Pebble Morphogenesis and Paleocurrent Pattern as Signatures of Tide- dominated Marginal Marine Environment: A Case Study from the Nanka Formation, Niger Delta Basin, Nigeria

Pebble Morphogenesis and Paleocurrent Pattern as Signatures of Tide- dominated Marginal Marine Environment: A Case Study from the Nanka Formation, Niger Delta Basin, Nigeria

Muogbo C.D, Aririahu D.C, Onuigbo E.N, Imoh D., Elomba U.F.

Department of Geological Sciences, Nnamdi Azikiwe University, P.M.B.5025, Awka

DOI: https://doi.org/10.51584/IJRIAS.2025.100800132

Received: 13 August 2025; Accepted: 22 August 2025; Published: 22 September 2025

ABSTRACT

Pebble morphometry and Paleocurrent analyses were carried out on Nanka Formation for the purpose of environmental discrimination. The morphometric analysis of four hundred and ten (410) pebbles from the formation indicated that pebble forms are dominanted by compact (50.05%), compact platy (27.42%) and compact bladed (10.09%). Other pebble forms comprise of platy (8.96%), compact elongate (1.92%), bladed (1.26%) and elongate (0.30%). This suggests dominance of fluvial pebbles over the beach ones. 88% of the pebbles plot near the upper parts of the Sphericity Form diagrams, thereby supporting dominance of the fluvial deposits.  The bivariate scatter plots of maximum projection sphericity index (MPSI) vs oblate- prolate index (OPI) and flatness index (FI) vs OPI show that over 95% of the pebbles indicated river process and less than 5% are beach pebbles. This result is consistent with the form diagrams for the pebbles. The occurrence of small proportion of the beach pebbles suggests that the ancient river flow reached the marginal marine environment. The Paleocurrent data indicated a bimodal- bipolar pattern in the northeasterly and southwesterly directions. Nanka Formation was laid down in a tide- dominated marginal marine environment.

Keywords: Morphometry, Azimuths, Nanka, Pebbles, Form, Sphericity

INTRODUCTION

Nanka Formation belongs to the Ameki Group of the outcropping Niger Delta Basin (Nwajide, 2022). Previous sedimentological work done on the formation focused on interpretation of the environment of deposition of the formation based on lithofacies, granulometric and Paleocurrent data (e.g Nwajide, 1979; Onuigbo et al., 2010; Oguadinma et al., 2014; Odumodu and Mode, 2014; Ekwenye and Onyemesili, 2018; Ogbe and Osokpor., 2021). The environment of deposition of the Nanka Formations has been interpreted as tidally dominated marginal marine environment.

Pebble Morphometry has been successfully applied in the interpretation of environment of deposition of sediments (e.g Nwajide and Hoque, 1982; Olugbemiro and Nwajide, 1997; Ogala et al., 2010; Okoro et al., 2012; Odumodu and Israel, 2014; Ocheli et al., 2018; Onyemesili and Odumodu, 2019; Madi and Ndlazi, 2020; Oluwajana et al., 2021; Ogbe et al., 2023; Ojong et al., 2023).

This paper will integrate pebble morphometry and paleocurrent data in the delineation of the environment of deposition of the Nanka Formation.

1.1 The Study Area

Umuawulu and environs lie between longitudes 7°3’36″E and 7°6’0″E and latitudes 6°5’0″N and 6°13’0″N. Town covered include Amawbia, Nibo, Nise, Mbaukwu, Agulu, Awgu, Umuawulu, Isiagu and Ndiagu. The study area is underlain by the Imo and Nanka Formations (Fig. 1) of the outcropping Niger Delta Basin.

Fig.1: Geologic Map of the Study Area (dark color represents Imo Formation and yellow, the Nanka Formation)

1.2 Regional Tectonics and Stratigraphic Setting

Evolution of the Niger Delta Basin has been linked to the development of the Benue Trough as a failed arm of a trilete (aulocogen) fracture system, during the break up of the Gondwana supercontinent and the opening up of the southern Atlantic and Indian Oceans in the Jurassic (Burke et al., 1972; Olade, 1975; Hoque and Nwajide, 1984; Benkhlil, 1989). Sedimentation commenced in the Benue Trough during the Aptian to early Albian. Two cycles of marine transgressions and regressions from the middle Albian to Coniacian filled the trough with mudrocks, sandstones and limestones (Murat, 1972; Hoque, 1977). The sediments underwent folding and uplifting into Abakaliki Anticlinorium with the simultaneous subsidence of the Anambra Basin and Afikpo sub basin during the Santonian tectonic episode (Murat, 1972; Burke, 1972; Obi et al., 2001; Mode and Onuoha, 2001).

Anambra Basin after installation was filled up with Campano- Maastrichtian to early Paleocene (Danian) sediments. Further subsedence led to the development of the Niger Delta Basin. Nwajide (2022) noted that the three southern Nigerian sedimentary basins are tier basins. Imo Formation is the basal lithostratigraphic unit of the outcropping Niger Delta and lateral equivalent of the Akata Formation in the subsurface Niger Delta. The formation is followed upwards by the Ameki Group, Ogwashi- Asaba and then the Benin Formation (Wright et al., 1985; Nwajide, 2022). Figure 2 shows the three southern Nigerian sedimentary basins, described by Nwajide (2022) as tier basins.

Fig.2: Stratigraphic succession in the Benue Trough, Anambra and Niger Delta basins (Short and Stauble,1967; Ekwenye et al.,2014).

MATERIALS AND METHOD

2.1 Pebble Morphometry

The long (L), intermediate (I), and short (S) axes of four hundred and ten (410) pebbles from the Nanka Formation were measured using a vernier caliper. The roundness of the pebbles was estimated using the chart by Sames (1966). The computed morphometric parameters are presented in Table 1

Table1: Mophometric Parameters and their formulars

The form of the Pebbles was determined using the sphericity form diagram of Sneed and Folk (1958).For the environmental discrimination, the bivariate scatter plots of M.P.S.I vs OP index, flatness index vs MPSI and sphericity form diagram were employed.

The bivariate plot of MPSI vs OPI (Dobkins & folk, 1970) is based on the proposition that 0.66 MPSI line best separates beach from river pebbles whereas values above 0.66 suggest fluvial origin.

2.2 Paleocurrent Analysis

Azimuths of sixty- six (66) planar cross beds measured from the Nanka Formation in the study area were plotted into a rose diagram using the class interval of 20°. The analysis provided significant insights into the paleoenvironment of deposition of the Nanka Formation.

RESULTS AND DISCUSSION

3.1 Pebble Morphometry

Figure 2 shows the pebbly sandstone unit of the Nanka Formation exposed along St. Lawrence Anglican Church Road Mbaukwu. The pebbles are sub- angular to sub- rounded in shape and of varying sizes and consists predominantly of quartz pebbles.

The values of the three axes (long, intermediate and short) of all the four-quartz pebble sets from the formation, their axial ratios, morphometric parameters and pebble forms are presented in Tables 2- 5. Averaged values of the pebble forms for the four pebble sets (Table 5) indicated that 50.05% of the pebbles are compact (C), 27.42% are compact platy (CP), 10.09% are compact bladed (CB), 8.96% are Platy (P), 1.92% are compact elongate (CE), 1.26% are bladed (B) and 0.3% are elongate (E). This shows that majority of the pebbles are compact, compact platy and compact bladed.

Fig. 3:  Pebbly sandstone of the Nanka Formation exposed along St. Lawrence Anglican Church Road Mbaukwu (Latitude: 6°9’8.88” N and Longitude: 7°4’5.66” E)

Table 2: The values of the three axes of Pebbles set 1, their axial ratios and morphometric parameters

C= 36.97%, CP= 26.89%, CB= 19.33%, P= 10.92%, CE= 3.36% and B= 2.52%

Table 3: The values of the three axes of Pebbles set 2, their axial ratios and morphometric parameters

C= 33.75%, P = 23.75%, CB= CP= 17.50%, CE= 3.75%, B= 2.50% and E= 1.25%

Table 4: The values of the three axes of Pebbles set 3, their axial ratios and morphometric parameters

C= 60.23, CP= 34.50%, CB= 3.51%, P= 1.17% and CE= 0.58%

Table 5: The values of the three axes of Pebbles set 4, their axial ratios and morphometric parameters

C= 69.23% and CP= 30.77%

Table 5: Averaged form values (%) for pebbles sets 1- 4

Dobkin and Folk (1970) and Gale (1990) noted that some particle form classes occur much more frequently in one environment than they do in another. Platy, very platy and very bladed are more diagnostic of beach, bladed and platy predominate in high energy beaches while blade dare most common in low energy beaches. Compact, compact bladed and compact elongate are most indicative of fluvial origin. The dominance of compact, compact platy and compact bladed forms in the investigated pebbles of the Nanka Formation suggests fluvial origin.

3.1.1 Sphericity Form Diagrams for the Pebble Sets

According to Dobkin and Folk (1970), beach pebbles plot towards the left and bottom parts of the sphericity form diagram whereas fluvial pebbles plot near the upper parts of the form diagram.

The form diagrams for the Pebbles of Nanka Formation is presented as Figure 4. Over 88% of the pebbles are compact, compact platy and compact bladed and they plot near the upper parts of the form diagrams. This agrees with the fluvial origin for the pebbles. However, less than 12% of the pebbles are platy, bladed and elongate.

Fig 4: Sphericity Form diagrams for Nanka pebbles (A) For pebble set 1(B) For pebble set 2 (C)For pebble set 3 (D)For pebble set 4

3.1.2 Bivariate Scatter Plots

The bivariate scatter plots of MPSI vs OPI for the four pebble sets in this study (Fig.5) show that over 98% of the pebbles were deposited by river process and less than 2% by beach. The bivariate Scatter Plots of FI vs MPSI (Fig. 6) show that over 95% of the pebbles are fluvial deposits while less than 5% are deposited by beach process. This is in line with the sphericity form diagrams for the pebbles.

Madi and Ndlazi (2020) noted that the occurrence of a small proportion of beach pebbles in a river dominated pebbles suggests that the river reached the marginal marine environment during it’s flow. Thus pointing the environment of deposition of the Nanka Formation to marginal marine.

Fig 5: A plot of MPS versus OPI after (Dobkins and Folk 1970)): (A) For pebble set 1(B)For pebble set 2 (C)For pebble set 3 (D)For pebble set 4

Fig 6: Plot of FI vs MPSI for Nanka Pebbles (A) For pebble set 1(B) For pebble set 2 (C)For pebble set 3 (D)For pebble set 4.

3.2 Paleocurrent Analysis

Table 6 shows the azimuths of the cross beds measured from the outcrops of the Nanka whereas Figure 7 is the rose plots of the cross-bed azimuths for paleocurrent evaluation. The rose diagrams indicated a bimodal- bipolar paleocurrent pattern which are in the northeasterly and southwesterly directions and thus, suggestive of tidal dominance.

Table 6ai: Azimuths of planar cross-beds from the Nanka Formation

Table 6aii: Azimuths of planar cross-beds from the Nanka Formation

Table 6bi: Azimuth and frequency distribution table

Table 6bii: Azimuth and frequency distribution table

Fig. 7: Rose diagrams of planer cross beds for the Nanka Formation: (A) For set 2 (B) For set 1.

CONCLUSION

Pebble morphometry and paleocurrent data employed in this study for environmental diagnosis of the Nanka Formation have shown that the formation was deposited in a tide- dominated marginal marine environment. This interpretation is in agreement with the results from the lithofacies and particle size distribution data employed by previous workers.

REFERENCES

1. Benkhlil, J. (1989). The origin and evolution of the Cretaceous Benue Trough, Nigeria. Journal of African Earth Sciences 8: 251- 282.
2. Burke, K. (1972). Longshore drift, submarine canyons, and submarine fans in development of Niger Delta. AAPG Bulletin 56: 1975- 1983.
3. Burke, K., Dessauvagie, T.F.G. and Whiteman, A.J. (1972). Geological history of the Benue valley and adjacent areas. In: Dessauvagie, T.F.G. and Whiteman, A.J. (Eds.): African Geology. Ibadan University Press, Ibadan, 187- 205.
4. Dobkins, J.E. and Folk, R.L. (1970). Shape development on Tahiti- Nui. Journal of Sedimentary Petrology 40: 1167- 1203.
5. Ekwenye,O.C. and Onyemesili, O.C. (2018). Unraveling the sedimentary facies of the tidal channel and tidal flat deposits within a microtidal estuarine setting: the Nanka Formation, southeastern Nigeria. The Pacific Journal of Science and Technology 19: 367- 388.
6. Ekwenye,O.C.,Nichols,G.J.,Collinson,M.,Nwajide,C.S.andObi,G.C.(2014).ApaleogeographicmodelforthesandstonemembersoftheImoShale,southeasternNigeria.JournalofAfricanEarthSciences96:190-211.
7. Gale, S.J. (1990). The shape of beach gravels. In: Douglas, W.L. and David, M. (Eds.); Practical Sedimentology, 114- 126.
8. Hoque, M. (1977). Petrographic differentiation of tectonically controlled Cretaceous sedimentary cycles, southeastern Nigeria. Sedimentary Geology 17: 235- 245.
9. Hoque, M. and Nwajide, C.S. (1984). Tectono- sedimentological evolution of an elongate intracratonic basin (Aulocogen): the case of the Benue Trough of Nigeria. Nigeria Journal of Mining and Geology 21: 19- 26.
10. Luttig, G. (1962). The shape of pebbles in the continental, fluviatile and marine facies. International Association of Scientific and Hydrogeology Publ. 59: 235- 258.
11. Madi, K. and Ndlazi, N.Z. (2020). Pebble morphometric analysis as signatures of the fluvial depositional environments of the Katberg Formation near Kwerela River around East London, Eastern Cape Province, South Africa. Arabian Journal of Geosciences 13(5):235
12. Mode, A.W. and Onuoha, K.M. (2001). Organic matter evaluation of the Nkporo Shale, Anambra Basin, from wireline logs. Global Journal of Applied Science 7: 103- 107.
13. Murat, R.C. (1972). Stratigraphy and paleogeography of the Cretaceous and lower Tertiary in southern Nigeria. In Dessauvagie, T.F.J. and Whiteman, A.J. (Eds.): African Geology, University of Ibadan Press, Ibadan, 251- 266.
14. Nwajide, C.S. (1979). A lithostratigraphic analysis of the Nanka Sand, southeastern Nigeria. Nigeria Journal of Mining and Geology 16: 103- 109.
15. Nwajide, C.S. (2022). Geology of Nigeria’s sedimentary basins. 2nd Ed. Albishara Educational Publ. Enugu, Nigeria. 693p.
16. Nwajide, C.S. and Hoque, M. (1982). Pebble Morphometry as an aid in environmental diagnosis: an example from the middle Benue Trough. Journal of Mining and Geology 19: 114- 120.
17. Obi, G.C., Okogbue, C.O. and Nwajide, C.S. 2001). Evolution of the Enugu Cuesta: a tectonically driven erosional process. Global Journal of Pure and Applied Science 712: 321- 330.
18. Ocheli, A., Okoro, A.U., Ogbe, O.B. and Aigbadon, G.O. (2018). Granulometric and pebble morphometric applications to Benin Flank sediments in western Anambra Basin, Nigeria: Proxies for Paleoenvironmental reconstruction. Environmental Monitoring and Assessment 190: 1- 17.
19. Odumodu, C.F.R. and Israel, H.O. (2014). Pebble for indices as Paleoenvironmental reconstruction tools for the Ogwashi- Asaba Formation, southeastern Nigeria. International Journal of Geology, Earth and Environmental Sciences 4: 149- 159.
20. Odumodu, C.F.R. and Mode, A.W. (2014). The Paleoenvironmental significance of Chondrites and other trace fossils from the Eocene Nanka Formation, southeastern Nigeria. Journal of Mining and Geology 50: 1- 9.
21. Ogala, J.E., Adaikpoh, E.O., Omo- Irabor, O.O. and Onotu, R.U., (2010). Granulometric analysis and pebble morphometric studies as indicators of depositional environments of the sandstone facies around Okanyan and environs in the Benin Formation, southwestern Nigeria. World Applied Journal 11: 245- 255.
22. Ogbe, O.B. and Osokpor, J. (2021). Depositional facies, sequence stratigraphy and reservoir potential of the Eocene Nanka Formation of the Ameki Group in Agu- Awka and Umunya, southeastern Nigeria. Elsevier Heliyon Journal 7(1): 1- 15.
23. Ogbe, O.B., Osokpor, J., Emelue, C.V. and Ocheli, A. (2023). Morphometric analysis of pebbles in verification of transport processes and interpretation of Paleoenvironment: A case study from the Ogwashi Formation (Oligocene), Niger Delta, Nigeria. Geologist 29(1): 21- 31.
24. Oguadinma, V.O., Okoro, A.U. and Odoh, B.I. (2014). Lithofacies and textural attributes of the Nanka Sandstone (Eocene): proxies for evaluating the depositional environment and reservoir quality. Journal of Earth Science and Geotechnical Engineering 4: 1- 16.
25. Ojong, R.A., Sugar, S.I., Odong, P.O., Otele, A. Chimezie, N. and Itowe, K. (2023). Pebble Morphometry and sieve analysis: tools in determining the depositional environment of sediments in Oron and environs. Global Journal of Geological Sciences 21: 201- 213.
26. Okoro, A.U., Onuigbo, E.N., Akpunonu, E.O. and Obiadi, I.I. (2012). Lithofacies and pebble morphogenesis: Keys to Paleoenvironmental interpretation of the Nkporo Formation, Afikpo sub- basin, Nigeria. Journal of Environment and Earth Sciences 2: 26- 37.
27. Olade, M.A. (1975). Evolution of Nigeria’s Benue Trough (Aulocogen): a tectonic model. Geology Margarine 12: 575- 583.
28. Olugbemiro, R. and Nwajide, C.S. (1997). Grain size distribution and particle morphogenesis as signatures of depositional environment of Cretaceous (non ferruginous) facies in the Bida Basin Nigeria. Journal of Mining and Geology 33.
29. Oluwajana, A.O., Ugwueze, C.U., Ogbe, O.B. and Egunjobi, K.J. (2021). Pebble morphogenesis of the Cretaceous conglomerates from the Abeokuta Formation near the Oluwa River, eastern Dahomey Basin, southwestern Nigeria. Mining- Geology- Petroleum Engineering Bulletin 36: 93- 104.
30. Onuigbo, E.N., Ajaegwu, N.E., Obiadi, I.I. and Okolo, C.M., 2010. Lithofacies analysis and textural characteristics of Eocene Nanka Sand, southeastern Nigeria. Natural and Applied Science Journal 2(1): 64- 74.
31. Onyemesili, O.C. and Odumodu, CF.R. (2019). Pebble Morphometric study of the Ogwashi- Asaba Formation at Ubakala and environs in southeastern Nigeria. Journal of Natural Sciences Research 9: 36- 50.
32. Sames, C.W. (1966). Morphometric data of some recent pebble associations and their application to ancient deposits. Journal of Sedimentary Petrology 36: 125- 142.
33. Short, K.C. and Stauble, A.J. (1967). Outline of Geology of the Niger Delta. AAPG Bulletin 51: 761- 779.
34. Sneed, E.D. and Folk, R.L. (1958). Pebbles in the lower Colorado River, Texas: a study in particle morphogenesis. Journal of Geology 66: 114- 150.
35. Wright, J.B., Hastings, D.A., Jones, W.B. and Williams, H.R. (1985). Geology and mineral resources of west Africa. George Allen and Unwin, London, 187p