PALAEOENVIRONMENTAL CONTEXTS FOR MICROBIAL COMMUNITIES FROM Fe-Mn CRUSTS OF MIDDLE-UPPER JURASSIC HARDGROUNDS (BETIC-RIFIAN CORDILLERA)

Abstract

The Middle-Upper Jurassic transition within the External Subbetic (Betic Cordillera) and the Jbel Moussa Group (Rifian Calcareous Chain) is characterised by numerous stratigraphic breaks recorded as palaeoreliefs, omission surfaces and hardgrounds. One common characteristic is the presence of microbial Fe-Mn crusts and Mn crusts. Three different contexts of microbially mediated Fe-Mn crusts can be distinguished: a) hydrogenetic surficial Fe-Mn crusts and macro-oncoids, b) hydrogenetic cryptic Fe-Mn crusts, and c) hydrothermal surficial Mn crusts. Encrusting foraminifera (Thurammina, Placopsilina, Tolypammina and nubeculariids) and fungi fed on bacteria, while the bacteria and fungi took advantage of the nutrient-rich foraminiferal excretions. The cryptic Fe-Mn crusts are also hydrogenetic crusts developed in the walls of submarine cavities and fractures in the External Subbetic at the time of the hardground development. They are endostromatolites consisting almost exclusively of Frutexites in the case of thin neptunian dykes, or serpulid-Frutexites assemblages in a large neptunian sill. This assemblage reflects the colonization of unfavourable environment (aphotic with low oxygenation) as a response to the photophobic behaviour of chemosynthetic and cryptobiontic microorganisms. Finally, the hydrothermal Mn crusts registered in the Rif are characterised by the record of bacillus-shaped bacteria and different type of filaments. The precipitation of manganese minerals is interpreted as induced by the chemo-organotrophic behaviour of benthic microbial communities, since manganese oxides may form as a result of the direct metabolic activity of bacteria in hydrothermal environments. Very well-preserved fungal hyphae evidence the presence of fungi that probably fed on bacterial communities and their by-products