An Upper Cretaceous mesophotic coral reef (Gosau Group, Eastern Alps, Austria): Significance for the palaeoreef record

Cretaceous reefs that record coral growth under mesophotic conditions apparently are very rare. Hereunder we present a middle-upper Coniacian to lower Santonian reef composed mainly of coral platestone that is located within the synorogenic wedge-top succession of the Eastern Alps (Gosau Group, Turonian to Ypresian) near Strobl am Wolfgangsee (Salzburg, Austria). In the studied reef, an exposed 15–18 m of coral platestone is overlain by a few meters of domestone-mixstone and bioclastic limestones. The platestones display a dense, continuous, interlocked fabric with a matrix of wackestone with planktic foraminifera and calcareous nannoplankton. Bioencrustation of coralla and boring traces are scarce. Hippuritids and radiolitids settled on the corals, but only few developed into adults. The dome/mixstones comprise massive and branched corals, show a packed fabric, and contain intercalated singles and clusters of rudists. In the reefal interval, one helioporid octocoral and 25 colonial scleractinian species (25 % pennular species) were identified. The top bioclastic limestones contain dasycladaleans, Miliolina, Textulariina and rare Rotaliina. The vertical succession from coral platestones to bioclastic limestones indicates shoaling. The platy-foliaceous shapes and packing of coralla in the platestones, and the scarce encrustation suggest constratal to low-superstratal growth under mesophotic conditions. In the dome/mixstones, toppled corals and rudists indicate episodic disturbance. In Jurassic seas mesophotic reefs were widespread, yet the studied reef is the first Upper Cretaceous example. This contrast in reef style may be attributable to: (1) intensified production and dispersal of chalk; (2) extinction/emigration of Jurassic microsolenids; and, (3) implicit description of Cretaceous mesophotic reefs.