THE GEOCHEMICAL AND MINERALOGICAL RECORD OF TRIASSIC-JURASSIC BOUNDARY EVENTS IN THE GSSP SECTION AT KUHJOCH (NORTHERN CALCAREOUS ALPS, AUSTRIA)

The Triassic-Jurassic boundary (TJB) is marked by one of the “big five” mass extinctions. Its GSSP has been recently established at Kuhjoch (Tyrol, Austria). The extinction and coeval rapid and severe environmental change is thought to be related to volcanism in the Central Atlantic Magmatic Province (CAMP). Direct evidence for this linkage was previously found in presumably volcanically-derived heavy and clay minerals, and heavy REE abundance at Kendlbachgraben, another TJB section within the Eiberg Basin in the Northern Calcareous Alps. To test if these anomalies were present basin-wide, we carried out similar analyses from the Kuhjoch section, complemented with high-resolution δ¹³C_org analyses from the critical interval.

The new δ¹³C data form a high-resolution segment of the previously established curve from this section and provide a detailed record of the onset of the initial negative carbon isotope excursion (I-CIE), which could be correlated worldwide.

Heavy REE enrichment is observed immediately below the I-CIE, in the same stratigraphic position as in the Kendlbachgraben section, differing from all other analyzed samples in the section, suggesting minor magmatic contribution to the sedimentary basin.

X-ray diffraction clay mineralogy revealed the presence of 15 Å smectite, expanding to 16.5 Å upon glycolation, as the dominant clay mineral parallel with the HREE enrichment. Illite and mixed layer illite/smectite are the dominant phases in the underlying beds, whereas kaolinite is dominant in the overlying beds which show no HREE enrichment.

Completely altered spherules and euhedral amphibole grains occur in the beds containing the HREE enrichment, similar to Kendlbachgraben. The alteration products of amphiboles are of chloritic-vermiculitic composition and 14 Å smectite for the spherules, with or without Ti-oxide alteration products. They are interpreted to derive from distal air-fallen volcanic ashes from CAMP eruptions, which suffered no terrestrial weathering and negligible aquatic transportation.

The new data provide trace element chemostratigraphical and clay mineralogical means to correlate a specific horizon immediately below the I-CIE, close to the TJB at the GSSP locality and further support the link between the extinction and I-CIE to CAMP volcanism.