Evaluating The Stratigraphy and Sub-Surface Structure Of Quaternary Sediments in Eastern Dahomey Basin, Southwestern Nigeria Using Electrical Resistivity Tomography

Abstract

The stratigraphic and structural delineation of the quaternary coastal deposit was carried out around  Ilaje community and its environs, eastern dahomey basin southwestern Nigeria with an intent to generate  subsurface lithologic map and depositional  attitude of the siliciclastic sediments around  the coastline. In order to predict the paleo-environment nature of the  quaternary deposit along the coastline understudy. 2-D resistivity data were acquired along two traverses in the study area. The dipole-dipole (Wenner) electrode array was engaged along the two traverses of length 300m each. The electrode separation ‘’a=10’’ and the number of layers ‘’N=5’’ were adopted. The center points of the four electrodes were plotted at each movement for N=5 and the pseudo-section for the acquired data along each traverse was carried out using resistivity inversion with the aid of DiproWin software. From the ERT pseudo-section, three (3) and four (4) geo-electric strata were inferred along the first and second profiles respectively. The resistivity range, from top to the base of the sequence, along the first profile is 932Ωm – 3343Ωm and 369Ωm – 470Ωm / 207Ωm - 354Ωm interpreted as quartz-rich sand and two-fold distinguishable silty sand strata (exhibiting a progressive increase in silt content with depth) respectively. The thickness range of quartz sand is 4m – 8m while that of the folded upper and lower silty sand extends to infinity. Along the second profile, the resulting 2-D resistivity structure also depicts a descending trend of resistivity variation which from top to base, delineated geo-electric strata with a resistivity range of 800Ωm – 4640Ωm interpreted as sand, 245Ωm – 350Ωm corresponding to silty sand, 119 Ωm - 204Ωm as sandy silt and between 48Ωm - 82Ωm for silt. In the aforementioned order, thickness ranges between 5m – 7m, 1m – 2m, 4m – infinity, and out rightly undeterminable for the last probed layer. The wavy undulations depicted on the 2-D resistivity structures are structurally interpreted as minor symmetrical folds. However, a recognizable lateral variation trend in lithological composition from the sandy northern to an interposed silty down south close to the regional river is indicative of a fluvial meandering depositional mechanism in the area. It can thus be qualitatively deduced from the above analysis; the probed section beneath the study is entirely composed of simple symmetrically folded siliciclastic deposits fining down the sequence deposited by a meander in a fluvial environment. High-grade silica sand can be readily exploited with very minimal processing to an average depth target of 6m. Further adoption of Vertical Electrical Sounding and seismic refraction surveys can improve the accuracy of estimation