Nannostructure and significance of fish coprolites in phosphorites

Abstract

Phosphatic coprolites (0.5-2 cm long) occurring in Cretaceous-Eocene phosphorites of Tunisia, Morocco, Mauritania, Senegal, and Egypt were investigated for their petrology by using SEM. They exhibit a homogeneous porous apatite structure with a few fish remains. The nannostructures of the coprolites consist of inframicron-sized and botryoid-type apatite microparticles. Spherical cavities and cavity-infill cavity-infilled inframicron-sized apatite globules are typical in the coprolites and are apparently formed by extracellular precipitation of phosphate around microbial organisms. We presume that the coprolites studied here may belong to fishes, whose excrements contain abundant organic matter and phosphate. Phosphatization of excrements appears to be a microbial process controlled by the microenvironment. The nannostructures observed in the coprolites investigated here differ significantly from the nannostructures 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 centimetre to decimetre thick coprolite beds in phosphorites indicate a lack of detrital input and strong bottom water currents at the sediment-water interface during deposition and subsequent phosphatization of the excrements.