PHOSPHORITES FROM THE OMAN MARGIN, ODP LEG-117

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 l evels poor in phosphatic grains but consisting of several brown to dar k-brown nodules; these mainly occur in Late Pliocene to Early Pleistoc ene sediments wherein some levels are sandy while others are clayey. T he phosphate grains occur as coprolites, faecal pellets, spherical and coated grains, micronodules, foraminifer infillings and bone fragment s. Ovoid to rod-shaped and botryoid-type apatite microparticles resemb ling phosphatized bacteria or coalesced bacteria are common in porous areas of all types of phosphate grains and nodules. Phosphatized extra cellular remnants such as polyhedral cell units and spherical cavities are also present. The compact structures seem to consist of tightly p acked ovoids or botryoids and/or to be formed by the overgrowth of pho sphate. The initial morphology of the grains/nodules was largely prese rved; the light-coloured dull grains are more porous, heterogenous and appear less evolved, whereas dark brown shiny grains are mostly compa ct, tend to be homogeneous and appear highly evolved. It appears that phosphatization took place within the initial supports and was control led by microenvironments, duration and source metals. The sediment lev els dominated by phosphate grains are bioturbated and associated with shallow water oxic conditions and lowest sedimentation rates. Bioturba tion probably favoured the production of different initial substrates which subsequently phosphatized. Light-brown nodules are formed by a r apid, early diagenetic process. The abundant nodule formation in the L ate Pliocene - Early Pleistocene sediments is favoured by the deepenin g of the Oman margin which took place during the Late Pliocene and the establishment of an oxygen minimum zone at about this time. Unlike th e Peru margin phosphorites, the Oman margin phosphorites lack conglome ratic nodules, phosphorite crusts formed in hardgrounds and thick phos phatic sandy beds and glauconites. Fe-recycling is not important in th e formation of Oman margin phosphorites. The reworked nature of organi c matter, the less pronounced oxygen minimum zone and high sedimentati on rates are probably responsible for the apparent absence of present day phosphorites on the Oman margin.