Development of coral reef-facies during the Viséan at Los Santos de Maimona, SW Spain

Abstract

Abstract The Siphonodendron Limestone at Los Santos de Maimona Basin is regarded as a reef structure built mainly by rugose corals. This unit is composed of biogenic marls and biostromal limestones containing abundant rugose corals and brachiopods, and frequent calcareous algae, tabulate corals, foraminiferans, bryozoans, echinoderms, ostracodes and molluscs. It is present throughout the basin, which is 12 km long and 3 km wide, but the thickness, development of the framework and distribution of organic components varies from SE (seaward) to NW (landward). It is 40 m thick in the SE, but its thickness reaches only 6 m in the NW. The unit shows a vertical evolution of lithological facies, from biogenic marls at the bottom up to biostromal limestones at the top. The main environmental factors controlling the development of the organic framework are the tidal regime, minor subsidence pulses and periodic storms. Arguments in favour of the reefal origin are structural (distribution, development and relationships between different facies of the Siphonodendron Limestone) and ecological (distribution and relationships of building organisms, environmental indicators, etc.). Objections to the reefal hypothesis (absence of reef crest and talus, low diversity, problematical wave resistance, biostromal nature of the upper beds, storm layers) are discussed. Building structures by corals, brachiopods and calcareous algae are briefly described.