Abstract
AbstractWe present a δ13Ccarb chemostratigraphy for the Late Ordovician Hirnantian Stage based on 208 whole-rock samples from six outcrops in the Oslo–Asker district, southern Norway. Our data include the Norwegian type section for the Hirnantian Stage and Ordovician–Silurian boundary at Hovedøya Island. The most complete record of the Hirnantian Isotope Carbon Excursion (HICE) is identified in a coastal exposure at Konglungø locality where the preserved part of the anomaly spans a c. 24 m thick, mixed carbonate–siliciclastic succession belonging to the upper Husbergøya, Langåra and Langøyene formations and where δ13Ccarb peak values reach c. +6 ‰. Almost the entire HICE occurs above beds containing the Hirnantia Fauna, suggesting a latest Hirnantian age for the peak of the excursion. The temporal development of the HICE in southern Norway is associated with substantial shallowing of depositional environments. Sedimentary facies and erosional unconformities suggest four inferably fourth-order glacio-eustatically controlled sea-level lowstands with successively increased exposure and erosion to the succession. The youngest erosional unconformity is related to the development of incised valleys and resulted in cut-out of at least the falling limb of the HICE throughout most of the Oslo–Asker district. The fill of the valleys contains the falling limb of the HICE, and the postglacial transgression therefore can be assigned to the latest part of the Hirnantian Age. We address the recent findings of the chitinozoan Belonechitina gamachiana in the study area and its relationship to the first occurrence of Hirnantia Fauna in the studied sections, challenging identification of the base of the Hirnantian Stage.
Highlights
The possibility of assessing the Hirnantian Stage and demonstrating the precise interrelationship of stage boundaries, biostratigraphy, sea-level trends and the temporal development of the Hirnantian Isotope Carbon Excursion (HICE) is hampered by the global rarity of complete sections: the HICE is to date recorded from several tens of localities worldwide, but few, if any, of the associated outcrops or core sections have a continuous sedimentary record due to the contemporaneous glacio-eustasy (e.g. Loi et al 2010; Ghienne et al 2014)
A dataset of 208 whole-rock carbon isotope samples from six sections in the Oslo–Asker district forms the basis for the first comprehensive documentation of the HICE in Norway
A tentative correlation of these cycles with latest Katian–Hirnantian sea-level cycles documented from the East Baltic area (Kiipli & Kippli 2020) and Morocco and Anticosti (Ghienne et al 2014) is proposed, but we note uncertainties primarily due to limitations in biostratigraphy
Summary
The Hirnantian Age in the latest Ordovician is of wide international interest due to its association with the peak of the early Palaeozoic Icehouse (EPI; Page et al 2007), the Late Ordovician Mass Extinction (LOME; Finnegan et al 2012; Harper et al 2014; Ling et al 2019; Rasmussen et al 2019; Wang et al 2019) and an associated major anomaly to the global carbon cycle, the Hirnantian Isotope Carbon Excursion (HICE; Brenchley et al 2003; Kaljo et al 2008; Bergström et al 2013; Subías et al 2015; Mauviel & Desrochers, 2016). The base of the Hirnantian is defined as the First Appearance Datum (FAD) of the graptolite Metabolograptus extraordinarius 0.39 m below the Kuanyinchiao Bed in the Wangjiawan North Section, Hubei province, China (Chen et al 2006; Gorjan et al 2012). Recent U–Pb dates from a continuous section at Wanhe in southwest China suggest that the duration of the entire Hirnantian Age is as little as 0.47 ± 0.34 Ma, and considerably shorter than previously understood (Ling et al 2019). Hi1 is defined as strata between the base of the Metabolograptus extraordinarius Graptolite Zone and the end of HICE. Hi2 is defined as strata between the end of the HICE and the base of the Akidograptus ascensus Graptolite Zone, marking the top of the Ordovician
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