Aretz et al_2011.pdf - ORBi - Université de Liège

_________________________________________________________________________________________________________
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
_________________________________________________________________________________________________________
Clear- and turbid-water coral assemblages from Barremian–Lower Aptian
of Bulgaria
Bogusław KOŁODZIEJ 1 , Vyara IDAKIEVA 2 , Marin IVANOV 2 & Vassil ZLATARSKI 3
1 Institute of Geological Sciences, Jagiellonian University, Oleandry str., 2a, 30-063 Kraków, Poland;
boguslaw.kolodziej@uj.edu.pl
2 Department of Geology, Paleontology and Fossil Fuels, Sofia University, Sofia, Bulgaria;
idakieva@gea.uni-sofia.bg, mivanov@gea.uni-sofia.bg,
3 131 Fales Rd., Bristol, RI 02809, USA, vzlatarski@yahoo.com
During Barremian–Early Aptian times several carbonate platforms existed on the northern Tethyan
margin in Bulgaria. In particular the Lovech Urgonian Group (Fig. 1A) in the Central Fore-Balkan contains
abundant and diversified corals (about 120 species). Occurrence of turbid- and clear-water environment is a
characteristic feature of this mixed siliciclastic-carbonate Urgonian system. Taxonomy of corals from
turbid-water environments (mainly from marls) where studied already in the 19 th century (F. TOULA), but
mostly since the 1960s (V. ZLATARSKI, V. TCHECHMEDJIEVA, V. IDAKIEVA). Recent studies were particularly
focused on corals from carbonate units (clear-water environments), coral palaeoecology and
palaeoenvironmental factors controlling development of coral assemblages.
Turbid-water coral assemblages form segment or cluster (matrix-supported) reefs and level-bottom
assemblages, rarely small frame (skeleton-supported) reefs. Usually massive colonies formed the core of the
isolated patch reefs, whereas phaceloid, dendroid or ramose corals grew on the periphery of the buildups
(IDAKIEVA & IVANOV 2002). These later, often still in life position, indicate constratal growth fabric (low syndepositional
relief). Corals are represented mainly by stylininans, cyathophorids, actinastreids, faviids
(Eugyra, Hydnophora, Felixygyra), clausastreids and microsolenines (e.g, IDAKIEVA 2003). Coral skeletons
show features typical for turbid-water and soft-substratum environments, such as growth anomalies due to
partial colony mortality, reorientation of growth direction (Fig. 1F), and the presence of colony and corallite
morphologies adapted to stressful factors of turbid-water settings (Fig. 1E, F; cf. Sanders & Baron-Szabo
2005). Platy and low domal colonies dominate as possible response to a soft, unstable substrate and
decrease in light intensity. Among them clausastreids (Fig. 1E) and Amphiaulastraea attain the largest size
suggesting their high resistance to sedimentation. Some corals as hydnophoroid-meandroid faviids show
significant morphological plasticity (from platy to branching growth forms). Small-sized colonies, common
encrustations by heterotrophic organisms (very rare microbial crusts) on the lower parts of coral colonies
(Fig. 1D), and rare on the upper ones indicate changes in sedimentation rate with intermittent rapid
sedimentation events of lethal or sublethal effects. Some localities show diversified coral assemblages and
high coral density, as result of low sediment input.
Level-bottom coral assemblages are dominated by solitary forms: Montlivaltia multiformis TOULA,
Axosmilia spp., Actinoseris alloiteaui BEAUVAIS & ZLATARSKI (Fig. 1J). An interesting assemblage is dominated
by abundant small fungiform colonies of the endemic species Dimorphocoeniopsis beauvaisorum ZLATARSKI
(ZLATARSKI 1967) (Fig. 1G–I), recently discovered also in the Vratsa Urgonian Group.
Clear-water coral bioconstructions (mainly biostromes) differ in significance of microbialites, presence
or absence of microencrusters, particularly phototrophic ones (Lithocodium aggregatum, Bacinella irregularis),
as well as in bioerosion rate, what were presumably controlled by sedimentation rate and trophic regime
(Fig. 1K–M). Chaetetids and other calcified sponges locally participate in frame construction. Rudists cooccur
with corals; however they usually form biostromes without or with rare corals. The Emen Fm, in
localities NW of Veliko Tarnovo, contains abundant and diversified corals of the suborder Pachythecaliina
associated with small rudists (Fig. 1M; KOŁODZIEJ et al., this volume). Lithocodium–Bacinella-dominated
facies (with rare corals) facies is poorly developed. Microbialite-dominated reefs do not occur.
Growth and demise of clear- and turbid water coral assemblages occurred in various eustatic, tectonic
and climatic contexts, which resulted in such environmental factors as water depth, nutrient and
siliciclastic input, oxygenation or hydrodynamics. The depositional pattern was controlled mainly by
83