CORALS TO RUDISTS - AN ENVIRONMENTALLY-INDUCED ASSEMBLAGE SUCCESSION

Corals and rudist bivalves commonly coexisted on the shallow carbonate platforms of the Cretaceous Tethyan Realm. Usually, the carbonate pla tforms show lateral and vertical biotic zonations. In general, the ver tical biotic zonation relates directly to a consistent lateral biofaci es pattern: coral or coral-rudist assemblages occupy more open zones o f the platform, and rudist-dominated assemblages, the more restricted zones. Several corresponding coral-to-rudist assemblage successions in carbonate platforms have previously been widely interpreted as ecolog ical successions. We document some coral-to-rudist assemblage successi ons, in a well preserved Upper Cretaceous section, in the Tremp area o f the South Central Pyrenean Unit, which are interpreted, by contrast, as environmentally-induced faunal replacements. A repealed assemblage succession in the eastern part of the section begins with a coral-dom inated assemblage, and passes up gradually to a mixed coral and rudist assemblage that is succeeded by a paucispecific unit of hippuritid el evator congregations. The lower coral-rudist units of such successions represent relatively more open, marine conditions, and the uppermost hippuritid unit relatively more restricted marine conditions. This ass emblage succession was formed in response to a change in the depositio nal setting caused by sediment accumulation. Two consecutive coral-dom inated assemblages, overlain by mixed coral and rudist assemblages, co nstitute a second kind of a sedimentary coral-to-rudist assemblage suc cession discussed in this paper. Its regular recurrence makes up the u ppermost part of the section in the East. The predominance of platy to low domal coral colonies in the lower units suggests that conditions were initially favourable for horizontal coral growth. As the habitat changed, these coral assemblages were replaced by massive-tabular to l arge domal coral colonies, with the large hippuritid Vaccinites, and w ere sometimes succeeded, in turn, by clustered hippuritid elevators. T he most important factor causing the replacement was probably the incr ease in sediment flux, associated with turbidity. This change in the d epositional environment eventually favoured the establishment of rudis t elevators, well adapted to moderate and intermittent to continuous a ccumulation of sediment.