Ferruginous microstromatolites related to Middle Jurassic condensed sequences and hardgrounds (Bucegi Mountains, Southern Carpathians, Romania)

摘要

Integrated analyses of ferruginous laminated crusts and macro-oncoids associated with Middle Jurassic (Bathonian-Callovian) hardgrounds and condensed horizons cropping out in the Bucegi Mountains (Southern Carpathians) allowed an assessment of their microbial origin and the paleoenvironmental context of their genesis. The ferruginous microstromatolites reveal different morphological types (or macrofabrics): ferruginous microstromatolites representing the hardgrounds crusts, ferruginous endostromatolites and oncoidal ferruginous microstromatolites. The last are associated with ooidal bioclastic grainstone, ooidal bioclastic grainstone-packstone, bioclastic ooidal packstone-grainstone, oncoidal floatstone and rudstone, stromatolitic bindstone, bioclastic wackestone-packstone and bioclastic wackestone microfacies. The host mineral of the ferruginous microbialites is calcite, but microbially induced iron oxyhydroxides (goethite and magnetite) prevail in the ferruginous laminae. Petrographical and scanning electron microscope (SEM) investigations revealed that these ferruginous microstromatolites were formed by the activity of microbial mats dominated by putative bacterial and fungal filaments. Locations with reduced or no sedimentation, in relatively deep-water, open-marine shelf environments, below fair-weather wave base or near to storm wave base, within the deep euphotic zone, were favorable for the hardening of the seafloor and the development of the microbial mats. The scarcity of an autochthonous benthic fauna and of burrowing, as well as the presence of framboidal pyrite suggest dysaerobic conditions. In such an environment, iron would have been in its soluble state (Fe2+) and the activity of micro-aerophylic iron-oxidizing bacteria appears to have been particularly intensive at the dysoxic-anoxic interface, inducing the precipitation of iron oxyhydroxides and the formation of diverse ferruginous microstromatolites.