oa Organic Matter Preservation and in Situ Humification in Phosphatic Pellets from Tunisian Phosphatised Sediments

The phosphatic "pellets", from Ypresian ore of Ras-Draâ, Tunisia, are rounded grains, with some hundreds of micrometers in mean diameter. These pellets are embedded in a sedimentary, more or less carbonated or shaly, matrix. The pellets studied in this ore contain between 28 and 38 % of P2O5 and between 0.30 and 1.62 % of TOC. Surprisingly – with regards to the age of the pellets – this organic matter (OM) is very rich in humics (>75.60 % of humic C). Curiously also, the OM present in the matrix of the pellets is by far not as rich in humic compounds (>21.42 % of humic C, % of TOC). The composition of the OM of the matrix is quite usual for a 50 My old, epicontinental, never buried (over a maximum of 200 m depth), sedimentary layer. It is a low-rank kerogen. Pellets appear therefore as chemically individual bodies whose diagenetic story has been quite different, and independent, of that of their embedding sediment. These bodies have been demonstrated to have derived from fish feces (Ben Hassen, 2007).