Elsevier

Quaternary International

Volume 657, 30 May 2023, Pages 77-91
Quaternary International

Sedimentological and palynological assessment of three wells in eastern Dahomey Embayment, southwestern Nigeria, West Africa

https://doi.org/10.1016/j.quaint.2021.07.010Get rights and content

Abstract

Integrated studies were carried out on one hundred and eleven (111) ditch samples retrieved from three wells drilled in Lagos area within the Dahomey Embayment, coastal southwestern Nigeria. The purpose of this study is to determine the provenance, heavy mineral assemblage(s), establish the lowstand - highstand episodes, Quaternary palaeoenvironment and date the encountered lithofacies which constitute sandstones, siltstones with thin beds of lignite. The textural parameters show that the sandstones are medium to fine grained with the lignite consisting of plant fragment remains. Petrographic analyses showed abundant detrital minerals with average modal composition of quartz (Q = 90%), feldspar (F = 2%) and rock fragments (RF = 4%). The QFR ternary plots revealed that the sandstones are quartz arenites of continental block provenance origin deposited under humid climatic setting. This lithofacies has undergone medium distance range of transportation and have been subjected to mechanical and chemical weathering prior deposition. The heavy mineral assemblages and the ratios of Zircon-Tourmaline (ZTR index) in the wells suggest matured sediments. The Apatite: Tourmaline (ATi) ratios showed variations within the studied wells indicating differences in rate of weathering. It is evident that the periods of residence on the floodplain despite similar provenances and fluctuations in ratios, suggest sea level changes. The fluctuations in apatite: tourmaline ratios with respect to weathering are attributed to rapid changes in climatic conditions during the floodplain residence. The high ATi values in sandstone were deposited during relative sea level highstand (transgression), while sandstones with low ATi were deposited during lowstand (regression) in Nigeria and along the West African region. The abundance species of Rhizophora sp., Avicennia africana, Acrostichu mauruem, Laevigatosporite sp., Combretaceae sp., Mitrogynaciliata, Sapotaceae sp., Disopyros sp., Elaeis guineensis and Alchronea spp. indicated depositional transition from mangrove to fresh water swamp due to the episodic highstand and lowstand regimes. The abundance of Podocarpus milianjanus and Retitricolporites hians and the absence of Miocene diagnostic forms suggest late Pleistocene (Tarantian = 0.012–0.126 Ma) deposition.

Introduction

Generally, the Dahomey Embayment is a transitional inland/coastal/offshore basin that stretches from southeastern Ghana through Togo and the Republic of Benin to southwestern Nigeria; it is separated from the Niger Delta by the Okitipupa Ridge and Benin Hinge Line, although the offshore extent is poorly defined (Obaje, 2009, Wilson and Willians, 1979; Onuoha, 1999, Coker and Ejedawe, 1987). The lithofacies emplacement follows an east-west trend. In the Republic of Benin, the geology is fairly well known (Billman, 1976; De Klasz, 1977). Okosun (1990) reported that Cretaceous strata are about 200 m thick in the onshore axis.

The study of the palynomorph evidence in an area can provide information on the climate of the area especially in the absence of body fossils. Heavy mineral assemblages are sensitive indicators of sediment provenance. A large number of heavy mineral species have been found in sandstones, many of which have restricted paragenesis that provide a positive indication of the mineralogical composition of the source region (Mange and Maurer, 1992). Differences in heavy mineral assemblages facilitate the categorization of sand bodies derived from different sources via different sediment transport pathways. Stratigraphic changes in heavy mineral assemblages provide a basis for correlation, independent of traditional biostratigraphic and log correlation methods.

The composition of heavy mineral assemblages is not, however, entirely controlled by source rock mineralogy, because the provenance signal can be modified by a variety of processes (weathering, abrasion, hydrodynamics, and diagenesis) that operate during the sedimentary cycle (Morton and Hallsworth, 1999). Apatite is a key mineral in establishing the effect of weathering the effects of weathering during floodplain residence, because it is stable during burial diagenesis, but unstable during weathering. Other minerals known to be unstable during weathering, such as olivine, pyroxene, and amphibole (Bateman and Catt, 1985; Lang, 2000), are also unstable during burial diagenesis (Morton and Hallsworth, 1999); by contrast, some minerals (notably sillimanite, andalusite, kyanite, and staurolite) are comparatively stable during weathering (Bateman and Catt, 1985; Velbel, 1999) but moderately to highly unstable during burial diagenesis. Variations in the abundance of apatite may be used as a guide to variations in the extent of weathering during sediment transport history, providing that the changes in provenance and hydrodynamic conditions are ruled out. However, this kind of study has not yet been conducted, or reported in any published article from the Nigeria Sector of the Dahomey Basin, hence this study attempts to apply these diverse methods to sediments from three wells, estimate the period of floodplain residence, establish the age(s) of the lithofacies, determine the palaeoenvironment and correlate the lignite unit occurrences within the well sequences.

Section snippets

Location and local geology of the study area

The study areas are from three wells within the Benin Basin, eastern Dahomey Embayment. These wells are drilled in Lagos within longitude of 3° 27″E and latitude of 6° 35″N. The samples were collected from Ikeja, Banana Island and Tin Can Island Apapa Wharf. These areas are linked by major and minor roads as well as the seaport of Apapa, Lagos. (Fig. 1, Fig. 2). The geology of the Lagos area has often been deciphered from various reports and publications made in the past by various workers in

Previous works

Extensive studies can be found on the stratigraphy of the Dahomey Embayment; however, studies about the Quaternary sediments are rare. Palynological and heavy mineral analyses have been used by different workers in the area to determine the chronology and environment of deposition as well as the provenance of the sediments respectively.

Salami (1987) reported from microfloral analysis that the basal units of the Araromi Formation, that are mainly shale in composition are considered to be the

Materials and methods

Samples were systematically collected across the study area from Ikeja (6° 36° N, 3° 21° E) via Banana Island (6° 27° N, 3° 27° E), and Tin Can Wharf (6° 27° N, 3° 21° E) via the GPS (Garmin Ventures HCx), with a total depth of approximately 116m, 108m and 220m respectively. The sampling intervals were 3m for wells 1 and 2 and 6m for well 3. The samples were subjected to the sedimentological and palynological analyses. The petrography, mineralogy, heavy mineral techniques followed standard

Lithostratigraphy

The lithostratigraphic sequences were described from the oldest units (basal units) to the youngest units (topmost units). The lowermost portion of Well-1 is comprised of soft layer black lignite with plant fragments. This layer is overlain by medium grained to fine grained sandstones (fining upwards sequence) unit of about 54m thick. This unit (sandstone) is directly overlain by soft, grey, and well indurated thin layer of siltstone (6m thick). Overlying this unit (sandstone) is siltstone (45m

Conclusions

The sedimentological and palynological studies of Quaternary sediments in the three wells drilled in the Lagos area of Dahomey Basin has shown that the lithofacies constitute conglomerate, sandstone, clay and siltstone facies. The sandstone indicated texturally and mineralogically mature quartz arenite with quartz having an average value of 91%, while feldspar and rock fragment ranged from 2% to 3% and 2%–4% respectively, most of which were altered via chemical and/or mechanical processes

Author contributions

Olugbenga A. Boboye: Conceptualization, Methodology, Investigation, Writing - Original Draft, Field data and sample collection, Funding acquisition, Supervision, Review & Editing. Christabel Chidiebere: Original Draft, Field data and sample collection and sample analysis.

Data availability

All necessary data set will be provided as per request.

Every other data set are available in Table 1, Table 2, Table 3A, Table 3B or upon request.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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