastic 'ore formation' in the Copperbelt of Zambia
No full textThe Copperbelt of Zambia is the world's largest sediment-hosted stratiform copper province, hosted in siliciclastic sediments of the Roan Group, which forms the basal part of the Neoproterozoic-Paleozoic Katanga Supergroup. Much of the ore deposition occurred between 880 Ma and 780 Ma, on a rimmed platform consisting of a carbonate barrier, a lagoonal basin and tidal flats grading into sabkhas in the hinterland. Various sedimentary structures developed in the ore formation at the Mindola Open Pit mine, are herein considered to be microbially induced and are identified as microbial shrinkage cracks, wrinkle structures, mat deformation structures, petees, concentric microfaults, and microbial mat chips. The occurrence of these structures in all ore formation units at the Mindola Mine suggests microbial mats grew on the paleo-sediment surface throughout deposition of the cupriferous succession. As these structures require cohesive layers, the mats were likely of the cyanobacterial type, that grew in the well aerated intertidal to lower supratidal zones. Cyanobacterial mats typically consist of a surface layer of filamentous cyanobacteria underlain by anaerobic, heterotrophic sulfate reducing bacteria (SRB). A distinct sulfide mineral zonation, developed in all major deposits of the Copperbelt, ranges from barren supratidal (sabkha) sediments, through chalcocite in the lower supratidal zone, to bornite followed by chalcopyrite in the intertidal zone, and pyrite in the subtidal zone and anoxic lagoonal depotcentre. This sequence of minerals can be modelled as a paragenetic sequence of mineralization resulting from the progressive reduction of a source fluid, indicating that geochemical conditions of ore formation, at least, are produced by the activity of SRB. (C) 2010 Elsevier Ltd. All rights reserved
Das Chausib-Turbiditbecken am Südrand des Damara-Orogens, Südwest-Afrika
No full textAm Südrand des Damara-Orogens ist innerhalb der Hakos Subgroup eine Turbiditserie von 1500 m Mächtigkeit entwickelt. Sie stellt als neudefiniertes Chausib Member eine Faziesvertretung der „Hakos-Quarzite“ (Chaibis Member) dar.
Die Zusammensetzung der Turbitbänke ist quarzitisch. Infolge metamorpher und tektonischer Überprägung ist eine Gradierung nach der Korngröße nicht mehr vorhanden; sie kann aber aus einer typischen Änderung des Stoffbestandes vom Liegenden zum Hangenden einer Bank abgeleitet werden.
Die an sich hochfaltbare Serie liegt in einer einfach gebauten Großstruktur von 10 km Wellenlänge vor; die Faltung wurde durch eine mächtige unterlagernde Quarzitfolge gesteuert. Einige für eine Wechselfolge kompetenter und inkompetenter Lagen charakteristische kleintektonische Phänomene werden beschrieben.
Für die paläogeographische Situation wird ein Modell entwickelt, nach dem die Chausib-Turbidite als Beckenablagerungen den Chaibis-Quarziten als gleichalte Schwellensedimente gegenüberstehen.A 1500 m thick turbidite series, developed within the Hakos Subgroup at the southern margin of the Damara Orogen, is defined as the Chausib Member. It represents a timeequivalent of the “Hakos-Quartzite” (Chaibis Member) in a different facies.
The turbidites are quartzitic. As a result of the metamorphic and tectonic overprint, they no longer show a grading of grain size. However, a former graded bedding can be inferred from a typical compositional change from the base to the top within each bed.
The turbidite series is potentially very susceptible to folding, but occurs in a simple structure with a wave-length of about 10 km. The style of folding was determined by an underlying, thick, quartzite unit. Several small-scale tectonic phenomena are described, which are characteristic for such an alternation of competent and incompetent layers.
A palaeogeographic model is envisaged in which the Chausib turbidites are basinal deposits bordering the time-equivalent sediments of a submarine rise, represented by the Chaibis quartzites.A la bordure sud de l'orogène de Damara, on trouve une série de turbidites d'une épaisseur de 1500 m intercalée dans le Hakos Subgroup. En tant que Chausib Member, nouvellement défini, elle représente un faciès équivalent des quartzites de Hakos (Chaibis Member).
Les bancs de turbidite ont une composition quartzitique. Mais la superposition métamorphique et tectonique ne permet plus de discerner le granoclassement qui peut cependant être déduit du changement typique dans la composition chimique et minéralogique du matériau d'un banc, du bas vers le haut.
La série dont la capacité de plissement est certaine, se présente avec une structure majeure simple, d'une longueur d'onde de 10 km, le plissement ayant été handicapé par une puissante série sous-jacente de quartzite. Quelques phénomènes microtectoniques caractéristiques d'une série interstratifiée de couches compétentes et incompétentes, sont décrits.
Un modèle de situation paléogéographique est développé, d'après lequel les turbidites de Chausib s'opposent comme dépôts sédimentaires de bassin contemporains, aux quartzites de Chaibis, considérés comme sédiments de seuil de même âge.На южном краю орогена Дамара внутри подгру ппы Hakos развилась серия Turbidite в 1500 м мощностью. Она представлена, как наново описанный член Chausib, фаций «кварцит ов Hakos».
Банки Turbit состоят из ква рцита. В следствие пер екрытия метаморфных и тектонических проц ессов распределение по зернистости не наблю дается. Но её можно вывести из типичного изменения состава от подошвы до кровли банки.
Сильное смятие в скла дки этих серий создал о крупные структурные единицы до 10 км. На характер скл адчатости повлияли м ощные слои кварцитовых пород, подстилающих е е. Описаны некоторые х арактерные чередования мелкотектонических образований.
Установлено, что Chausib-Turbidite от ложились в глубине ба ссейна, а кварциты Chaibis оказались пороговыми отложени ями того же возраста. Реконструирована па леогеография данног о бассейна
Mat-related sedimentary structures in Neoproterozoic peritidal passive margin deposits of the West African Craton (Anti-Atlas, Morocco)
No full textProterozoic inliers in the central Anti-Atlas mountains expose predominantly siliciclastic sedimentary successions deposited in peritidal zones along the Neoproterozoic continental margin of the West African Craton (WAC). The low-grade metamorphic and modestly deformed sediments contain a wealth of sedimentary structures related to the former presence and activities of microbial mats and respective physicobiological processes. The well-preserved structures include wrinkle structures, erosion marks, microbial sand chips, spindle-shaped and subcircular microbial shrinkage cracks, and possibly gas domes and cabbage-head structures. Thin sections exhibit mat fragments and dispersed grains of hematite/limonite after pyrite in fine-grained quartzitic storm deposits. Post-storm layers frequently consist of matrix-supported sand-sized to silt-sized grains and are overlain by argillaceous veneers including isolated silt-sized grains and black carbonaceous laminae. The muddy veneers are considered to represent compacted stacks of microbial mats (biolaminites), which colonized and biostabilized storm and post-storm layers, and thus prevented them from eroding. In the absence of grazing and burrowing organisms and at suitable depositional and hydrodynamic conditions, it may be expected that Proterozoic microbial mats extensively grew from the supratidal to the intertidal zones and occasionally, e.g. behind protective barriers, in the subtidal zone and beyond. Mat-related structures, however, need specific conditions for their formation and preservation: Wrinkle structures, erosion marks, and microbial sand chips require tractional currents and soon deposition and burial, respectively. Such conditions are preferably met in intertidal and supratidal zones. Spindle-shaped and subcircular cracks require mat shrinkage due to either desiccation or "syneresis". Crack propagation implies progressive shrinkage, while superposition of crack generations indicates repeated alternation between mat exposure and flooding. Respective conditions prevail in the upper intertidal zone. Gas domes and cabbage-head structures are related to the production of gas from decaying organic material beneath a scaling cohesive mat. They may be some of the first features formed during genesis of petee structures in the intertidal to lower supratidal zones. Mat-related structures may serve as sensitive facies indicators once their modes of formation are revealed. (C) 2002 Elsevier Science B.V. All rights reserved
Siliciclastic biolaminites indicative of widespread microbial mats in the Neoproterozoic Nama Group of Namibia
No full textSiliciclastic biolaminites are thinly laminated sediments resulting from the interaction of epibenthic microbial mat growth and sediment supply. A thick succession of ancient siliciclastic biolaminites consisting of alternating thin layers of siltstone and sericitic argillite is reported here from the Ediacaran Vingerbreek Member (Schwarzrand Subgroup) of the Nama Group in Namibia. The succession was laid down in a low-energy, intertidal environment, behind a sheltering sand barrier or shoal. Intercalated are sandstone beds and bodies originating from storm events or representing tidal channel fills. The participation of microbial mats in the accretion of the siliciclastic biolaminites is indicated by mat growth and destruction structures, like 'elephant skin', circular cracks with involute margins, and sand clasts, and by microstructures like 'coated grains' and isolated silt-sized grains 'trapped' in mat-derived sericitic argillite. Typical features of the biolaminites are isolated, tepee-like, antiformal structures and isolated upturned packages of the laminated sediment, overlain by undisturbed layers. By comparison with modern examples, these structures are interpreted as upturned margins of major shrinkage cracks developed once in thick microbial mats, partly involving underlying biolaminated sediment. Microbial mats inducing accretion of siliciclastic biolaminites act as barriers and retain sediments, locally leading to sizeable sedimentary successions, in the intertidal zone. This may be expected particularly in the Precambrian where mats were widespread due to the absence of predators. (C) 2007 Elsevier Ltd. All rights reserved
Wind-induced mat deformation structures in recent tidal flats and sabkhas of SE-Tunisia and their significance for environmental interpretation of fossil structures
No full textPhysical processes acting on leathery and cohesive microbial mats that grow in tidal flats produce a large variety of mat deformation structures (MDS). Among these processes are strong winds which sweep episodically or continuously wide and protected areas of intertidal-supratidal zones covered with microbial mats. Wind-induced MDS occur when a mat layer covering the intertidal zone is floating or loosely attached to the underlying sedimentary layers. Observed MDS triggered by wind shear in recent intertidal to supratidal flats include: i) tearing and breaking up of mats into fragments and pieces of distinct size and shape, ii) network of folds and crumpled structures related to warping and creeping of soft mats, iii) flipped-over edges along shrinkage cracks and tears, iv) rolled-up mat edges and v) wind-blown mat fragments, scattered over the supratidal zone. The observed structures association forms a succession starting from simple tearing and breaking of a mat by wind forces and subsequent crumpling and folding. With continuous strong wind shear acting upon mat surfaces, most of the flipped-over edges are oriented in the direction of wind and form along tears and crack margins: they may evolve into rolled-up edges forming thick cigar-like bodies including both mat and thin sediment layers ('jelly roll'). Dried and non-biostabilised mat fragments are ripped off, transported landward and scattered over upper supratidal and sabkha zones. Within and intertidal-supratidal profile, the structures display a zonality which is controlled by the cohesive behaviour of mats and water-saturation of both mats and underlying sediment substrate. In the absence of recorded physical sedimentary features within the peritidal deposits, recognition and preservation of similar wind-induced mat deformation structures appear critical for environmental interpretation and indicate aeolian processes and an intertidal to supratidal flat setting, flooded intermittently during spring tide or storm events and periodically during high tide. (C) 2011 Elsevier B.V. All rights reserved.VW-Foundation [1/78 706
Setting and sedimentary facies of late Proterozoic alkali lake (playa) deposits in the southern Damara belt of Namibia
No full textThe late Proterozoic Damara Belt of Namibia has evolved from an elaborate system of continental rifts in which the basal portion (Nosib Group) of the Damara Sequence was deposited. In the southern rift, situated at the southern margin of the Damara Belt, the Nosib Group is represented by coarse elastic sediments (Kamtsas Formation) and fine-grained partly dolomitic deposits (Duruchaus Formation). Both formations occur, with interfingering relationships over nearly the total length of the rift.
In the Geelkop Dome area the pelitic-dolomitic Duruchaus Formation includes, in its upper part, a sequence of sediments that are characterized by cyclical deposition, high sodium contents, abundant albite pseudomorphs after primary evaporite minerals, and concordant solution and collapse breccias. This 300 m thick sequence has been interpreted as deposits of an alkali lake or playa complex.
Based on the playa lake models of Eugster and Hardie (1975) and Rowlands et al. (1980) four distinct facies have been distinguished in the upper part of the evaporitic Duruchaus Formation: (1) the submergent lake facies, represented by laminated shales containing layers of laminated siltstone; (2) the submergent/emergent mud flat facies, represented by laminated siltstone and calcareous shale with layers of dolomitic mudstone, calcareous shaly siltstone with scapolite and laminated sandy albitic dolomite with abundant pseudomorphs of albite after primary evaporite minerals (e.g. shortite, thermonatrite, borax); (3) the exposed saline crust facies, characterized by solution breccias, “albitolite” and “albitolite” breccias, all originating from former salt crusts; and (4) the elastic marginal facies, represented by quartzites of fluviatile and partly aeolian origin.
From identified primary and diagenetic to metamorphic minerals, and from the composition of several generations of fluid inclusions, it is concluded that the brines of the alkali lake were of a NaHCO3(KBClS) type. Material creating the extremely alkaline environment was derived from eroded acid-to-basic magmatic basement rocks and from coeval alkali-rich volcanism.
During tectogenesis the evaporite sequence was overridden by nappes approaching from the closing Damara geosyncline. Saline crust horizons were thereby partly mobilized and the resulting mush of dolomite and crystal fragments was squeezed into the thrust planes of the nappes where they acted as lubricants which enhanced late-orogenic thrusting along the southern margin of the Damara Belt
Neoproterozoic trace fossils vs. microbial mat structures: Examples from the Tandilia Belt of Argentina
No full textThe Tandilia Belt in northeast Argentina includes a Neoproterozoic sequence of sediments (Sierras Bayas Group), in which the Cerro Largo Formation, ca. 750 Ma in age, forms a siliciclastic, shallowing upward succession of subtidal nearshore to tidal flat deposits. Trace fossils Palaeophycus isp. and Didymaulichnus isp. have been described from the upper part of this succession. Specific sedimentary structures consisting of round-crested bulges, arranged in a reticulate pattern, and networks of curved cracks are associated with the trace fossils. These structures are considered to be related to epibenthic microbial mats that once colonized the sediment surface. They reflect stages of mat growth and mat destruction, if compared to analogous structures in modem cyanobacterial mats of peritidal, siliciclastic depositional systems. Also the trace fossils are interpreted as mat-related structures, partly fort-ning components of networks of shrinkage cracks, partly representing the upturned and involute margins of shrinkage cracks or circular openings in desiccating and shrinking, thin microbial mats. The definition of Didymaulichnus miettensis Young as a Terminal Proterozoic trace fossil is questioned, and it may be considered to interpret the 'bilobate' structure as the upturned, opposite margins of microbial shrinkage cracks which have been brought back into contact by compaction after burial. (C) 2007 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved
Wrinkle structures - a critical review
No full textIn this paper, a variety of so-called 'wrinkle structures' is reviewed in an attempt to help distinguish between crinkly decorations arising from physical processes that acted on siliciclastic bedding surfaces, and true microbially induced 'wrinkle structures'. Two types of small-scale, microbially induced sedimentary structures are prominent due to their distinct geometry and mode of occurrence: (1) 'elephant skin' textures, characterized by reticulate patterns of sharp-crested ridges forming mm- to cm-scale polygons, occurring on argillite or argillaceous veneers above fine-grained sandstone and likely reflecting growth structures of microbial, mats (2) 'Kinneyia' structures, characterized by mm-scale flat-topped, winding ridges and intervening troughs and pits, sometimes resembling small-scale interference ripples. 'Kinneyia' structures usually occur on upper surfaces of silistone/sandstone beds, themselves frequently event deposits, and are thought to have formed beneath microbial mats. Additionally, more linear variations of mat growth structures, partly resembling small-scale 'alpha-petees' may be developed. Finally, some wrinkly structures resulting from tractional mat deformation or mat slumping are occasionally preserved. These may appear as arcuate belts of non-penetrative, small-scale folds or as wrinkled bulges on otherwise flat surfaces. 'Wrinkle structures' as indicators for the former presence of mats gain in importance if other mat-related structures are additionally observed in the same clastic succession, e.g. 'sand chips' (sandy intraclasts) or spindle-shaped or sinuously curved to circular sand cracks, frequently combined in networks. Furthermore, appropriate lithologies and facies are required. For instance, if compared with the distribution of modem cohesive microbial mats, laminated siltstone/argillite with intercalated siltstone/sandstone beds representing event deposits in tidal flat successions would be compatible with microbial mat development. Within a variety of physically induced small-scale wrinkly structures, miniature load structures may, above all, be misinterpreted as microbially induced 'wrinkle structures', due to their similar size and appearance, and their comparatively frequent occurrence. (c) 2006 Elsevier B.V. All rights reserved
Bio-sedimentary structures evolving from the interaction of microbial mats, burrowing beetles and the physical environment of Tunisian coastal sabkhas
No full textMicrobial mats are widespread biotic systems in Tunisian coastal sabkhas. In these environments, staphilinid beetles are important bioturbators, making vertical burrows and depositing characteristic hillocks of excavation pellets on the mat surface. The intimate interplay between microbial and beetle activities results in a structured sedimentary surface. Small photosynthetic domes or bulges develop which in turn become microbially overgrown and gypsum-encrusted. Other examples are mat expansion domes above vertical burrows, and excavation pellets overgrown by biofilms trapping aeolian sand grains. In many cases the structures are too complex to explain clearly their formation. Since modern microbial mat-related surface structures form the base for interpretation of fossil record, it is essential to decipher what is contributed by the different structuie-forming factors
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