STRUCTURAL AND PALEOSTRESS ANALYSIS OF THE UPPER CRETACEOUS AFIKPO SYNCLINE, SOUTH-EASTERN NIGERIA.

1.      INTRODUCTION

1.1.  Study Background

Even though a century of geological studies have enabled a broad understanding the geology of the Benue Trough, it was only in the latter part of the 20^th century that a picture of the structural framework, within which the Benue trough evolved, began to emerge. The controversies surrounding the tectonic evolution of the Benue Trough have been largely resolved, with the overwhelming evidence leaning towards the interpretation of the so called French school of structural Geologists which sees the Benue trough as a collection of pull apart basins related to transcurrent or strike-slip movement along deep-seated  basement shear zones of Pan African origin reactivated as oceanic transform faults (Benkhelil, 1982, 1989; Guiraud, 1993). This view is supported by field evidence in the Northern Benue trough where the climate and the nature of the sedimentary units allow for classic geologic study. In the Southern Benue Trough, the fine grained nature of most of the units and the dense vegetation as a result of a wet tropical climate have hindered field studies and created a missing link in the proper explanation of the structural framework of the basin.

The Afikpo area offers a unique opportunity to study and understand the deformational processes and to determine the tectonic stresses active in the southern Benue Trough as the highly indurated nature of the sediments allow for an abundance of outcrops that is unmatched anywhere else in the region.  

1.1.  Geographical Setting

1.1.1.     Location and Accessibility

The study area is bounded by latitudes N 5° 51´ and N 6° 03´; and longitudes E 7° 51´ and E 8° 06´ (Figure 1) and covers the areas around Afikpo town, Amaseri, Ohaozara, Akpoha and Abomege in Ebonyi State of South Eastern Nigeria. Itigidi, Ediba, Itegeve and Adadama communities in Cross River state also fall within the study area. Access to the area is through the roughly east-west Afikpo-Okigwe road which connects the Okposi-Amaseri-Amoso road at Amaseri. On the outskirts of Afikpo town, this road connects with the Northbound Abakaliki road passing through Akpoha and Abomege. The eastern side of the study area is accessible through the Abomege-Ugep road which passes crosses the Cross River at Itigidi on its way to Ugep and Calabar to the south of the study area. Other minor roads link the smaller interior villages from these major roads. The major roads are tarred while the minor roads are at best graded and may not be accessible in the peak of the rainy season. 

1.1.2.     Physiography

1.1.2.1.          Topography

The study area can be divided into roughly two regions, northern and southern, with different topographic styles (Figure 2). The northern region has a lower elevation on average (less than 100 m) and is characterized by ridge and swale topography. The ridges are most prominent towards the Western part of the study area around Amaseri and Ibi where they trend NE-SW. The ridge profiles are asymmetrical with a gentle south eastern dip slope and a steeper north western scarp slope. They are related to the indurated sandstones which comprise them.

Towards the eastern part of the study area the trend of the ridges gradually rotate from NE-SW to NW-SE with dip slopes facing southwest around Ameta-Oziza. In the eastern part of the study the ridges are less prominent and can be seen to be folded about a NE-SW axis from digital elevation models (Figure 2).

The southern part of the study area is has a much higher elevation (above 100 m) and is more rugged than the Northern region. These hills are dissected by the Cross River which has cut gorges with moderately steep slopes at Itigidi and Oziza.

1.1.2.2.          Drainage

The Cross River is the largest river passing through the study area (Figure 1). It originates in Cameroon, where it takes it is called the Manyu River and flows southwards through the study area to the Atlantic ocean (“Cross River (Nigeria),” 2013). The Aboine River is one of the major tributaries of the Cross River. It passes through the study area from north to south where it joins the Cross River. Its path seems to be controlled by NNW-SSE lineaments. The Asu River is a West-East flowing tributary of the Aboine River. Its flow direction is controlled by the alignment of the ridges and the river path is limited to one of the large shale swales through which the river meanders and forms a wide flood plain before joining the Aboine River at Akpoha. These rivers, though perennial, show a large variation between peak flow (usually at the end of the rainy season) and ebb flow (at the end of the dry season) where they are reduced to a bare trickle. Their banks provide in the dry season, very good exposures of the shale units otherwise hidden in other locations.  Other minor smaller streams are also controlled by the ridge and swale topography giving a roughly trellis drainage pattern along with the Aboine and Asu rivers.

Figure 1: Accessibility and Drainage Map of the Study Area (Modified from Google Maps™)

Figure 2: Shuttle Radar Topographic Mission (SRTM) based Digital Elevation Model (DEM) of the Study area. Legend height is in metres. Grid is in decimal degrees.

1.1.3.     Climate and Vegetation and Soils

The study area enjoys a warm tropical climate with relatively high temperatures (27° C on average) throughout the year and two seasons – the rainy or wet season that lasts from March – November in the South and from May to October in the north; and the dry season that occupies the rest of the year. The rainy season has two period of maximum rainfall separated by a short relatively dryer period in August (the August break).

The vegetation in the study area is derived Guinea savannah with relict forest. This area was originally the drier part of the high forest. Due to bush burning and overgrazing, cultivation and hunting activities over a long period in the area, most of the high forest trees were destroyed and the forest replaced with a mixture of grasses and scattered trees. However, along the streams and in wet low-lying swales were surface water accumulates there are still some traces of forests. The study area forms a part of the Cross-Niger transition forests ecoregion south-eastern Nigeria, located between the Niger River on the west and the Cross River on the east.

Flora is dominated by grasses such as Pennisetum, Andropogon, Panicum, Chloris, Hyparrhenia, Paspalum and Melinis. These tall grasses are characteristic of the Guinea savanna proper. Trees include Afzelia (Palm) and Borrasus.

1.1.4.     Human Geography

The part of the study area west of the Cross River is predominantly Igbo. The traditional language is the Afikpo dialect of the Igbo language in the regions around Afikpo town. In the regions around Itigidi the dialect is the Agbo dialect. The eastern side of the Cross river is dominated by the Bahumono, which includes seven villages: Ediba, Anong, Usumutong, Afafanyi, Igonigoni, Ebom, and Ebiriba.

Afikpo has been an important town since colonial times when it was a divisional headquarters and is currently the seat of the Afikpo-North Local Government Area of Ebonyi state. Itigdi used to be an important ferry crossing point until the completion of the Itigidi Bridge (Error! Reference source not found.). Most of the people in the region are subsistent farmers making use of the relatively fertile swales to plant food crops like Cassava and sometimes rice crops.   

1.2.  Previous Work

Geological study of the Nigerian rocks began with the establishment of the Geological Survey of Nigeria (GSN) in colonial times. Early research was motivated by –among other things- the potential of the discovery of mineral deposits (e.g. coal in and around Enugu and Tin Ore in the Jos Plateau) (Simpson, 1954; Wilson & Bain, 1928). Early accounts of the stratigraphy of the Benue were given by Falconer et.al. (1911) Wilson and Bain (1928), Tattam (1944) McConnel (1949), Farrington (1952) and Simpson (1954) as well as Shell (1957) who, as part of initial preliminary geological exploration work published a number of maps at a scale of 1:250000.

It was after independence and the establishment of the first universities that more detailed studies began to be carried out in association with the Nigerian Geological Survey. Reyment (1965) gave a detailed lithostratigraphic and biostratigraphic description of the Cretaceous rocks of the Benue trough establishing formal units, some of which are still in use today.  These descriptions have been built upon by further work undertaken in the Southern Benue (Murat, 1972; Ojoh, 1992; Petters, 1980; Umeji, 2000); in the Central Benue trough (Offodile, 1976) and in the Northern Benue trough (Allix & Popoff, 1983; Benkhelil, 1982; Guiraud, 1993; Maurin et al, 1986; Maurin & Guiraud, 1993) Also a description of the distribution and nature of magmatic occurrences in the Benue trough have been attempted (Caen-Vachette & Umeji, 1983; Hossain, 1981; Obiora & Umeji, 2005). A summary of the stratigraphy of the Nigerian sedimentary basins has been provided by  Kogbe (1989) and Nwajide (2013).

Geophysical investigations over the years have also provided very important data on the regional structure, morphology and salient features of the Benue trough. Of note are Gravity surveys undertaken by Cratchley and Jones (1965), Ajakaiye and Burke (1973), and Adighije (1981); as well as Magnetic surveys carried out by Ofoegbu (1988)  and  Ofoegbu & Onuoha (1991).

Studies have also been carried on the stratigraphy and sedimentology of the Afikpo Syncline in particular, establishing the different facies and environments of deposition as well as reservoir potential (Amajor, 1987; Banerjee, 1980; Odigi & Amajor, 2009b).

Along with detailed descriptions of the Benue Trough came attempts to explain its origin and evolution beginning with pre-plate tectonic ideas (Cratchley & Jones, 1965), early plate tectonic theories of a tensional aulacogen related to triple junction development over a hotspot (Burke et al,  1971; Thiessen et al,  1979; Wright, 1968) and current wrench controlled pull apart theories related to oceanic fracture zones (Benkhelil et al, 1989; Benkhelil, 1982, 1989;  Guiraud, 1993). Current understanding places the evolution of the trough within the context of a set of genetically related continental basins: The West and Central African Rift system (WCARS) (Binks & Fairhead, 1992; Bosworth, 1992; Fairhead et al, 2013; Genik, 1992; Guiraud & Bosworth, 1997; Guiraud & Maurin, 1992; Maurin & Guiraud, 1993).

Our understanding of joints and fractures have also evolved from the earliest 19^th century attempts to characterize them through their geometry and pervasiveness. Early attempts included the work of Gilbert (1882) and Woodworth (1896). Griffith (1921) laid down the experimental and theoretical framework of modern fracture mechanics by developing concepts that account for stress concentration at fracture tips. Anderson (1951) laid out the different classes of faulting in relationship to the principal stress directions. This is idea formed the foundation of paleostress inversion methods using fault and fracture data. Summaries of the advances and progress in our understanding of fractures can be found in the works of Engelder (1987) and Pollard and Aydin (1988).

The method of stress inversion of fault slip data was developed by Jacques Angelier (Angelier & Mechler, 1977; Angelier, 1994; Célérier et al., 2012) based on Byerlee’s (1978) assumption that the slip direction on a fault is the direction of the maximum resolved shear stress. Computer based methods to handle the inversion of numerous fault slip data were then developed (Angelier & Mechler, 1977; Delvaux & Sperner, 2003; Delvaux, 1993; Delvaux et al., 1997; Etchecopar et al, 1981) including the software TENSOR™ developed by Damien Delvaux (Delvaux & Sperner, 2003; Delvaux, 1993) which is used in this work.  

1.3.  Objective of Present study

The primary of objectives of this study were as follows:

1. Analysis of the deformational structures in the Pre-Santonian Southern Benue trough and post-Santonian deposits in the Afikpo syncline. The analysis would involve a description of the form and geometry of the different structural features, an analysis of the deformational movements responsible for the structures.
2. Paleostress estimation from analysis of fracture data using the method developed by Angelier (1994). The primary focus would be on isolating the Santonian related paleostress as well as paleostress related to pre-Santonian and post Santonian deformational events.
3. Explanation of the Pre-Santonian, Santonian and Post-Santonian tectonic phases in the light of Regional Tectonics.

1.4.  Workflow

The primary source of data used in this study was field based. Preliminary reconnaissance field studies and detailed field studies were carried over a 4 week period at the end of the long dry season. The emphasis was observing and describing the different structural features and measuring relevant geometrical and physical features. Field studies were aided by data from LANDSAT™, GeoEye™ and SPOT™ satellite images provided by Google Earth™. Also Digital Elevation Models acquired from Shuttle Radar Topographic Mission data sets were used to access lineaments and determine structural control.

Preferred directions of the fractures were established using stereographic projection (OpenStereo™) in order to classify fractures into different sets and systems for subsequent analysis and stress inversion.

Stress inversion was carried out using TENSOR™ software (Delvaux & Sperner, 2003) to isolate the dominant stress direction in the pre-Santonian Santonian and post-santonian deposits.