NANOSTRUCTURE AND SIGNIFICANCE OF FISH COPROLITES IN PHOSPHORITES

Phosphatic coprolites (0.5-2 cm long) occurring in Cretaceous-Eocene p hosphorites of Tunisia, Morocco, Mauritania, Senegal, and Egypt were i nvestigated for their petrology by using SEM. They exhibit a homogeneo us porous apatite structure with a few fish remains. The nannostructur es of the coprolites consist of inframicron-sized and botryoid-type ap atite microparticles. Spherical cavities and cavity-infilled inframicr on-sized apatite globules are typical in the coprolites and are appare ntly formed by extracellular precipitation of phosphate around microbi al organisms. We presume that the coprolites studied here may belong t o fishes, whose excrements contain abundant organic matter and phospha te. Phosphatization of excrements appears to be a microbial process co ntrolled by the microenvironment. The nannostructures observed in the coprolites investigated here differ significantly from the nannostruct ures of other studied phosphatic grains (pellets and coated grains) in the same samples which are mostly heterogeneous and consist of ovoid- type apatite particles. In the case they are not reworked, the centime tre to decimetre thick coprolite beds in phosphorites indicate a lack of detrital input and strong bottom water currents at the sediment-wat er interface during deposition and subsequent phosphatization of the e xcrements.