Papers by Gerard J B Germs
Frontiers in Earth Science
The recovery of microfossils from Proterozoic rocks is commonly challenging because of metamorphi... more The recovery of microfossils from Proterozoic rocks is commonly challenging because of metamorphism. In this study, an application of different methods usually applied on Phanerozoic rocks to test efficiency on recovering microfossil from Proterozoic units is presented. Chemical, physical, and biological factors can influence the recovery of microfossils, thereby becoming a barrier for biostratigraphic and paleoecological studies. Furthermore, low-cost projects with a reduced amount of sample collected, such as drill core sampling, need to optimize the preparation time and sample needed for different analyses. To overcome this challenge, the classical procedure of mineralized microfossil preparation, the palynological technique, and the study of clay mineralogy with the analyses of diagenetic alteration and the search for possible microfossils in thin sections were combined. Three Proterozoic lithostratigraphic units were selected to develop an integrated procedure for preparing sam...
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Precambrian Research, 2022
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Goldschmidt2022 abstracts, 2022
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Geologische Rundschau
Zusammenfassung Aus dem Witwatersrandbecken von Südafrika wird eine proterozoische, schichtparal... more Zusammenfassung Aus dem Witwatersrandbecken von Südafrika wird eine proterozoische, schichtparallele Störungszone beschrieben. Diese Störungszone wird von Pseudotachyliten dominiert, ihre jüngsten Tektonite enthalten daneben Quarzadern und Kataklasite, die jünger sind als die mit ihnen räumlich verknüpften Mylonke. Sowohl tektonische Reduktionen als auch Verdoppelungen haben ihre Ursache in der vorherrschend Nord bis West gerichteten Überschiebungszone. Die deformierten Gesteine sind wahrscheinlich nicht während eines
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South African Journal of Geology, 2014
Heavy mineral rich-sandstones (“black sands”) of the Lower Cambrian upper Fish River Subgroup in ... more Heavy mineral rich-sandstones (“black sands”) of the Lower Cambrian upper Fish River Subgroup in Namibia were used to reveal the provenance of the detrital material. The host rocks of the black sands from the Haribes and Rosenhof Members are mainly composed of partially rounded quartz, feldspar (K-feldspar > plagioclase), polycrystalline quartz, few siltstone fragments and labile altered or metamorphosed volcanic lithoclasts. Geochemical proxies are similar to those from typical foreland deposits and point to slightly recycled upper continental crustal composition, e.g.: Th/Sc ratios between 1.23 and 1.95, Zr/Sc ranging from 24 to 38 and La/Sc from 4.8 to 10.8. Heavy mineral addition of up to 30% does not significantly change Th/Sc, La/Sc and La/Th values, but slightly changed the Zr/Sc ratios, which range from 38 to 53. The heavy mineral-enriched beds (50 to 75% of the whole rock), display nearly similar La/Sc ratios compared with other lithotypes. They are 9x enriched in rare earth elements and show a significantly stronger negative Eu-anomaly, and an enrichment of heavy over light rare earth elements, as well as higher Zr/Sc ratios (82 to 299), typical for reworked passive margin or foreland deposits. The chemical composition of separated heavy minerals point to a relatively strong influence of metapelitic and metamorphosed mafic sources, which were not identified using geochemical data. Mineral chemical analyses of chromian spinels point to island arc and mid-ocean ridge basalts as protoliths, whereas rutiles were derived from metamorphosed mafic and pelitic rocks, and abundant garnet (mainly almandine) derived from high-grade and low-grade metapelitic sources. Titanite shows dominance of felsic and mafic plutonic rocks as sources, while epidote point to low-grade metamorphic protoliths. Magnetite was eroded from felsic plutonic and/or mafic metamorphosed rocks. Previous works using U-Pb detrital zircon dating show typical Namaquan and Pan-African age populations as expected for a collisional tectonic setting. Palaeocurrents indicate that the main source region of the heavy and the host rock minerals is within the Damara Belt which resulted from the convergence of Congo and Kalahari Cratons. Although the exact age of thrusting is debated, the data presented suggest the exhumation of metamorphosed crustal fragments and plutonism related to the Damara orogeny during the deposition of the upper Nama Group (ca. 520 Ma).
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The late Precambrian to Early Cambrian Nama Group of southwest Africa (Namibia) has yielded addit... more The late Precambrian to Early Cambrian Nama Group of southwest Africa (Namibia) has yielded additional trace fossils. The succession is divided, in ascending order, into the Kuibis, Schwarzrand and Fish River Subgroups. In the Kuibis Subgroup Bergaueria sp. is recorded. In the Schwarzrand Subgroup Brooksella sp., ?Chondrites sp., Diplocraterion sp., Nereites sp., and Sko- lithos sp. occur near the base while ?Diplichnites sp., Neonereites biserialis Seilacher, 1960 and N. uniserialis Seilacher, 1960 occur only high up, in the Nomtsas Formation. The Fish River Subgroup has yielded Enigmatichnus africani n. ichnogen., n. ichnosp., Phycodes cf. P. pedum Seilacher 1955 and Skolithos sp. On the basis of this material and the trace and body fossils described by Germs (1972a, b, c; 1973a, b) it is inferred that the Kuibis Subgroup is most probably Vendian, the Schwarzrand Subgroup below the Nomtsas Formation may be Vendian or Tommotian, while the Nomtsas Formation and Fish River Subgroup a...
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Developments in Precambrian …, 2009
... de la Plata-Pampia). Although earlier rifting events have been noted, most authors agree that... more ... de la Plata-Pampia). Although earlier rifting events have been noted, most authors agree that the rift–drift transition took place no earlier than 550 Ma ( Van Staal et al., 1998 ; Cawood et al., 2001 ; Puffer, 2002 ). All in all, six ...
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A preliminary biostratigraphic correlation of the A rroyo del Soldado, Cango Caves and Nama Group... more A preliminary biostratigraphic correlation of the A rroyo del Soldado, Cango Caves and Nama Groups is postulated on the basis of organic-walled microfossils. The occurrence of a lowdiversity, high abundance assemblage of organic-wal led microfossils in the Cango Caves Group of South Africa is reported for the first tim e. The assemblage comprises the following species: Bavlinella faveolata, Leiosphaeridia minutissima , L. tenuissima , Chuaria circularis, Micrhystridium cf. M. tornatum, Soldadophycus bossii , S. major, Myxococcoides p. A and B, Glenobotrydion aenigmatis , Siphonophycus robustum and Coniunctiophycus conglobatum . While the assemblage preserved in the Nooitgedagt M ember is strongly dominated by Bavlinella faveolata, the association occurring in the overlying Kombui s Member is dominated by Soldadophycus bossii . Up section (Groenefontein and Huis Rivier Formati ons), sphaeromorphic acritarchs dominate. Around 70 % of the above mentioned species occur in the Arroyo del...
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Journal of Paleontology, 1972
... 2) in which the trace and body fossils associated with the various sedimentary units are repr... more ... 2) in which the trace and body fossils associated with the various sedimentary units are represented. ... 2, fig. 3). TEXT-FIG. 2-Stratigraphical section of the Nama Group in the southern part of South-West Africa with summarized evidence of trace and body fossils. 866 Page 4. ...
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The late Precambrian to Early Cambrian Nama Group of southwest Africa (Namibia) has yielded addit... more The late Precambrian to Early Cambrian Nama Group of southwest Africa (Namibia) has yielded additional trace fossils. The succession is divided, in ascending order, into the Kuibis, Schwarzrand and Fish River Subgroups. In the Kuibis Subgroup Bergaueria sp. is recorded. In the Schwarzrand Subgroup Brooksella sp., ?Chondrites sp., Diplocraterion sp., Nereites sp., and Skolithos sp. occur near the base while ?Diplichnites sp., Neonereites biserialis Seilacher, 1960 and N. uniserialis Seilacher, 1960 occur only high up, in the Nomtsas Formation. The Fish River Subgroup has yielded Enigmatichnus africani n. ichnogen., n. ichnosp., Phycodes cf. P. pedum Seilacher 1955 and Skolithos sp. On the basis of this material and the trace and body fossils described by Germs (1972a, b, c; 1973a, b) it is inferred that the Kuibis Subgroup is most probably Vendian, the Schwarzrand Subgroup below the Nomtsas Formation may be Vendian or Tommotian, while the Nomtsas Formation and Fish River Subgroup are...
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Precambrian Research
Abstract Chromium isotopes are powerful in recording redox fluctuations in marine environments, p... more Abstract Chromium isotopes are powerful in recording redox fluctuations in marine environments, potentially tracing past atmospheric oxygenation and continental weathering. We present δ 53 Cr signals with 87 Sr/ 86 Sr, δ 13 C and δ 18 O, and element abundances in carbonates of two shallow marine successions of the Otavi Group, Namibia, stretching over four stages from preglacial setting to retreat of the late Neoproterozoic Ghaub glaciation (∼635 Ma). The fluctuating but generally heavy δ 53 Cr signals indicate oxygenated shallow seawater prior to the glaciation (stage 1). Sustained high δ 13 C carb signatures at that time suggest organic matter formed a greater proportion of buried carbon, which implies a net increase in oxidant flux to the surface. The δ 18 O signals fluctuate, with a tendency towards lighter values approaching the Ghaub Fm. Correlating δ 53 Cr and δ 13 C carb values indicate disruptions affecting the carbon cycle and oxygen availability (stage 2). Prior to the Ghaub diamictite, a sequence of 87 Sr/ 86 Sr in the range typical of late Neoproterozoic seawater occurs with high Sr concentrations, low Mn/Sr and low Mg/Ca. Enhanced detrital input after the Ghaub glaciation (stage 3) indicates elevated weathering rates likely accompanied by enhanced nutrient influx. The δ 53 Cr–δ 13 C carb patterns are resolved in a two-step recovery towards the late glacial aftermath (stage 4), which is characterized by values similar to those of the early preglacial interval. There, a sequence of postglacial limestones also records typical late Neoproterozoic seawater 87 Sr/ 86 Sr ratios. The carbonate δ 53 Cr signatures at the base of the postglacial sequence are characterized by values even below the range of bulk silicate Earth (BSE). We hypothesize that this is due to (i) redox cycling of Cr in seawater, e.g. by (partial) reduction of Cr(VI) during microbial-mediated cap dolostone deposition and/or at the redox boundary of redox-stratified seawater and/or due to (ii) increased contribution of negatively fractionated Cr associated with an enhanced input of detrital-derived Cr from postglacial weathering and/or subaqueous hydrothermal influx. Previously it was demonstrated that carbonate δ 53 Cr signatures trace changes in continental weathering and provide valuable indication of fluctuating seawater redox conditions prior to and after one of the major syn-Marinoan glaciations. Furthermore, changes in δ 53 Cr values might scale with shifts in bioproductivity.
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Precambrian Research
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Developments in Precambrian Geology, 2009
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Precambrian Research
Chromium isotopes constitute a powerful paleoenvironmental tracer recording fluctuations of atmos... more 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.
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The Journal of Geology
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Scientific reports, Jan 14, 2018
Trace fossils of sediment bulldozers are documented from terminal Ediacaran strata of the Nama Gr... more Trace fossils of sediment bulldozers are documented from terminal Ediacaran strata of the Nama Group in Namibia, where they occur in the Spitskop Member of the Urusis Formation (Schwarzrand Subgroup). They consist of unilobate to bilobate horizontal to subhorizontal trace fossils describing scribbles, circles and, more rarely, open spirals and meanders, and displaying an internal structure indicative of active fill. Their presence suggests that exploitation of the shallow infaunal ecospace by relatively large bilaterians was already well underway at the dawn of the Phanerozoic. Efficient burrowing suggests coelom development most likely linked to metazoan body-size increase. These trace fossils are the earliest clear representatives so far recorded of sediment bulldozing, an activity that may have had a negative impact on suspension-feeding and/or osmotroph communities, as well as on matgrounds, representing early examples of ecosystem engineering and trophic-group amensalism. The o...
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South African Journal of Geology
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Geology, Feb 1, 2000
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Developments in Precambrian Geology, Dec 31, 2009
Neoproterozoic shelly fossils include calcareous (Cloudina, Namacalathus, Namapoikia, Sinotubulit... more Neoproterozoic shelly fossils include calcareous (Cloudina, Namacalathus, Namapoikia, Sinotubulites), siliceous (vase-shaped microfossils, sponge spicules, Tindir scales), phosphatic (Waltheria, Vendoconularia) and agglutinated skeletons (Titanotheca). Vase-shaped microfossils are the oldest, potentially useful skeletal fossils for Neoproterozoic biostratigraphy, especially for Cryogenian subdivision. The Cloudina Range Zone is erected, which could be used to characterise the youngest, still unnamed series of the Ediacaran. Titanotheca
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Papers by Gerard J B Germs