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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Coral assemblages of Cenozoic mixed carbonate-siliciclastic settings:
sediment-resistant corals and their constructional and depositional facies
Francesca R. BOSELLINI & Giulia SILVESTRI
Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, Modena, Italy;
francesca.bosellini@unimore.it, 36222@studenti.unimore.it
In many regions of modern oceans coral reefs, traditionally associated with well-lit, oligotrophic,
shallow waters and high-energy conditions, develop also in areas where terrigenous sediments are an
important component of the sedimentary environment. Despite, in fact, relatively high turbidity conditions,
restricted light penetration, presence of often mobile terrigenous sediment substrates, active coral growth is
well developed (WOOLFE & LARCOMBE 1999; PERRY 2005) and many coral bioconstructions in the past are
actually associated with marly and mixed carbonate-siliciclastic sediments (SANDERS & BARON-SZABO 2005).
Their reconstruction, however, largely refers to uniformitarian assumptions based on present-day classic
clear-water reef models, and there are a limited number of studies and limited data, especially for the
Cenozoic, concerning their detailed structure and composition, depositional patterns and relationship with
main controlling factors such as light penetration, turbidity, sedimentary input, hydrodynamic energy,
nutrient supply.
Two main case studies are investigated in detail and correspond to delta and fan delta systems
respectively of the Upper Eocene of the Jaca Basin (southern Pyrenees, Spain) (MORSILLI et al., submitted)
and of the Oligocene of the Tertiary Piedmont Basin (northwestern Italy). In these areas extensive fieldwork
with detailed coral facies mapping has been performed together with biofacies and microfacies analyses. A
non-conventional approach based on microtaphofacies analysis has been also applied in order to unravel
the depositional history of these coral assemblages which are often dominated by coral rubble deposits.
Even in situ bioconstructions result largely affected by taphonomic processes such as encrustation and
bioerosion.
Results, compared with other fieldwork and literature data, reveal the occurrence of distinctive coral
assemblages and facies characterizing mixed carbonate-siliciclastic settings during Eocene to Miocene time
in the Mediterranean region. These can be distinguished on the base of their taxonomic composition,
growth forms, taphonomic signatures and associated sediments, and include both coral growth fabrics
(such as platestone, domestone and pillarstone) and rubble deposits (floatstone and rudstone).
Constructional features highlight that these assemblages represent an alternative product of the
carbonate factory with respect to “clear-water” reefs. Large part of bioconstructions can be defined as
cluster reefs (sensu RIDING 2002), are usually characterized by a low topographic relief and rare coral
interlocking. Taphonomic alteration and especially bioerosion is high, also concurring to determine
fragmentation of corals and definitely the formation of rubble.
The examined contexts are consistent with generally mesophotic, low-energy conditions, with
occasional higher energy episodes responsible of sediment resuspension and consequent turbidity, or
connected to coarse sediment run-off and successive sediment accumulation. Among others, corals able to
face sediment accumulation, to tolerate periods of sediment veneer, to cope with turbidity/reduced water
transparency, include poritids like Porites and Goniopora, Actinacis, typical branching corals such as
Stylophora and the phaceloid Caulastrea, platy agariicids like Cyathoseris, and faviids such as Antiguastrea,
Tarbellastraea, Favia, Colpophyllia, Diploria. The genus Acropora, displaying mostly a branching growth form,
is also common, especially during the Oligocene, suggesting a capacity to grow within low-energy and
turbid waters.
Taphonomic processes and signatures (abrasion, fragmentation, bioerosion, encrustation), whose effect
is basically related to surface residence time of corals, appear to be a promising tool for better
understanding the response of corals to the influence of sedimentary input resulting in water turbidity
and/or sediment accumulation.
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