| 1. |
- Agic, Heda, et al.
(författare)
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Late Ediacaran occurrences of the organic-walled microfossils Granomarginata and flask-shaped Lagoenaforma collaris gen. et sp. nov.
- 2022
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Ingår i: Geological Magazine. - : Cambridge University Press. - 0016-7568 .- 1469-5081. ; 159:7, s. 1071-1092
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Tidskriftsartikel (refereegranskat)abstract
- New occurrences of flask-shaped and envelope-bearing microfossils, including the predominantly Cambrian taxon Granomarginata, are reported from new localities, as well as from earlier in time (Ediacaran) than previously known. The stratigraphic range of Granomarginata extends into the Cambrian System, where it had a cosmopolitan distribution. This newly reported Ediacaran record includes areas from Norway (Baltica), Newfoundland (Avalonia) and Namibia (adjacent to the Kalahari Craton), and puts the oldest global occurrence of Granomarginata in the Indreelva Member (< 563 Ma) of the Stahpogieddi Formation on the Digermulen Peninsula, Arctic Norway. Although Granomarginata is rare within the assemblage, these new occurrences together with previously reported occurrences from India and Poland, suggest a potentially widespread palaeogeographic distribution of Granomarginata through the middle-late Ediacaran interval. A new flask-shaped microfossil Lagoenaforma collaris gen. et sp. nov. is also reported in horizons containing Granomarginata from the Stahpogieddi Formation in Norway and the Dabis Formation in Namibia, and flask-shaped fossils are also found in the Gibbett Hill Formation in Newfoundland. The Granomarginata-Lagoenaforma association, in addition to a low-diversity organic-walled microfossil assemblage, occurs in the strata postdating the Shuram carbon isotope excursion, and may eventually be of use in terminal Ediacaran biostratigraphy. These older occurrences of Granomarginata add to a growing record of body fossil taxa spanning the Ediacaran-Cambrian boundary.
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| 2. |
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| 3. |
- Appel, Peter W.Uitterdijk, et al.
(författare)
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Unusual features caused by lightning impact in West Greenland
- 2006
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Ingår i: Geological Magazine. - : Cambridge University Press. - 0016-7568 .- 1469-5081. ; 143:5, s. 737-741
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Tidskriftsartikel (refereegranskat)abstract
- Two lightning impacts are described from an area near the Inland Ice in West Greenland. The first lightning blasted an outcrop of metacherts. It subsequently split into two branches, which traversed rock outcrops and boulders, leaving behind two white almost straight lines, 30 m and 14 m long, respectively, where all lichens and plants were burned away. On the white lines the upper few millimetres of the traversed boulders were melted to a glass which subsequently peeled off by thermal expansion to leave a rough surface. Magnetic investigation of an amphibolite boulder found on the white line showed that a strong electric current indeed traversed the boulder. A few years later a second lightning impacted on a mountaintop close to the first impact. The second lightning left a trail on the rock surface covered by a thin layer of glass. The glass displays spectacular colours ranging from metallic blue to red, yellow and green.
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| 4. |
- Aswad, Khalid J. A., et al.
(författare)
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Cr-spinel compositions in serpentinites, and their implications for the petrotectonic history of the Zagros Suture Zone, Kurdistan Region, Iraq
- 2011
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Ingår i: Geological Magazine. - 0016-7568 .- 1469-5081. ; 148:5-6, s. 802-818
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Tidskriftsartikel (refereegranskat)abstract
- Accessory chrome spinels are scattered throughout the serpentinite masses in two allochthonous thrust sheets belonging to the Penjween–Walash sub-zone of the northwestern Zagros Suture Zone in Kurdistan. Based on field evidence, the serpentinites are divided into two groups: (1) highly sheared serpentinites (110–80 Ma), which occupy the lower contact of the ophiolitic massifs of the Upper Allochthonous sheet (Albian–Cenomanian age), and (2) ophiolitic mélange serpentinites of mixed ages (150 and 200 Ma) occurring along thrust faults on the base of the volcano-sedimentary segment (42–32 Ma) of the Lower Allochthonous sheet. The Cr-spinels of both groups show a wide range of YCr (Cr/(Cr + Al) atomic ratio) from 0.37 to 1.0, while the XMg (Mg/(Mg + Fe2+) atomic ratio) ranges from 0.0 to 0.75. Based on the Cr-spinel compositions of the entire dataset and in conjunction with back-scattered electron imaging, from core to rim, three spinel stages have been recognized: the residual mantle stage, a Cr-rich stage and a third stage showing a very narrow magnetite rim. These three stages are represented by primary Cr-spinel, pre-serpentinization metamorphosed spinel and syn- or post-serpentinization spinel, respectively. The chemical characteristics of primary (first-stage) Cr-spinels of both serpentinite groups indicate a tectonic affinity within a fore-arc setting of peridotite protoliths. The second stage indicates that Cr-spinels have undergone subsolidus re-equilibration as a result of solid–solid reaction during pre-serpentinization cooling of the host rock. Here the primary Cr-spinel compositions have been partly or completely obscured by metamorphism. During the third stage, the Cr-spinels have undergone solid–fluid re-equilibration during syn- or post-serpentinization processes. Both the second and third stages point to diachronous metamorphic paths resulting from continuous tectonic evolution influenced by either slow or fast uplift of mantle protoliths. In the fast metamorphic paths, the primary chrome spinels are flanked by a very narrow magnetite rim. The presence of two groups of distally separated serpentinites with different emplacement ages and fore-arc tectonic affinity could indicate that the closure of the Tethys Ocean culminated in two fortuitous subduction processes.
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| 5. |
- Aziz, Nabaz R. H., et al.
(författare)
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Contrasting Settings of Serpentinite Bodies in Northwestern Zagros Suture Zone, Kurdistan Region, Iraq
- 2011
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Ingår i: Geological Magazine. - 0016-7568 .- 1469-5081. ; 148:5-6, s. 819-837
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Tidskriftsartikel (refereegranskat)abstract
- Protrusions and lenses of serpentinite–matrix mélanges occur at several places along the thrust faults of the Zagros Suture Zone. They separate the lower allochthonous thrust sheet, the ‘Lower Allochthon’ (i.e. Walash–Naopurdan nappe), of Paleocene–Eocene age from sediments of the Arabian platform and the upper thrust sheet of Mesozoic, ophiolite-bearing terranes termed the ‘Upper Allochthon’ (i.e. Gemo–Qandil nappe). The serpentinite–matrix mélanges occur mostly as stretched bodies (slices) on both sides of the Lower Allochthon (Hero, Halsho and Pushtashan (HHP) and Galalah, Qalander and Rayat (GQR)). Their overall chondrite-normalized rare earth element (REE) patterns form two main groups. Group One exhibits enrichment in the total REEs (> 1 × chondrite) whereas the Group Two pattern shows depletion (i.e. < 1 × chondrite). Bulk-rock MORB-normalized profiles of Group Two are almost flat in the MREE–HREE region with flattening profiles in the Gd–Lu range (> 3 times the MORB composition). In comparison with Group One, Group Two has extremely high REE content and displays variable depletions in the moderately incompatible high-field-strength elements (HFSEs) (Zr, Hf, Y) relative to their adjacent REEs. The REEs in the GQR serpentinite–matrix mélanges have a noticeably high LREE content, and a positive Eu anomaly, and their HREE content never reaches more than 1 × chondrite (i.e. < 0.01 to 1 × chondrite). The latter indicates that the hemipelagic sedimentary, melt-like components (i.e. high LREE, U/La, La/Sm and low Ba/Th) control the geochemical peculiarities of this type of serpentinite. The HHP serpentinite–matrix mélanges, however, are either equally divided between the two REE pattern groups (e.g. Hero, Halsho) or inclined towards Group One (e.g. Pushtashan). Contrary to GQR serpentinites, the variation in HHP serpentinite–matrix mélanges spans a compositional spectrum from U/La-rich to more Ba/Th-rich. Such ratio variations reflect the large variation in these two subducted sedimentary components (i.e. carbonate and hemipelagic sediment mix). The obvious differences in the trace element signatures of the GQR and HHP serpentinite–matrix mélanges might be related to plate tectonic parameters such as convergence rate, subduction age and thickness and type of subducted slab. It is more likely that the influx of subducted components to the mantle wedge relied heavily on the composition of the sedimentary inputs. These vary considerably with time from the relatively deepwater hemipelagic sediments (Qulqula Radiolarite Formation) to platform carbonate sediments (Balambo limestone). The trace element signatures of the GQR and HHP serpentinite–matrix mélanges might suggest multi-staging of the allochthonous sheet emplacement on the Arabian platform sediments.
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| 6. |
- Bicknell, Russell DC, et al.
(författare)
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Malformed individuals of the trilobite Estaingia bilobata from the Cambrian Emu Bay Shale and their palaeobiological implications
- 2023
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Ingår i: Geological Magazine. - : Cambridge University Press (CUP). - 0016-7568 .- 1469-5081. ; , s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- Malformed trilobite specimens present important insight into understanding how this extinct arthropod group recovered from developmental or moulting malfunctions, pathologies, and injuries. Previously documented examples of malformed trilobite specimens are often considered in isolation, with few studies reporting on multiple malformations in the same species. Here we report malformed specimens of the ellipsocephaloid trilobite Estaingia bilobata from the Emu Bay Shale Konservat-Lagerstätte (Cambrian Series 2, Stage 4) on Kangaroo Island, South Australia. Ten malformed specimens exhibiting injuries, pathologies, and a range of teratologies are documented. Furthermore, five examples of mangled exoskeletons are presented, indicative of predation on E. bilobata. Considering the position of malformed and normal specimens of E. bilobata in bivariate space, we demonstrate that the majority of malformed specimens cluster among the larger individuals. Such specimens may exemplify larger forms successfully escaping predation attempts, but could equally represent individuals exhibiting old injuries that were made during earlier (smaller) growth stages that have healed through subsequent moulting events. The available evidence from the Emu Bay Shale suggests that this small, extremely abundant trilobite likely played an important role in the structure of the local ecosystem, occupying a low trophic level and being preyed upon by multiple durophagous arthropods. Furthermore, the scarcity of malformed E. bilobata specimens demonstrates how rarely injuries, developmental malfunctions, and pathological infestations occurred within the species.
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| 7. |
- Bingen, B., et al.
(författare)
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U-Pb geochronology of the syn-orogenic Knaben molybdenum deposits, Sveconorwegian orogen, Norway
- 2015
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Ingår i: Geological Magazine. - 0016-7568 .- 1469-5081. ; 152, s. 537-556
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Tidskriftsartikel (refereegranskat)abstract
- Paired isotope dilution – thermal ionization mass spectrometry (ID-TIMS) and secondary ion mass spectrometry (SIMS) zircon U–Pb data elucidate geochronological relations in the historically important Knaben molybdenum mining district, Sveconorwegian Orogen, south Norway. This polyphase district provided c. 8.5 Mt of ore with a grade of 0.2%. It consists of mineralized quartz veins, silica-rich gneiss, pegmatites and aplites associated with a heterogeneous, locally sulphide-bearing, amphibolites facies gneiss called Knaben Gneiss, and hosted in a regional-scale monotonous, commonly weakly foliated, granitic gneiss. An augen gneiss at the Knaben I deposit yields a 1257±6 Ma magmatic zircon age, dating the pre-Sveconorwegian protolith of the Knaben Gneiss. Mineralized and non-mineralized granitic gneiss samples at the Knaben II and Kvina deposits contain some 1488–1164 Ma inherited zircon and yield consistent intrusion ages of 1032±4, 1034±6 and 1036±6 Ma. This age links magmatism in the district to the regional 1050–1020 Ma Sirdal I-type granite suite, corresponding to voluminous crustal melting during the Sveconorwegian orogeny. A high-U, low-Th/U zircon rim is present in all samples. It defines several age clusters between 1039±6 and 1009±7 Ma, peaking at c. 1016 Ma and overlapping with a monazite age of 1013±5 Ma. The rim records protracted hydrothermal activity, which started during the main magmatic event and outlasted it. This process was coeval with regional high-grade Sveconorwegian metamorphism. Molybdenum deposition probably started during this event when silica-rich mineralizing fluids or hydrous magmas were released from granite magma batches. An analogy between the Knaben district and shallow, short-lived porphyry Mo deposits is inappropriate.
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| 10. |
- Bremer, Oskar, 1985-, et al.
(författare)
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Vertebrate microremains from the upper Silurian Winnica Formation of the Holy Cross Mountains, Poland
- 2018
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Ingår i: Geological Magazine. - 0016-7568 .- 1469-5081. ; 155:7, s. 1523-1541
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Tidskriftsartikel (refereegranskat)abstract
- Vertebrate microremains from the upper Silurian Winnica Formation in the Holy Cross Mountains, Poland are described from the Winnica and Rzepin sections. Both sites record the uppermost part of the Supianka Member, but represent different depositional environments. The Winnica samples come from a low-energy environment, while the Rzepin sample was taken from a high-energy, oolitic facies. Both sites contain thelodonts Thelodus parvidens, Thelodus trilobatus, an anaspid cf. Liivilepis and a number of acanthodian scales of 'nostolepid', poracanthodid and 'gomphonchid' types. Notable differences between the sites are the addition of the osteostracan Tahulaspis cf. ordinata, the thelodont Paralogania ludlowiensis and acanthodian scales identified as Nostolepis gracilis in the Rzepin section. Placing the vertebrate faunas within the vertebrate biozonation established for the Silurian proved difficult. The suggested late Ludlow age for the Supianka Member based on sequence stratigraphical and chemostratigraphical correlations cannot be definitely confirmed or refuted, but a late Ludfordian age seems the most plausible based on invertebrate and vertebrate faunas. The much lower abundance of poracanthodid acanthodians in the Rzepin sample supports the notion of Poracanthodes porosus Zone as a deep-water equivalent to a number of vertebrate biozones. The presence of P. ludlowiensis only in the oolitic sample confirms a long temporal range, but restricted environmental distribution for this taxon.
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