Late Jurassic coral/microbial reefs from the northern Paris Basin – facies, palaeoecology and palaeobiogeography

During the late middle Oxfordian, patch reefs grew on the northern margin of the Paris Basin. According to the facies analysis of the reef and inter-reef sediments, the environment was a warm, clear and agitated sea with highly episodic sedimentation. The bioherms were a short-lived phenomenon during the third phase of regional reefal development. Sequence stratigraphically, they are associated with a highstand system tract. Volumetrically and trophically dominant organisms were microbes now represented by massive clotted leiolite; ‘stalactitic’ hemispheroids with purely thrombolitic texture are restricted to open caves. Corals were of structural, reef-building importance due to their rapid upward growth. The patch reefs are characterised by thickets of ramose corals which developed a very open framework. In the vicinity of these patch reefs, though in hydrodynamically higher-energy environments, grew thickets of more stoutly branched corals; however, they are rarely preserved in situ and are generally represented as abundant coral rubble. The reef taxa are characterised by the notable absence of several groups (e.g. oysters, serpulids, bryozoans, pectinids) occurring at other localities where reefs of similar age developed in similar environmental conditions. The reefs also have strikingly modern aspects to them, in particular the presence of cryptic elements within caves and a sponge-dominated borer association. Dwellers belong to various life-form types although encrusting taxa are exceedingly rare. This may be explained by the presence of soft microbial films on most surfaces. The palaeoecological analysis suggests that the major controls on faunal composition and high diversity were elevated nutrient levels, highly episodic sedimentation and probably seasonal environmental disturbances. Structural and functional aspects of the reef community (grazers trigger framebuilders, borers trigger binders, binders hamper borers) allow ecological comparisons to be made with contemporaneous, as well as Recent, reefs. The unique combination of ecological factors resulted in a specialised, previously undescribed, community which differs from both Tethyan and northern localities in various aspects; these include cavities with cryptofauna, prominence of grazing gastropods and high faunal diversity in a microbially dominated build-up.