Role of microbial processes in the genesis of Jordanian Upper Cretaceous phosphorites

Abstract

Abstract Samples representing granular and laminated phosphorites from the main Upper Cretaceous deposits of Jordan, have been examined by SEM for microbial structures. Globular clusters and sheaths, postulated to have formed by bacterial activity or possibly as inorganic precipitates, have been found only within cavities and in the cementing phosphatic material around phosphate grains of the granular phosphorites. These structures are interpreted to have originated in association with percolation of ground-water late in the history of the sediments, rather than as a consequence of primary grain-forming processes. Organic-rich phosphate grains are believed to have acted as a food source for the bacteria and are, therefore, the cause of the microbial structures and not the result of them. On the other hand, empty cylindrical and/or spherical sheaths, interpreted to be of primary, cyanobacterial (blue-green algae) origin, were readily observed in the laminated phosphorites. These phosphorites are interpreted to have been deposited as algal mats in shallow, subtidal to intertidal environments.