Original paper

The selachian fauna (sharks and rays) of the phosphate series of Ndendouri-Ouali Diala (Matam, Western Senegal): Dating and paleoenvironmental interests

Sambou, Bernard Siguendibo; Sarr, Raphaël; Hautier, Lionel; Cappetta, Henri; Adnet, Sylvain

Abstract

The phosphate series of Matam (located at the North-East of Senegal) consists of a 10 meters alternation of soft phosphate levels, sand horizons and phosphatic limestones. While they were only mentioned in some preliminary geological works, isolated teeth of selachians are nevertheless common in these phosphate deposits. Detailed fieldwork of 9 new well-drillings allowed us to establish the first exhaustive list of the Matam selachian fauna with their biostratigraphical distribution through five main fossiliferous levels spanning the Thanetian-Early Lutetian period. Their importance for biochronological, paleoenvironmental purposes and correlations with the other Eocene phosphate deposits worldwide is noteworthy. The Matam selachian is represented by 38 species belonging to 34 genera, 16 families and 6 orders, which allow us to define different biochronological associations. The first association (e.g.characterized by Eotorpedo hilgendorfi and E. jaekeli) is associated with the sands of the top of Gorgol Fm. that constitute the ground level of exploited phosphate deposits and testify of a clear marine influence during the Late Thanetian. The three following associations were recovered in the main levels of exploited phosphate of the Matam Fm. They contain a richer and more diversified fauna dated to ?Lower Ypresian and are characteristic of neritic environments too. The last associa- tion (top of the Matam Fm.) overlies the phosphate layers and is characterized by a drastic decrease in selachian richness and diversity. Dated to the Lower Lutetian, this last faunal association may be indicative of a marine embayment in Matam area and contrary to western Senegal where phosphate deposits begin. If all these associations appear typical of the eastern margins of central Atlantic, the light observed faunal turnover across the ten-meter sequence suggests productivity changes in phos- phate depositions during the Late Thanetian-Early Lutetian period.

Keywords

paleoenvironmentmatamselachiansbiostratigraphyphosphates