Geology
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Item Geophysical siting of boreholes in crystalline basement areas of Africa(Pergamon Press Ltd, 1992) Olayinka, A.I.This paper assesses the effectiveness of surface geophysical methods namely electrical resistivity, electromagnetic, seismic refraction, magnetic, gravity and induced polarization for groundwater exploration in crystalline basement complex areas. Most of these geophysical techniques can provide quantitative information on the characteristics of the weathered zone which relate to the occurrence of an economic aquifer. The critical factors in the choice of a particular method include the local geological setting, the initial and maintenance costs of the equipment, the speed of surveying, the manpower required as field crew, the degree of sophistication entailed in data processing to enable a geologically meaningful interpretation, and anomaly resolution. The particular advantages and limitations of each technique are highlighted. Several case histories from Nigeria and the rest of Africa indicate that electrical resistivity (both vertical sounding and horizontal profiling) is the most widely used, followed by electromagnetic traversing. These are often employed in combination to improve upon the percentage of successful boreholes. Due to the high cost of equipment, large scale of the field operations and difficulties in data interpretation, seismic refraction is not widely adopted in commercial- type surveys. Similarly, magnetic, gravity and induced polarization are used only sparinglyItem Error in depth determination from resistivity soundings due to non-identification of suppressed layers(Elsevier Science, 1997) Olayinka, A.I.The magnitude of errors in the determination of depth to bedrock from Wenner and Schlumberger resistivity sounding curves, caused by the nonidentification of a suppressed layer, has been investigated. The principal objective is to evaluate how the layer thicknesses and resistivities affect the accuracy of depth estimates. In the computations, the intermediate layer in a 3-layer model, in which the resistivity increases with depth, is removed and the 2-layer sounding curve that is electrically equivalent to the 3-layer curve is generated. The results indicate that there is a possibility for large depth underestimations when the resistivity contrast between layers 1 and 2 is very large. This is manifested in a steeply rising terminal branch on the sounding curve. There is a slight decrease in the depth underestimation as the resistivity contrast between layers 2 and 3 increases. Conversely, if the intermediate layer is fairly thick and the resistivity contrasts are not too large, the best-fit 2-layer curve shows large deviations from the 3-layer curve, in such cases, the intermediate layer can be identified, resulting in reliable depth estimates. A field example from Nigeria is presented in which the sounding data has been interpreted so as to account for a prebasement layer of intermediate resistivity, indicative of a fractured granite.Item Electromagnetic profiling for groundwater in precambrian basement complex areas of Nigeria(1990) Olayinka, A.I.Ground electromagnetic profiling, using a Geonics EM34-3 instrument, has been employed to identify areas of high conductivity in a Precambrian basement complex terrain of Nigeria. Field examples, conducted as part of a rural water supply programme, are presented. They indicate that the apparent conductivities are generally lower than about 60 mmho m-1. Subsequent borehole drilling suggests a good correlation between high EM34 anomalies, deep weathering and high well yield (> 1 1 s-1). On the other hand, boreholes sited on conductivity lows penetrated a thinner regolith with relatively lower yields.Item Electromagnetic profiling and resistivity soundings in groundwater investigation near Egbeda- kabba, Kwara state(1990) Olayinka, A.I.As part of a borehole siting programme for rural water supply in a Precambrian crystalline basement terrain, 36 Offset - Wenner electrical soundings and about 4.5 line-kilometres of ground conductivity profiling with a Gconics EM34-3 equipment were made near Egbeda-Kabba. Kwara Slate, Nigeria. The EM34-3 measurements provided a rapid reconnaissance tool in identifying high conductivity anomalies thought to be due to deep weathering and/or bedrock fissuring. The apparent conductivities arc generally less than 50 mmho m'1. A quantitative interpretation of the vertical sounding data indicated that the resistivity of the weathered zone varies over a wide range, from about 10 to 200 ohm.m, and that the overburden is generally less than 40m thick. Three abstraction wells, each having a yield of about 11s"1, were drilled at deeply weathered sites (depth to bedrock > 20m) identified from the surface geophysical measurements.Item MINERALOGICAL AND GEOCHEMICAL STUDIES OF AGBAJA IRONSTONE FORMATION NUPE BASIN, CENTRAL NIGERIA(1994-07) ABIMBOLA, A. F.Agbaja Ironstone Formation of the Lokoja district, Central Nigeria occurs within the Upper Cretaceous sedimentary sequences of the NW-SE trending Nupe Basin. Three lithostratigraphic units were delineated; Lokoja Sandstone (the oldest), Patti Formation and Agbaja Ironstone Formation. Lokoja Sandstone rests unconformably on the basement complex and grades from conglomerate to arkosic sandstone. Patti Formation consists of intercalations of sandstone, siltstone and carbonaceous mudstone units. Agbaja Ironstone Formation is made up of colites, pisolites and bog ore. Textural studies show a very finely to very coarsely skewed mesokurtic sands for Lokoja Sandstone and leptokurtic for Patti Formation. ZTR index value ranges from 61 to 81% for Lokoja Sandstone and 83 to 95% for the Patti Formation. Higher ZTR index value for Patti Formation indicates a relative higher mineralogical maturity over the Lokoja Sandstone. These sedimentological studies indicate a basement source for the sedimentary materials deposited under low energy short fluvial regime, that evolved into two facies; alluvial fans and flood basin. Four petrographic varieties of the Agbaja Ironstone Formation were identified; ooidal pack-ironstone, pisoidal packironstone, detrital mud-ironstone and breccia mud-ironstone Kaolinite ooids are spherical, oblong and fragmented, usually with pseudomorphs of goethite after pyrite at the core. Goethite pisoids cemented in kaolinitic to goethitic matrix are elliptical, oval, subspherical in shape and composite in nature. Constituent minerals of the mud-ironstone are kaolinite, quartz and heavy minerals. Paragenetic studies show that pyrite, siderite, kaolinite, quartz, mica and heavy minerals are the initial minerals of the ironstone while secondary enrichment minerals are goethite, hematite, goyazite-crandallite, bolivarite and boehmite. Chemistry of these minerals further classified them into ferritic kaolinite, kaolinitic ferrite and ferrite. Si02 ranges between 23.71 and 56.41 % and AI203 between 22.01 and 36.54% in all the unferruginized portions of the Agbaja ironstone. But both components can jbe as low as 0.22% in the ferruginized equivalent. Si02 and AI203 contents are highest in the mud-ironstone varieties and lowest in the pack-ironstone varieties. Fe203 content increases from about 1.59% in the unferruginized to about 97.54% in the ferruginized equivalents, for all the petrographic varieties. The highest concentration of Fe203 is in the pisoidal pack-ironstone. P20S values range between 0% in the unferruginized to 5.10% in the ferruginized portions, and often increase with increasing Fe203 contents. MgO, CaO, MnO, Na20 and K20 are generally less than 1.0% in all the petrographic varieties. Element mobility during ferruginization indicates that iron enrichment is accompanied by a parallel depletion in Si02 and AI203, moderate to extreme reduction in CaO, MgO, MnO, Na20 and K20 and a significant gain in P205. This relationship confirms the supergene nature of the replacement process (desilicification, removal of alumina and iron enrichment process (ferruginization). Sedimentological, mineralogical and geochemical studies provided evidences for a kaolinitic precursor for the ironstone deposit, contrary to the earlier proposed chamositic precursor. Also the presence of pseudomorphs of pyrite in nuclei and as incorporation into the concentric laminae of ooids are reliable indicators for a possible accretionary model for the formation of ooids and pisoids prior to ferruginization. Two ferruginization periods unrelated to lateritization were established; first won enrichment is supply due to oxidation of the initial pyrite/siderite and reconcentration of absorbed FeOOH in the lattice structure of kaolinite; second period is linked to the presence of bacteria (framboids) which is oxidised to form iron phosphate complex in the bog ore. The Fe and P were remobilised into the underlying pack-ironstone by descending meteoric water.Item MINERALOGICAL INVESTIGATION OF CORROSION PRODUCTS IN PROCESSING MACHINES(THE NIGERIAN SECURITY PRINTING AND MINTING COMPANY (NSPMC) LIMITED, LAGOS RESEARCH AND DEVELOPMENT DEPARTMENT, 1997) Nton, M. E.Item Sedimentology and depositional environment of Awi formation Calabar flank, southeastern Nigeria(Journal of mining and geology, 1999) Nton, M.E"A sequence of conglomerates, sandstones, siltstones, claystones carbonaceous shales and mudstones which rims the Oban Massif in the Calabar Flank, constitutes the Early Cretaceous (probably Aptian) Awi Formation In the southeastern Nigeria. Field studies and laboratory analyses were conducted on these sediments to determine their provenance and depositlonal environment. Field observations show that the sediments are grently dipping (.< 16°) in a southwesterly direction. They vary in thickness from thin to very thick beds and are laterally, discontinuous. The pattern of sedimentation is cyclic with fining upward sequences. The sediments are textually immature with angular to subangular grains dominating, thus indicating short distance transportation. The sandstones are medium to coarse-grained, poorly sorted, mostly leptokurtic, fine skewed and unimodal in distribution. They are subarkoslc with more than 70% quartz which are of Igneous and metamorphic origin while the associated claystones contain kaolinite. Heavy mineral assemblages show the presence of predominantly zircon, tourmaline, rutile, garnet and staurolite with a range of 66.7 to 96%. This indicates mature to superrnature sandstones and can be attributed to wet climatic conditions. The carbonaceous shales which grade into mudstone In some cases are non-fossillferous, poorly laminated, pyritized and rich in lignitised wood. All these indicate that sediments of Awi Formation were derived from the Oban Massif and deposited in environments ranging from channel lag and point bar to flood plain. "