FIELD RELATIONS AND LITHOLOGY UNIT DESCRIPTION
GEOLOGY OF IVA VALLEY
A total outcrops of six were observed in the study area during the geologic field mapping.
The sedimentary succession of the area studied, comprises three dominant lithologic units. The stratigraphic sequence are as follows;
Unit C: Friable, cross bedded, poorly sorted sandstone.
Unit B: Black-dark grey coaly sandstone, siltstone and shale unit.
Unit A: Fissile, black to light grey shale and siltstone shale unit.
These were delineated based on outcrop studies, lithofacies analysis and structural characteristics of the mapped rock units.
The oldest of the units is the unit A overlain by Unit B and followed by Unit C which is the youngest.
LITHOLOGIC UNIT DESCRIPTION
The sedimentary succession of the study area consists of three sedimentary formations which were deposited during the Campanian and Maastrichtian times of the upper Cretaceous era.
Unit A underlies the plains east of the Enugu escarpment. It consists of finely laminated, dark grey carbonaceous shale with thin layers of fine grained sandstone, soft grayish blue dark grey mudstones and with occasionally beds of white sandstone. Sandy shale are seen at various location like IN2 on latiude6 27.010minN , longitude7 28.040minE and IN5 on latitude 6 28 .138minN, longitude 7 28.290minE .
The shale is jointed and laminated. The shale are carbonaceous and contain no minerals filling the ripple marks and joint as seen at location IN5.
At the lower base are siltstone and heterolites intervals which display several joint sets. Exposed along Enugu-Onitsha express way, the shale is well eroded, dark grey in colour.
The ironstone are brownish in colour. The dip amount is 44.5 and the direction is in the SW direction. The formation is commonly exposed along deeply incised river valleys at the base of the Enugu escarpment. The abundance of plant fragment associated with molluscan remains in the fossiliferous bands and the nature of the fauna indicate that the formation was deposited in Brackish water.
Unit B overlies unit A, it consist of sandstone, siltdstone, shales, mudstone and sandy shales with coals seam at several horizons. The sandstone which are fine to mediun grained, white or yellow in colour are normally well beded and ripple laminated as observed at the locations IN1 on latitude 628.172minN,, EIB/09/05. The shale and mudstone are dark blues grey and grade into the sandstone. Observed at the most area are sandstone that are weathered to siltstone and sandy shale is observed at location EIB/09/01. The average dip amount is 4o in the SW direction.
Ekulu Rivers cuts across the area towards the Eastern part of the ma. The coal bearing Mamu Formation overlies the Enugu shale. The coal seam in this Formation are interstratified with the sandstone, siltstones and in a fairly rhythmic succession. The bedding of this unit is usually irregular especially where heterolith of sandstone, siltstone, coal and shale are in about equal proportions.
Unit C is the youngest unit of the Enugu Formation. It lies above unit B. it is strongly cross bedded and loosely consolidated. The sands are generally very thick especially in the less eroded area in the vicinity f Ngwo as observed at EIB/09/12, and EIB/09/10. This unit is composed mostly of unconsolidated poorly sorted and friable sandstone which are white to pale grey in colour. The sandstone unit occupied the North western and south western part of the study area as observed at locations EIB/09/11, EIB/09/7, EIB/09/10 AND EIB/09/12.
The average dip amount is 4 in the SW direction with a strike of about 125N. the rock may be strongly cross bedded with forest laminae making an angle up to 16 with the major bedding planes.
TABLE2: SEDIMENTARY SUCCESSION OF THE STUDY AREA.
Source: Simpson (1955), Egboka and Uma (1985)
|UPPER CRETACEOUS||UPPER MAASTRICHIAN||AJALI
CORRELATION OF THE UNITS
Lithological and structural characteristic of this unit which includes; finely laminated dark grey carbonaceous shale with thin layers of fine grained sandstone, siltstone and ironstone. Joints, laminations, faulty, ripple marks, nodules etc, are some structures found in this unit. This correlates it to the Enugu shale of Simpson (1954), Reyment and Barber (1956) and Reijer et al (2001). Lithological, this unit is similar to shale mudstone unit of Eungu shale.
Lithological and structural characteristics of this unit which includes; distinctive assemblage of sandstone, shale siltstone and sandy shale which seams at several horizons. Burrows, joint ripples lamination, cross-breeding etc. are some structures found in this unit. This correlation it to the lower coal measures (Mamu Formation) Reyment (1956) and Simpson (1954).
Lithological and structural characteristics of this unit which includes; friable, medium grained, poorly sorted, white sandstone broken by occasional thin beds of hale structures like cross-bedding etc. are found in this unit. Thus unit C is correlable to the continental false bedded sandstone (Ajali sandstone) Reyment (1965) false bedded sandstone of Nwaajide and Raijer (2001) and continental white sandstone of Simpson (1945).
mine about 1.5km Ezionye petrol stations, Enugu-Onitsha express road.