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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The Hettangian corals of the Isle of Skye (Scotland): an extreme ecosystem
just after the Triassic-Jurassic boundary crisis
Mélanie GRETZ 1 , Bernard LATHUILIERE 2 & Rossana MARTINI 1
1 Department of Geology and Paleontology, University of Geneva, 13 rue des Maraîchers, 1205 Geneva,
Switzerland; Melanie.Gretz@unige.ch
2 UMR CNRS 7566, Géologie et Gestion des Ressources Minérales et Energétiques (G2R), Université de
Nancy I, France; bernard.lathuiliere@g2r.uhp-nancy.fr.
The Triassic-Jurassic boundary (T-J boundary), estimated at ~200 million years ago, was a critical
transition where profound biotic and environmental changes occurred. This transition is considered as one
of the big five mass extinction events of the Phanerozoic. The causes remain controversial. Many authors
report that the end of Triassic was marked by an important increase in pCO2 which induced a global
warming. The source of the CO2 emission was probably the Central Atlantic Magmatic Province (CAMP),
one of the largest igneous provinces of the world which was occasioned by the break-up of the Pangea.
Other scenarios have been proposed to explain the mass extinction including meteoric impacts and a sealevel
change.
HAUTMANN (2004) has suggested that the important increase of the atmospheric CO2 at the end of the
Triassic caused a CaCO3 undersaturation of the oceans which consequently induced a biocalcification crisis.
Thus, the reef communities, and especially the corals suffered high extinction rates. Before the recovery of
reef building during the Dogger (LATHUILIÈRE & MARCHAL 2009), the Lower Jurassic is characterized by a
“reef gap”. Thereby Hettangian outcrops showing genuine frameworks of colonial corals are very scarce
and essentially concentrated in Western Europe (western Tethys).
This work focuses on the study of a Hettangian outcrop containing coral beds which is situated on the
Isle of Skye (Ob Lusa locality, Highlands, Scotland). In comparison with their counterparts, these coral
communities lived in a very high latitudinal area (around 40° N latitude). This area was situated in the
eastern Inner Hebrides Basin. In a wider context, the Hebrides Basin was constituted during the Jurassic by
a network of shallow seaways that connected the Tethys in the South to the Boreal ocean in the North
(DORÉ 1992). In order to better understand the paleoenvironmental conditions of this north-western part of
the Tethys during the Triassic-Jurassic reef crisis, it is especially important to make detailed observations in
the field as well as analyses related to paleotemperatures, paleoecology and paleogeographic setting of the
corals of the Isle of Skye.
At Ob Lusa, six distinct coral beds were found in the studied outcrop. The coral associations are
monogenic, belonging to Heterastraea, a massive cerioid genus. The first bed exhibits relatively well
developed colonies that constituted small bioconstructions, whereas the other beds display very small
colonies completely drowned in the matrix. Thereby, the studied corals were sampled in this first coral bed.
Their morphology and size can vary (Fig. 1) but the general growth fabric is dominated by platy colonies.
This kind of growth fabric is defined as a platestone (INSALACO 1998). The more surprising characteristic of
those specimens, and especially for the platy corals, is the growth pattern. Indeed, many samples do not
show the classical growth polarity because they are bifacial; it means that the corallites grew toward both
sides of the colonies (Fig. 2). The possibility that they were rolled has been rejected because the corals
clearly form in situ bioconstruction.
The sedimentological study revealed that those corals lived in a shallow environment with fluctuating
paleoenvironmental conditions (e.g., sea-level, currents and maybe climate). Moreover, the facies show that
the area was influenced by a strong siliciclastic input. The corals developed during periods with a lower
input but the quantity of quartz is nevertheless not insignificant. It is also important to notice that the
general faunistic assemblage of the studied outcrop is low diversified and is mainly composed of
allochthonous bioclasts. The paucity of other organisms (e.g., foraminifers, bivalves, sponges, etc.), which
are more tolerant and normally abundant in reefal environments, is striking because corals are generally
thought to have a lower environmental tolerance.
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