Original paper
Carbon isotope stratigraphy, carbonate sedimentology, and microfossil record across the Triassic–Jurassic boundary in the Northern Calcareous Alps – Fonsjoch and Schloßgraben sections, Austria
Mette, Wolfgang; Korte, Christoph; Quante, Ella; Zierl, Tamara
Newsletters on Stratigraphy Volume 56 Number 2 (2023), p. 225 - 255
137 references
published: Mar 13, 2023
published online: Sep 27, 2022
manuscript accepted: Jul 14, 2022
manuscript revision received: Jul 1, 2022
manuscript revision requested: May 30, 2022
manuscript received: Mar 30, 2022
ArtNo. ESP026005602003, Price: 29.00 €
Abstract
Carbonate sedimentology, microfossil palaeontology and carbon isotope stratigraphy have been studied at the Triassic–Jurassic boundary in the Northern Calcareous Alps at the Fonsjoch and Schloßgraben sections in Austria. The carbonate microfacies and microfossil results of the Upper Rhaetian at Schloßgraben are in accordance with previous research indicating a deep subtidal environment within the Eiberg Basin. The so called event bed (T-Bed) is characterised by a successive diversity decrease and disappearance of ostracod taxa which was most probably caused by oxygen deficiency. Lithofacies and micropalaeontologic data from the uppermost Rhaetian at Fonsjoch are indicative of a shallow subtidal environment and existence of a nearby carbonate platform edge. Low diversity ostracod assemblages of juvenile Cytherellidae occurring above the event bed point to a second interval of suboxic conditions in the latest Rhaetian of the Eiberg Basin. For both sections, the late Rhaetian bulk carbonate carbon isotopes of about 1 ‰ reflect the near primary signal of the ancient seawater, and the well-known and prominent initial negative δ13C excursion is present, starting in the limestones of the uppermost Eiberg Member and reaching a minimum within the Schattwald Beds. The δ13Ccarb values remain low in the Schattwald Beds in both sections and this might be explained by diagenetic alteration, because it contrasts the δ13Corg trend which returns to heavier values here. However, δ18O versus δ13Ccarb cross-plots for the lower beds of the Tiefengraben Member and the Schattwald Beds show only negligible correlation supporting an interpretation that at least the carbonate carbon isotope values represent near primary signals also in this stratigraphic level. Accepting this interpretation, the new results imply that the primary trends of the δ13Ccarb and δ13Corg are different, suggesting that the bulk organic matter sources have changed temporarily. These results further indicate that organic carbon isotope trends must be regarded with caution when used as a chemostratigraphic tool, and especially the positive carbon-isotope excursion at the Triassic-Jurassic boundary does not necessarily represent a time-correlative chemostratigraphic marker.
Keywords
Triassic-Jurassic boundary • Northern Calcareous Alps • Carbon isotopes • Oxygen isotopes • Carbonate sedimentology • Microfossil record