Mesophotic coral buildups in a prodelta setting (Late Eocene,southern Pyrenees, Spain): a mixed carbonate–siliciclastic system

摘要

Lower Priabonian coral bioherms and biostromes, encased in prodelta marls/clays, occur in the A?′nsa-Jaca piggyback basin, in the South Central Pyreneanzone. Detailed mapping of lithofacies and bounding surfaces ontophotomosaics reveals the architecture of coral buildups. Coral lithosomesoccur either isolated or amalgamated in larger buildups. Isolated lithosomesare 1 to 8 m thick and a few hundred metres wide; clay content within coralcolonies is significant. Stacked bioherms form low-relief buildups, commonly20 to 30 m thick, locally up to 50 m. These bioherms are progressively youngerto the west, following progradation of the deltaic complex. The lowermostskeletal-rich beds consist of bryozoan floatstone with wackestone to packstonematrix, in which planktonic foraminifera are abundant and light-relatedorganisms absent. Basal coral biostromes, and the base of many bioherms,consist of platy-coral colonies ‘floating’ in a fine-grained matrix rich inbranches of red algae. Corals with domal or massive shape, locally mixed withbranching corals and phaceloid coral colonies, dominate buildup cores. Thesecorals are surrounded by matrix and lack organic framework. The matrixconsists of wackestone to packstone, locally floatstone, with conspicuous redalgal and coral fragments, along with bryozoans, planktonic and benthonicforaminifera and locally sponges. Coral rudstone and skeletal packstone, withwackestone to packstone matrix, also occur as wedges abutting the buildupmargins. Integrative analysis of rock textures, skeletal components, buildupanatomy and facies architecture clearly reveal that these coral buildupsdeveloped in a prodelta setting where shifting of delta lobes or rainfall cyclesepisodically resulted in water transparency that allowed zooxanthellate coralgrowth. The bathymetric position of the buildups has been constrained fromthe light-dependent communities and lithofacies distribution within thebuildups. The process-product analysis used here reinforces the hypothesisthat zooxanthellate corals thrived in mesophotic conditions at least during theLate Eocene and until the Late Miocene. Comparative analysis with someselected Upper Eocene coral buildups of the north Mediterranean area showsimilarities in facies, components and textures, and suggest that they also grewin relatively low light (mesophotic) and low hydrodynamic conditions.