Porifera-microbialites of the Lower Liassic (Northern Calcareous ...

Triassic-Jurassic Boundary Event 99
9. Triassic-Jurassic Boundary Event
In the last decade, research on the Triassic-Jurassic mass extinction was strongly enforced,
currently by the IGCP-Project 458 - Triassic-Jurassic Boundary Events. The T-J event was
characterized rather by several local changes than by one big turn-over, hence, global effects
have been discussed controversially (Hallam 1989, 2002; Marzoli et al. 1999; McElwain et al.
1999; Tanner et al. 2001; Hesselbo et al. 2002; Olsen et al. 2002; Ward et al. 2001; a.o.). Eustatic
sea level changes caused high extinction rates especially in the shelf areas and particular low-
stands were responsible for a lot of shallow marine sequences displaying a hiatus in
sedimentation just where the event should have been documented. A relation between the
Triassic-Jurassic boundary event and carbon isotope excursions in δ 13 Ccarb and δ 13 Corg has also
been mentioned from several localities (Germany: McRoberts et al. 1997; Turnsek et al. 1999;
Hungary: Pálfy et al. 2001; Canada: Ward et al. 2001; England: Hesselbo et al. 2002).
Disregarding those of apparently diagenetic origin (Morante and Hallam 1996; Ward et al.
2001), a hiatus in the fossil record often prevents exact stratigraphical correlations, thus, a lot of
work still has to be done correlating the few present data from all over the world in order to gain
insight into the driving forces of the event (Guex et al. 2002; Pálfy et al. 2001).
At Steinplatte/Plattenkogel locality a new ammonite found of Psiloceras sp. from the spiculite
base let allocate the stromatactis limestones to the Lower Hettangian. This also corresponds with
the geochemical data, supposing that the isotopic pathway in sediments at Plattenkogel hill
displays the beginning of the same amplitude that is shown in Adnet quarries.
Nevertheless it remains unknown so far whether the δ 13 C-excursions at Adnet and Steinplatte
show only one amplitude of probably a series of little fluctuations or just the end of a bigger
carbon anomaly. Anyway, it should be considered that in contrast to some deep-water settings
the isotopic records from shallow marine sediments of the Tethyian region could have been
overprinted by several local paleoenvironmental changes. At Steinplatte/Plattenkogel hill, for
example, an effect of meteoric diagenesis seems to decrease upward from the capping facies
(Stanton and Flügel 1989) to the cements of the stromatactis cavities (Mazzullo et al. 1990). The
following succession is characterized by constantly increasing water depths so that an emergence
later than the origin of the spiculite layer is unlikely. Probably the sediments of the Lower
Liassic sequence were influenced later by percolating waters that were trapped first in Upper
Triassic time and then reactivated during Liassic burial.