BIOSTRATIGRAPHIC CHARACTERIZATION OF THE CRETACEOUS DEPOSITS OF THE SAN-PEDRO MARGIN OF DRILL HOLES K1 AND K2 OF BLOCK KL: PALEOENVIRONMENT IMPLICATION

The KL block studies was carried out the eastern part of the San Pedro margin, it has an area of 2034 km 2 with a water depth varying from 500 to 2750 m with two probings (K1 and K2).The objective of this work is to carry out a biostratigraphic and paleoenvironmental study based on the associations of planktonic and palynomorphicmicrofauna in the formations of the KL block boreholes. From a lithological point of view, the base of the boreholes generally comprises alternating limestone and argillite, very fine to fine grained quartz sandstone. Its upper part is overlain by claystoneinterbedded

The KL block studies was carried out the eastern part of the San Pedro margin, it has an area of 2034 km 2 with a water depth varying from 500 to 2750 m with two probings (K1 and K2).The objective of this work is to carry out a biostratigraphic and paleoenvironmental study based on the associations of planktonic and palynomorphicmicrofauna in the formations of the KL block boreholes. From a lithological point of view, the base of the boreholes generally comprises alternating limestone and argillite, very fine to fine grained quartz sandstone. Its upper part is overlain by claystoneinterbedded with limestone, silstone and siliceous cemented sandstone and alternating claystone, medium to coarse grained sand and siliceous cemented quartz sandstone. The Albian is determined by the presence of the species Ticinellamadecassiana. The Cenomanian is identified by the micropalaeontological assemblages composed of Globigerinelloides spp., Guembelitria spp., Hedbergella spp., Hedbergelladelrioensis, Globigerinelloides bentonensisandLoeblichella cf. hessi. The Turonian is based mainly on the species Hedbergellaplanispira, Heterohelixmoremani and Whiteinella archaeocretacea. The Early Senonian is characterized by associations of species (Hedbergellasp, Hedbergella cf. delrioensis, Buliminacrassa and Whiteinella baltica) and palynomorphs (Proteaciditestienabaensis, Odontochitinacostata, Odontochitinaporiferaand Tricolpites sp). The roof of the Campanian is known by the association of the palynomorph (Trichodinium castanae) and the microfossil (Gaudryina cretacea) The Maastrichtian is highlighted by the associations composed of species Rzehakina epigona fissistomata, Rzehakina minima, Plectina lenis, Reophax duplex, Reophax pilulifera, Reophax globosus, Gaudryina pyramidata and Afrobolivina afra) and palynomorphic species (Andalusiella gabonensis, Cerodinium granulostriatum and Palaeocystodinium australinium). All the micropalaeontological data coupled with those of the microfaunas make it possible to envisage a depositional environment of the internal platform type with continental influence on an external platform.
The oil fields exploited in the Ivorian Offshore Basin have as reservoir rock, generally, sandstone banks of Cretaceous age (Bié, 2012, Koffi 2017). Therefore, the biostratigraphic characterization of the KL block will allow us to determine the ages of the geological formations east of the San-Pedro margin of the Côte d'Ivoire offshore basin, which are still poorly known.
These ages are determined on the basis of the first appearance of certain forms associated with the last appareance of other species in the direction of sedimentation.

Geological overview-
The Ivory Coast, with a surface of 322462 Km 2 belongs to the old platform of Zest Africa. It is formed by two geological units of unequal surface that are: -the base of Côte d'Ivoire belonging to the West African craton and more precisely to the Man ridge. It includes the Archean domain in the West and the Paleo-Proterozoic domain in the East (Bessoles, 1977).
-The sedimentary basin occupying the southern part of the territory is composed of continental and marine deposits. It owes its origin to the opening of the South Atlantic in the Lower Cretaceous 106 Ma ago. This basin is crossed by two major faults of ENE-WSW direction. It represents more than 3/5 of the coastline and is divided into a crescentshaped onshore area and an offshore area which is subdivided into two margins (Abidjan and San-Pedro) where oil exploration takes place (Aka, 1991; Monde, 1997).

Study area-
Ours study area is located in the eastern part of the San Pedro margin in the KL block ( Figure 1). This block an area of 2034 Km 2 with a water depth varying from 500 to 2750 m with two(2) holes (K1 and K2): 1. Borehole K1 is located to the North -East of the KL block. It is a submerged hole with a final depth of 3592 m. 2. Hole K2 located to the South-West of the KL block. The final depth of this submerged hole is 4556m.

Materials And Methods:-
The material used consists of 190 samples distributed as follows: 90 for the K1 survey and 100 for the K2 survey.

Micropalaeontological method-
A quantity of approximately 40g of sample to be studied is taken (and crushed if necessary in a mortar). This sample is then poured into a plastic beaker which is marked with the number of the sampled cuttings.
Each sample is then attacked with hydrogen peroxide, i.e. 10% hydrogen peroxide. The mixture (hydrogen peroxide + sediment) is stirred with a rod until the reaction is complete. The product is left to stand for an average of 12 hours.
There follows a series of preparations for the study which consists of washing, drying, bagging the samples and sorting the microfossils.
Palynological study methods-Each sample of excavated material will undergo a palynological preparation which consists in destroying all the mineral phases to keep only the organic phase generally constituted of sporopollinic or palynomorphic material.
The technique used is the standard technique of palynology laboratories using strong acids (HCl and HF) to extract the palynomorphs. The laboratory preparation for palynological analysis begins with sampling, which consists of taking 20g of sample and putting it in a 2-liters beaker numbered according to a particular dimension, thus avoiding errors. Then comes the chemical treatment phase which aims to eliminate all the mineral matter contained in the sediments. Finally, palynological slides are made.

Dating by stratigraphic fossils-
It is based almost exclusively on the microfossils and palynomorphs contained in the formations. This is possible thanks to the use of the distribution scale of planktonic foraminifera (Figure 2, 3 and 4) of Caron (1985); as well as the work of palynology of Digbehi et al. (1997).

Biostratigraphic species and depositional environments-
The presence or absence of foraminifera can give information on: -The position of the deposit in relation to the shoreline; -The proximity of the shoreline, the depth; -The physico-chemical conditions of the sea water (salinity, oxygenation and temperature). The morphology of planktonic foraminifera can also provide information on the paleoenvironment (Caron, 1983).

Biostratigraphy of the Cretaceous formations-Lower Cretaceous-
The Lower Cretaceous is marked by a single stage, the roof of the Albian in borehole K1 was identified at 3414 m by the last appearance of the species Ticinella madecassiana. The associations of microfaunal species (Ticinellaprimula  and  Ticinellamadecassiana), and palynological species (Afropollisjardinus, Callialasporitesdampieri, 0ligosphaerdium Complex, Classopollisbrasiliensisand Classopollisclassoides, Vitreisporitespustulosus, Elaterosporitesprotensus) justify this age. The Albian is located in the interval 3595 to 3414 m ( Figure 2).
292  Cenomanian-Cenomanian roof of borehole K1 was identified by the last appearance of the palynomorph Classopolisclassoides at elevation 3344m. The abundance of small planktonic foraminifera dominated by the genus Hedbergella and Classopollis spp. Classopollisspinosus, Cretaceaporitesmulleri and Cupuliferoidaepollenitesminutus justify a Cenomanian age located between intervals 3414 and 3344 m.

Turonian-
It is not highlighted in hole K1 due to the lack of biostratigraphic markers.
Turonian roof in hole K2 is highlighted by the presence of the palynomorph Foveotricolpitesgiganteusat 4370m. Also the presence of the planktonic foraminifer Hedbergellaplanispira justifies the existence of Turonian age between 4391 and 4370m. 294 Early Senonian-The identification of the Early Senonian in Hole K1 is essentiqlly based on palynomorphs. The top of this age is highlighted by the last appearance of the species Oligosphaeriduim complex at 3271 m. Palynological associations such as Odontochitinacostata, Oligosphaeridium spp., Ariadnaesporitesverrucatus attest to a Early Senonian age between 3344 and 3271m.
The roof of Early Senonian is determined by the disappearance of the palynomorph Oligosphaeridium sp. at 4316 m in borehole K2. Palynological (Cretacaeporitesmuelleri, Oligosphaeridium complex, Hexaporotricolpitesemelianovii) and micropalaeontological (Neobuliminasubregularis) associations attest to aEarly Senomanian age of the 4370 to 4316m interval.

Campanian-
Campanian in borehole K1 is essentially based on the association of the palynomorphs and a few microfauna. Indeed, the last appearances of palynomorphs identified at elevation 2868m such as Subtilisphaera spp. and Alterbidinium minus allowed the identification of the roof of this zone. This interval consists of an association of palynomorphs such as trichodinium castanae, Xenascusceratioides, Achomosphaeraramuliferagabonense, Spinidiniumdensispinatum and benthic calcareous and agglutinated foraminifera Afrobolivina afra, Buliminellaquadrilobata, Plectina lenis, Siroplectamminanavarroana, and Reophax globosus. A Campanian age is assigned to the interval 3271 to 2868 m.

Maastrichtian-
The last appearance of the palynomorph Dinogymniumspp affected the Maastrichtian roof at elevation 2750 m. Palynological association consisting of Proteaciditessigali, Palaeocystodinium gabonensis, Palaeocystodinium Australinum, Andalusiella mautheiSenegaliniumcerodinium, Senegalinium spp. EchitriporitestrianguliformisandDinogymniumacuminatun support this age. Maastrichtian ranges from 2868 to 2750 m in borehole K1.Maastrichtian is 180 m high in borehole K2 and shows no micropalaeontological markers. The palynological species have allowed us to highlight the roof of this stage. Indeed, at elevation 3510 m, the last appearances of the palynomorphs Senegaliniumbicavatum, Palaeocystodinium australinum and P. gabonense indicate the Maastrichtian roof. In addition, an association of the palynological species Palaeocystodinium species, Palaeocystodiniumgolzowense, P. lidiae, Gabonisporisvigourouxii and Alterbidiniumminud characterizes this age. Maastrichtian is located in the interval 3690 to 3510m. 296 Correlative analysis of Cretaceous drill holes-Boreholes K1 and K2 are located withim an anticline system in the KL block of the San-Pedro margin ( Figure 6). They are located to the northeast and southwest, respectively. More specifically, K1 is located on the hinge and K2 is located on the western flank of the anticline. However, a correlative lithostratigraphic West-East analysis can be performed.
-Albian of holes K1 and K2 has a similar style of deposition. It is marked by alternating argillite, sand, and minor proportions of limestone with sandstone interlayers (K2). The accessory minerals present are carbonaceous debris and pyrites. All its lithological characteristics are typical of shallow and reducing environments with variable energy. From west to east, terrigenous contents are low on the western flank (hole K2) and high on the hinge (hole K1). The thickness of the Albian is variable (93-181m). It is thick in the North-East with a thickness of 181m and narrows in the South-West (99m). -Cenomanian, its thickness increases from West to East with relatively equivalent thicknesses for K1 and K2 (about 70m). It is characterized by an alternation of sandstone, claystone and limestone in all the boreholes.
-Turonian is absent in borehole K1 and is present in a flake in borehole K2 where it is composed of mudstones with limestone layers. As Turonian is poorly represented in the entire Ivorian basin, its absence in borehole K1 could be explained by Turonian regression which strongly eroded the deposits of the Ivorian margin and could accentuate the erosion or non-deposition phenomena thus creating an unconformity. -Early Senonian, its thickness increases from west to east. It is composed of alternating limestones and argillites. In addition to these two formations, sandstones and sands are encountered in the K1 boreholes. The presence of claysandstone deposits (K1) and the local enrichment of limestone (K2) are possibly related to the regression (fall in sea level) that took place during the post-rift phase of the Ivorian sedimentary basin. This drop caused a strong erosion of Early Senonian deposits.
-Campanian, has a lithology consisting of alternating sandstone, sand and mudstone. Its thickness decreases from west to east. -Maastrichtian, is essentially clayey with sandstone and limestone in all the drill holes. Accessory minerals such as pyrite and glauconite are present in this stage. It is characterized by a rise in sea level due to eustatic variations, attested to by the abundance of mudstones in this zone. Moreover, the presence of glauconite confirms this hypothesis, because it develops in open marine environment and far from active sedimentation zones, preferably during long periods of non-deposition due to relative sea level rise. Table I highlights the synthesis of these stages.  The absence of benthic foraminifera indicates unfavorable conditions for life on the bottom. This microfauna, which is not very diversified, is made up of Hedbergella, which are organisms that develop and invade the inner to middle shelf where they can withstand variations in salinity and temperature (Leckie, 1987). The presence of limestone topped by alternating thick mudstone and thin sandstone beds would show that the energy of the environment is variable. The presence of glauconite and pyrite confirms that the depositional environment is marine and reductive. All these characteristics concern holes K1 and K2. The depositional environment during Cenomanian would vary from internal platform to medium and reductive.

Paleoenvironment of Turonian deposits-
It is essentially clayey with a limestone background with the presence of glauconite and carbonaceous debris, which would reflect a low-energy marine environment with continental influence. The microfauna and palynoflora are in poor condition. The depositional environment would be of the internal platform type with a condition influence for hole K2.

Paleoenvironment of EarlySenonian deposits-
The presence of pyrite, glauconite and limestone would indicate a deep and reductive marine environment (boreholes K1 and K2) with conditions less favorable to the proliferation of microfauna, hence the near absence of both benthic and planktonic species in this stage. This interval is represented by a low thickness; this could be due to erosion. This erosion of Early Senonian probably occurred during the early oceanic expansion phase with the first frank marine trangression (Cenomanian -Early Senonian) (Chierici, 1996). The depositional environment would be between the internal and extrenal platform.  Maastrichtian is composed of the palynological genera that are: Senegalinium and Palaeocystodinium typical of this age. It appears from the work of Bie (2012) that some authors such as Oloto (1989) and Jain and Millepied (1975) have shown that the species Palaeocystodinium australiniumis a characteristic species of Maastrichtian in Senegal and Nigeria. In addition, Helene and Cabrera (2003) stated that the species Senegaliniumbicavatum, Palaeocystodinuimaustralinumand Palaeocystodinium golzowense are good markers of the Late Maastrichtian.

Conclusions:-
The biostratigraphic characterization of the San-Pedro margin has highlighted the Lower and Upper Cretaceous stages. These stages range from Albian to the Maastrichtian. All stages are present with the exception of the Turonian in borehole K1.
Lithological correlations based on facies show that the KL block of the San Pedro margin is composed of five (5) main lithological facies, namely: a clay facies, a sandy facies, a sandy facies and a limestone facies. Clay, sand and sandstone dominate the lithologies described. They occur in the from of alternating layers and the terrigenous formations (sandstons and stand) encountered date from Albian to Campanian for holes K1 and K2.