Palynological Age Control and Paleoenvironments of the Paleogene Strata in Eastern Dahomey Basin, Southwestern Nigeria

Paleogene deposits are extensively exposed in the Eastern Dahomey (Benin) sedimentary Basin in southwestern Nigeria. Outcrop logging and sampling of lithological sections were systematically carried out at the Ibese Quarry, in order to re-establish the age of sediments and reconstruct their depositional environment using samples rich in organic-walled microfossils. Two formations were recognized; the Ewekoro and Akinbo Formations. Two main lithological units were identified; limestones and carbonaceous shales. Minor lithologies include ferrugineous sandstones and glauconite. Results from the palynological examination show that terrestrial palynomorphs (spores and pollen) dominate over the marine dinoflagellates cyst species in the samples recovered from the Ewekoro Formation, while samples from the Akinbo Formation recorded high abundance with less diversity of the marine dinoflagellates cysts over the terrestrial sporomorph. Age determination/correlation was achieved based on selected stratigraphic index taxa recovered. The samples from Ewekoro Formation were dated as late Early Paleocene to early Middle Paleocene, based on the presence of the following pollen key-taxa: Proxapertites operculatus, Retidiporites magdalenensis, Spinizonocolpites baculatus, Mauritidiites crassiexinus, Scabratriporites simpliformis, and Echitriporites trianguliformis. The samples from the Akinbo Formation were assigned late Middle Paleocene to late Paleocene, based on the presence of the following pollen key-taxa: Proxapertites operculatus/cursus, Grimsdalea polygonalis, Retibrevitricolpites triangulatus, Psilatricolporites operculatus, Retistephanocolpites williamsi, Bombacidites sp., Apectodinium homomorphum, and Apectodinium quinquelatum. The environmentally significant palynomorph species indicated differences in paleodepositional environments, ranging from shallow marine in the Ewekoro Formation at the base, up the stratigraphic sequence, to marginal marine (estuarine) depositional environment in the overlying Akinbo Formation.

Several works, including [15][16][17][18] , have discussed the stratigraphy of Dahomey Basin. The basin consists of Late Cretaceous to Cenozoic sediments of about 2100 m thick, but thickens markedly into the offshore and thins beneath the deep-water area. The Nigerian sector of the Benin Basin thins into the Basement Complex of the Okitipupa ridge. According to 19 , the basin was cut off from the eastern side, i.e. the Anambra Basin and the Niger Delta Basin by a ridge of the crystalline basement (the Okitipupa ridge). The Dahomey Basin fill can broadly be classified into two: the Cretaceous Abeokuta Group, which comprises of the Ise, Afowo, and Araromi; and the Cenozoic units consisting of Ewekoro, Akinbo, Oshosun and Ilaro Formations 17,20 (Fig. 3).
The Ewekoro Formation is traceable over a distance of 320 km continuously from east of Accra, eastward through the whole of the Dahomey Basin 21 . It is a WNW-ESE trending unit, and consists of light brown to grey arenaceous limestone, with fragmentary bioclast and terrigenous material increasing towards the base 22 .
Overlying the Ewekoro Formation is the Akinbo Formation, which consists of shale and clay, and a basal glauconitic band, which forms a prominent horizon that separates it from the topmost unit of the underlying Ewekoro Formation 20 . The Akinbo Formation is overlain by Oshosun Formation, which consists of phosphate-rich, green-grey clay and shale with sandstone interbeds 7 . Overlying the Oshosun Formation are the Ilaro and Benin Formations, which are characterized predominantly by coarse-grained sandstone; and sandy beds of estuarine, deltaic and continental origins.

Description of stratigraphic sections
The study area is delimited by latitudes 06°59.440′N and 07°00.75′N, and longitudes 003°01.614′E and 003°01.692′E. It lies within the Cenozoic sequence in the Eastern Dahomey (Benin) Basin, southwestern Nigeria (Figs. 1 and 2). Two formations (Ewekoro and Akinbo), were exposed in the studied area. The sections (Figs. 4A,B and 5A,B) exposed at the Ibese Quarry comprise of two main lithological units, which include limestones and carbonaceous fissile shales, interbedded with a glauconitic band and ferruginized sandstones. The outcrop logging and sampling were systematically carried out from base to top. A total of 12 field samples, distributed as follow, were collected and studied: Ewekoro Formation (8 samples) and Akinbo Formation (4 samples) (Fig. 2). The sections are described as follows: Unit 1: Northern Mine (NM). This section is exposed in the Northern Mine (NM) at the Ibese Quarry  www.nature.com/scientificreports www.nature.com/scientificreports/ macrofossils. This is overlain by 3 m thick hydrated, dirty white limestone, with few macrofossils, followed by 2 m thick white limestone. Overlying this unit is 1 m thick green glauconitic band, which has a sharp contact with the underlying limestone unit. Above the glauconitic unit is about 10 m thick sequence of grey to dark, fissile shale, interbedded with approximately 1 m thick ferruginized sandstone unit. The fissile shale forms a sharp contact with the underlying glauconitic unit (Fig. 5A,B).

Methods
The methods employed in the study included laboratory sample processing and transmitted light microscopic logging. Twelve (12) samples of limestone and shale were subjected to palynological sample processing for their palynomorph content ( Table 1). The sample preparation was carried out, using the conventional method of acid maceration technique for recovering acid-insoluble organic-walled microfossils from sediments. Each sample was thoroughly cleaned to remove field contaminants. 10 g of each sample was weighed out in a standard weighing balance and gently crushed with agate mortal and piston. The crushed sample was digested for 30 minutes in 40%  www.nature.com/scientificreports www.nature.com/scientificreports/ concentrated hydrochloric acid for removal of carbonate, and 72 hours in 48% concentrated hydrofluoric acid to remove silicates. The digested sample was diluted with distilled water and sieve-washed through 10 microns nylon mesh. The sieve-washed 10 g residues equivalent was partitioned into two parts, 5 g each, for oxidation and for kerogen assessment. The 5 g residue extract was oxidized for 30 minutes in 70% conc. HNO 3 and 5 minutes in Schulze solution to render the fossils translucent for transmitted light microscopy. The acid-free oxidized residue was rinsed in 2% concentrated KOH solution to neutralize the acid; and swirled to remove the resistant coarse mineral particles and undigested organic matter. The swirled residue was collected on the sieve and stained with Safranin -O to increase the depth of contrast for microscopic examination and photography.
The stained residue (aliquots) was dispersed with polyvinyl alcohol, dried on cover-slips and mounted in petro-poxy resin. One slide was made from each sample, scanned and logged under the transmitted light microscope. Light photomicrographs were taken with a Leica III binocular microscope. Table 1, shows the absolute occurrence and distribution of palymomorph counts present in the examined samples from the Units 1 and 2. The carbonaceous fissile shale samples recorded moderately rich palynomorph assemblage while the white indurated limestone samples yielded moderate to low palynomorph counts. The claystone (NM02), argillaceous limestone (SWM02), glauconite (SWM 05), and ferruginized sandstone (SWM07) samples were barren of microfossils ( Table 2). The white indurated limestone samples recorded more terrigenous (spores and pollen) species than the marine dinoflagellate cysts. Among the terrigenous species, pollen was the most abundant and diverse. The fissile shale samples also produced more terrigenous species (52-53%) than the marine microplanktons (48-47%). The pollens (39-41%) predominate over spores (12-13%) and were also more abundant and diverse (Fig. 8). The freshwater algal spores yielded very low count. Among the marine dinoflagellate species, the peridiniacean species, with proximate cyst affinity, were more abundant and diverse than the gonyaulacaceans species (Table 1) (Figs. 6 and 7).

Summary and conclusion
Palynological study of the Paleogene succession exposed at the Ibese quarry, within the Eastern Dahomey (Benin) Basin has been undertaken. Two formations, the Ewekoro and Akinbo, were identified in the area of study. The result from the palynological analysis of collected field samples revealed a late Early Paleocene -early Middle Paleocene age for the Ewekoro Formation, and late Middle Paleocene -late Paleocene for the Akinbo Formation. Palynomorphs of environmental value indicated differences in paleodepositional environments,