Characterization of Depositional Environment within Parts of Benin Formation, Akwa Ibom State, Nigeria Using Vertical Electrical Sounding Technique

This study presents a comprehensive interpretation of the depositional environment within the Benin Formation in Akwa Ibom State, Southeastern Nigeria, based on Vertical Electrical Sounding (VES) data integrated with borehole lithologic information. This study investigated the spatial variability of subsurface lithologies and their electrical resistivity signatures to reconstruct sedimentary facies and depositional settings. The VES data, acquired across key local government areas, reveal stratigraphic layering indicative of a fluviodeltaic depositional system, dominated by coarse- to fine-grained sand, gravel beds, and interbedded clay lenses. The lithostratigraphic interpretation delineates proximal braided fluvial facies transitioning into meandering river systems with floodplain deposits. The presence of resistivity values ranging from <100 Ωm (indicative of clay and silty sands) to >1500 Ωm (characteristic of coarse sand and gravel) supports the identification of high-energy channel fills and overbank deposits. Emphasis was placed on correlating resistivity signatures with sedimentary facies and curve types to delineate fluvial depositional systems and hydrogeological conditions. The VES data revealed stratigraphic sequences comprising high-resistivity coarse sands and gravels, interlayered with low-resistivity clay lenses. Curve types such as KHK, HK, and KH dominate the area, indicating multiple aquifer zones, including confined, semi-confined, and unconfined systems. These hydrostratigraphic interpretations support a fluviodeltaic model transitioning from braided to meandering systems, with implications for groundwater availability and environmental planning. The results provide a valuable framework for water resource management and geotechnical assessment in this region. This study offers insight into the paleoenvironmental conditions that control sediment supply and distribution, informing hydrogeological modeling and resource exploration within the region.

Introduction

The Benin Formation, which forms the uppermost stratigraphic unit of the Niger Delta Basin, is a prominent continental deposit with significant hydrogeological and geotechnical importance. Dominated by coarse-grained sandstones, gravels, and minor clay interbeds, it was deposited in a high-energy fluvial environment and underlies much of the Akwa Ibom State [1], [2]. The formation serves as the principal aquifer system in the region, supporting domestic, agricultural, and industrial water supply needs [3], [4]. Its near-surface occurrence also makes it critical for land-use planning and infrastructure development.

Despite its importance, there remain substantial gaps in our understanding of the internal facies architecture and depositional dynamics of the Benin Formation, especially in areas with limited borehole control. Traditional methods such as lithologic logging and sedimentological field mapping are spatially limited and often inadequate for reconstructing subsurface depositional environments [5], [6].

Geophysical techniques, particularly Vertical Electrical Sounding (VES), have gained traction as effective tools for subsurface investigations because of their ability to resolve lithologic variations and stratigraphic discontinuities [7], [8]. However, the potential for detailed sedimentological interpretation remains underexplored in the context of the Benin Formation. There is a pressing need to integrate VES data with sedimentological models to better delineate facies transitions, channel fills, and aquifer geometry, especially in rapidly urbanizing regions, such as Akwa Ibom [9], [10].

This study aims to fill this gap by employing VES surveys across selected areas in Akwa Ibom State to characterize the depositional environment of the Benin Formation. By correlating resistivity variations with known sedimentary facies, this study sought to construct a depositional model that enhances the understanding of sediment transport processes, groundwater occurrence, and geotechnical suitability of the subsurface. The results provide a scientifically grounded framework for groundwater exploration, hazard mitigation, and sustainable land-use planning in the region.

Study Area

The study area lies between latitudes 4°36′N and 5°19′N and longitudes 7°27′E and 8°19’E and covers approximately 4,080.5 km² (Fig. 1). It is located within the sub-equatorial climatic region of Nigeria, with a total annual rainfall of more than 300 cm to 400 cm and a temperature range of 25°C to 28°C. The elevation ranges between 0 and 110 m above sea level (ASL). The Western Sector of the Lower Cross River Basin is alluvium in the coastal areas, while the rest of the environment is the Benin Formation [11]–[13]. The study focused mainly on the segment covered by the Benin Formation.

Fig. 1. Geologic map of Akwa Ibom State highlightingthe study area.

Geologic and Hydrogeologic Setting of Akwa Ibom State

Akwa Ibom State lies within the southeastern portion of the Niger Delta Basin and is predominantly underlain by the Benin Formation, with minor exposures of the Agbada and Imo formations at depth. The Benin Formation, also referred to as the coastal plain sands, extends inland from the Niger Delta through the State and comprises predominantly unconsolidated sands and gravel units, with intercalations of clay and silt layers. Geologically (Fig. 1), the state is situated within a region dominated by Cenozoic sedimentation that is primarily of fluvial origin. The lithologic sequences observed in this region are reflective of a prograding deltaic system [14], where river-dominated sedimentation occurs in high-energy environments, often resulting in the deposition of thick sand bodies and gravel beds. Toward the south and southwestern flanks of the State, the sediment grain size tends to be fine-grained, indicating a transition from proximal to distal fluvial environments.

Hydrogeologically, the Benin Formation forms the principal aquifer system within Akwa Ibom State, yielding moderate to high quantities of groundwater. The aquifers are typically unconfined to semi-confined, occur at shallow depths (10–50 m) and are composed of highly permeable sands and gravels. Groundwater recharge is primarily due to direct rainfall infiltration, facilitated by the high porosity of the formation. However, the heterogeneity in lithology results in variable aquifer transmissivity and potential vulnerability to contamination.

The region experiences a humid tropical climate with high annual rainfall (2000–3000 mm), that supports continuous recharge and sediment mobility. This hydrological setting reinforces the dynamic nature of the depositional processes shaping the Benin Formation.

As subsequent sections will show, the geoelectric layering obtained from the VES data aligns with the stratigraphy expected from a fluviodeltaic depositional model. Thus, the geological context serves as a critical backdrop for interpreting the resistivity profiles and their implications for depositional environments.

Overview of the Benin Formation

The Benin Formation, also known as the coastal plain sands, represents the youngest lithostratigraphic unit of the Niger Delta complex. It spans a significant portion of southern Nigeria and is particularly well-developed in the Akwa Ibom State. The formation is generally dated to Oligocene to Recent and reflects a major episode of continental sedimentation resulting from the uplift and erosion of the adjoining hinterland.

Lithologically, the Benin Formation comprises predominantly coarse- to medium-grained, poorly sorted sands with variable gravel content, interbedded with minor clay and silty clay layers. The sandy layers are often friable and loosely consolidated, with characteristic cross-bedding and occasional pebble lags indicative of high-energy depositional conditions. The Clays are typically found at depth or as thin lenses, representing overbank or floodplain deposits within predominantly fluvial systems.

The sedimentary structures and grain size variations observed within the Benin Formation suggest deposition by braided and meandering river systems. Braided river facies are marked by thick sand units with high resistivity values (often exceeding 1500 Ωm), whereas meandering facies show alternations of sand and clay, producing variable resistivity responses. These environments reflect an episodic sediment supply controlled by climatic variability, tectonic subsidence, and basin geometry.

The thickness of the Benin Formation in the Akwa Ibom State ranges between 100 and 400 m, with VES data indicating multilayered sequences consistent with stacked channel fills and paleosol horizons. The lateral heterogeneity observed in the resistivity values across the study area correlates well with variations in sediment provenance, flow regime, and depositional energy.

As the principal aquifer system in the region, the Benin Formation not only supports the water supply but also influences land stability, foundation conditions, and waste infiltration potential. Therefore, understanding its depositional characteristics has wide-ranging implications for groundwater resource management and environmental sustainability.

Methodology–VES Survey and Lithologic Interpretation

The methodology adopted in this study integrates geophysical field surveys using the Vertical Electrical Sounding (VES) technique with borehole lithologic correlation to determine the depositional environment of the Benin Formation. VES was conducted across multiple locations in Akwa Ibom State using the Schlumberger electrode array configuration (Fig. 2), which provides an effective vertical resolution and penetration for identifying lithological discontinuities.

Fig. 2. Schlumberger electrode arrangement [6].

A total of 80 VES stations were surveyed, with electrode spacings ranging from 1 to 600 m depending on site-specific conditions and accessibility. The field data were acquired using an Ohmega resistivity meter, and the apparent resistivity values were plotted against half-current electrode spacing (AB/2) on a bi-logarithmic scale (Fig. 3). These data were interpreted using partial curve matching and computer-assisted inversion methods using WinResist software, allowing for the analysis and interpretation of layers based on their resistivity (Table 1). An analysis of the 80 VES points across the study area is summarized in Table II.

Fig. 3. VES curve for AK004 (Sample curve).

Table I. Table of Analysis and Interpretation (Drilling Guide for AK004)

S/N	ρ (Ωm)	Thickness (m)	Depth (m)	Geologic description
1.	1160.1	2.6	2.6	Poorly sorted wet topsoil
2.	254.9	8.2	10.8	Wet lateritic soil (not saturated)
3.	1443.9	63.9	74.7	Medium to fine grained sand (saturated)
4.	598			Fine grained sand with clay intercalations (saturated)

Table II. Summary of VES Data and Interpretations

VES NO.	Location	Latitude	Longitude	No. of layers	ρ1 (Ωm)	ρ2 (Ωm)	ρ3 (Ωm)	ρ4 (Ωm)	ρ5 (Ωm)	d1 (m)	d2 (m)	d3 (m)	d4 (m)	Depth to aquifer (m)	Curve type
AK001	Ikot Ebak, Oruk Anam1	4.76708	7.60419	5	1514.7	4428.2	1586.1	1024.2	2053.1	1.2	3.4	25.3	71.9	51	KQH
AK002	Ibesit Numj Ikot Oruk Anam	4.79839	7.68839	5	713.6	482.8	246.8	1267	2470.7	6.2	27.1	68.6	141	48	QHA
AK003	Ntak Ibesit Oruk Anam	4.89228	7.74739	5	99.6	2570.9	263.8	1527.3	939.9	0.4	1.6	11	178.1	53	KHK
AK004	Essien Udim 1	5.12208	7.61328	4	1160.1	254.9	1443.9	598		2.6	10.7	74.7		64	HK
AK005	Essien Udim 2	5.10819	7.75228	4	363.4	1343	1746.5	857.3		1.5	19.5	130.5		68	AK
AK006	ABAK 1	5.04628	7.73650	4	157.7	40.9	1305.5	425		1.3	11.7	87.4		45	HK
AK007	ABAK 2	4.97419	7.80700	4	1477.5	143.3	501.9	1872.2		2.8	55.4	109.4		44	HA
AK008	Abak 3	5.07250	7.79878	4	372.7	116.9	598.4	57		2.5	9	60.5		46	HK
AK009	Ubon Ukwa, Obot Akara	5.19178	7.55875	5	728.3	1294.6	739.7	1288.3	577.9	1.2	3.5	17.2	88	73	KHK
AK010	Nto Eton, Obot Akara	5.20264	7.59483	5	635.7	159.2	915.7	397.7	283.5	2.5	9.3	88	173.9	76	HKQ
AK011	Nto Edino Housing Estate, Obot Akara	5.25317	7.59892	4	317.3	756.3	1400.5	538.9		2.6	18.5	103		63	AK
AK012	Abak Oko, Ikot Ekpene	5.13978	7.73017	4	388.7	121.7	2063.2	227.9		3.3	10.1	62.5		56	HK
AK013	Abiakpo Edem Idim, Ikot Ekpene	5.14908	7.70328	4	683.9	282.5	1860.7	757.8		1.3	8.7	79.4		35	HK
AK014	Ifuho, Ikot Ekpene	5.18269	7.68439	4	177.3	691.1	1210	981		1.6	10	100		38	AK
AK015	Ibesikpo 1	4.91500	7.95600	4	439	1964.7	940.2	607		2.2	35	112.3		29	KQ
AK016	Ibesikpo 2	5.02500	8.06800	4	214.8	53	22.2	277.2		1.7	6.2	75.9		31	QH
AK017	Ibesikpo 3	4.97553	7.94942	5	1167.7	237	2896.2	1231.1	851.1	1.9	5.8	26.6	90.1	26	HKQ
AK018	Ikot Edebe, Nsit Atai	4.87889	8.13528	5	1459.6	890.9	1971.2	953.4	1663.8	1.4	4.6	16.2	129.9	19	HKH
AK019	Ibakang, Nsit Atai	4.81142	8.01453	4	1707.7	714.9	1835.7	610.8		2.5	10.6	96.2		16	HK
AK020	Esa Ekpo, Mkpat Enin	4.74308	7.73553	4	1274.3	3014.7	1750.1	502.4		8.1	134.4	188.1		21	KQ
AK021	Technical college Mkpat Enin	4.69894	7.76978	4	623.5	173.3	415.3	829.2		2.9	12.6	148.6		22	HA
AK022	AKSU Road	4.61706	7.76217	4	236.6	1383.9	251.4	886.4		1	9	94.8		17	KH
AK023	Afaha Atai	4.64181	7.84850	4	226.9	1470.3	395.5	1099.6		1.3	44.4	117.1		31	KH
AK024	Abat	4.62264	7.86142	4	147.7	2968.2	1515	2513.4		3	24.8	113		33	KH
AK025	Ikot Akan	4.61494	7.67108	4	1445	2125.7	855.8	1126.4		3.5	12.4	163.5		29	KH
AK026	Ukwo, Eket	4.66217	8.05856	4	64.2	2351.2	127.4	869.2		6	42.7	199.1		22	KH
AK027	Ikot Nkebek	4.72800	8.03506	4	577.6	2998.8	529.4	327.6		2.7	25.1	152.8		31	KQ
AK028	Ikot Ekpene Udo	4.69189	7.94303	5	650.6	957.7	1895.4	612.6	1076.4	1.7	9.4	29.8	113	29	AKH
AK029	Ebughu	4.68917	8.03506	4	160.9	1297	2910	1791.8		10.2	40.7	179.5		29	AK
AK030	Mbo	4.68250	8.25508	4	1425.6	148.6	1781.3	973.7		2.8	69.6	241.5		22	HK
AK031	Ewang, Mbo	4.75600	8.28700	4	545.6	194.1	1722.1	1077.3		2.5	14.6	222.8		19	HK
AK032	Obio Iyata	4.71600	8.16700	5	264.4	66	2174.1	1080.4	786.6	2.2	5.7	51.5	177.1	20	HKQ
AK033	Ika	5.00639	7.53150	4	524.6	212.6	1828.7	226.6		6.4	17.6	100.8		87	HK
AK034	Etim Ekpo 1	4.99900	7.64569	4	259.7	505.7	896.4	1292.6		12.4	72.2	208.5		49	AA
AK035	Etim Ekpo 2	4.94728	7.55128	4	227.2	806.9	1212.1	340.4		2.3	23.4	175.3		50	AK
AK036	Ukanafun 1	4.88333	7.57119	4	191.8	1181.2	906.8	1584		11.9	40.5	175.2		50	KH
AK037	Ukanafun 2	4.92669	7.66728	4	214.7	1256.1	307.5	135.1		1.3	32.6	137.4		54	KQ
AK038	Ikot Ebak, Oruk Anam	4.79945	7.69845	4	1860	2465.4	936	2900.3		1.6	15.7	83.8		41	KH
AK039	Oruk Anam 2	4.98237	7.76740	4	701.2	349	2890	1940		10.6	99.4	306.5		34	HK
AK040	Oruk Anam 3	4.88600	7.60450	5	105.6	2402.8	268.6	1521	952.8	0.4	1.9	11.3	179.7	44	KHK
AK041	Ikot Udo Mbang, Ukanafun	4.88600	7.60453	4	609.5	1322.4	130.9	4245.9		2.5	54.6	112.3		47	KH
AK042	VES 14	4.77269	8.19442	4	1375.7	2953.7	257	900.6		18.8	51.8	110		24	KH
AK043	VES15	4.84489	8.13219	4	1632.1	2996	524	1579.4		1.5	36.3	118		48	KH
AK044	Nduk, Ikot Abasi	4.65961	7.69292	4	779.2	2999	709.5	412.3		0.4	9.2	95.4		23	KQ
AK045	Oruefong Oruko	4.69936	8.16161	4	2098.3	1247.4	815.3	291.8		1.5	63	178.8		44	QQ
AK046	Utine Ndung, Okobo	4.78253	8.12589	4	498.2	2846.3	540.2	1385.9		0.9	33.4	86.7		54	KH
AK047	Nsit Atai	4.77300	8.04958	4	1402.6	744.4	155.4	2676.5		1.3	17	66.9		34	QH
AK048	Ikot Okpudo, Nsit Ubium	4.70894	8.02550	4	1422,2	1897.1	315	770.6		9.1	28.6	95.1		47	KH
AK049	Okobedi, Okobo	4.84489	8.13219	4	647	2837.7	858.7	1500		1.1	27.1	164.5		48	KH
AK050	Udung Uwe, Urue Offong Oruko	4.69936	8.16161	4	1181.3	2895	632.5	1205.6		5.5	20.8	184.4		39	KH
AK051	Akwata, Esit Eket	4.66042	8.11925	4	255.4	909.5	307.8	1441.9		4.4	22.7	91.5		19	KH
AK052	Ikpa Town, Esit Eket	4.65700	8.04500	4	225.8	2337.9	1050.1	484		1.4	105.3	131.3		20	KQ
AK053	Eket (Girls Secondary School)	4.65960	7.69290	5	63.7	1584	165.2	983	163.8	0.6	4.1	14.1	75.1	25	KHK
AK054	Nduke, Ikot Abasi	4.64270	7.64000	5	1011	2953.6	603	1087.5	398.9	1	8.6	65.1	89.5	29	KHK
AK055	Atan Ikpe, Edemaya Clan, Ikot Abasi	4.64000	7.64000	5	672.9	2538.9	901.7	294.7	1184.9	1.1	5.6	32.7	123.2	28	KQH
AK056	Inen Ekeffe, Oruk Anam	4.78672	7.55947	4	1100.8	2680.5	1597.5	378.4		1.2	9.3	59.6		49	KQ
AK057	Ikot Udo Obobo, Ukanafon	4.88250	7.54108	4	1371.2	2677.8	483.6	1012		1	44.6	78.9		54	KH
AK058	Ikot Ntot, Mkpat Enin	4.72328	7.71489	4	1300.5	2893.1	843.1	443.1		2.5	8.2	80.5		28	KQ
AK059	Nung Oku Ekanem, Onna	4.69378	7.85011	4	924.4	1776	538.6	1144		1.1	16.5	125.2		22	KH
AK060	Ikot Akan Uran	4.95750	8.04111	4	1234.6	1668.1	988	1500		1.7	21.8	194.6		22	KH
AK061	Anakpa, Uran	5.00989	8.03900	4	318.1	577.9	1202.3	834.6		0.7	40.2	72.1		42	AK
AK062	Idu Uran Estate	5.03058	8.00386	4	292.3	2581.6	1039.5	627.3		1.8	54.4	132.2		25	KQ
AK063	Nnkak Abak, Ukanafon	4.96197	7.70558	4	1397.3	541.4	2169.4	811.7		4.1	11.2	96		47	HK
AK064	Nto Udo, Midim, Essien Uduim	5.04650	7.62917	4	969.2	1895.3	1079	781.1		3.5	72.1	92		52	KQ
AK065	Ekasara, Ekpene Ukpa, Etinan	4.95508	7.86786	4	1020.2	2874.5	1959.8	1319.3		3.1	21.8	135.8		20	KQ
AK066	Ndon Eyo1, Etinan	4.86647	7.81944	4	329.5	118.2	1416.3	779.2		1.2	5.1	169.7		28	HK
AK067	Ikot Abasi 1, Ekinan	4.85872	7.85328	4	936	1912.9	1297.9	760.6		1.1	16	81.9		27	KQ
AK068	Ibesikpo Asutan	4.86731	7.99308	4	1254.6	2966.3	329.4	1001.9		1.1	32.6	93.7		24	KH
AK069	Mbio Etene, Nsit Ubium	4.71475	7.96653	4	1326.4	2631.4	1076	740		3.1	19.2	132.2		33	KQ
AK070	Omum Unyam, Etim Ekpo	4.96533	7.55831	4	490.7	2091.2	1390.3	890.2		1.4	82.9	162.4		75	KQ
AK071	Ikot Uko, Ika	4.97250	7.47069	4	437.3	1800.9	2904	630.8		1.8	14.7	105.9		88	AK
AK072	Ikot Akpan, Obio Okot, Etinan	4.78481	7.80283	4	1293.6	163.4	2453.3	1207.5		9	19.5	150.9		33	HK
AK073	Ikot Udo, Ika	5.03189	7.53147	4	479.8	1506.7	2549.6	717		1	14.1	78.2		94	AK
AK074	Ikot Umoessien Odorikot, Essien Udium	5.13653	7.55506	3	1904.1	2684.5	877.3			7	68.2			72	K
AK075	Usaka Annang, Obot Akara	5.29031	7.55028	4	1016	1989.8	753.1	964.6		1.5	46.6	95.6		69	KH
AK076	Obot Akara	5.27439	7.60217	4	469.5	1045.3	2606.4	1496.4		1.2	22.1	142.1		73	AK
AK077	Ikot Ideh, Obot Akara	5.23117	7.56692	4	265.9	2893.8	1653.8	1014.4		1.8	14.1	116.9		73	KQ
AK078	Notridam Sec Sch. Essein Udium	5.18603	7.61956	4	1946.5	1303.9	2577.1	1420.6		6.5	11	104.5		70	HK
AK079	Mbiabon Ikot Ntem, Uyo	4.99575	7.97131	4	272.6	42.7	2695.7	119.9		2.8	4.8	100.3		54	HK
AK080	Obon Street Off IBB, Uyo	5.01675	7.91342	4	882.6	417.3	994.5	1932.1		2	70.4	115.8		48	HA

The resulting geoelectric sections were classified into three to five distinct subsurface layers based on resistivity contrasts, which corresponded to varying lithologies: topsoil, sandy clay, clay, fine-medium sand, and coarse sand with gravel. Resistivity values for clay ranged between 10–100 Ωm, while clean sands and gravels recorded values exceeding 1000 Ωm, depending on moisture content and grain packing. To validate the geoelectric interpretations, lithologic logs from nearby boreholes were compared with the VES-derived layer characteristics (Figs. 4 and 5).

Fig. 4. Representatives of vertical electricity sounding results showing correlations with borehole logs from the same location: (a) BH2 correlating with VES AK017, (b) BH3 correlating with VES AK061, (c) BH6 correlating with VES AK067, and (d) B18 correlating with VES AK06714. VES curve for AK004 (Sample Curve).

Fig. 5. Spatial view of VES and Borehole locations.

This comparative approach enabled better identification of sedimentary facies such as point bars, levees, floodplain deposits, and channel lag fills. Areas where the resistivity curves indicated significant lateral heterogeneity were further analyzed for potential paleo-channels and depositional boundaries.

This combination of VES data with stratigraphic logs and hydrogeological context provides a robust framework for deducing the spatial distribution and characteristics of the depositional environments within the Benin Formation.

Lithofacies and Sedimentary Structures

The interpretation of the VES data in conjunction with borehole lithologic logs revealed a diverse assemblage of sedimentary structures and facies associations indicative of a complex fluvial depositional system within the Benin Formation. The major lithofacies identified included massive sands, cross-bedded sands, gravel beds, and clay intercalations. These results are consistent with depositional features formed under varying hydrodynamic conditions typical of braided and meandering fluvial systems.

The resistivity values across the study area ranged from less than 100 Ωm for clay-rich zones to more than 1500 Ωm for gravely, well-sorted sand units. These values suggest a spectrum of depositional settings, from low-energy floodplain and overbank environments to high-energy channel deposits. For instance, VES stations that recorded resistivity profiles with sharp increases in the second or third geoelectric layer typically corresponded to coarse-grained, well-drained sand units, interpreted as fluvial channel fills or point bar deposits. In contrast, profiles showing thin topsoil and very low resistivity in deeper layers were aligned with floodplain fines and abandoned channel fills.

The cross-bedding observed in sand units from core samples and borehole records correlates with lateral accretion deposits in meandering stream environments. The occurrence of laminated clay lenses interbedded within sand layers supports the hypothesis of periodic flooding events and the subsequent settling of suspended sediments in overbank settings. These depositional structures signify rhythmic alternation between high and low-flow phases, which are characteristic of fluvial systems influenced by seasonal precipitation patterns.

Facies associations based on VES-derived stratigraphy also support the vertical stacking of channel deposits over floodplain clays, indicating a progressive aggradation process. In certain areas, particularly along the lower Imo River axis, the resistivity profiles reflect the presence of stacked high-resistivity units separated by thin, low-resistivity clay lenses. These are interpreted as migrating channels with occasional avulsions and floodplain development.

This analysis of lithofacies and associated resistivity signatures provides critical insight into the sedimentological framework and paleoenvironmental conditions governing sediment deposition in the Benin Formation.

Depositional Environment Models from VES Profiles

The synthesis of VES-derived resistivity profiles with lithologic data enabled the delineation of depositional environments across Akwa Ibom State within the Benin Formation. Three principal depositional models have emerged: proximal braided river systems, meandering channel systems, and floodplain-overbank environments. Each model corresponds to characteristic VES signatures and inferred lithologies, and their spatial distributions reflect the fluvial dynamics and paleo-drainage patterns across the State.

Proximal Braided River Model

This model is typified by very high resistivity values (>1500 Ωm), thick sand units, and minimal clay intercalation. The VES curves exhibit sharp resistivity contrasts with massive sand units dominating the geoelectric sections. These profiles correspond to areas such as Itu, Ibiono Ibom, and northern Uyo, where coarse sediments dominate because of their proximity to high-energy alluvial fans and source regions. The lithologic logs in these zones confirmed the presence of thick, gravelly sand beds with poorly sorted textures, indicative of rapid sediment deposition in braided stream channels.

Meandering Fluvial Model

In areas such as Abak, Oruk Anam, and Etinan, the VES data exhibited moderate resistivity values (400–1200 Ωm) with alternating sequences of sand and clayey layers. These profiles are interpreted as point bar and overbank deposits typical of a meandering river system. The presence of cross-bedded sand interlayered with clay supports lateral accretion processes and periodic flooding events. These environments are characterized by better sorting, stratification, and more continuous aquifer horizons.

Floodplain and Overbank Environments

The southern portions of the State, such as Eket and Mkpat Enin, show VES profiles with predominantly low resistivity values (<100–300 Ωm) and thin sand layers underlain by thick clay deposits. These are interpreted as floodplain or backswamp environments associated with fine-grained sedimentation under low-energy conditions. Clay dominance limits aquifer productivity but plays a crucial role in regulating contaminant transport and groundwater flow paths.

Transitional Zones

Some VES profiles, particularly in transition zones such as Ibesikpo and Ikot Ekpene, display complex layering with interdigitating sand and clay units. These hybrid profiles suggest avulsion-prone environments or abandoned channels, where both high-energy and low-energy sediments are juxtaposed because of lateral channel migration.

By correlating resistivity signatures with known sedimentary processes, this depositional model framework provides an interpretative base for reconstructing paleoenvironments and guiding future groundwater and land-use assessments.

Zonation of Depositional Settings Based on VES and Stratigraphy

The depositional environments within the Benin Formation in Akwa Ibom State exhibit regional variability that aligns with both the geomorphological gradients and subsurface lithological architecture. Through the integration of VES data, lithologic logs, and resistivity mapping, distinct depositional zones were delineated based on the sediment texture, resistivity ranges, and inferred depositional energy.

Zone A–Northern Fluvial Belt (High-Energy Proximal Zone)

This zone encompasses areas such as Itu, Ibiono Ibom, and parts of Northern Uyo. The VES profiles in this region revealed thick, coarse-grained sand and gravel deposits with resistivity values exceeding 1200 Ωm, indicating deposition in a proximal braided river environment. These high-energy deposits suggest intense channel migration and a high sediment load, often associated with seasonal flash floods and steep gradients.

Zone B–Central Fluvial-Meandering Transition Zone

Covering Abak, Ikot Ekpene, and Etinan, this zone exhibited alternating resistivity profiles ranging between 300 and 1000 Ωm. The lithological sequences suggest a shift from coarse channel deposits to finer-grained floodplain sediments. Stratigraphic logs support the presence of point bar sands, lateral accretion deposits, and interlayered clay lenses. This zone is interpreted to be a meandering river plain with frequent avulsions and overbank flooding.

Zone C–Southern Floodplain Complex

In the coastal regions of Eket, Esit Eket, and Mkpat Enin, the VES profiles predominantly show low resistivity values (<300 Ωm), indicating the presence of clayey floodplain and swamp deposits. These areas represent the distal parts of the fluvial system where the energy conditions are low and fine-grained sediments dominate. The thickening of clay sequences southward supports the hypothesis of sediment sorting and hydrodynamic settling away from the channelized flow.

Zone D–Structural Transition and Interfingered Facies

Certain areas, such as Ibesikpo Asutan and Onna, present stratigraphic complexity characterized by rapid vertical changes in resistivity and layer thicknesses. These are interpreted as zones of facies interfingering, possibly controlled by fault-related subsidence or paleochannel shifts. VES interpretations suggest alternating fluvial and overbank depositions in structurally influenced landscapes.

This zonation framework provides a spatial perspective on how fluvial processes shape the subsurface architecture of the Benin Formation in Akwa Ibom State. The identification of these zones has practical implications for groundwater exploration, land-use planning, and geotechnical investigations, particularly in areas susceptible to erosion or subsidence.

Implications for Groundwater Resource Development

The depositional characteristics and stratigraphic architecture of the Benin Formation, as revealed by the VES data, have significant implications for groundwater resource development in Akwa Ibom State. The hydraulic properties of aquifers in this region are directly influenced by the depositional environment and textural composition of the sedimentary facies.

Aquifer Productivity

High-resistivity zones associated with thick, well-sorted sand and gravel, particularly in the northern fluvial belt (Zone A), indicate aquifers with high porosity and permeability. These areas serve as reliable groundwater reservoirs and are suitable for the development of high-yield boreholes. The VES profiles confirmed multilayered aquifer systems, with vertical hydraulic continuity that enhances recharge and sustainable extraction.

Groundwater Vulnerability

In contrast, the floodplain and swampy zones in the southern regions (Zone C) exhibited thick clay units that acted as confining layers. Although these reduce the aquifer vulnerability to surface contamination, they also limit the volume of extractable groundwater and impede recharge. In transition zones (Zone D), where interbedding of sand and clay is common, aquifers are semi-confined and may be susceptible to contamination from nearby surface sources, especially in densely populated or industrial areas.

Recharge Potential and Water Table Dynamics

The meandering river facies in the central zones (Zone B) displayed moderate resistivity with interlayered sand-clay sequences, indicating semi-confined to unconfined aquifers. These zones exhibit a moderate recharge potential owing to their relatively balanced permeability. The presence of laterally extensive point bar deposits suggests distributed recharge zones along paleo-river courses, promoting sustainable water table conditions.

Well Design and Drilling Strategy

Understanding the depositional context allows for optimized borehole siting and design. In high-energy zones, wells can be designed with deeper screens to tap into thick, basal gravel aquifers. In structurally complex or floodplain zones, shallow wells may require protective casing and filtration systems to manage fine sediments and minimize clogging.

Environmental and Geotechnical Concerns

Spatial variability in depositional environments also affects the engineering behavior of the subsurface. Areas dominated by clay-rich facies pose challenges for construction and septic infiltration systems, whereas gravelly zones are more stable and drainable. Moreover, depositional heterogeneity has implications for contaminant transport, with coarse-grained units promoting rapid migration and finer units acting as contaminant sinks.

Overall, detailed characterization of depositional environments using VES data enhances the predictive understanding of aquifer properties, recharge dynamics, and water quality risks.

Resistivity Curve Types and their Implications on the Study Area

Analysis of resistivity curves from the acquired VES data provides crucial insights into lithological variations, aquifer properties, and hydrogeological characteristics. In Akwa Ibom State, the Benin Formation is a dominant lithostratigraphic unit, primarily consisting of sand and gravel with occasional clay intercalations. The resistivity curve types observed in this region are instrumental in understanding the groundwater occurrence and flow dynamics. The Benin Formation is known for its high permeability, making it an excellent groundwater reservoir, although variations in the resistivity curves highlight the complexity of subsurface hydrogeology. The distribution of curve types in the study area is shown in Fig. 6.

Fig. 6. Distribution of curve types.

The curve types are related to hydrogeology, and the types of areas they represent are as follows:

Each of these curve types represents different subsurface conditions that influence groundwater potential and hydrogeological interpretations.

VES Curve Type and its Implications in the Study Area

The analysis of Vertical Electrical Sounding (VES) data across various locations in Oruk Anam, Akwa Ibom State, revealed a range of curve types that reflect the complex subsurface lithology and aquifer characteristics typical of the Benin Formation. These curve types—classified based on resistivity signature offer insights into the distribution, confinement, and productivity of groundwater-bearing formations.

At Ikot Ebak, the VES result was characterized by a KQH-type curve, indicating a subsurface system composed of alternating layers of sand and clay. This configuration suggests the presence of a semi-confined aquifer situated between less permeable clay units. Such a setting promotes the development of confined aquifers capable of storing substantial water reserves beneath protective clay caps, which act as barriers to surface contamination. For optimal groundwater exploitation, boreholes in this area should be drilled sufficiently deep to access more productive confined layers.

A QHA-type curve is observed in Ibesit Nung Ikot, reflecting a more complex subsurface with alternating permeable and impermeable layers. This arrangement created multiple water-bearing zones, each potentially separated by clay-rich beds. The presence of these stratified aquifers necessitates careful borehole planning and depth targeting to ensure efficient water extraction while minimizing the risk of vertical contamination.

Similarly, at Ntak Ibesit, the presence of a KHK-type curve indicates interbedded sand and clay sequences that result in both confined and unconfined aquifers [15]. The variability in aquifer types makes this site highly promising for groundwater development, provided that borehole designs are optimized to target the most productive and least vulnerable aquifer zones.

Several VES stations, AK004, AK006, and AK008, exhibited an HK-type curve, which denotes the presence of near-surface unconfined aquifers. These aquifers are generally easy to access and suitable for small- to medium-scale water supply schemes. However, their shallow nature makes them susceptible to seasonal fluctuations and surface contamination. Thus, boreholes in these zones must be adequately cased and sealed to ensure the water quality and sustainability.

In contrast, VES stations AK019, AK023, AK024, and AK025 showed a KH-type curve, suggesting deeper confined aquifers overlain by clay-rich formations. These zones are particularly favorable for high-yield and long-term borehole development, as the confined nature of the aquifers offers better protection from contamination and more stable water supply conditions.

In contrast, AK016 and AK047 present a QH-type curve, which features a decreasing resistivity profile typically associated with clay-rich formations or potential saline water intrusion. These areas are less favorable for groundwater development, as low permeability and possibly poor water quality can limit borehole yield and usability. Borehole siting in such locations should be approached cautiously and supported by further hydrochemical analyses [16].

Another noteworthy curve type is the HKQ-type, observed at AK017 and AK032, indicating alternating layers of high and low resistivity materials. This pattern implies the presence of confined aquifers at depth, making these sites suitable for deep borehole drilling to exploit secure and productive groundwater zones [17].

These interpreted curve types correlate well with the known characteristics of the Benin Formation, which comprises coarse-grained sand, gravel, and minor clay intercalations. The alternating resistivity patterns seen in the VES data reflect the sand-clay sequences typical of this formation, which are known for their high permeability and groundwater storage potential. Groundwater flow within the Benin Formation is largely lateral, as confining clay layers inhibit vertical movement, a fact supported by the dominance of HK, KH, and KHK curve types in the study area.

Importantly, the presence of QH and QQ curve types in selected zones highlights the occurrence of clay-rich formations that may serve as aquitards or present risks of low-yield and poor-quality water. These areas require careful site selection and additional investigation before borehole development to avoid resource mismanagement.

Overall, the diversity of VES curve types across Akwa Ibom State indicates significant variations in subsurface conditions, which in turn influence aquifer geometry, groundwater flow patterns, and borehole productivity. The predominance of the KHK, HK, and KH curves signifies the availability of substantial and relatively secure groundwater resources. Conversely, zones dominated by QH and QQ curves require a more detailed hydrogeological assessment to mitigate risks of water scarcity and contamination. A careful and integrated interpretation of VES data is thus essential for informed groundwater management, ensuring that borehole development aligns with both the geological reality and long-term sustainability goals.

Discussion in Context of Previous Studies

The findings of this study align with, and extend, the existing geological and hydrogeophysical research on the Benin Formation and the broader Niger Delta Basin. Several researchers have emphasized the significance of depositional settings in influencing aquifer characteristics and environmental behavior [8], [18], [19]. The identification of braided and meandering fluvial facies from VES data in Akwa Ibom State corroborates earlier sedimentological interpretations that classify the Benin Formation as a predominantly continental fluviodeltaic sequence.

Ajakaiye [20] and Archibald and Bochetti [21] have demonstrated the effectiveness of geophysical techniques, including resistivity and gravity data, in delineating sedimentary structures in the Niger Delta. This study builds on these foundations by using high-resolution VES profiles to distinguish depositional facies with enhanced clarity and relate them directly to aquifer potential. The variability in resistivity values observed here reflects the sediment sorting and lithologic discontinuities identified by other authors, including Akpan et al. [22], who mapped similar heterogeneities in the Calabar Flank.

Furthermore, the classification of zones into braided, meandering, and floodplain environments mirrors the work of Ehirim and Ebeniro [23], who explored fluvial influences on aquifer behavior using geoelectrical methods. The correlation between high-resistivity zones and aquifer productivity reaffirms the hydraulic relevance of the depositional setting and supports the approach adopted by Ilevbare and Ogundana [24] for assessing hydrogeological potential via VES.

The structural complexity noted in Zone D aligns with regional tectonic interpretations provided by Ajibade and Wright [25], who identified that crustal deformation trends influence sediment deposition across southern Nigeria. This implies that depositional processes within the Benin Formation are not solely driven by surface hydrology but are also modulated by subsurface tectonics, especially near fault zones and structural highs.

These findings underscore the utility of integrated geophysical approaches for reconstructing depositional environments to inform sustainable water resource planning. In particular, the recognition of aquifer vulnerability in transitional zones reinforces the findings of Aladejana et al. [26] and Adebiyi et al. [27] for increased geophysical monitoring in densely populated and industrialized regions.

The interpretation of VES data across Akwa Ibom State revealed significant variability in both lithologic composition and hydrogeological conditions. When coupled with resistivity curve types, these insights provided a robust reconstruction of the depositional framework of the Benin Formation.

The Benin Formation, characterized by poorly sorted sand, gravel, and clay intercalations, reflects a high-energy fluvial regime that evolves into floodplain environments. Resistivity values above 1000 Ωm are associated with coarse-grained channel fills, while low values (<100 Ωm) reflect clay-dominated floodplain deposits. These resistivity ranges align well with the identified VES curve types and their corresponding implications.

For instance, KHK-type curves, common in areas such as Ntak Ibesit, signify interbedded sand and clay units indicative of alternating energy conditions typical of meandering rivers. These settings support multiple aquifers, often separated by confining clay layers, aligning with the floodplain-meandering transition zones mapped in the study [28].

Similarly, HK- and KH-type curves, such as those found in VES AK004, AK019, and AK025, confirmed the presence of unconfined and confined aquifers. These curves correspond to proximal braided channels and meander belts where clean sand dominates. The presence of HKQ- and QHA-type curves implies stratigraphic complexity, often observed in tectonically influenced or transitional depositional environments, such as those near Ibesikpo and Ikot Ekpene.

In contrast, QH-type curves indicate zones of poor groundwater potential, often corresponding to clay-rich environments or areas with saline intrusions. These areas are consistent with low-resistivity floodplain deposits in the southern sector of the study area and require careful assessment for borehole development.

These curve interpretations not only validate lithostratigraphic correlations but also enhance the understanding of aquifer dynamics. The dominance of favorable curve types (KHK, KH and HK) underscores the substantial groundwater potential of the Benin Formation. Nevertheless, the presence of QH and QQ curves flag areas that require detailed hydrogeological evaluation owing to poor yield or contamination risks.

Thus, the integration of the VES curve typology with geological and resistivity data has reinforced the zonation of depositional environments and provides a predictive model for groundwater exploration and management in the region.

Conclusions and Recommendations

Conclusions

This study presents a comprehensive interpretation of the depositional environment within the Benin Formation in Akwa Ibom State, utilizing Vertical Electrical Sounding (VES) data and borehole lithologic information. The analysis revealed a stratigraphically complex and regionally variable subsurface characterized by braided river deposits, meandering fluvial sediments, and floodplain clays. These depositional environments have a significant influence on the groundwater distribution, aquifer productivity, and vulnerability to contamination.

High-resistivity values correspond to clean, coarse-grained channel fills, indicating zones of high aquifer potential. Conversely, low-resistivity floodplain deposits dominate the southern part of the study area and exhibit low permeability but provide natural confinement. Transitional zones demonstrate structural and depositional complexities, reinforcing the importance of site-specific investigations in hydrogeological and engineering applications.

The incorporation of curve-type classification (such as KHK, HK, KH, QH, and HKQ) has proven instrumental in correlating resistivity profiles with aquifer geometry and depositional environments. Most notably, high-yield aquifer zones are associated with the KHK, HK, and KH curves, confirming thick, clean sand sequences typical of high-energy depositional settings. Conversely, the QH and QQ curves indicate zones of clay dominance or possible saline intrusion, resulting in low groundwater productivity.

The integration of these datasets improves our understanding of subsurface heterogeneity and enhances hydrostratigraphic mapping, with direct applications in groundwater development, land-use planning, and geotechnical design.

The integration of VES and lithologic data proved to be a robust method for identifying lithofacies and reconstructing depositional systems, thereby enhancing the accuracy of groundwater resource assessments and land-use planning strategies.

Recommendations

Based on the findings of this study, several key recommendations are proposed to guide groundwater development, environmental management, and future research in Akwa Ibom State. Priority should be given to the high-resistivity zones located in the northern and central parts of the State for the development of high-yield boreholes, as these areas are characterized by thick, permeable sand and gravel layers that are highly suitable for large-scale water supply. To ensure long-term sustainability, regular geophysical and hydrogeological monitoring should be implemented, particularly in transitional and floodplain zones where aquifers are more vulnerable to degradation and contamination. Furthermore, complementary geophysical methods such as seismic and gravity surveys are recommended to refine the structural framework of the region, especially in Zone D, where the presence of interfingering facies may indicate tectonic influences that affect subsurface geometry. Environmental regulations and groundwater abstraction policies must also consider variable recharge potential and aquifer sensitivity across different depositional environments to avoid overexploitation and ensure resource sustainability. Finally, future research should include detailed sedimentological and geochemical investigations to validate and enhance the depositional models developed in this study. These efforts will also aid in assessing broader implications for hydrocarbon prospectivity, engineering geology, and sediment dynamics in the context of ongoing climate change.