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PLEISTOCENE ROLLING STONES OR LARGE BRYOZOAN NODULES IN A MIXED SILICICLASTIC-CARBONATE ENVIRONMENT (RHODES, GREECE)

¹ 1. UMR 5125 CNRS PEPS, Université Lyon 1, 69622 Villeurbanne Cedex, France
² 2. UMR 5243 CNRS, Université Montpellier II, Géosciences Montpellier, CC 60, pl. Eugène Bataillon, 34095 Montpellier Cedex 05, France
³ 3. University of Athens, Department of Historical Geology and Paleontology, Panepistimiopolis, Athens 15784, Greece
⁴ Pierre.Moissette{at}univ-lyon1.fr

Spectacular accumulations of abundant bryozoan nodules (bryoliths) occur in the early Pleistocene coarse-grained, mixed carbonate-siliciclastic deposits of the Cape Arkhangelos Calcarenite of Rhodes, Greece. These bryoliths are always associated with numerous bivalves (mostly ostreids and pectinids) and serpulid worms, together with other rarer skeletal organisms. The spheroidal bryoliths vary in diameter from 1 cm to 16 cm, and those with a more ellipsoidal shape range from 0.8 cm to 22 cm in length and from 0.7 cm to 16 cm in width; an elongated bryolith of exceptional size (40 cm) was even found. The bryoliths are composed almost exclusively of the anascan cheilostome Calpensia nobilis and are nucleated on gravels, pebbles, or bivalve shells. After settlement of bryozoan crusts, the bryoliths increased in size by repeated overturning of these mobile hard substrates by longshore currents. The bryoliths developed at depths of 20–40 m on a flat shoal between emerged areas where bottom currents episodically accelerated during storms. The currents also transported the bryoliths, together with other skeletal elements, into deeper settings (40–80 m) where they could continue their growth, upward or downward, or be colonized by other organisms.