UNIV ROUEN,DEPT GEOL,F-76821 MONT ST AIGNAN,FRANCE
Oceanologica Acta (0399-1784) (Gauthier-Villars), 1995 , Vol. 18 , N. 3 , P. 289-307
The phosphorite levels present in the ODP cores of the Oman margin are of two types: (a) sediment levels dominated by phosphate grains with a few white to light-brown nodules; these are sandy and predominantly occur in Upper Miocene to Early Pliocene sediments; and (b) sediment levels poor in phosphatic grains but consisting of several brown to dark-brown nodules; these mainly occur in Late Pliocene to Early Pleistocene sediments wherein some levels are sandy while others are clayey. The phosphate grains occur as coprolites, faecal pellets, spherical and coated grains, micronodules, foraminifer infillings and bone fragments. Ovoid to rod-shaped and botryoid-type apatite microparticles resembling phosphatized bacteria or coalesced bacteria are common in porous areas of all types of phosphate grains and nodules. Phosphatized extracellular remnants such as polyhedral cell units and spherical cavities are also present. The compact structures seem to consist of tightly packed ovoids or botryoids and/or to be formed by the overgrowth of phosphate. The initial morphology of the grains/nodules was largely preserved; the light-coloured dull grains are more porous, heterogenous and appear less evolved, whereas dark brown shiny grains are mostly compact, tend to be homogeneous and appear highly evolved. It appears that phosphatization took place within the initial supports and was controlled by microenvironments, duration and source metals. The sediment levels dominated by phosphate grains are bioturbated and associated with shallow water oxic conditions and lowest sedimentation rates. Bioturbation probably favoured the production of different initial substrates which subsequently phosphatized. Light-brown nodules are formed by a rapid, early diagenetic process. The abundant nodule formation in the Late Pliocene - Early Pleistocene sediments is favoured by the deepening of the Oman margin which took place during the Late Pliocene and the establishment of an oxygen minimum zone at about this time. Unlike the Peru margin phosphorites, the Oman margin phosphorites lack conglomeratic nodules, phosphorite crusts formed in hardgrounds and thick phosphatic sandy beds and glauconites. Fe-recycling is not important in the formation of Oman margin phosphorites. The reworked nature of organic matter, the less pronounced oxygen minimum zone and high sedimentation rates are probably responsible for the apparent absence of present day phosphorites on the Oman margin.