Biofabrics as Dynamic Indicators in Nummulite Accumulations

Biofabric interpretation of skeletal accumulations consisting of "biologically standardized" nummulite tests (larger foraminifera) is based upon (1) the ratio of small-bodied A-forms (megalospheric) and large-bodied B-forms (microspheric), and (2) on their packing and imbrication features. In the Eocene Mokattam Formation of Egypt, Nummulites gizehensis occurs in limestones with four basic biofabrics. In wackestones, the nummulites "float" throughout the matrix, and A-forms strongly dominate over B-forms, approximating the ratio reported for undisturbed assemblages. Packstones, however, reflect various degrees of sedimentary reworking and winnowing of the original substratum by physical processes. For packstones with edge-wise imbrication and A-forms strongly dominating over B-forms, in situ winnowing of fine-grained matrix by waves can be inferred. In contrast, packstones composed of imbricated B-forms suggest hydraulic sorting and selective removal of A-forms by currents. Packstones composed almost entirely of A-forms represent transport-sorted allochthonous deposits. Biofabric analysis thus allows us to understand better the dynamics of skeletal accumulations such as nummulite banks, whose commonly complex and extensive buildups can form hydrocarbon reservoirs.