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GEOLOGY, ECOLOGY, AND LANDSCAPES 2020, VOL. 4, NO. 4, 251–256 INWASCON https://doi.org/10.1080/24749508.2019.1633218 RESEARCH ARTICLE Heavy mineral analysis of Eocene sands and sandstones of Nanka Formation, Cenozoic Niger Delta petroleum province a b Osuwake Omini Etimita and Francis Thomas Beka a b NABDA, RCBBR Centre, University of Port Harcourt, Port Harcourt, Nigeria; Department of Geology, University of Port Harcourt, Port Harcourt, Nigeria ABSTRACT ARTICLE HISTORY Received 24 November 2018 The heavy minerals present in lithologic sand and sandstone units of Nanka Formation are Accepted 14 June 2019 mainly zircon, rutile, tourmaline, apatite, staurolite, and opaque minerals (Goethite, hematite, ilmenite), which are present dominant in weathered sandstones when analysed using petro- KEYWORDS graphic varietal studies to infer provenance, diagenesis and source area weathering history. Heavy mineral; diagenesis; The low-diversity and relative abundance of heavy mineral assemblages are controlled by provenance; weathering; provenance and diagenetic processes, and these present limitations in accurate reconstruc- transportation; mineral tion of the paleo-history of these lithologic units using heavy minerals analysis alone. These assemblages lithologic Eocene units are derived from intensely weathered granitic and metamorphic source rocks and they are composed dominantly of matured polycyclic sediments. Introduction condition (Mange-Rajetzky, 1995; Hawthorne & Henry, 1999;Morton, Knox,&Hallsworth, 2002; Heavy minerals are vital in provenance determination Morton & Hallsworth, 1999;Morton et al., 2004; and understanding of diagenetic processes. Heavy Garzanti et al., 2013). minerals speciﬁc gravity (sg) is greater than 2.89 Zircon morphology has been used as (Bromoform) and they have been studied for various a petrogenetic indicator (Pupin, 1976), and chrome interest by many researchers such as Pettijohn, Potter, spinel composition is reliable for provenance char- and Siever (1973), Morton (1985), Adekola, Akinlua, acterisation (Power, Pirrie, Andersen, & Wheeler, Ajayi, Adesiyan, and Ige (2018), Mange and Wright 2000). Hurst and Morton (2001) stated the relative (2007), Morton (1984), Joevivek, Chandrasekar, and abundance of heavy minerals and its abundance Shree (2018), Mange and Maurer (1992), Morton and ratios. The hydraulic and diagenetic equivalent Hallsworth (2007), Morton (1991), Weltje and von heavy mineral pairs were recommended for prove- Eynatten (2004), and Nwajide (2013). nance studies (Hurst and Morton, 2001;Morton Heavy mineral composition is inﬂuenced by and Hallsworth, 1994). The geochemistry of heavy source area lithology, pre-depositional, and post- minerals reﬂects its provenance (Mange and depositional eﬀects on sediments. Generally, heavy Morton, 2007; Totten & Hanan, 2007; Yang, Jung, minerals form accessory components or essential Choi, & Li, 2001; Zack, von Eynatten, & Kronz, rock minerals and they are less than 3% in sands 2004). Ilmenite is an important economic heavy when highly impacted by weathering. Heavy mineral mineral which has been utilised for provenance assemblages are inﬂuenced by physical properties ﬁnger printing (Asiedal et al., 2000a;Grigsby, (size, shape, and density) that aﬀect selective sorting 1992;Pirkle, Pirkle,&Pirkle, 2007; Pownceby and due to hydraulic eﬀects, and each assemblage con- Bourne, 2006) tains diverse heavy minerals with unique grains that The aim of this study is to evaluate the heavy miner- convey its paleo-history. The reliability of heavy als present in sand and sandstone units of Nanka mineral analyses is aﬀected by source area climate, Formation in other to infer its sediment provenance, physiographic settings, hydraulic factor, diagenesis, diagenesis and weathering history with emphasis on the pH, and abrasion during transportation (Garzanti physical properties which encompasses mineral colour, et al., 2013). inclusions, twinning, striations, overgrowth, zoning, Garnet and tourmaline are mostly used for pro- grain sizes, roundness, etc. They need to evaluate these venance studies due to their relative abundance in heavy minerals properties petrographically at higher sediments, chemical and mineralogical diversity, magniﬁcations (Greater than 40 μm used by previous diﬀerent rock origin, physical and chemical resis- studies in the study), and identify the dominant mineral tance, and stability under severe geological CONTACT Osuwake Omini Etimita email@example.com Department of Geology, University of Port Harcourt, East-West Road, Choba, Port Harcourt P.M.B 5323, Nigeria © 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the International Water, Air & Soil Conservation Society(INWASCON). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 252 O. O. ETIMITA AND F. T. BEKA Figure 1. Map of the study area. composition of lithologic samples using X-ray diﬀrac- Methodology tion analyses in other consolidate present knowledge of Lithology samples were obtained from various out- heavy minerals in this study area (Figure 1). crop locations within the study area and they were analysed mainly on micro-scale using polarising microscopes. Each sample was disaggregated, sieved Geological settings (0.06–0.25 mm size fraction), and the minerals were The study area is located in Anambra state and lithol- separated in funnel using bromoform (sg = 2.89) to ogies evaluated are part of the outcropping units (Table extract heavy minerals. The extracted minerals were 1) of the Cenozoic Niger Delta (Nwajide, 2013). They then mounted on slides. Generally, a relatively large are mainly exposed in gullies and burrow pits. The bulk of samples was chosen for disaggregation to general lithologies in the area are sands, sandstones, obtain suﬃcient amount of heavy mineral grains for heteroliths, and claystones (Nwajide, 1980). this analysis. GEOLOGY, ECOLOGY, AND LANDSCAPES 253 Table 1. Stratigraphy of outcropping units of the Cenozoic Results and discussion Niger Delta (Nwajide, 2013). The heavy minerals (Figure 2) that are dominant in Age Lithostratigraphy units the sandstones are opaque minerals (O), zircon (Z), Oligocene – Present Benin Formation Oligocene – Miocene Ogwashi to Asaba Formation rutile (R), and tourmaline (T) while the sands have Eocene – Early Oligocene Ameki Group Nanka Formation zircon, rutile, tourmaline, apatite (A) and staurolite Ameki Formation Paleocene – Early Eocene Imo Formation (SQ). The heavy minerals present in these litho-units are continuously altered and gradually replaced by (2a) (2e) (2b) (2f) (2c) (2g) (2d) (2h) Figure 2. Photomicrograph of heavy minerals in sands and sandstone units of Nanka Formation. 254 O. O. ETIMITA AND F. T. BEKA opaque minerals, especially in weathered sandstones presence of inclusions (Figure 2(d,e,h)). They are and near surface (Figure 2(a,e)). vitreous, transparent to translucent, pleochroic with The opaque minerals show diverse habit but they yellow brown to pale yellow colour dominance. This are commonly cubic or rhombohedra. The heavy shows that the tourmaline may be irons or magne- minerals are rarely striated with some having octahe- sium bearing (Mange & Maurer, 1992). dral faces, uncommonly well-developed prisms with The sillimanite (S) grains are rare, colourless, penetrative twin that are typically lamellar. white, greenish, and transparent to translucent pris- The relative abundance of heavy mineral is greatly matic grains (Figure 2(b,c)). The apatite identiﬁed inﬂuenced by post-depositional conditions such as dis- shows hexagonal prism or pyramidal habit and is solution and iron replacement in sandstones. commonly transparent to subtranslucent, colourless, Consequently, less stable minerals are removed while and white to grey (Figure 2(f)). ultra-stable minerals like zircon, rutile, and tourmaline Generally, Zircon grains are the most stable non- are mainly preserved. The absence of unstable minerals opaque heavy mineral observed in this lithologic unit probably infers that the sediments source area lithology is (Figure 3). These grains have subangular to sub- a low-grade metamorphic terrain, or it may imply that rounded edges and they are transparent to translu- the sediments are matured, polycyclic, and highly cent, microfractured, inhomogeneous, and impacted by diagenetic dissolution or changes. dominantly pitted with obvious discontinuous zones. The presence of dominantly red wine coloured, The microfractures observed may have resulted non-rounded, and euhedral rutile grains with mainly from grain to grain collision during sediment trans- pyramidal terminations infers high-grade meta- portation or they are products of diagenesis. The morphic source area (Figure 2(d–g)). These pyrami- grains are dominantly sharp euhedral crystals with dal terminations commonly have simple to repeated prismatic and anhedral fragment imprints. This zir- contact twins showing geniculate or cyclic forms con grain abundance suggests granitic and meta- (Figure 2(d)). The rutile present shows irregular or morphic source rock derivations. Goethite was conchoidal breakage patterns (Figure 2(f,g)). signiﬁcantly identiﬁed in disaggregated sandstones The tourmaline has variable habit with dominantly grains sizes that are less than 2 μm using X-ray rounded triangular cross section and they show the diﬀraction analysed (Figure 4). (3a) (3b) Figure 3. Photomicrograph of zircon (Z) grains showing inhomogeneity. Figure 4. Typical X-ray diﬀraction patterns of sandstones <2 µm size fraction. GEOLOGY, ECOLOGY, AND LANDSCAPES 255 Funding This work was supported by the Petroleum Technology Development Fund [2016/2017 LSS PhD Scholar]. ORCID Osuwake Omini Etimita http://orcid.org/0000-0002- 0022-5774 References Adekola, S. A., Akinlua, A., Ajayi, T. R., Adesiyan, T. A., & Figure 5. Microphotograph of staurolite (SQ) inclusions in Ige, D. O. 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Geology Ecology and Landscapes – Taylor & Francis
Published: Oct 1, 2020
Keywords: Heavy mineral; diagenesis; provenance; weathering; transportation; mineral assemblages
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