LA GESTIÓN DE RIESGO DE CUMPLIMIENTO

4.3.1 Regional geology

Middle to Upper Albian age Asu River Group sediments, described by Benkhelil (1989) and (Ojoh 1990) crop-out in and around the core of the Abakaliki Anticline in the Lower Benue Trough. The Middle Albian deep marine sedimentary sequence - about 1400 m thick - is made up o f sequences o f slumps and turbidites. Each sequence is composed of basal sandstone slumps capped by finely laminated shales and silts or turbidites. This deep marine environment passed progressively into a shale-dominated shelf in the Upper Albian. The Upper Albian is composed of ammonite-rich, black shales with minor siltstones, limestones and sandstones.

Various sedimentary structures are present throughout the Asu River Formation. These include folded cross-bedded lamination, convolutions and micro-breccias. Other deformations are related to pore water expulsion triggered by seismic waves and fluidised intrusions; for example, sand hydro-plasticity, liquefaction, fluidisation and compaction. The distribution and intensity o f the deformation structures indicate that they were produced by earthquakes associated with syn-deposition tectonic activity.

4.3.2 Local geology

The Asu River Formation occurs throughout much of central and southern Oju. The geological and hydrogeological nature o f the Asu River Formation sediments was investigated at three sites: Odubwo village (BGS2, 2a, b), the Water Aid compound (BGSl) and Elim Bakery (BGS39). Detailed geological logs are summarised in Figure 4.3.

The Asu River Formation comprises moderately hard splintery mudstones and laminated coarse siltstone to very fine sandstone. The sediments have been lithified by burial diagenesis (Lott 1998). Much o f the hardened mudstone has a distinct slatey cleavage and contains disseminated iron pyrite. These mudstones are commonly convoluted and folded. Seismites (rocks that have been disturbed by tectonic activity during deposition) and stress-deformed ammonites are also present. Below 10-15 mbgl the mudstone contains many fractures; many of which are filled with calcite. Cores from BGS2a show iron oxide coating fracture surfaces.

H yd ro g eo lo g ica l investigation s in Oju and Obi BG S1 O) E t 10 20 30 40 50 60 70 B G S 2 v~- ■. B G S 2A WHft Ferruginous soil Clay W eathered mudstone

Hard splintery m udstone

Calcite veining

Fractures

First main w ater strike

Figure 4.3 Borehole logs for the A su R iver Formation at Odubwo and the Water Aid O ffiee.

The top few metres of the mudstone are weathered and contain thin layers of kaolinite clay. Thin ferruginous soils cover much of the outcrop areas. From the few boreholes drilled, it appears that extensive weathering does not extend more than several metres. The shales are mainly composed of kaolinite and illite clays. Rocks of this formation are exposed within stream beds and gullies along the northern flanks of the Wokum Hills where they crop out as hard dark grey lithified mudstones with interbedded hard quartzitic sandstones and thin limestones. The rocks have been folded into a series of tight NE-SW trending structures.

4.3.3 H yd rogeology

The Asu River Formation has favourable hydrogeology for developing groundwater. Groundwater occurs within fractures below 10-15 m. These fractures appear to be widespread. The Asu River Formation sediments underwent lithification due to burial diagenesis and so are moderately hard. Fractures therefore generally remain open. There

H yd ro g eo lo g ica l in vestigation s in Oju a n d O bi

is no intergranular porosity or permeability within the mudstones. Shallow wells or boreholes that do not penetrate the fracture zones have poor yields.

The two deep boreholes at Odubwo and BGS39 at Elim Bakery all gave transmissivity values of about 4 m^.d'\ Subsequent drilling of 28 boreholes in the Asu River Group commissioned by Water Aid, demonstrated that fractures are widespread. Each borehole could be equipped with a handpump, although the local contractors did not measure yield or aquifer properties satisfactorily (MacDonald 2001). Tests in the 11-m deep borehole (BGSl) drilled at the Water Aid compound gave a transmissivity o f less than 0.3 m^.d'* - this borehole was too shallow to penetrate the highly fractured zone. Water in this borehole (and the adjacent hand dug well) derives primarily from the ferruginous soil.

Two measurements o f storage coefficient were made at Odubwo from 4-hour long constant rate tests with observation borehole data, both giving values of 0.0005.

4.3.4 Geophysical investigations

Geophysical surveys were undertaken at Odubwo and at the Water Aid office in Oju, including 7.5 km o f EM34 surveys, 3 km o f magnetic profiling and 2 resistivity VES.

1. Apparent conductivity measured using the EM34 is generally low, 5-30 m.Sm'* (Figure 4.4).

2. Apparent conductivity increases with longer inter-coil spacings (and therefore deeper penetration). However, with very deep penetration (40-m coil orientated horizontally) conductivity reduces.

3. Resistivity VES prove a resistive soil overlying a 10-20 m thick moderately resistive (40-70 D.m) layer that overlies more resistive bedrock. The lower resistivity weathered zone is due to water filled fractures and clay content. A low resistivity weathered layer restricted to a thickness of about 20 m, explains variation in the measured conductivity with inter-coil spacing and coil orientation.

4. Magnetic profiling showed the presence o f magnetic rocks at several locations. Unfortunately time did not permit these sites to be investigated by exploratory drilling.

H ydrog eo lo g ica l investigations in Oju an d O bi 10-m v-coil 10-m h-coil 20-m v-coil 20-m h-coil 40-m v-coil 40-m h-coil -100 100 200 300 400 500 600 700 800 D is ta n c e (m) B G S 2 B G S2A 0 m 40 m Weathered mudstone Highly fractured Hard mudstone

Figure 4.4 EM 34 surveys at various intercoil spacings in the A su River Formation at

Odubwo. A schem atic g e ological interpretation is also given from drilling and the geophysical surveys.