STRATIGRAPHY AND SEDIMENTOLOGICAL STUDIES OF THE ALBIAN TO MAASTRICHTIAN SEDIMENTS AROUND LAFIA TOWN, LAFIA SHEET 231NW MIDDLE BENUE TROUGH, NIGERIA

The middle Benue Trough is one of the three segments of the larger NE-SW trending Benue Trough. As is the case with the Upper Benue Trough, there does not seem to be any recent work dedicated to the Geology of the Middle Benue Trough. Recent works on the stratigraphy and sedimentological studies of the Albian to Turonian sediments of the middle Benue Trough, covered areas around Keana and Awe respectively (all within sheet 232 NW). This work covered Areas within Lafia (Sheet 231NW). The study area is located southeast of Lafia town. It is accessible by some major roads which includes: Lafia to Makurdi, Doma to Agyaragu and Lafia to Keana. Reconnaissance study was first carried out in the study area on a scale of 1:100000 followed by a detailed field mapping on a scale of 1:50,000. In the course of the field work, the lithostratigraphic successions in the study area was established, sedimentary structures were identified and samples collected for petrographic and sieve analysis. Stratigraphic study in the study area indicates the occurrence of three formations namely; the fluvial Lafia Formation (Campano-Maastrichtian), the shallow marine Awgu Formation (Turonian-Coniacian) and the transitional Awe Formation (Albian-Cenomanian). A total of 12 samples were collected for granulometric analysis, and 11 samples for Petrographic analysis. The Lafia Formation consist of ferrugenised sandstone at the base, clay and clayed loosed sands. Petrographic and sieve analysis revealed that the Ferrugenised sandstone range from fine-medium grain. It is moderately sorted, strongly fine skewed and is a Quartz arenite. More than 60% of the crystals are monocrystalline which is indicative of igneous provenance. The Awe Formation consists of fine-grained white sandstone, with some intercalations of white clay and laminated shale. The sandstones are fine- grained and poorly-moderately sorted which coupled with the bivariate plot that indicates a transitional conditions between shallow marine and fuvial systems. The sandstones of the Awe Formation range from Arkose to Subarkose. The presence of monocrystalline quartz and plagioclase feldspars are all indicators of igneous provenance. The Awgu Formation consists of sandstones that are fine grained, very poorly sorted and strongly fine-skewed. The sandstones of the Awgu Formation are Quartz Arenites. There are ostracodes, echinoderms, and ammonites fossils within the shaley limestone and limestones of the Awgu Formation. This is an indication of a shallow marine environment of deposition. The Sedimentary structures in the study area include; beddings, mudcracks, faults, joints and veins and unconformity. The bedding planes are indications of break in deposition of the sediments while the mudcracks indicates long exposure to a dry warm climate. Other Methods of studies such as aeromagnetic studies, palynological studies and diagenetic studies should be carried out for more geological information of the study area to properly characterise the Middle Benue Trough

 

 

 

 

 

 

The Benue Trough is a sediment-filled elongated NE-SW trending depression that originated from the opening of the Atlantic Ocean. It is a failed arm of a rift-rift-rift triple junction developed over a Cretaceous hotspot located below the present-day Niger Delta (fig 4). (Cratchley and Jones 1965; Burke et al., 1970). This trough has been divided into three sections namely Upper, Middle and Lower Benue Trough by Cratchley and Jones (1965). The structural and stratigraphic settings of the trough have attracted the attention of geologists and many hypotheses have been advanced for its origin (King, 1950; Lees, 1952; Carter et al., 1963; Cratchley and Jones, 1965; Dessauvagie, 1969; Burke et al., 1970; Olade, 1975; Offodile, 1978). The rift tectonic/strike model seems the most widely accepted mode of origin. The Nigerian Benue Trough is an intracratonic rift structure whose evolution is related to the Early Cretaceous opening of the South Atlantic Ocean and the Gulf of Guinea (Akande et al., 2011). Fig. 1 is the map of Nigeria showing sedimentary basins.

Fig.1: Map of Nigeria showing Sedimentary Basins (Obaje, 2009)

The aim of this work is to study the stratigraphy and sedimentological characteristics of the Albian to Maastritchtian sediments around Lafia town and their sedimentary structures.

Grain analysis was carried out in order

 

The study area is located within the middle Benue Trough of Nigeria. It lies between latitudes 8o13' N to 8o28' N and Longitudes 8o32' E to 8o47' E. The study area covers about 770km2 (Fig 2). The study area was accessible by the major roads that extends from Lafia to Obi, Lafia to Makurdi and other minor roads. The study area is located in Nasarawa State of Nigeria as seen in Fig. 2 below

Fig.2: Map of Nigeria Showing the Study Area (Not to scale)

The study area consist of undulating lowland with most parts of the State (Nasarawa State) lying within the Benue Trough. The highland areas of the State occurs towards the north, notably in Wamba, Eggon and Akwanga Local Government Areas. The Eggon hills has an average height of about 1200m above sea level

The study area is drained by numerous fast flowing streams/rivers that take their source from the Jos Plateau and flow through the area to the River Benue. The drainage pattern is dendritic (Fig. 3)

 

The study area occur within the tropical humid climate. The climate can be characterised into two seasons: the wet and the dry seasons. The wet season starts from the beginning of May and ends in October while the dry season begins by November and ends in April. During this dry `season, harmattan begin by December and ends in February. The annual rainfall range from 200mm to 1100mm. About 90% of the rain falls between May and September with the peak of rainfall between July and August.

Temperatures are generally high during the day particularly between the months of March and April. The mean monthly temperature in the study area ranges between 200C and 340C with the highest temperature being in the months of March/April and the lowest temperatures being in the months of December/January.

The vegetation is of the guinea savannah type. This is a transition zone between the forest region of the south and the true savannah of the north; therefore, it has both the characteristics of the forest and the grassland. The vegetation on the hilly part of the study area is made up of shrubs and isolated trees. Trees of economic value, including locust bean, shea butter, mango, citrus and banana are scattered across the study area.

There are several tribes within the study area. In Lafia, the largest town, there are different tribes with almost equal proportion. These tribes includes Kanuri, Tiv, Alago, Koro, Eggon, Hausa, Fulani and Gwandara. Southwards towards Giza town, the most dominant tribe is the Koro tribe around the towns of Akelekwu and Agyaragu. This is followed by the Tiv tribe that live around the towns of Ajor, Kadarko, Uchin, and Giza. The third dorminant tribe is the Alago around the towns of Keana, and Giza. The dominant occupations of the people in the area are farming and cattle rearing. The Farmers produce yams, guinea corn, maize e.t.c. The Fulani tribe on the other hand are involved in milk and meat production.