362 JOUR.GEOL.SOC.INDIA, VOL.90, SEPT. 2017 JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.90, September 2017, pp.362-370 Variations in Soils Derived from an Erodible Sandstone Formation and Factors Controlling their Susceptibility to Erosion and Landslide Chukwuebuka Emeh* and Ogbonnaya Igwe Department of Geology, University of Nigeria, Nsukka, Nigeria E-mail: chubylingy@yahoo.com*, Chukwuebuka.emeh.pg79017@unn.edu.ng ABSTRACT Environmental degradations caused by erosion and landslide within an area in the South-eastern part of Nigeria were assessed, and also an attempt to characterize the underlying soils based on their degree of susceptibility to erosion and landslide. Factors affecting soil erodibility such as physical and chemical properties of soil, as well as vegetation density and slope gradient was determined in 20 different areas. Simple linear regression and principal component analysis were employed to relate the factors controlling erosion and landslide to the erosion and slide densities, and to determine the pattern that exists in the data and as well express the data in such a way as to highlight their similarities and differences. Four vertical horizons and two horizontal zones of soil were distinguished based on their degree of weathering as well as their chemical and mineralogical composition; hence the soils were classified into six different types based on their degree of susceptibility to erosion and landslide. Variations in their degree of susceptibility to erosion and landslide were majorly controlled by its chemical and mineralogical composition rather than its particle size distribution. INTRODUCTION Erosion and landslide are the major natural geologic hazard causing degradation of arable lands, structural collapse, flooding, damage of pipelines, and distortion of other communication utilities in the south- eastern part of Nigeria. These natural geologic hazards have been attributed to the prevailing geologic, geomorphologic, and climatic conditions within the area. The geology and hydrogeology of the area have been studied by various workers (Ogbukagu, 1979; Nwajide, 1979, 1992; Egboka et al., 1989; Egboka et al., 1990; Uma & Onuoha, 1986; Onwuka et al., 2004; Igwe et al., 2013). Nanka sands (Eocene), medium- to coarse-grained Nsukka sandstone and Ajali sandstone (Cretaceous) of the Anambra-Imo basin region have been identified to be more prone to erosion and landslide within this region (Akpokodje et al., 2010). The particle size distribution of the soils derived from these sediments was identified as the major factors controlling its erodibility (Obi & Asiegbu, 1980; Akpokodje et. al 1986; Okagbue & Ezechi 1988; Okagbue, 1988; Hudec et al. 2005), which is accelerated further by the increase in rainfall amount and intensity (Eze, 2007; Igwe et al, 2013), and also by the undulating nature of the terrain (Ofomata, 1965, 1988, Nwajide & Horque, 1979); hence causing deeply incised valleys of about 157m in length, 50m in width, and 5m in depth in one rainy season on the slope (Akpokodje et al., 2010). The gullies are further widened by the slumping/sliding of the side walls of the slope into the valleys which occurs mostly after heavy rainfall (Eze, 2007; Igwe et al, 2013) as the soils loose its shear strength due to increase in pore water pressure which eventually leads to liquefaction of the slope materials (Igwe et al, 2013; Igwe and Fukuoka, 2014; Sassa et al., 2003; Sharma et al., 2016). Other factors that affect erosion and landslide in this region include human activities like over-grazing, bush burning, and poorly designed drainage system of the highways (Akpokodje and Ekeocha, 2010; Ofomata, 1988, Okagbue & Uma, 1987). Several control mechanism which includes construction of drainage systems, slope stabilization, afforestation, and proper land use (Ofomata, 1988, Okagbue & Uma, 1987, Nwachukwu, 1988, Igbozurike, 1993; Ijioma, 1988, Okanigbo, 1980) have been used to check the devastating effect of these geologic hazards and have so far proved ineffective in some areas. Meanwhile, Egashira et al., (1983) revealed that erodibility is an inherent soil characteristics which cannot readily be controlled, and also Garland (1995), Smith (1999), and Laker (2004) in their work proved that the study of erosion is more site specific by observing that the universal soil loss equation (USLE) model is ineffective on South African soils. Noting that the ineffectiveness was because the model was designed using European and American soil erodibility data. Therefore, the reason why erosion control mechanisms were partially effective in south-eastern Nigeria could be a result of a holistic application of the control mechanisms in all the area without considering the variations in the soils physical and chemical properties, as well as variations in other factors controlling soil erodibility and landslide such as vegetation density and slope gradient; hence erosion and landslide continue to pose major challenge to the environment in this region. In order to solve these environmental challenges, it is imperative to understand the variations in the physical and chemical properties of the soils as well as the degree of influence of the other factors controlling soil erosion and landslide within the area; hence selecting a site specific control mechanism. Thus the objective of this work is to assess the various factors affecting the erodibility of soils derived from the Cretaceous Ajali sandstone which underlies large portion of the major highways within the region and have been associated with devastating erosion and landslide; and also an attempt is made to characterize them based on their susceptibility to erosion and landslide. STUDY AREA Climate The study area is in Enugu state, south-eastern Nigeria which lies within longitude 6°24'16.7"-6°22'17.2" and latitude 7°23'36.3"- 7°26'16.4" (Fig. 1) in the tropical climate zone with maximum temperature of 30.64°C in March and a minimum of 15.86°C in December. The total annual rainfall is 1580.66mm with the lowest rainfall of about 16mm occurring in February and the highest about 350mm in July (Eze, 2007). DOI: 10.1007/s12594-017-0725-5 | 0016-7622/2017-90-3-362/$ 1.00 © GEOL. SOC. INDIA