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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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