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Sofie Lindström, Geological Survey of Denmark and Greenland (Denmark)
Karen Dybkjær, Geological Survey of Denmark and Greenland (Denmark)
Bas van de Schootbrugge, Goethe University Frankfurt (Germany)
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The Triassic-Jurassic boundary (T/J; 199.6 Ma) is associated with perhaps one of the five largest mass extinctions during the Phanerozoic, and marked negative excursions in both carbonate and organic δ13C, as reported from extensively studied strata in Europe and North America. Here we provide high resolution palynological records, geochemistry and sedimentology, from Denmark and Sweden that enable us to correlate terrestrial and marine events across the T/J boundary.
In the Danish Basin T/J boundary strata comprise siliciclastic sedimentary rocks deposited in fluvio-deltaic to shallow marine environments along the northern low-gradient shelf of a sea that covered large parts of NW Europe. The arid to semi arid conditions that had prevailed during most of the Triassic was replaced by a humid subtropical climate during the Rhaetian. Extensive forest mires developed along this flat northern coast of the sea, in particular at the onset and end of marine transgressions.
The sedimentological and palynofloral successions in Denmark and Sweden are remarkably uniform, and provide excellent correlation between terrestrial and marine events across the T/J boundary. Typical Rhaetian spore-pollen floras of the Ricciisporites-Limbosporites Zone coincide with an interval within the Rhaetogonyaulax rhaetica Zone that contains the suessiacean dinocyst Lunnomidinium. During the latest Rhaetian maximum flooding event, marked by deposition of a black bituminous shale and mass occurrences of the dinocysts R.rhaetica and/or D. priscum, terrestrial palynofloras can be assigned to the Ricciisporites-Polypodiisporites Zone. This latest Rhaetian marine black shale and associated on-shore terrestrial coals are abruptly terminated by the deposition of a light grey clay and coarse siliciclastics in more marginal settings, and further offshore a corresponding conspicuous ca 8 m thick light grey clay-/siltstone interval. During this "grey clay" interval the palynoflora changes drastically. The assemblages become less diverse and palynomorphs are generally sparse. Typical Rhaetian spores and pollen, i.e. Ricciisporites tuberculatus, Semiretisporis gothae and Polypodiisporites polymicroforatus are still present, and Deltoidospora is often frequent. The most striking palynological feature of the "grey clay" interval is the abundant sphaeromorphs of varying size and preservation. Reworked elements, including Silurian acritarchs and chitinozoans, Carboniferous and Early-Middle Triassic spores and pollen are also present. The interval is succeeded by shales, heteroliths and sandstones containing typical Hettangian palynofloras dominated by Pinuspollenites minimus and trilete spores.
Preliminary C-isotope results suggest that the top of the "grey clay" interval is associated with the onset of the main negative δ13C excursion, but on-going δ13Corg analyses of two cored wells from Denmark and Sweden will provide additional geochemical data on the T/J transition in the Danish Basin.
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