Biostratigraphy and Paleogeography of Late Campanian–Early Maastrichtian (Nkporo Formation) Sections in Awgu, Southeastern
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RESEARCH ARTICLE Biostratigraphy and P aleogeography of Late Campanian–Early M aastrichtian (Nkporo F ormation) Sections in Awgu, Southeastern Nigeria Ezike Onyemaobi R*, Ajaegwu Norbert E Ezike Onyemaobi R, Ajaegwu Norbert E.Biostratigraphy and paleogeo encompasses both field and laboratory exercises. The field work involved graphy of Late Campanian–Early Maastrichtian (Nkporo Formation) sections detailed field mapping, outcrop logging and rock sample collection for in awgu, Southeastern Nigeria. J Environ Geol. 2021;5(3):1-16. laboratory utilization. The obtained shale samples were processed in the laboratory for palynological and micropaleontological analysis in order to determine the age and compartimerize the analyzed sections through age ABSTRACT range chart and biozonations. The Owelli Sandstone was interpreted to be Surface Late Cretaceous sediments of the Nkporo Shales of Anambra Basin deposited in a tidally influenced estuary while Nkporo Shale was interpreted have been studied for their foraminifera and palynomorphs contents in to be deposited in a Shore face zone environment. The results from both order to interpret depositional architecture associated with the formation palynological and micropaleontological analysis of Nkporo Formation dated using lithofacies parameters and hence date the shale samples of the the analyzed sections Late Campanian to Early Maastrichtian. formation. The Environments of depositions were interpreted through integration of lithology, textures, sedimentary structures and boundary Key Words: Biostratigraphy; Palynology; Foraminifera; Late Campanian-Early contacts between one bed and another (facies parameters). The work Maastrichtian; Biozonation; Nkporo Formation; Estuarine environment; Shoreface zone. INTRODUCTION Basin contains over 2000 m. of cretaceous marine, paralic and deltaic facies sediments in outcropping sections which extended into adjacent tectonic The Anambra Basin is one of the three tectonic subdivisions of the Southern basins such as the Benue Trough, the Afikpo Syncline and the Calabar Flank Nigeria Sedimentary Basin as recognized [1]. Anambra Basin is a region [2]. Detailed lithostratigraphic compilations, such as Simpson [1, 3-6] among within the Southern Benue Trough bounded by the Abakaliki Anticlinorium others have been presented for the Cretaceous Sediments of the Southern in the east, the Benin hinge line in the west, the Niger hinge line in the Benue Trough including the Anambra Basin. north and the Niger Delta hinge line in the south (Figure 1) [2]. The first of the depositional cycles occurred in the pre-Albian times, and was confined However, the bulk of biostratigraphic studies on surface sediments mainly to the Benue Trough. The second depositional phase dominated in Anambra Basin were based on foraminifera studies. Palynological the Anambra Basin as well as narrow Afikpo synclinorium. The Anambra information is limited for most of the formations in Anambra Basin [2]. The Figure 1) Tectonic Subdivisions of the Southern Nigeria Sedimentary Basin showing the Anambra Basin. Department of Geological Sciences, Nnamdi Azikiwe University, Awka, Nigeria Correspondence: Ezike O R, Department of Geological Sciences, Nnamdi Azikiwe University, Awka, Nigeria, Tel: +234- 8080890766, E-mail: [email protected] Received: March 17, 2021, Accepted: April 02, 2021, Published: April 07, 2021 This open-access article is distributed under the terms of the Creative Commons Attribution Non-Commercial License (CC BY-NC) (http:// OPEN ACCESS creativecommons.org/licenses/by-nc/4.0/), which permits reuse, distribution and reproduction of the article, provided that the original work is properly cited and the reuse is restricted to noncommercial purposes. For commercial reuse, contact [email protected] J Environ Geol Vol Vol 5 No 3 April 2021 1 Ezike Onyemaobi R, et al. significant among the few palynological publications in this basin are those of Basin consists of highly fissile dark grey-black shales with occasionally clay Van Hoeken-Klinkenberg [2, 7-11], on the “coal measures” facies within the and siltstone nodules. Samples were collected from three outcrop sections Anambra Basin. This article therefore serves as a complimentary publication in Anambra Basin. Two of these outcrops are located along Awgu-Mmaku on biostratigraphic literatures on Late Campanian-Early Maastrichtian road and one of the outcrops is located at Community Secondary School exposures in Anambra Basin at Awgu areas. Mmaku. Both outcrops belong to Nkporo Shales (Figure 3). In all, six samples which yielded a rich microfossil assemblage including pollen, spores GEOLOGIC SETTING AND SAMPLE LOCALITIES and foraminifera species were studied. Chronostratigraphic and palaecologic investigations have been carried out MATERIALS AND METHODS on various cretaceous sequences of the Benue Trough using ammonites calcareous nannoplanktons, especially coccoliths and palynomorphs [4, 9, 11- The study is located at Awgu and its environs in Enugu state within 20]. The Earlier workers took the Nkporo Group as the oldest sedimentary the western flank of Abakaliki Anticlinorium, South Eastern Nigeria. deposits in the Anmabra Basin [21, 23]. But recent works opened up the Detailed field mapping was carried out with the aid of the accessibility possibility of having pre-Santonian Sediments, at least at the northwestern (topographic) map (Figure 3), GPS for locating the study sections in the part of the basin [23]. The Nkporo Group forms the basal facies of the Mid map and compass clinometer for determining the trend of the geologic to Late Cretaceous Sedimentary cycle in the southern Benue Trough and boundaries. The laboratory materials used include: transmitted light Anambra Basin, deposited during Late Campanian. The Nkporo Group is binocular microscope, SedLog version 3.0, excel spread sheet and other made up of three members: Afikpo/Owelli Sandstone, Nkporo and Enugu relevant journals for studying of the previous works in the study area. The shales. The arenaceous facies of the Afikpo and Owelli Sandstone are boundaries were inferred from the gradual changes in lithology, vegetation laterally equivalents to the Nkporo Formation in the Afikpo and Anambra and topography. However, the boundaries between one formation and Basin respectively [24]. other were inferred based on the gradational changes in lithofacies. The shale samples collected from three outcrops along Awgu-Mmaku Road and The Nkporo Group is essentially marine sediments deposited by the third Community Secondary School, Mmaku were analyzed for foraminifera and transgressive cycle within the Anambra Basin. Its deposition ended in the palynomorph contents. Available sandstone materials were unsuitable for Early Maastrichtian in both Abakaliki- Anambra and Afikpo synclines foraminifera and palynological analysis due to their extreme weathering and (Figure 2). The Enugu shales are made up of carbonaceous shales and coals low matrix contents respectively. Outcrop logging was carried out at different with upper half deposited in lower flood plain and swampy environment that locations where they are exposed as a result of road cut, erosion and stream overlie the Nkporo Formation [25]. It is found in the North of Awgu and channels. The observation and recording of the features started from the exposed at Milliken Hills at Enugu, restricting the facies to the central and base of the exposures. The geological features recorded are: The lithotypes, Northern parts of the Basin [26, 27]. The sediments have a poorly developed bed thickness, grain size, colour, mineral composition, nature of contact foreshore and shore face with extensive coastal swamps, and were assigned between one bed and the other, the sedimentary structures which include Campanian to Lower Maastrichtian (Figure 2), based on diagnostic species of physical, biogenic and chemical, attitude of the bed and cross bed azimuth. palynomorphs such as Cingulatisporites ornatus and Tricolpites tienebaensis The intensity of bioturbations at different beds was noted while illustrations [28]. Indicated that the deposition of the sediments of the Enugu/Nkporo of their structures were made on the field notebook. Dilute HCl were Formations reflect a funnel-shaped shallow marine setting that graded into applied to beds suspected to contain carbonates for possible confirmation. channeled low energy marshes[25]. Samples were prepared for foraminifera and palynomorphs contents. The The Late Cretaceous Sections studied from the Anambra Basin in Awgu area foraminifers’ preparation was by washing and treating samples of small lie approximately between latitudes 6° 0’ N-6° 8’ N and longitudes 7° 26’- quantity (about 20 g-30 g) of the outcrop sample with one teaspoonful of 7° 30´ E (Figure 3). These rocks which comprise of bioturbated sandstone anhydrous sodium carbonate for thorough disintegration. This was followed and occasional siltstones belong to the Owelli Sandstone member while the by sieving the samples into coarse, medium and fine fractions which were Nkporo Shale which is distributed in the northwestern part of the Anambra later stored in well-labeled sample bags. The samples were finally subjected to mounting procedures. The palynomorphs preparation followed the proper routine process as Carbonate digestion Crushing to 2 mm in a mortar, adding 10% HCL which was stirred until effervescence ceased, thereby allowing it to settle for 5 minutes. Removal of silicates 200 ml of concentrated Hydrofluoric Acid (HF) was added.