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 platforms show lateral and vertical biotic zonations. In general, the vertical biotic zonation relates directly to a consistent lateral biofacies pattern: coral or coral-rudist assemblages occupy more open zones of 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 ecological successions. ument some coral-to-rudist assemblage successions, in a well preserved Upper Cretaceous section, in the Tremp area of the South Central Pyrenean Unit, which are interpreted, by contrast, as environmentally-induced faunal replacements. A repeated assemblage succession in the eastern part of the section begins with a coral-dominated assemblage, and passes up gradually to a mixed coral and rudist assemblage that is succeeded by a paucispecific unit of hippuritid elevator 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 assemblage succession was formed in response to a change in the depositional setting caused by sediment accumulation. nsecutive coral-dominated assemblages, overlain by mixed coral and rudist assemblages, constitute a second kind of a sedimentary coral-to-rudist assemblage succession discussed in this paper. Its regular recurrence makes up the uppermost 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 large domal coral colonies, with the large hippuritid Vaccinites, and were sometimes succeeded, in turn, by clustered hippuritid elevators. The most important factor causing the replacement was probably the increase in sediment flux, associated with turbidity. This change in the depositional environment eventually favoured the establishment of rudist elevators, well adapted to moderate and intermittent to continuous accumulation of sediment.