Nannostratigraphic Study of K/Pg Limit Formations of Cote d’Ivoire Offshore Sedimentary Basin of (West Africa)

Study of calcareous nannofossils in the interval 1044 -1445 m of ZNB-1X well located off Abidjan in the offshore part of Cote d’Ivoire sedimentary basin revealed a low diversity of nannoflora. The material used consistits mainlay of drill cuttings that undergone a three step classic treatment nanofloral study allowed to identify three intervals: Selandien (Middle Paleocene) characterized by species Toweius selandianus, Toweius crassus, Toweius rotondus Danian (Lower Paleocene) based on assemblage composed of Neochiastozygus modestus, Cruciplacolithus primus, and Thoracosphaera sp. and Maastrichtian reconized by the assemblage of species Micula murus, Cribrosphaera ehrenbergii, Retecapsa crenulata, Archangelskiella maastrichtiana, Quadrum sissinghi and Micula concava. This nannoflora study that also hights the K/Pg boundary associated with both lithological and calmetric data indicates an inner shelf depositional environment.


Introduction
In recent years PETROCI has developed new techniques to better characterize the boundaries of the stages and to establish a biostratigraphic model typical of the Ivorian basin. Some research works where carried out in palynology and micropaleontology; in particular, those reported by the recent work [1,2] even gave details on lamellibranchs, gastropods fauna as well as Cretaceous and Tertiary series. The present work is the few ones conducted in nanostratigraphy on the Côte d'Ivoire sedimentary basin after those of [3] realized on Ghana Côte d'Ivoire Ride. This work aims to answer to a number of scientific questions mainly (i) In what consist different lithologies crossed by wells and the characteristic nanofossils described with in different deposits. (ii) How evolved the depositional environments? The main objective is to establish the evolution of depositional environments based on analysis of calcareous nannofossils identified in the cuttings analyzed. More specifically, it will be necessary to: (i) compile the lithological and calcimetric assessment of the deposits; (ii) inventory all the nanofossils encountered and described in order to establish a local nannostratigraphic scale for this part of the Côte d'Ivoire offshore sedimentary basin and especially in the area located in figure 1.

Material and Methods
The material used consists exclusively of drill cuttings on which nannofossils extraction standard technics were carried out using main conventional steps. 1 to 2 g were sampled, dried in an oven at 80°C and then ground in an agate mortar and stored in a pulp. A pinch of the sample is spread on the lamella by adding a water drop using a peak until the wavelets are homogeneously distributed on the lamella. The slide is dried using a hardened adhesive (loctite 835) by depositing blade and slide under a UV lamp. The blades thus obtained were studied using a Nikon petrographic microscope (Ellipse LV100 POL). Nannostratigraphic interpretations were based on the first and last appearances of the chronostratigraphic biozonal species used according [4, 11and 14] modified by [5] for Cretaceous and that of [6] for Tertiary.

Lithostratigraphy
The interval 1061 to 1445 m of ZNB-1X well is composed of a single lithological unit with very glauconitic clays with carbon debris and with intercalation of limestone (figure 2). The limestone (CaCO 3 ) content in the samples is relatively low with a peak at 1308m.
The Selandian: (1079-1116 m) was attributed by the presence of Toweuis selandianus, a characteristic species (Martini Top NP4-NP5, 1971) and associated here with two other Toweius rotundus and T. crassus.

Discussion
In recent years, significant progresses were recorded in biostratigraphy, leading to the establishment of several scales, in particular those [4,5,6 and 13] of synchronous biozonations we have a global scale. Indeed, the discovery of the Cretaceous / Palaeogene (K/Pg) boundary with in the present studied area is supported by the work carried out by [7] south-east of Petén basin (Guatemala). Generally, Toweius selandianus, Toweius crassus, Toweuis rotondus, characteristic of the Paleocene (Selandian) encountered with in Cote d'Ivoire basin are markers of this interval in many other basins. The Paleocene was identified by the presence of Cruciplacolithus intermedius and Neochiastozygus modestus as described by [8] at the base of Danian. In the offshore basin in Norwegian Sea, works of [9] revealed the Danian base through the assemblage of Biantholithus sparsus, Thoracosphaera sp. and Prinsius dimorphosus. Works carried out on Cote d'Ivoire offshore basin (unpublished internal report, Petroci) showed that Neochiastozygus perfectus and Fasciculithus bitectus species characterized lower Palaeocene. Maastrichtian was identified by [12] Micula murus, Cribrosphaerella ehrenbergii, Retecapsa crenulata, Archangelskiela maastrichiana in Cote d'Ivoire basin, whereas in Tunisia this stage was revealed by the association composed of Micula prinsii, Acuturris scotus, Tranolithus exiguus, Biscutum notaculum, Ceratholithodes aculeus, Microrhabdulus decoratus. In addition, work in the Brazilian Basin also show that the species Micula staurophora, Cribrosphaerella ehrenbergii, Prediscosphaera cretacea, Quadrum sissinghii and Watznauria barnesiae, Lithraphidites quadratus and Micula murus indicate Maastrichtian as well as present results. Despite some differences in the assemblages composition of the nannoflora, the most important species described in this work are known and described in many peri-Atlantic basins. The Cretaceous / Paleocene boundary corresponds to periods of climatic and oceanographic changes and coincides with major biological crises. This idea is shared through works of [10] performed on basins of Cameroon, Gabon, Senegal and Nigeria indicating that the associations characterizing the Upper Cretaceous are totally different from the Palaeocene species. The complete disappearance of species at the end of Maastrichtian could be one of the consequences of this biological crisis.

Conclusion
The study of the Calcareous nannofossils from the interval 1061 to 1445 of the ZNB-1X well lead to record a nannoflora of low abundance and diversity. This study allowed to propose a nannostratigraphic scale and to determine the depositional environments. The lithology of the interval studied consists of a single unit composed of very glauconitic clay with carbonaceous debris and limestone intercalation clay. The nannoflora reveals the Cretaceous / Palaeogenic boudary and defines three stages: