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Sökning: WFRF:(Calner T)

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  • Cramer, Bradley D., et al. (författare)
  • U-Pb (zircon) age constraints on the timing and duration of Wen lock (Silurian) paleocommunity collapse and recovery during the "Big Crisis"
  • 2012
  • Ingår i: Geological Society of America Bulletin. - 0016-7606. ; 124:11-12, s. 1841-1857
  • Tidskriftsartikel (refereegranskat)abstract
    • High-precision isotope-dilution U-Pb (zircon) dating was conducted on three volcanic ash fall (bentonite) samples from the Swedish island of Gotland, and on a fourth bentonite from the West Midlands, England. Zircons from the Ireviken, Grotlingbo, Djupvik (Gotland), and Wren's Nest Hill-15 (West Midlands) bentonites yielded weighted mean Pb-206/U-238 ages of 431.83 +/- 0.23/0.67 Ma, 428.45 +/- 035/0.73 Ma, 428.06 +/- 0.2110.66 Ma, and 427.86 +/- 032/0.71 Ma, respectively (analytical/total uncertainties). These biostratigraphically well-controlled age dates effectively bracket the Wenlock Epoch of the Silurian Period and provide control for the duration of one of the major Paleozoic biotic events and associated perturbations to the global carbon cycle (the "Big Crisis" or lundgreni event- graptolites; the NIulde Event-conodonts; the Mulde excursion-carbon isotopes). These new data suggest an older and shorter duration for the recalibration of the Wenlock Series and demonstrate that the cascade of biological and chemical events that took place during the Big Crisis happened on time scales of tens to hundreds of thousands of years.
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  • Lehnert, O., et al. (författare)
  • New Ordovician-Silurian drill cores from the Siljan impact structure in central Sweden: an integral part of the Swedish Deep Drilling Program
  • 2012
  • Ingår i: Gff. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 134:2, s. 87-98
  • Tidskriftsartikel (refereegranskat)abstract
    • New drill cores from the largest known impact structure in Europe, the relict of the Siljan meteorite crater, provide new possibilities to reconstruct Early Palaeozoic marine environments and ecosystems, and to document changes in sedimentary facies, sea level and palaeoclimate in Baltoscandia. The impact crater is an important target of the project "Concentric Impact Structures in the Palaeozoic" within the framework of the "Swedish Deep Drilling Program". Two core sections, Mora 001 and Solberga 1, have been analysed. The sedimentary successions of these core sections include strata of late Tremadocian through late Wenlock ages. Our preliminary studies show not only that several of the classical Palaeozoic units of Sweden are represented in the area, but also that other significantly different facies are preserved in the Siljan district. An erosional unconformity representing a substantial hiatus occurs between Middle Ordovician limestone and a Llandovery-Wenlock (Silurian) shale succession in the western part of the Siljan structure and suggests an extended period of uplift and erosion. This may be related to forebulge migration due to flexural loading by the Caledonian thrust sheet to the west. Thus, this part of Sweden, previously regarded as a stable cratonic area, presumably was affected by the Caledonian collision between Baltica and Laurentia.
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  • Lehnert, Oliver, et al. (författare)
  • Palaeokarst formation in the early Palaeozoic of Baltoscandia – evidence for significant sea-level changesin a shallow epicontinental sea
  • 2013
  • Ingår i: Proceedings of the 3rd IGCP591 Annual MeetingLund, Sweden, 9–19 June 2013. - Lund : Lund University. ; , s. 169-170
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • In the Lower Palaeozoic sedimentary succession of Sweden palaeokarsts have been reportedfrom different stratigraphic levels in the Silurian strata of Gotland by Calner(2008; see references therein). Until last year there were no records of Cambrian karstsand in the Ordovician only the basin-wide Katian palaeokarst horizon in the UpperOrdovician Slandrom Limestone has been described in detail (Calner et al. 2010a). Theunconformities and disconformities on top of the slightly older Kullsberg mounds inquarries located in the Siljan impact structure (Dalarna) presumably represent an earlierregression and karstic development (Calner et al. 2010b). Beside these reports, there isonly the statement by Nielsen (1995) that karst may have formed at the top of theDarriwilian Komstad Limestone. During the last two years, however, several new andsignificant palaeokarst surfaces have been detected in the Cambrian–Ordovician successionsof Sweden (Lehnert et al. 2012).At Kakeled Quarry (Västergötland), a palaeokarst cave with a breccia fill (large, angularOrsten clasts in a dark limestone matrix) is exposed beneath a ‘Middle Cambrian’palaeokarst surface (Jiangshanian Stage) located close to the top of the Kakeled LimestoneBed of the Alum Shale Formation (Lehnert et al., 2012). In the karstic pockets,a mass occurrence of Orusia lenticularis occurs. These shallow-water brachiopods originallysettled on hard substrates after a major regression exposing, regionally, the seafloors of the alum shale basin. Their reworking and concentration in the conglomeraticbed overlying the irregular palaeokarst surface reflects deposition during transgressionin extremely shallow marine environments.A younger karst surface is exposed in Tomten Quarry at Torbjörntorp (Västergötland).In two dimensions in the quarry wall it resembles the “Schrattenkalk”, but rockslabs cut vertically and parallel to bedding planes display a karren system that resembles“Napfkarren” or cockling features. Trilobites of the Furongian Ctenopyge bisulcata andC. linnarssoni zones occur in the 1–2 cm thick, glauconitic packstone bed that overliesthe palaeokarst surface and which represents the upper Tremadocian BjørkåsholmenFormation. The associated stratigraphic gap comprises the six uppermost trilobite zonesof the Furongian plus most of the Tremadocian. Darriwilian conodonts with reworked170older material within a limestone bed slightly above the glauconitic packstone bed indicateyet another substantial gap in the succession.In the new Tingskullen core from northeastern Öland, another palaeokarst surfacewith grikes and evidence of repeated exposure marks the top of the upper TremadocianObolus conglomerate (?) or a lower limestone part of the Djupvik Formation (“CeratopygeShale”). This palaeokarst surface is overlain by glauconitic limestone of the KöpingsklintFormation and inferably reflects the global Ceratopyge Regressive Event(CRE).At the base of the Lanna Limestone in the Siljan area, palaeokarst is associated withthe Dapingian Blommiga Bladet (‘flowery sheet’) hardground complex, which can becorrelated across most of Baltoscandia.The basin-wide palaeokarst in the Katian Slandrom Limestone (Calner et al. 2010a)no longer marks the youngest Ordovician karst record. Recently, Hirnantian karst cavesand solution cavities filled with greenish marls of the Glisstjärn Formation have beenrecognized in sections of the Boda Limestone in the Siljan Ring structure (Dalarna).Solution and karst cave formation reflects an interval of the regression during theHirnantian glaciation and the youngest period of subaerial exposure during the Ordovician.Some earlier sedimentary models suggesting that Baltoscandia was flooded by a deepepicontinental sea are challenged by the discovery of multiple palaeokarst developmenttogether with other shallow-water features. Instead, palaeokarst formation impliessubaerial exposure during a number of major regressions.
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  • Munnecke, Axel, et al. (författare)
  • Ordovician and Silurian sea–water chemistry, sea level, and climate: A synopsis
  • 2010
  • Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : Elsevier BV. - 1872-616X .- 0031-0182. ; 296:3-4, s. 389-413
  • Tidskriftsartikel (refereegranskat)abstract
    • Following the Cambrian Explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans, the Ordovician and Silurian periods witnessed three subsequent major biotic events: the Great Ordovician Biodiversification Event, the end-Ordovician extinction (the first animal extinction and second largest of the five mass extinctions of the Phanerozoic), and the Early Silurian postextinction recovery. There are currently no simple explanations for these three major events. Combined extrinsic (geological) and intrinsic (biological) factors probably drove the biodiversifications and radiations, and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography, palaeoclimate and sea-water chemistry. The present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the Ordovician and Silurian, including sea-level changes, geochemical proxies (δ13C, δ18O, 87Sr/86Sr) of the ocean waters, and the evolution of the atmosphere (oxygen and carbon dioxide content).
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