International Journal of Advanced Research and Publications ISSN: 2456-9992

Provenance, Diagenesis And Paleogeographyof The Late Cretaceous Sediments, Benin Flank (Western Anambra Basin).

Ocheli Azuka, Aigbadon Godwin Okumagbe, Ocheli Paul Chukwujindu

Nnamdi Azikiwe University, , Dept. of Geological Sciences, PH-07036717589

Wesley University of Sciences and Technology, Ondo, Dept. of Applied Geology, PH-08069537273 [email protected]

Delta State Unversity, Abraka, Dept. of Geology, PH-07039551597 [email protected]

Abstract: Integrated textural, thin section, heavy mineral species and paloecocurrent study of Late Cretaceous rocks of the Benin Flank, Nigeria were used to determine textural characteristics, mineralogical composition, paleocurrent direction, provenance and diagenetic history of the rocks in the study area. Textural analysis revealed that the sandstones are of two source derivatives and texturally sub- mature.Thin section analysis revealed the sandstones of the study area to be quartz arenite, sublithic arenite and subarkosic arenite. Petrographic analysis revealed that the sandstones are mineralogically matured and provenance of igneous and metamorphic rocks. Paleocurrent analysis shows bimodal patterns with primary mode in the NE-SW direction and secondary mode in the E-W direction indicating sediment derivation from igneous and metamorphic rocks of the western Nigerian Basement Complex and uplifted . The sandstoneshave undergone compaction, cementation and authigenetic diagenetic change on the basis of mineralogical composition as well as reaching intermediate (Locomorphic) stage based on the corroded quartz grains.

Keywords: Diagenesis; Late Cretaceous; Paleocurrent; Petrographic; Provenance.

1 Introduction current diagrams plotted and interpretation of results The western Anambra Basin, where the study area lies obtained from the laboratory studies. This study willalso corresponds to the western complimentary syncline to the serve as an academic document for local and international emergent Anticlinorium in the Lower Benue purposes as well as increasing the confidence level of the Trough, Southern Nigeria[1]. The area of study is underlain interpretations obtained. by sediments of Cretaceous age which were deposited at the Flank of the basin and exposed in gullies, roadcuts, quarry 2 Location and Accessibility sites and river channels. The rocks bear evidence of their The area of study is within the Cretaceous sediments. It is original mineralogicalcomposition, heavy mineral species situated in the western arm of Anambra Basin, Nigeria. The and cross-bed azimuths. These indicators serve as primary study area lies between latitudes 7000'30” N and 7012'30” N tool for determining provenance as well as their diagenetic and between longitudes 60 08' 30” E and 60 44’ 30” E (Fig. history. It is obvious that detailed information on the 1), bordered to the North by Aigiere, to the south by Egboto, provenance and diagenetic changes within any rock is of to the west by Ekpeshi and to the east by Agenebode. The invaluable significance in source area and diagenetic study area is traversed by both major and minor roads such evaluations and other aspects of surface geological studies. as the Auchi – Okene expressway, Auchi – Imiegba road Unfortunately, past studies of this nature within the Anambra enhanced the accessibility to the outcrop locations. Within Basin often relate to regional basis [2]. Moreso, the area, footpaths made accessibility area easier. previousprovenance and diagenetic studies from the western Anambra Basin have not been well documented and 3 Geological Setting published hence it forms the major focus of this study. The Anambra Basin has received enough attention Provenance study of sandstones has been used on both particularly within the past thirty years. The Benin Flank maturity index (ZTR Index).The integration of petrography where the study area lies is the western arm of the Anambra and paleocurrent data of sedimentary rocks have been used Basin which extends west wards across the River Niger from to reveal the source rocks of the sedimentary basins and Agenebode, running westward through Fugar, Auchi, Ifon, paleoclimatic conditions [3]and were applied in this study. Okada and Ohosuonlapping on to the Okitipupa High which This study involves the determination of the paleocurrent forms the subsurface boundary between the Anambra Basin direction for the study area by measuring the azimuths and and Dahomy Basin to the west. It rims the western Basement dips of cross-beddings and then systematic collections of Complex North of the area and the Niger Delta to the South sandstone. Representative samples of these rocks collected [4]. The Late Cretaceous Post Santonian Formations present were further subjected to various laboratory studies which in the central parts of the Anambra Basin have pinched out included textural analysis, thin section analysis and heavy midway into the Benin Flank area. This could be attributed mineral separation.Provenance, maturity index anddiagenetic to either erosion or non-deposition [5]. The lithostratigraphic changes of parts of the western Anambra Basin rocks have units in the Benin Flank are hereunder discussed; been established and documented in this study using the combination of cross-bed azimuth measurements, rose

Volume 1 Issue 4, Oct 2017 110 www.ijarp.org International Journal of Advanced Research and Publications ISSN: 2456-9992 3.1 The LokojaBassange Formation (Upper Campanian) [6] were first to identify the LokojaBassange Formation. According to [7], it represents the east of the River Niger and South of the confluence that directly overlying the basement.

0 1 11 0 1 11 6 08 30 E 6 44 30 E 0 1 11 0 1 11 7 12 30 N T h. Sc 7 12 30 N o ALImieg O Imekeh. 1 k lu ba Sc e AL h. AL n 3 2 e AL Okpek 4 ALpe 5 River AL 6 Ojo Sc AL 7 AL h. 12 Imak Unuo a ke Ogbi da AL Afara 13 AL Udo 8 ni AL Ogbo Avia Iviot AL AL Sc 14 na ha 9 Ogbo 16 A h.Sc Sc u h. na c AL h. AL Sc hi AL 10 Sc 21 - 15 h. h. AL Ayogw I AL 19 Ugbe bi 20 iri AL lo no 11 Irekp RALJattu AL o 17 a 23 a Sc Afash Fugar d AL AL h. io 22 18 AUCH AL 24 C AL Sc I h. Sc 25 h. h.

Jedda Iyakpe 0 1 11 7 00 30 N 0 1 11 0 1 11 7 00 30 N 6 08 30 E 0 1 11 6 44 30 E

Key

Major Roads Sample Location

Minor Roads River Bridge

Towns/Villag 0 2 4k e m

Fig. 1: Map Showing Sample Locations and Accessibility.

It underlies the Mamu Formation in the study area. Its 3.2 The Mamu Formation (Lower Maastrichtian) contact with the basement-complex rocks, stratigraphic The Mamu Formation outcrops extensively in the Anambra position and areal extent has been used to assign it to the Basin, Southern Nigeria. The Formation was first published Nkporo Group. by [8] and was named the “Lower Coal Measures”. Lithologically, it consists of an alternating succession of coarse to fine-grained sandstones, dark shale, coal seams at various horizons and thin beds of limestone towards the top [9]- [10]. The thickness of theMamu Formation is about Volume 1 Issue 4, Oct 2017 111 www.ijarp.org International Journal of Advanced Research and Publications ISSN: 2456-9992 610m [11]. Maastrichtian age was assigned to the Mamu 4.3 Heavy Mineral Analysis Formation based on fauna evidence, [12]. The depositional Thirty (30) disaggregated samples were selected for heavy environment of the Mamu Formation ranges from paludal minerals separation to reveal the composition of opaque and with channels to marginal marine [9], [13] - [15]. The Mamu non -opaque heavy mineral species in the rocks collected Formation is underlain by the Nkporo Group and overlain by from the study area. Preparation was based on the method the Ajali Formation. described by [26]. Examination and identification of the heavy minerals were carried out under a transmitted light flat 3.3 The Ajali Formation (Mid-Maastrichtian) stage petrographic microscope on the basis of their optical Ajali Formation was originally mapped as white false- properties. The number, size and shape of the different bedded Sandstone and eagle rock sandstone [16] - [17] opaque and non-opaque minerals were noted; the percentage renamed it as the False-Bedded Sandstone. [9] was the first of these minerals was also estimated. Maturity index or to use the name “Ajali Sandstone” to it. The Ajali Formation “ZTR index” [27]) was calculated using the formula stated comprises of white, friable, coarse-grained, moderately to by [27]. The ZTR index is used as a scale for the estimation poorly sorted, cross-bedded sandstone with locally thin beds of the degree of modification, or maturity of the entire heavy of variegated, rarely carbonaceous claystones[9], [13]. Some mineral assemblages of the sandstone. of the claystones may appear to be beautiful plant impressions. Burrows, borings and other biogenic structures 4.4 Paleocurrent Analysis also occur. The stratigraphic position of the Formation and The ancient current direction of the transporting medium for its field relations with the underlying Mamu Formation and the sediments in the study area was achieved through overlying Nsukka Formation suggest Mid-Maastrichtian age measuring the azimuths and dips of cross-beddings for the Formation [9]. Depositional environment has been usingcompass. The cross-bed azimuthmeasurements were inferred to be fluviatile with a stretch intodeltic regimes [18]. plotted as rose paleocurrent diagrams at each sample location. A paleocurrent rose diagram was also plotted for 3.4 The Nsukka Formation (Upper Maastrichtian - the overall azimuth measurements to obtain a regional Paleocene) paleocurrent direction. The Nsukka Formation was originally delineated as stratigraphically synonymous to the Upper Coal Measures 5 Result [8], [9], [19] which spans the Maastrichtian to the earliest Paleocene while Murat [5] considered it as Maastrichtian – 5.1 Result of the Grain size Analysis Danian age. It conformably overlies the Ajali Formation and The results obtained from the grain size analysis was used to occurs on the gentle western slope of the transition from the plot histograms involving individual frequency percentage Udi Plateau to the Niger - Anambra lowlands. It reflects the against grain size (Ø) for the formations in the study area beginning of the “Sokoto transgression” [20]. According to (Fig. 2) which show unimodal and bimodal sources. The [9], the Nsukka Formation laps on the Ajali Formation and unimodal shows that the sediments were derived from one overlies the crystalline basement in Okitipupa area of Ondo source while bimodal shows sediments derivative from two State. The Formation consists of alternations of sandstones, sources. shales and coal seams. At the top of the sequence thin limestone occurs. It attains a thickness of 350m [21] - [24]. 5.2 Results ofthe Petrographic Analysis [9], [25] used sedimentological evidence to suggest that the Petrographic studies carried out to assess the mineralogical Nsukka Formation represented fluvial-deltaic sedimentation composition of the sandstones revealed that they are that began close to the end of Maastrichtian and continued composed of 80.8% quartz, 2.9% feldspar, 1.9% rock during the Paleocene. [13] inferred the depositional fragments, 1.7% mica, 7.6% matrix, 3.4% cement and 1.7% environment of Nsukka Formation from surface exposure to unfilled void for the Ajali Formation (Tables 1 and 2), 77.6% be similar to that of Mamu Formation, which is of quartz, 4.4% feldspar, 3.7% rock fragments, 2.0% mica, strandplain marsh origin with occasional fluvial incursions. 5.2% matrix, 4.6% cement and 2.5% unfilled void for the Mamu Formation (Tables 1 and 2) and 74.5% quartz, 5.8% 4 Materials and Methods feldspar, 5.6% rock fragments, 1.6% mica, 6.5% matrix,

4.1 Grain Size Analysis The grain size analysis was aimed at determining grain size distribution of the sediments in the study area. Fewsamples were selected for the grain size analysis using sieving method. Their weight percentages were determined and used to plot histograms.

4.2 Petrographic Analysis Thirty (30) thin sections were made from selected lithologic units to reveal the mineralogical composition of the rocks. The thin section preparation was based on the method described by [26].The prepared thin sections were examined under a flat stage petrographic microscope for mineral identification and estimation of their relative abundance. Photomicrographs of diagnostic properties were also taken.

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Sample No.: AL20/SST01 Pattern: Unimodal

) Sample No.: AL6/SST/01

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Sample No.: AL21/SST/01 Sample No.: AL3/SST/01 Pattern: Bimodal Pattern: Unimodal

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Fig. 2: Histogram Plots of Cumulative Frequency against Grain Size (Ø-scale)

4.3% cement and 1.7% unfilled void for the LokojaBassange basis of QT/F + RF ratio [28] - [30]. From Table 2, the Formation (Tables 1 and 2). The sandstones are sub-angular quartz arenites show a maturity index of 27.6 – 31.3 while to angular with few sub-rounded grains (Plate 1). On the that of the sublithicarenites and subarkosicarenites are 5.0 – basis of framework composition of quartz (Q), feldspar (F) 15.9 and 2.5 – 18.6which are interpreted as supermature and and rock fragment (RF), (Table 2) shows the recalculated submature. According to [31], quartz arenites are both (normalized) percentage frequency of the detrital framework mechanically and chemically stable, the sublithicarenites are grains. The average percentage of polycrystalline quartz both mechanically and chemically less stable whereas (QP) ranges from 8.9% – 9.7%. According to [28] mode of subarkosicarenites are mechanically stable and chemically sandstone classification, the sandstones aregenerally arenites less stable (Fig. 3). since they are composed of less than 15% matrix. The ternary diagram (Fig. 3) plotted for the sandstones in the study area shows that the sandstones are quartz arenites, sublithicarenites and subarkosicarenites. The mineralogical maturity index (MMI) of the sandstone is calculated on the Volume 1 Issue 4, Oct 2017 113 www.ijarp.org International Journal of Advanced Research and Publications ISSN: 2456-9992

Table 1:Result of Thin Section Analysis Showing the Mineralogical Composition of the Sandstones in the Study Area.

MQ: Monocrystalline Quartz KP: Potassium Feldspar PQ: Polycrystalline Quartz TR: Total Rock Fragment TQ: Total Quartz PF: Plagioclase Feldspar M: Metamorphic Fragments DM: Detrital Mica I: Igneous Fragment CEM: Cement UV: Unfilled Void

The ternary diagram (Fig. 4) of the compositional framework 5.3 Results of the Heavy Mineral Assemblages grain plotted shows that the sandstones in the study area are Eleven (11) heavy mineral species have been recognized from both igneous and metamorphic sourcesunder humid (Table 3), which include both non-opaque (zircon, paleoclimatic condition. Most of the sediments in the study tourmaline, rutile, staurolite, sillimanite, kyanite, garnet, area have two provenances, the continental block provenance hornblende, apatite, epidote) and opaque minerals (Fig. 7). and the recycled provenance (Fig. 4). Inthe ternary plot the compositionalframework grains are plotted in the 5.4 Results of Paleocurrent Analysis craton interior and recycle orogeny fields (Fig.5). It implies The rose current plot (Fig. 8) at most parts of the study area that the sandstones in the craton interior and recycle orogeny exhibit unimodal patterns with low variability, while few fields are mineralogically matured derived from igneous and locations show bimodal patterns. The unimodal patterns metamorphic sources fragment (QT/F+RF) (Fig. 6) has been indicate sediment transport along NE – SW direction, and the used to diagnose the interior and recycle orogeny fields are bimodal patterns reveal both NE – SW and E – W directions. mineralogically matured derived from igneous and metamorphic sources fragment (QT/F+RF) (Fig. 6) has been used to diagnose the paleoclimate regime of provenance [3], [32]. It shows that the sandstones in the study area are characterized by semi-humid to humid paleoclimatic regimes.

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Table 2:Recalculated Framework Composition of the Sandstones in the Study Area

Quartz arenites: Range from 27.6 – 31.3 Subarkosicarenites: Range from 2.5 – 18.6 Maturity Index Sublithicarenites: Range from 5.0 – 15.9

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F

Fig. 4: Ternary plot of framework modes for the sandstones in the study area.

QT

Craton Interior

Transitional Recycled Orogen continental

Dissected Arc Transitional Arc Basement Uplift Undissected Arc F RF

Fig. 5: Ternary Diagram for the sandstones in the study area.

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3. 3.5

0 RF PQ/F+ 2. 25. 10. 15. 0. 05. -0 .5 0 5 10 15 20 25 30 35 0 Arid 0 QT/F+RF0 0 0 0 Semi-Arid Semi-Humid Humid

Fig.6: Binary plot of PQ/RF Versus QT/F+RF for the sandstones in the study area.

Opaque Minerals Total No. of Non Opaque Fig. 7: HistogramMinerals plot showing the percentage of opaque and non-opaque minerals in the sandstones of the study area.

The overall (regional) current vector obtained by plotting Volume 1 Issue 4, Oct 2017 117 www.ijarp.org International Journal of Advanced Research and Publications ISSN: 2456-9992

Fig. 8: Paleogeographic Model of the Study Area.

The overall (regional) current vector obtained by plotting all pattern of grain size distribution suggest sediment derivation cross bed azimuthal values on a single current plot shows a from two sources lying North- east (NE) and easterly (E). fan – shaped pattern (Fig. 8). The fan – shaped pattern These are similarly confirmed by regional (overall) fan – largely indicate sediment transport from NE to SW direction shaped patterns exhibited by the sandstones in the study area. and few sediments transport from E to W direction. The The angular to sub-angular shape of the quartz grains (Plate paleocurrent direction of the sandstonesreveals that the study 1) and the higher percentage of opaque heavy minerals (Fig. area was drained by both a major ancient river flowing in NE 7) depicts a shorter distance of transportation. This is – SW direction, and a minor stream flowing E – W direction supported by the occurrence of feldspar and rock fragments (Fig. 8). These ancient rivers couldhave been responsible for in some of the sandstones because feldspar and rock the transportation of clastic sediments from the Pre – fragments hardly survive long distance of transportation as a Santonian igneous and metamorphic rocks of the western result of their chemical instability. Sub-rounded quartz grains Nigerian Basement Complex and uplifted Benue Trough. are not ruled out as observed in some samples. This may indicate relatively longer distance of transportation. The 6 Discussion of Results occurrences of staurolite, kyanite, hornblende and polycrystalline quartz grains in the sandstones are excellent 6.1 Provenance indicators of metamorphic source [27], [38] - [39]. Provenance analysis of sediments is aimed at deciphering the Polycrystalline quartz has been proved to be a good indicator source area for the sandstones in the study area. It is of provenance [40] - [42]. The framework composition of the achieved by considering grain texture, sandstones in the study area reveals very high quartz content, mineralogicalcomposition, heavy mineral assemblages and and low feldspar and rock fragment contents respectively. paleocurrent directions [33] - [37]. The unimodaland fan – The rock fragments are predominantly igneous and shapedpaleocurrent patterns ( Fig 8) of NE –SW direction metamorphic chips, which if based on their angularity, suggest sediment derivation from a source area lying in the suggest a shorter distance of transportation. The presence of NE Parts of the Study area. The unimodal pattern of grain the igneous and metamorphic chips strongly indicates size distribution also depicts a single source. The bimodal igneous and metamorphic derivatives. The ternary diagram Volume 1 Issue 4, Oct 2017 118 www.ijarp.org International Journal of Advanced Research and Publications ISSN: 2456-9992 (Fig. 4) of the compositional framework grain data plotted can easily be amendable to compaction. T he porosity shows that the sandstones in the study area are from both reduction could be gravitational compaction (overburden igneous and metamorphic sources. Two distinct provenances, pressure) and post–depositional introduction of matrix and the continental block provenance and the recycled orogeny cement. Evidence is seen in the contacts between quartz provenance have been revealed for the sediments in the study which are mainly straight to concavo-convex. [28] suggested area (Fig. 5). The paleoclimatic regime is diagnosed [3], that in an initial deposit, the contacts between grains are [32]. Accordingly, the chemical weathering influence over basically point or tangential in nature. Overgrowth in quartz mechanical destruction of sandstone components has been grains is anevidence of authigenesis observed in the noted by[43]. Chemical weathering under humid and hot sandstones of the study area. The source of the silica (quartz) climate destroys feldspar and rock fragments much faster overgrowth in the sandstone samples could be pressure than mechanical weathering even under short transport and solution as it is evidenced from concavo – convex and high topographic relief [3]. Polycrystalline quartz grains are sutured contacts. Other sources of the silica include not easily destroyed by chemical weathering as mechanical diagenetic changes in inter-bedded shale and mixed – layer weathering does. This may be attributed to the relatively smectite – illite to pure illite in mudrocks [45] - [46]. higher percentage of polycrystalline quartz in the sandstones Haematite (iron oxide) is the main cementing agent on of the study area. The sandstones in the study area are framework grains which appear as shapeless void fillers. It characterized by semi–humid to humid paleoclimatic shows varying degree of adherence to detrital grains often regimes (Fig. 6).The maturity index of the sandstones in the ranging from a loose, or no contact at all, to a very close study area is determined using textural characteristics, adherence along a clearly observable boundary. The mineralogical composition, and the calculated ZTR The corroded quartz grains, the haematite cement, and the shape of the grains also determines the textural maturity of unrecrystallized clay matrix suggest that the diagenetic burial the sandstone. The angular to sub-angular (poor roundness) of the sandstones in the study area has reached intermediate indicate textural immaturity of the sandstone, while the few (Locomorphic) stage. sub-rounded grains indicate that they are sub-mature. Accordingly, the sandstones in the study area are texturally Conclusion immature to submature. In the ternary plot, the compositional Finally, the bulk of the sediments were derived from the framework grains plotted in the craton interior and recycled igneous and metamorphic rocks of the western Nigerian orogeny fields. These sandstones plotted in the craton fields Basement Complex and uplifted Benue Trough.The rocks are are chemically matured sandstone derived from relatively texturally immature to sub mature, minerological and igneous and metamorphic sources, supplemented by recycled chemically matured. Threediagenetic changes occur in the sands from associated platform or passive margin basins. sandstones as compaction, authigenesis and cementation. The mineralogical maturity of the sandstones was calculated Thethesandstones have undergone intermediate using the mineralogical maturity index (MMI) of [28] - [29], (locomorphic) stage of diagenetic burial. the quartz arenites show a maturity index of 27.6 – 31.3 while that of sublithicarenites and subarkosearenites are 5.0 Acknowlegdements – 15.9 and 2.5 – 18.6 respectively. This reveals that the Our profound gratitude goes to Delta State University, sandstones in the study area are mineralogicallysubmature to Abraka, Federal University of Science and Technology, supermature. The high ZTR index values and the high Akure. contents of quartz grains in the sandstone of the study area suggest that the sandstones are matured. Finally, the bulk of the sediments were originally derived from the igneous and REFRENCES metamorphic rocks of the western Nigerian Basement [1] O. 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