Upper Jurassic microsolenid biostromes of northern and central Europe: facies and depositional environment

Abstract

Microsolenid biostromes are a common and important reef type in many Upper Jurassic reefal sections. Comparative study of these biostromes in western Europe allows the characterisation of this distinct biostrome type in terms of its taxonomic composition, palaeoecology and sedimentary regime. The biostromes are characterised by a coral community of relatively low generic diversity, being dominated by the Microsolenidae family of corals. The coral skeletal biovolume is generally high and is overwhelmingly dominated by platy ecomorphotypes. Sedimentologically these biostromes developed in low energy marl and limestone regimes which were subject to low sedimentation rates and varying degrees of siliciclastic influx. In contrast to the coral fauna the associated fauna shows a high generic diversity, in particular a rich assemblage of echinoids and bivalves. In addition the coral plates provide an ideal habitat for a well developed and strongly polarised encrusting and boring fauna. Top surfaces show limited post-mortem encrustation by serpulids and cementing bivalves, and are also bored by lithophagan bivalves. The undersides of the plates show a highly developed and diverse cryptic fauna of serpulids, thecidean brachiopods, bryozoans, calcareous sponges and sclerosponges. It is concluded that the main control on the development of these biostromes was low light intensities, though low energy level and low background sedimentation rates were also important. It is suggested that relatively high nutrient levels may have also played a significant role. The conditions for microsolenid biostrome development prevail in relatively deep-water environments, and hence these are the first coral-dominated reefal structures to have developed in deeper water shallowing upward sequences. The corals in these biostromes have adapted to these conditions by developing a strongly platy growth form, but also reducing their growth rates substantially. The microsolenids are particularly adapted to these environments in two ways: (1) They secreted an extremely porous calcium carbonate skeleton, which in environments where calcification rates are slow, such as deep water environments, is a distinct advantage; (2) In response to the limited potential of phototrophy in these environments the microsolenids have developed a pennular septal structure which is suggested to be a means of improving heterotrophic feeding. Trophic analysis of the reef fauna reveals that the nutrient limitation model developed for present-day coral reefs cannot be applied to these biostromes. It is demonstrated that the Microsolena and the present-day agariciid, Leptoseris show a number of significant morphological and ecological similarities.