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- Brosse, Morgane, et al.
(författare)
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Conodonts from the Early Triassic Microbialite of Guangxi (South China) : Implications for the Definition of the Base of the Triassic System
- 2015
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Ingår i: Palaeontology. - : Wiley. - 0031-0239 .- 1475-4983. ; 58:3, s. 563-584
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Tidskriftsartikel (refereegranskat)abstract
- We describe a new Early Triassic (Griesbachian) succession of conodont faunas from a high-resolution sampling of the basal Early Triassic microbial limestone and the base of the overlying unit at the Wuzhuan section (Nanpanjiang Basin, Guangxi, South China). The microbial limestone records the earliest phase of the Early Triassic biotic recovery after the end-Permian mass extinction. For the first time, rich conodont faunas are reported from within the microbialite. The faunas from Wuzhuan are largely dominated by anchignathodontids, including several Isarcicella species, which were previously documented only from strata above the microbialite. A total of 14 conodont species assigned to three genera is recorded from the Wuzhuan section. Starting from the base of the microbialite upwards, several species are sequentially added to the conodont assemblage. The alpha diversity peaks at the top of the microbialite. The conodont record in the considered microbialite interval at Wuzhuan is presumably unaffected by local ecological changes. It therefore more likely represents an evolutionary rather than an ecological pattern. We compare the Wuzhuan's conodont record with a well-supported phylogenetic model and suggest that the sequence of first occurrences at Wuzhuan is the closest to the true' sequence of evolutionary events that took place during this Griesbachian radiation of anchignathodontids. Based on comparisons with the GSSP section at Meishan, we suggest further that the first occurrence of Hindeodus parvus in Meishan does not correspond to its first appearance datum.
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- Ebbestad, Jan Ove R., et al.
(författare)
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Terminal Ordovician stratigraphy of the Siljan district, Sweden
- 2015
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 137:1, s. 36-56
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Forskningsöversikt (refereegranskat)abstract
- Integration of new isotopic data and earlier biostratigraphic information from eight sections through the terminal Ordovician (Pirgu and Porkuni stages) of the Siljan district, Sweden, allows a more precise correlation of sections in terms of biostratigraphy and carbon isotope dating. Four levels with positive delta C-13 excursions are identified (from bottom) - the Moe, an unnamed excursion, Paroveja and Hirnantian Carbon Isotope Excursion (HICE). The delta C-13 values through the Boda Limestone are 1-2 parts per thousand higher than usual in Baltica, only the values for the HICE remains within what is expected. Background values increase from 1.5 parts per thousand in the bottom of the core of the Boda Limestone up to 3 parts per thousand in the top of it. The HICE is identified in five of eight sections and the main peak falls according to inferred correlation within the Metabolograptus persculptus Biozone, at or close to the Hindella beds in the Upper Boda Member. The late Katian (Pirgu) age of Holorhynchus in the Siljan district is clear and its co-occurrence with the chitinozoan Belonechitina gamachiana in Estonia supports a Katian age for this zone. The base of the Ozarkodina hassi Biozone may occur within units B-C of the Upper Boda Member and in the upper part of the Loka Formation and most likely is correlated with the M. persculptus Biozone. The Hirnantia-Dalmanitina faunas reported from the lowermost part of the Loka Formation and units B-D of the Upper Boda Member seem to range through all the Hirnantian, but detailed morphological studies allow to distinguish an older (=extraordinarius) and a younger (=persculptus) fauna.
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- Egenhoff, Sven, et al.
(författare)
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Sedimentology and sequence stratigraphy of a pronounced Early Ordovician sea-level fall on Baltica - The Bjorkasholmen Formation in Norway and Sweden
- 2010
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Ingår i: Sedimentary Geology. - : Elsevier BV. - 0037-0738 .- 1879-0968. ; 224:1-4, s. 1-14
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Tidskriftsartikel (refereegranskat)abstract
- The Bjorkasholmen Formation consists of interbedded carbonates, shales, and glauconitic beds and is characterized by heavy bioturbation and few preserved sedimentary structures. The unit shows five facies shale, glauconitic packstone, and three predominantly mud-dominated carbonate facies. Carbonates and shales are arranged in small-scale deepening-upward cycles. A minimum of fourteen of these small-scale cycles are recognized in the Bjorkasholmen Formation. They are arranged in stacks of 3 to 5, forming a total of four medium-scale cycles separated by decimeter-thick shale units. Based on the predominance of mud-rich facies the succession is interpreted to have been deposited in an overall tranquil setting during one mayor sea-level fall and subsequent initial rise of third order. Time-estimates suggest that the 14 small-scale cycles fall into the Milankovitch band of precessional forcing, and the overriding medium-scale cycles likely represent short eccentricity. The sequence stratigraphic interpretation shows that the Bjorkasholmen Formation is characterized by failing stage, lowstand and initial transgressive systems tracts. Consequently, the contact between the Bjorkasholmen and the underlying Alum Shale Formation represents the basal surface of forced regression. The maximum regressive surface is defined by a hiatus in the Oland sections and by shallow-marine packstones within mud-rich distal ramp carbonates in Norway. The top of the Bjorkasholmen Formation represents a flooding surface at the base of the transgressive systems tract. A comparison of time-equivalent successions worldwide suggests that the Bjorkasholmen Formation represents a tectonically-enhanced lowstand with two overriding short-term Milankovitch eustatic signals. Although deposition of the Bjorkasholmen Formation coincides with the initiation of a foreland basin in the Caledonides of Norway it remains unclear how these tectonic movements may have lead to the widespread Bjorkasholmen lowstand during the Early Ordovician. It is Suggested in this Study that a combination of compressional forces from Avalonia and the Caledonian margin may have acted in concert to produce an uplift of larger parts of the Baltica plate for a time-span of approximately 0.5 Myr.
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- Egenhoff, Sven, et al.
(författare)
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Tectonically-Induced Lowstand on an Ordovician Ramp – the Björkåsholmen Formation of Scandinavia
- 2008
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Ingår i: 2008 Houston Annual Meeting (5–9 October 2008).. - : Geological Society of America. ; , s. 393-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The Tremadoc Björkåsholmen Formation is an approximately one meter thick unit consisting of several carbonate beds with intercalated shales and some glauconite content in its upper part. It is underlain by the Cambrian-Ordovician Alum Shale Formation throughout Scandinavia, and in Norway and westernmost Sweden overlain by the Lower- to Middle Ordovician Tøyen Formation. The Björkåsholmen Formation is remarkably widespread in Scandinavia with relatively little variation in thickness or faunal composition. Internally, the unit consists of thin beds at the base with thicker ones in the center and again thinner beds towards the top. Thin beds are generally mud-rich while thicker ones display packstones. In the Norwegian sections the grain-rich facies have been mostly diagenetically overprinted, whereas eastern Swedish localities are characterized by much better facies preservation regardless of lithology. The overall mud-rich facies of the Björkåsholmen Formation reflects deposition on a low-inclined ramp environment. Coarse-grained sediments represent the proximal facies while increasing mud content shows deeper water middle to outer ramp deposition. Facies trends indicate that the Björkåsholmen Formation consists of at least five regressions and successive transgressions with a maximum lowstand represented by its coarse-grained central portion. As the Björkåsholmen Formation is sandwiched between two prominent outer shelf shales in the Norwegian and westernmost Swedish localities it also represents the most pronounced sea-level lowstand in the Early Ordovician of Scandinavia.This lowstand is within the upper part of the Aorograptus victoriae Biozone and the basal Kiaerograptus supremus Biozone. Deposition of the Björkåsholmen Formation therefore likely represents a fairly short time-span. However, this remarkably strong lowstand in Scandinavia is not paralleled by base-level falls in Bolivia, China or Newfoundland. It is therefore suggested that the Björkåsholmen Formation reflects a tectonic rather than a eustatic event which is restricted exclusively to Baltoscandia.
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