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Paleo Environmental Studies for Stream Sediments around Awe-Obi Area North Central Nigeria
Journal of Geology & Geophysics

Journal of Geology & Geophysics
Open Access

ISSN: 2381-8719

+44 20 3868 9735

Research Article - (2015) Volume 4, Issue 2

Paleo Environmental Studies for Stream Sediments around Awe-Obi Area North Central Nigeria

Gajere JN1*, Abaa SI2 and Kana JE1
1Department of Natural Sciences, Nasarawa State Polytechnic, P.M. B. 109 Lafia, Nasarawa State, Nigeria
2Department of Geology and Mining, Nasarawa State University, P.M.B. 1022 Keffi, Nasarawa State, Nigeria
*Corresponding Author: Gajere JN, Department of Natural Sciences, Nasarawa State Polytechnic, P.M. B. 109 Lafia, Nasarawa State, Nigeria, Tel: +2348052900806 Email:

Abstract

A stream’s sediment load is typically deposited, eroded, and redeposited many times in a stream channel, especially during climatic variations such as flooding. During this process of deposition, erosion and redeposition the chemistry of stream sediments changes. Understanding the past environmental conditions under which these sediments have been subjected to are of great significance to geologist interested in exploring for mineral and petroleum resources. Paleo Environmental studies conducted on 17 stream sediment samples in parts of Awe-Obi area using chemical index of alteration (CIA) and chemical index of weathering (CIW) revealed that the area is intense to extremely weathered with calculated values between 67 and 99. The mineralogical index of alteration (MIA) was studied to evaluate the degree of mineralogical weathering. The calculated values revealed that Awe-Obi is intermediate, intense-extremely weathered. Index of compositional variability (ICV) was used in this study as measure of sediment maturity. The calculated ICV values showed all the samples as immature except sample SSB8 which has a value of 0.765515. Paleo-Oxygenation condition of the study area was conducted using Ni/Co ratio. This revealed that all the samples were deposited in an oxygenated condition.

Keywords: Paleo-environment; Stream sediments; Awe-Obi

Introduction

Stream sediments are composites of rock and soil material eroded upstream in a catchment area. A stream’s sediment load is typically deposited, eroded, and redeposited many times in a stream channel, especially during climatic variations such as flooding. During this process of deposition, erosion and redeposition the chemistry of stream sediments changes. Understanding the past environmental conditions under which these sediments have been subjected to are of great significance to geologist interested in exploring for mineral and petroleum resources. Various geochemical proxies have been proposed to estimate the intensity of chemical weathering in continents.

Geochemical weathering proxies make use of the changes of bulk rock geochemical composition caused by chemical alteration. A very simple proxy is the Ruxton Ratio R [1] given by the SiO2/Al2O3 ratio. The ratio assumes that Al2O3 remains immobile during weathering so that changes in R reflect silica loss as a proxy for total element loss. Although Ruxton Ratio may be useful when weathering profiles on rocks of felsic and intermediate composition are considered, it was found to be poorly correlated to the actual weathering grade of a silicate rock [2]. As in hydrolytical weathering, i.e. the transformation of feldspar to clay minerals and the coincident mobility of the main cations is a major process of chemical, Parker [3] considered it more useful to mirror changes in Na+, K+, Ca2+ and Mg2+ and created a weathering index (WIP, Weathering Index of Parker). Although, WIP is considered the most appropriate for application to weathering profiles on heterogeneous and homogeneous parent rock, some implicit assumptions rendered the index limited in application. The WIP implicitly assumes that all Ca2+ in a silicate rock is contained in silicate minerals. More problematic still is the lack of consideration of a relatively immobile reference phase like Al2O3 in the formula which would help to monitor relative changes of composition of the relevant mineral components. The disadvantages of the WIP are overcome in the Chemical Index of Alteration (CIA) using whole rock geochemical data of major element oxides [4]. It represents a ratio of predominantly immobile Al2O3 to the mobile cations Na+, K+ and Ca2+ given as oxides.

Geology

The area of study falls within the Middle Benue Trough and stratigraphically it is composed of continental and marine sediments, represented by sandstone, shale and the limestone of the Asu River Group [5,6]. The succession upward is the Awe formation which consists of transitional sandstone, shale, siltstone and limestone and the fluviatile sandstone of the Keana Formation. Major lead-zinc-baryte mineralization is associated with the mineralized hydrothermal vein that is a consequence of the tectonic rifting that led to the emplacement of the Benue Trough which was in part controlled by transcurrent fault activity [7]. These faultings are thought to have been responsible for creating pathways through which hydrothermal veins of possibly magmatic origin [8] or remobilized meteoric waters enriched in Pb, Zn and Na percolated. Two sets of fractures were recognized in the Keana Azara area, the former of which are associated with the Pb- Zn mineralization while the later was associated with the baryte mineralization [5].

Methods

A total of Seventeen (17) samples were collected in the study area (parts of Awe-Obi North-Central Nigeria) (Figure 1). The samples collected were laid out in pre-numbered evaporating dishes to dry and then placed in low temperature oven and maintained at 105°C for 12 hours. Each sample was disaggregated and homogenized by the use of agate pestle and mortar [9]. A 100 mesh screen was used. This is because in environmental research with the purpose of assessment of total elemental concentration, it is necessary to use a broader screen value compared to that needed in mineral exploration [10]. The homogenized samples were passed through 100 mesh nylon screen. This helped extract metals from the <100 mesh fractions which is considered highly adsorptive fraction. The nylon screen was used to avoid contamination. The samples were digested by the use of aqua regia based on standard methods described in Fletcher (1981) [9]. 0.5 g of the screened samples were weighed out and placed in 20 ml beaker. 10 ml of aqua regia was added and stirred and was gently boiled on hot plate to a volume of 25 ml. 10 ml of deionized water was added and gently boiled to the volume of 5 ml. This was kept to cool and there after filtered into 50 ml measuring cylinder. Beaker and filter paper were washed into the cylinder to a level of 12.5 ml. Deionized water was added to make up to 25 ml. The samples were analyzed for major and trace elements using the Inductive Couples Plasma Optical Emission Spectrometer (ICPMS) technique in Acme Laboratory Ltd Vancouver, Canada.

geology-geosciences-points-study-area

Figure 1: Sample map showing sample points in the study area.

Chemical Index of Alteration

Chemical index of alteration (CIA) was proposed by Nesbitt and Young [4] as a measure of the role played by chemical weathering in the production of clastic sediments. CIA is calculated using this equation: CIA=(Al2O3/Al2O3+CaO*+Na2O+K2O) × 100. (Where CaO* is calcium in silicates). CaO* was corrected using the formula (CaO*=CaO-P2O5 × 10/3). From this formula, if the resultant CaO* value is less than Na2O, CaO* is assumed to be equal to CaO.

The above equation yields CIA values between 50 and 60 for incipient weathering, between 60 and 80 for intense weathering and more than 80 for extreme weathering. CIA for different samples within the study area is given in Table 1 (Figure 2).

Samples CIA value Remarks
SSB6 70.40066 Intense weathering
SSB7 67.98449 Intense weathering
SSB8 78.44681 Intense weathering
SSB9 73.66754 Intense weathering
JSSA 01 91.52102 Extreme weathering
JSSA 02 86.92504 Extreme weathering
JSSA 03 90.20776 Extreme weathering
JSSA 04 96.02682 Extreme weathering
JSSA 05 97.78803 Extreme weathering
JSS 6 82.41515 Extreme weathering
JSSS 1 78.99933 Intense weathering
JSSS 2 87.24457 Extreme weathering
JSSS 3 83.25819 Extreme weathering
JSSS 4 83.52478 Extreme weathering
JSSS 5 83.34418 Extreme weathering
JSSS 7 84.53855 Extreme weathering
JSSS 8 90.46489 Extreme weathering

Table 1: Chemical index of alteration table showing CIA values across samples in parts of Awe-Obi area.

geology-geosciences-Profiles-CIA

Figure 2: Profiles of CIA value across samples of the study area.

Chemical Index of Weathering

Chemical index of weathering is an improved measure of the degree of weathering experience by a material relative to its parent rock. The principal difference between CIA and CIW is that the former treats K strictly as mobile component whereas the latter does not. The value of this index increases as the degree of weathering increases, and the difference between CIW index values of the silicate parent rock and soil or sediment reflects the amount of weathering experienced by the weathered material [11]. CIW for stream sediments were calculated using the formula:

CIW=(Al2O3/Al2O3+CaO+Na2O) × 100)

From this formula, chemical index of weathering, values in the range of 50 to 60 indicate incipient weathering, values between 60 to 80 indicates intermediate weathering, and values above 80 indicate extreme weathering. Calculated values of chemical index of weathering are presented on Table 2 (Figure 3).

Samples CIW Remarks
SSB 6 89.51964 Extreme
SSB 7 94.46959 Extreme
SSB 8 92.81807 Extreme
SSB 9 85.49214 Extreme
JSSA 01 98.57771 Extreme
JSSA 02 95.79366 Extreme
JSSA 03 95.90149 Extreme
JSSA 04 98.62526 Extreme
JSSA 05 99.64495 Extreme
JSS 6 94.63207 Extreme
JSSS 1 89.1634 Extreme
JSSS 2 95.21836 Extreme
JSSS 3 93.66418 Extreme
JSSS 4 92.393 Extreme
JSSS 5 93.10734 Extreme
JSSS 7 91.99573 Extreme
JSSS 8 94.64388 Extreme

Table 2: CIW values for stream sediments.

geology-geosciences-Profiles-CIW-value-across

Figure 3: Profiles of CIW value across samples of the study area.

Mineralogical Index of Alteration

Because CIA ranges from 50-100 and cannot be directly applied for normative calculations, thus it is necessary to calculate the mineralogical index of alteration given by the formula.

MIA=2 × (CIA-50)

The mineralogical index of alteration evaluates the degree of mineralogical weathering, that is, the transformation ratio of a primary mineral into its equivalent alteration mineral. MIA yields value between 0-100, and reflects incipient weathering (MIA<20), intermediate (MIA=20-60) and intense to extreme (MIA>60) mineralogical transformation. The value of 100 means complete transformation. MIA values for different samples within the study area are represented on the Table 3 (Figure 4).

Sample MIA Remarks
SSB6 40.80133 Intermediate
SSB7 35.96897 Intermediate
SSB8 56.89361 Intermediate
SSB9 47.33508 Intermediate
JSSA 01 83.04203 Intense to extreme
JSSA 02 73.85008 Intense to extreme
JSSA 03 80.41552 Intense to extreme
JSSA 04 92.05363 Intense to extreme
JSSA 05 95.57606 Intense to extreme
JSS 6 64.83029 Intense to extreme
JSSS 1 57.99866 Intermediate
JSSS 2 74.48914 Intense to extreme
JSSS 3 66.51638 Intense to extreme
JSSS 4 67.04955 Intense to extreme
JSSS 5 70.68835 Intense to extreme
JSSS 7 69.0771 Intense to extreme
JSSS 8 80.92978 Intense to extreme

Table 3: Mineralogical index of alteration showing MIA values across samples in parts of Awe-Obi area.

geology-geosciences-study-area

Figure 4: Profiles of MIA value across samples of the study area.

Index of Compositional Variability

Because minerals show resistance to weathering, index of compositional variability can be used as a measure of sediment maturity. This index measures the abundance of alumina relative to other major cations in a rock or mineral. ICV for sediments was calculated using the formula:

ICV=(Fe2O3+K2O+Na2O+MgO+MnO+TiO2)/Al2O3

This formula excludes silicates so as to eliminate the problem of quartz dilution. Using this formular, ICV>1 indicates immature sediments, while ICV values <1 indicates matured sediments. Calculated ICV values for samples are presented on Table 4 (Figure 5).

Samples ICV Remarks
SSB 6 1.513434 Immature
SSB 7 2.375354 Immature
SSB 8 0.765515 Mature
SSB 9 1.407367 Immature
JSSA 01 2.523827 Immature
JSSA 02 1.931619 Immature
JSSA 03 3.053004 Immature
JSSA 04 3.83714 Immature
JSSA 05 3.148043 Immature
JSS 6 1.80792 Immature
JSSS 1 2.244322 Immature
JSSS 2 3.134128 Immature
JSSS 3 2.195364 Immature
JSSS 4 4.118853 Immature
JSSS 5 3.821462 Immature
JSSS 7 2.88721 Immature
JSSS 8 1.224251 Immature

Table 4: Calculated ICV values for samples in parts of Awe-Obi area.

geology-geosciences-samples-study-area

Figure 5: Profiles of ICV value across samples of the study area.

Paleo-oxygenation Conditions

Several trace elemental ratios have been used to discriminate paleo oxygenation conditions for sediments. Kroner et al. [12] used the Ni/Co ratio to discriminate whether sediments were deposited in an oxygenated conditions or a reducing condition. Based on their ratio, Ni/Co ratio <5 suggest an oxidizing condition of deposition while Ni/ Co ratio >5 suggest a sub oxidizing to reducing conditions [13]. This ratio was determined for samples (Table 5).

Samples Ni/Co Remarks
SSB 6 1.172297 Oxidizing
SSB 7 0.585417 Oxidizing
SSB 8 1.663636 Oxidizing
SSB 9 1.034014 Oxidizing
JSSA 01 2.776579 Oxidizing
JSSA 02 2.138081 Oxidizing
JSSA 03 2.512357 Oxidizing
JSSA 04 3.961965 Oxidizing
JSSA 05 4.472934 Oxidizing
JSS 6 1.39521 Oxidizing
JSSS 1 2.757603 Oxidizing
JSSS 2 3.51325 Oxidizing
JSSS 3 2.271298 Oxidizing
JSSS 4 3.242424 Oxidizing
JSSS 5 2.75134 Oxidizing
JSSS 7 1.963595 Oxidizing
JSSS 8 0.073232 Oxidizing

Table 5: Ni/Co ratio for samples in parts of Awe-Obi area.

Discussion

Chemical index of alteration for stream sediments in Awe-Obi revealed most sediment have been subjected to extreme weathering. Only a single sample had experience intense weathering. Further evaluation of weathering using chemical index of weathering revealed all samples had undergone extreme weathering. However, evaluation of weathering using the mineralogical index of alteration revealed samples had undergone intermediate weathering and intense to extreme weathering, most samples showed intense to extreme weathering characteristics with five samples showing intermediate weathering characteristics. High CIA, CIW and MIA values are indication that sediments have been subjected to severe alterations under humid climate. Inferences on maturity of these rocks using index of compositional variability (ICV) revealed most samples are immature (ICV values greater than 1). This study also inferred that the paleo-oxygination conditions for sediments using Ni/Co ratio revealed an oxidizing environment of deposition of the samples. It must be made clear here that the application of chemical index of alteration (CIA), chemical weathering index (CIW) and mineralogical index of alteration (MIA) of heterogeneous profiles of Awe-Obi was done with extreme caution, accepting its limitations, because this study too just as previous study, assume aluminium is immobile during the process.

Conclusion

The Chemical Index of Alteration (CIA) [4] is the most widely applied and most indicative of the available weathering indices. From this study, one can conclude that the sediments from the study area have been subjected to varying degree of weathering and were deposited in an oxidizing environment. Furthermore, the ICV values for sediments in the study area suggest most sediments are immature. The high CIA, CIW and MIA values are indications that the sediments have been subjected to severe alteration under humid climatic condition.

References

Citation: Gajere JN, Abaa SI, Kana JE (2015) Paleo Environmental Studies for Stream Sediments around Awe-Obi Area North Central Nigeria. J Geol Geosci 4:197.

Copyright: © 2015 Gajere JN, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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