An Early Cretaceous mesophotic coral ecosystem built by platy corals (middle Aptian, Southern Carpathians, Romania)

Abstract

A low relief middle Aptian (Gargasian) reef built by platy corals is described from the Lower Cretaceous succession of the Reşiţa–Moldova Nouă zone (Southern Carpathians, SW Romania). Two coral-bearing units, 16–17 and 38–42 m thick, discontinuously cover ca. 1100 m. This is an unusually thick fossil reef to be built by platy corals. The coral units are underlain by bioclastic limestones interlayered with thin rudist-chaetetid biostromes, separated by a 15–30 m thick interval of bioclastic limestones and overlain by upper Aptian conglomerates. Mostly dense, rarely sparse, platestones are composed of a low diversity coral assemblage mainly represented by the suborder Microsolenina. Small, branching corals are very rare. The matrix mainly consists of fine bioclastic-peloidal packstones and wackestones. Dominance of microsolenine corals, their flattened morphology, the presence of epibionts on coral undersurfaces and occurrence of red algae Sporolithon rude and Polyastra alba indicate that low-light was the main factor controlling reef growth. As with most fossil reefs dominated by platy corals (e.g., Upper Jurassic microsolenid reefs), the Reşiţa reef can be considered an analogue of modern reefs from mesophotic coral ecosystems (MCEs) in relatively deep water settings. The mesophotic or perhaps oligophotic environment was characterised by low background sedimentation, and high nutrient level as evidenced by abundant bioerosion traces. Matrix sediment and rare fragmentation of coral skeletons indicate moderate water movement. In contrast with common Barremian–lower Aptian coral reefs, younger Lower Cretaceous reefs in Romania are very rare. This reflects the general demise of carbonate platforms in the northern Tethyan domain during the early Aptian Oceanic Anoxic Event (OAE1a).