Crystalline Basement Areas Research Articles

Application of environmental isotopes to groundwater recharge studies in a semi-arid fractured crystalline basement area of Dodoma, Tanzania

The distribution of 18O and 2H in various water sources indicates that groundwater recharge is due to local rainfall occurring within the basins. Groundwater recharge takes place under a bypass flow mechanism and matrix diffuse flow and is 3% and 2% of the long-term mean annual rainfall of 550 mm for the Makutapora and Hombolo basins, respectively. Chloride mass balance indicates that 60% and 40% of the total groundwater recharge takes place through macropores and matrix flow, respectively. Sporadic variations in 18O, 2H and chloride among adjacent boreholes suggest existence of a discrete fractured aquifer and/or dominance of local recharge. The relationship between δ 2H and chloride indicates that groundwater salinization is due to the leaching of surficial and soil salts during high intensity rainfall, which causes high surface runoff and flash floods. It has been concluded that the isotopic and chemical character of groundwater in fractured semi-arid areas may provide the most effective complementary means of groundwater recharge assessment and therefore is very useful in the management of the water resources.

Groundwater survey methodology in crystalline basement areas; example of the district of Korhogo (Ivory Coast) (in French) : Savane, I; Affian, K; Koli Bi, Z; Cougny, G Int Assoc Engng Geol BullN44, Oct 1991, P79–88

Groundwater survey methodology in crystalline basement areas; example of the district of Korhogo (Ivory Coast) (in French) : Savane, I; Affian, K; Koli Bi, Z; Cougny, G Int Assoc Engng Geol BullN44, Oct 1991, P79–88

Geophysical Interpretation and Modelling of Three-Dimensional Structure in the Duchess Area, Mount Isa, Australia

In most areas of polydeformed crystalline basement, interpretation of structure in the third dimension is based solely on structural observations of incomplete surface exposures. One way to alleviate this problem is to use geophysical data and rock-property data to place additional constraints on structural interpretation. Geophysical data can provide information on the surface distribution of rock units, and can be used to constrain forward models of structural interpretations. Most available geophysical modelling routines, however, do not lend themselves to specification of complex structure in a geologically meaningful way.The Duchess area in the Mount Isa Inlier has been used as a test area for a structural and geophysical modelling package which allows specification and forward modelling of a multiphase deformation history. This study combines the input of field-based structural mapping, rock-property measurements, three-dimensional structural interpretation, and interpretation of a high-resolution aeromagnetic survey. The aim of the study was to use magnetic data to place constraints on the detailed three-dimensional structural interpretation of the area.The deformation history of the Duchess area consists of an early layer-parallel foliation and associated high-strain zones, which are overprinted by a regional east-west shortening event which resulted in the formation of upright folds and a steep foliation, with north-trending axial surfaces. These structures are overprinted by a series of northwest, northeast and north-trending faults which are also associated with east-west shortening.The approach in this study was to produce a preliminary three-dimensional interpretation based on surface mapping and magnetic interpretation. The results of this stage were then used, with constraints from magnetic susceptibility data, in a forward model of the magnetic anomaly field of the interpreted structure. The results of the model were then used to modify iteratively the interpretation until it was consistent with both surface observations and magnetic data.This approach has allowed additional constraints to be placed on: a) the subsurface geometry of folded magnetic units; b) absolute sense of displacement on faults; c) the surface and subsurface geometry of granitic bodies; and c) the nature and extent of deep structural features not exposed at the surface.

Geophysical siting of boreholes in crystalline basement areas of Africa

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 sparingly.

Detection of groundwater flow in fractured media through remote sensing techniques — some Nigerian cases

About a decade or two ago, groundwater supply was always associated with regions of sedimentary basins and consequently, exploration for groundwater was carried out almost exclusively in such regions. However, recent advances in exploration techniques have shown that groundwater can be found in abundance even in rocky terrains devoid of deep sedimentary covers. The occurrence of groundwater in these crystalline basement areas is due primarily to secondary porosity and permeability created by the presence of discontinuities in such regions. The discontinuities in and through which groundwater occurs and flows are in form of fractures. The extent, width, roughness and nature of the fractures; their patterns of distribution in a given area; frequency of occurrence and type and amount of infilling in these fractures determine the amount of groundwater stored and the case with which it can be extracted for possible domestic and/or commercial uses. Most of the fractures occur deep within the basement complex and thus their detection by means of conventional geological field mapping techniques is not very feasible. However, remote sensing techniques such as radar imageries can be conveniently used to detect these fractures or zones where they occur. By carefully analysing imageries obtained through remote sensing, favourable sites could be selected for the location of boreholes that could tap the groundwater in these areas. This paper deals with the techniques for detection of fractures and siting of boreholes in rocky terrains. Few cases in which remote sensing techniques have been successfully used in the detection of groundwater in parts of southwestern and northern Nigeria are also cited.

Borehole siting in crystalline basement areas of Nigeria with microprocessor-controlled resistivity traversing system : Olayinka, A; Barker, R Ground WaterV28, N2, March–April 1990, P178–183

Borehole siting in crystalline basement areas of Nigeria with microprocessor-controlled resistivity traversing system : Olayinka, A; Barker, R Ground WaterV28, N2, March–April 1990, P178–183

Borehole Siting in Crystalline Basement Areas of Nigeria with a Microprocessor‐Controlled Resistivity Traversing System

AbstractA Microprocessor‐controlled Resistivity Traversing (MRT) System using the Wenner configuration has been developed to measure the distribution of resistivity in the subsurface down to depths of 200 m. The data are presented as a contoured apparent resistivity cross section, a type of electrical image, from which useful information about the structure of the subsurface can be obtained. Quantitative interpretation consists of iterative computer modeling using a two‐dimensional finite‐difference method.Measurements conducted in Kwara State, Nigeria, in borehole siting surveys show that MRT surveys are an important aid in locating areas of deep weathering and fissure zones in structurally complex basement areas. The resistivity information obtained allows an appreciation of the subsurface structure which cannot be obtained easily with other geophysical techniques.

Variscan events: early Palaeozoic continental rift metamorphism and late Palaeozoic crustal shortening

Summary Variscan events are interpreted in terms of a geodynamic process of long duration. It began in the early Palaeozoic, possibly in the late Precambrian in some regions, with widespread rifting of the continental lithosphere. Granulite facies metamorphism, widely in evidence in the Ordovician and Silurian, coincides in time with igneous activity and with continuous accumulation of sediment at the surface. That association is taken to indicate continental rift metamorphism above anomalous regions of the mantle. Folding and metamorphism of what is now regarded as the basement began early in the Devonian. By Upper Devonian at the latest wide areas of crystalline basement had been deeply exposed by erosion. The orogenic crustal shortening which began early in Devonian time induced intensive development of nappe tectonics involving the basement rocks. This resulted in deep-reaching crustal imbrications, especially well shown at the Moldanubian-Saxothuringian zone and Saxothuringian-Rhenohercynian zone boundaries, which evolved to carry crystalline basement rocks towards their foreland regions over distances greater than 100 km. During the course of these nappe developments folding of the adjacent sedimentary troughs proceeded. A geodynamic model of the northern flank of the central European Variscan orogen is presented. (Rb/Sr data are given using λRb 87 = 1.42 × 10 −11 y −1 . The error limits are taken from the original papers, original data are given in brackets.)