Oligophotic coral bioherms in a terrigenous prodelta setting (Late Eocene, Southern Pyrenees, Spain)

Recent studies in the present oceans have shown that corals and coral reefs can thrive in episodically or permanently turbid waters with moderate to high terrigenous sediment supply. Many coral bioconstructions in the past are associated with marly and mixed carbonate-siliciclastic sediments suggesting that an actualistic approach (shallow, clear and oligotrophic waters associated with high energy settings), where zooxanthellate corals are able to build up to sea level, cannot be truly applied.The study area is located in the southern part of the Jaca Basin, a piggyback basin of the South Pyrenean foreland. Here, a late Eocene thick pile (1 km) of marls and some interbedded limestone (Arguis Fm.) passes up section to westward prograding shallow marine and deltaic deposits (Belsue-Atarés Fm to Yeste-Arrés Fm) and to continental clays, sandstones and conglomerates (Campodarbe Fm). Within the Arguis Fm, coral-rich bodies deposited during the early Priabonian, interfingered and covered by clays and sandstone. Individual coral lithosomes are mostly nodular to wavy, 1 to 8 m thick, boundaries are irregular to sharp, and clay content is significant. They stack vertically, forming low relief lens-shaped bioherms, commonly 20-30 m thick, up to 50 m. These bioherms grew within prodelta marls, being progressively younger basinward. Main bio-lithofacies are: 1) floatstone to rudstone with large fragments of corals (Cyathoseris, Actinacis, Colpophyllia, Stylophora, Caulastrea, Astrocoenia, Goniopora, Astreopora, Siderastrea, Alveopora, Agathiphyllia), branching and encrusting red algae, bryozoans and scattered bivalves, encrusting foraminifera (eg. Miniacina sp.), with wackestone to packstone matrix; 2) coral framestone with branching to meandroid corals in growth position (Caulastrea, Actinacis, Cereiphyllia, Leptoria); 3) wackestone to mudstone with platy corals encrusted by bryozoans and laminar coralline red algae, rare planktonic foraminifera; 4) grainstone to rudstone with branching coralline red algae, bryozoans and abundant miliolids, small rotalids, ostreids fragments; 5) sandstone with abundant nummulitids and coral fragments, gastropods, bryozoans, bivalves and coralline red algae fragments. Bio-lithofacies 4 and 5 are mainly developed on the flanks of the bioherms. In some outcrops, Acanthochaetetes, a demosponge that seems to be characteristic of reduced light conditions in cryptic or fore reef settings, is also present. Among larger foraminifera, the presence of Asterocyclina, Heterostegina, Operculina and the abundance of the encrusting genus Miniacina could also suggest an oligophotic paleoenvironment.These bioherms, located inside prodelta marls, grew in a relatively low to moderate energy setting, below the fair-weather wave base. Our results highlights the contrast with present-day “classic” reef models, and suggests that by the late Eocene reef coral assemblages where able to thrive in relatively deep (oligophotic), low-energy and turbid conditions.