The Dahomey Embayment spans the continental margin of the Gulf of Guinea, covering the Volta delta in Ghana to the west and the Okitipupa ridge/Benin hinge line to the east [8] [9] . It’s a marginal pull-apart basin or marginal sag basin [10] that developed in the Mesozoic sequel to the separation of African from the South American plates [11] [12] . This separation, accompanied by basement fracturing accounted for the early rifting stage during Jurassic to Early Cretaceous and the development of several marginal sub-basins [9] .

The eastern Dahomey Embayment (Figure 1) has been studied both on outcrop scale as well as from core holes by various workers such as [2] [4] [8] [9] [13] - [16] , amongst others. The stratigraphy of the Nigerian sector of the embayment can be broadly divided into two: the Cretaceous Abeokuta Group (comprising Ise, Afowo and Araromi Formations) and the Cenozoic units (comprising Ewekoro, Akinbo, Oshoshun, Ilaro, and Benin Formations) [2] [9] (Figure 2). However, this study is focused on the Ewekoro limestone Formation.

This formation is made up of fossiliferous shelly limestone of about 12.5 meters thick, which tends to be sandy at the base [1] . It has been divided into three micro-facies namely: the sandy biomicrite lower unit; the shelly biomicrite grading into biomicrosparite middle unit that consists mainly of pure limestone making up the bulk of the

Ewekoro Formation and; the shelly biomicrite and Algal biosparite upper unit [1] . It’s of Paleocene age based on fossil evidence (foraminifera and ostracods) and deposited in a shallow marine environment [16] [17] .

Field study revealed five limestone beds, labeled E to A spanning the 15 sections logged (Figure 1 and Figure 3); although a sixth bed, F (which is quite silty with large amount of quartz) was recognized belonging to the underlying Araromi Formation. Bed E is the oldest bed among the limestone units. It’s a light brown sandy algal bed with fossils embedded in sparry calcite cement. Bed D is light brown, shelly biomicritic and inter-fingered Bed E which is more friable, sandy algal biosparite, with the cementing material mostly sparry calcite. Bed C is dark grey, calcareous unit that comprised algal biomicrite facies. Bed B is grayish to brown in colour, hard with greater tenacity than the overlying bed and comprised sandy algal biomicrite embedded in micrite cement. Bed A is a red phosphatic limestone (typifying the intra-sparite facies) that is crystalline textured with localized quartz fillings within vugs and caves induced by migrating acidic fluids.

The concentrations (in wt% of oxides) of major elements in the limestone samples are shown in Table 1. The

limestone samples analyzed are enriched (>1 wt%) in CaO (33.7 - 59.99 wt%, average 47.29 wt%) and slightly depleted to enriched in SiO₂ (0.47 - 31.21 wt%), Al₂O₃ (0.47 - 3.23 wt%), MgO (0.39 - 3.15 wt%), Fe₂O₃ (0.51 - 3.43 wt%) and SO₃ (0.10 - 2.27 wt%). However, they are depleted (<1 wt%) in the alkalis (Na₂O and K₂O), TiO₂, MnO, P₂O₅ and Cr₂O₃. LOI varies between 17.64 - 45.10 wt percent. The values of CaO and MgO tend to decrease down the Formation with increasing silica content probably due to the presence of non-carbonate input transported from adjacent continental sources.

A marked inverse negative correlation exists between SiO₂ and CaO contents of the limestones (Figure 4). A generalized geochemical variation model for the area representing the SiO₂ distribution is depicted in Figure 5. A bed-by-bed spatio-temporal geochemical distribution of the concentrations of CaO and SiO₂ is discussed below in super-positional order.

The silica content varies widely (0.47 - 3.21 wt%, Table 1); an indication that the adjacent basement complex rocks of southwestern Nigeria may have sourced varied amounts of these detrital impurity and/or its dissolved component in the shallow marine environment. Geochemical plots (Figure 4) of SiO₂ against CaO for all the samples analyzed clearly revealed a similar and unique trend marked by increase in CaO with corresponding decrease in SiO₂ contents and vice versa. This SiO₂-CaO negative correlation can be attributed to chemical diagenetic replacement. At the shallow part of the upper continental crust where silica is readily made available through weathering and erosion, calcite reacts with it to form a calc-silicate at low temperature and pressure. Also, at all stages of diagenesis, dissolved silica (derived from dissolution of siliceous tests of marine organisms) replaces calcite. Fluvial silica input often augment dissolved silica in the basin at the unset of shallow marine condition. However, CaCO₃ production dominates as shallow marine conditions become fully established and silica supply from the hinterland reduces. Hence the higher the SiO₂ input, the more the continental influence, whereas the CaCO₃ production signals shallow marine incursion.

The low alumina content confirms a low index of weathering of the alumino-silicates such as feldspars and micas in the adjacent basement areas during transportation and deposition prior to diagenesis [18] . Fe₂O₃ is usually

derived from intense chemical weathering of heavy mineral such as the ferromagnesians. Its low value indicates that the environment of deposition is a reducing one that does not favour the precipitation of Iron (II) to Iron (III) and thus leached away [18] . SO₃ is low probably because anoxic conditions prevailed in such quiet, low energy environments and there is rapid rate of sulphate reduction.

Todd [19] presented a petrogenetic classification of carbonate rocks that involved the standard ratio, Ca/Mg and reciprocal ratio, Mg/Ca. The class limits of the standard ratio, Ca/Mg are: > 100 - 39.0, 39.0 - 12.3, 12.3 - 5.67, 5.67 - 1.86, 1.86 - 1.50, 1.50 - 1.22, and 1.22 - 1.00 expressed as limestone, magnesian limestone, dolomitic limestone, dolomitized limestone, calcareous dolomite, dolomite and magnesian dolomite respectively. Also, the class limits of the reciprocal ratio, Mg/Ca are: 0 - 0.03, 0.03 - 0.08, 0.08 - 0.18, 0.18 - 0.54, 0.54 - 0.67, 0.67 - 0.82 and 0.82 - 1.00 also expressed as limestone, magnesian limestone, dolomitic limestone, dolomitized limestone, calcareous dolomite, dolomite and magnesian dolomite respectively. The standard and reciprocal ratios of Ca and Mg composition of the Ewekoro limestone are shown in Table 2. More than 79% of the samples are classified as “pure” limestone and about 20% are magnesian limestone according to the method of [19] . The Ca/Mg ratio has implications for the stability conditions of the depositional environment that led to the formation of the carbonate(s) [20] . Naturally, the Mg/Ca ratio increases during evaporation of sea water, especially under saline environmental conditions. Considering the Ca/Mg and Mg/Ca ratios (Table 2) it can be concluded that the relative rate of evaporation of sea water and the palaeo-salinity condition was low, as such limestone was deposited more at the expense of dolomite. However, intermittent increase in rate of sea water evaporation and salinity resulted to the deposition of the few magnesian limestones.