THE HIRNANTIAN ISOTOPE CARBON EXCURSION (HICE) IN SOUTHERN NORWAY

We present a carbon isotope stratigraphy for the Katian-Hirnantian of the Oslo-Asker District of southern Norway. Primary target has been to identify the Hirnantian Isotope Carbon Excursion (HICE) in order to understand its degree of preservation, and thus illustrate the stratigraphic completeness of the Norwegian successions and better relate the sedimentary succession here to the latest Ordovician changes in biota and climate. Some of these data were presented at the IGCP 591 meeting in Ghent, Belgium, in 2016, and we herein expand our data set. Our results reveal a two-peak-structure of the Norwegian HICE, separated by strata with distinctly lower δ¹³C_carb values; a stratigraphic signature that is previously known from e.g. southern Sweden, China, as well as from the Monitor Range and Vinni Creek sections in the United States. In Norway, the pre-excursion δ¹³C_carb values scatters between 0-1‰ and occur in calcareous shale and siltstone of the upper Skogerholmen Fm [a minor rise to ca 1.5‰ within more limy parts of this formation may be interpreted as the Katian Paroveja Excursion although more data are needed to confirm this]. The rising limb of HICE is steady and uninterrupted over an at least 10-m-thick limestone-marl succession in the upper half of the overlying Husbergøya Fm. These strata reflect a shallowing of the depositional environment and appear to be conformable with no evidence of a stratigraphic gap. δ¹³C_carb values continue to increase and stabilize at 5.5-6‰ in the overlying 1.5-m-thick Langåra Fm, which yields a rich shelly fauna including corals. These values represent a first peak in the HICE and thereby imply the presence of the Metabolograptus extraordinarius Zone. The succeeding Langøyene Fm is rich in quartz sand and records successively lower δ¹³C_carb values up-section until a minimum of ca 3‰ is reached in the upper half of that formation. The topmost part of the Langøyene Fm is a sandy oolite throughout much of the Oslo-Asker District. In this unit δ¹³C_carb values again rise to above 5‰. This second peak in the HICE suggests that the sandy oolites belong in the M. persculptus Zone. The falling limb of the HICE is cut out at a major sequence boundary at the top of the sandy oolite in most sections, but is identified in up to 10-m-thick incised valley successions cutting through the oolites at Hovedøya and Brønnøya.