Abstract

Recents sediments have been sampled on the Pater Noster Platform during three operations.

All the sediments are medium to coarse grained carbonate sand. On the reef islands and in their vicinity sands are made of coral fragments, red algae, Molluscans and Foraminifera (mainly Calcarinidae and dominating Amphistegina).

Between the reef complexes, in the wide channels where currents are active, sediments are formed by benthic Foraminifera (up to 90 %) with minor percentages of green algae, Bryozoa, Echinoderma and Molluscans. In the Foraminifera the most represented forms are Calcarinidae, Soritidae and Elphidiidae; Alveolinella and Miliolidae are frequent. Near the great central platform, Operculinidae are in increasing percentage.

In some sheltered lows, calcareous algae (Halimeda) are very abundant and their fragments may represent 80% of the sediment.

In conclusion, on that carbonate platform, corals are just a source of bioclasts on the reef islands or in their close vicinity.

Everywhere else, due to the open marine circulation, the sand is formed mainly by forams, often the larger ones. It is one of the first models found on a large area, corresponding to the foraminiferal limestones of the past (Nummulite limestone for example).

On the oriental slope and medium deep terraces of Massa Lima, sediments are rich in planktonic Foraminifera and in Coccoliths, glauconite may be abundant. This facies may be taken as a model for the glauconitic chalk of the past. Often, this facies is disturbed by turbidites made of shallow calcarenites from the platform, with benthic forams and algal debris.