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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Palaeosoil draping a remarkable Disphyllum autobiostrome: a case study
from the Middle Frasnian, Southern Belgium.
Delphine RENSONNET 1 & Edouard Poty 2
1 Fugro Geoconsulting Belgium, Bruxelles, Belgium; drensonnet@fugro.be
2 Service de Paléontologie animale et humaine, Département de Géologie, Université de Liège, Bat. B18,
Allée du Six-Août, Sart Tilman, B-4000 Liège, Belgium; e.poty@ulg.ac.be
The lower part of the Middle Frasnian Lustin Formation – also known as the “Biostromal Unit” –
consists of shallow-water limestones that crop out in the northern part of the Namur-Dinant Basin
(Southern Belgium). The unit is densely populated by disphyllid corals and stromatoporoids and present
well-established stacking patterns. In the western part of the Vesdre-river area, the Prayon section recorded
plurimetre-thick shallowing-upward sequences. From base to top, a typical sequence shows: (1) a rudstone
with coarse debris of rugose and tabulate corals and stromatoporoids in a wackestone to packstone matrix
(parabiostromes), (2) boundstone with lamellar to tabular stromatoporoids in an slightly-argillaceous
mudstone matrix with some rugose and tabulate corals, (3) bioturbed mudstone to bioclastic wackestone
with brachiopods, gastropods, crinoids, Amphipora and algae, (4) wackestone to packstone with small-sized
gastropods and ostracods to floatstone-rudstone with Amphipora, (5) laminated stromatolitic mudstone. But
one of the sequences comprises an autobiostrome mainly composed of Disphyllum goldfussi and Alveolites
suborbicularis, some of them being a little disturbed and sometimes overturned, capped by an argillaceous
palaeosoil. The exceptional outcropping setting of this autobiostrome allows for a semi-quantified
palaeoecological study. Skeletal features and morphologies of corals demonstrate a significant turbidity,
with seasonal variations and a high rate of sedimentation. The biostrome was buried and preserved in-situ
by a cinerite. The latter evolves later to a palaeosoil. Although palaeosoils are known in the Lustin
Formation, the calcrete layer in Prayon is unique and evidences the development of an arborescent
palaeoforest.
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