Aretz et al_2011.pdf - ORBi - Université de Liège

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Kölner Forum Geol. Paläont., 19 (2011)
M. ARETZ, S. DELCULÉE, J. DENAYER & E. POTY (Eds.)
Abstracts, 11th Symposium on Fossil Cnidaria and Sponges, Liège, August 19-29, 2011
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Middle and Late Frasnian (Late Devonian) sea-level changes in southern
Belgium: their consequences on corals and brachiopods
Edouard POTY, Julien DENAYER & Bernard MOTTEQUIN
Service de paléontologie animale et humaine, Département de géologie, Université de Liège, Bât. B18, allée
du 6 Août, B 4000 Liège 1, Belgium; E.Poty@ulg.ac.be, julien.denayer@ulg.ac.be, bmottequin@ulg.ac.be
In the Namur-Dinant Basin (southern Belgium), the end of the Middle Frasnian recorded the
development of an extensive carbonate platform and large stromatoporoid-coral reefs, which was induced
by a significant progradation during the highstand systems tract (HST) of a third-order sequence. The
subsequent fall of the sea level caused their emersion and the end of the carbonate production (POTY &
CHEVALIER 2007).
The rise of the sea level (Fig. 1) during the following sequence (Aisemont sequence sensu POTY &
CHEVALIER 2007) was marked by a huge input of argillaceous sediments more or less mixed with
carbonates, which progressively covered the previously exunded reefs and carbonate platform (DENAYER &
POTY 2010). At the onset of the sea-level rise, the tabulate and rugose coral faunas were identical to those
previously known in the carbonate and shaly facies, comprising numerous thamnoporids (Tabulata) and
fasciculate (e.g. Disphyllum) and cerioid (e.g. Hexagonaria) disphyllids (Rugosa). However, the latter
disappeared during the transgressive systems tract (TST) (Early rhenana conodont Biozone) and were
replaced by representatives of the family Phillipsastreidae (e.g. POTY 1999), such as Frechastraea and
Phillipsastrea (Fig. 1). The maximum of the TST is characterized by the development of argillaceous deposits
with dysaerobic–anaerobic facies (occurrence of lingulid brachiopods and bivalves of the “paper pecten”
morphotype), which can be correlated with the Lower Kellwasser Event (LKW) (POTY & CHEVALIER 2007).
Therefore the initial decline among corals and stromatoporoids was not related to the LKW sensu stricto,
which happened later and induced strictly no extinction in corals and brachiopods, as the taxa occurring
after the first crisis were still present above the LKW.
Later, in the Late rhenana Biozone, rugose corals progressively disappeared along with tabulates, before
the Upper Kellwasser Event (Fig. 1). Therefore, the gradual coral extinctions during the Late Frasnian were
not the result of sudden events, but were triggered by a relatively continuous degradation of the
environments, including the local development of dysaerobic–anaerobic facies. The latter, in conjunction
with an increase of turbidity due to huge detrital inputs, were not responsible for coral and stromatoporoid
extinctions but prevented local recolonization. In southern Belgium, rugose corals were absent from the
linguiformis Biozone to the Early crepida Biozone in which rare small solitary forms re-appeared (see
MOTTEQUIN et al. 2011 for more details).
In parallel, the brachiopod decline occurred in three steps within the interval spanning the Early rhenana
Biozone to the linguiformis Biozone (Fig. 1). Most brachiopod orders suffered severely and the major losses
occurred at the top of the Late rhenana Biozone (e.g., MOTTEQUIN 2008a). These extinction episodes were
linked principally to diachronous regional facies changes related to transgressions. For example, atrypids
and pentamerids became extinct within the Late rhenana Biozone in the shallow parts of the Namur-Dinant
Basin whereas they had already disappeared at the top of the Early rhenana Biozone in its distal part, just
before the deposition of the dark shales of the Matagne Formation indicative of hypoxic bottom conditions
(GODEFROID & HELSEN 1998, MOTTEQUIN 2008b). Contrary to corals, post-extinction brachiopod recovery
was rapid in the basal Famennian but, despite their great abundance, their diversity was quite low. New
cosmopolitan genera appeared at this time especially among the athyridids, rhynchonellids, and spiriferids
concomitantly with new species of pre-existing orthid and orthotetid genera (e.g., MOTTEQUIN 2008a, c).
DENAYER, J. & POTY, E. (2010): Facies and palaeoecology of the upper member of the Aisemont Formation (Late
Frasnian, S. Belgium): an unusual episode within the Late Frasnian crisis. - Geologica Belgica, 13: 197-212.
GODEFROID, J. & HELSEN, S. (1998): The last Frasnian Atrypida (Brachiopoda) in southern Belgium. - Acta
Palaeontologica Polonica 43: 241-272.
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