Friday, 5 October 2012

Taking a short road trip through Oke-Mesi Fold Belt (Part 1)

Straddling the boundary between Ekiti and Osun States of Nigeria, West Africa, is a narrow gigantic  ridge popularly known as Effon Ridge.
Effon ridge stretches along the entire north-south boundary of the two states, spaning a distance of a about 45km and a maximum width of approximately 18km. The ridge sits within the Oke-Mesi Fold Belt of  the Basement Complex rocks of southwestern Nigeria. The petrology of this metasedimentary fold belt which also houses a great segment of the popular Ifewara Fault, had been an important subject of discussion among numerous researchers over time. The geology of the ridge is a subtly interesting one as the ridge features a long narrow ‘valley’ sandwished between two enormous ridges. The 'two arms of the ridge' and the ‘valley in-between' are of two slightly different lithologies emplaced along with numerous interesting structures.
So, there pops up our question: How was this beautiful ridge formed?
Ok, let's first of all take an east-west road trip across the ridge. 
We will access the ridge from the east through Ita-Ido pass...let's go there... ;) ;)

- Satellite image showing Okemesi Mega Fold (Red Box). Image Source- Google Earth
Before we get to Ita-Ido pass, the lithologies are mainly amphibolites with outcrops of migmatite gneises and granites in some places. At Ita-Ido pass, a road-cut reveals the constituent lithology of the eastern ridge, shown in the picture below. It is composed of massive quartzite. The quartzites have been steeply folded with a conspicuous overall dip to the east.

As one moves past the eastern ridge and slowly descends into the gently undulating central valley, the lithology changes into quartz-schist (foliated quartzite) as shown below:

 As one transits into Effon-Alaaye and Oke-Mesi towns- located on the western ridge, we again encounter the massive quartzites.

What exactly is happening here?
Effon Ridge is composed of the Effon Psaamitic formation which is essentially the major fold structure within the Oke-MEsi fold belt. The psaamite formation have been folded antiformally along a NNE-SSW strike, with the rocks on the eastern ridge dipping to the east, and on the western ridge dipping to the west as well as to the east in different  parts. The later had been noted to suggest the presence of minor folds on the main Okemesi fold structure. The fold was formed during the deformational phase of the Pan-African orogeny (550±100 m.a.). Some past workers believe that the psaamitic rocks are overthrusts on the amphibolites (Hubbard, 1975) while some others believe that the rocks were formed along a collisional suture zone (Turner, 1983). 
Well, a further observation of the structures of the fold in relation to the Ifewara Fault bounding it to the west, may give us a better understanding of the origin of the great Okemesi Fold.  To be continued in Part 2 of this post.

- A sketch of Oke Mesi mega-fold and cross-section.

- A simplified model describing the process of folding and subsequent weathering that produced the twin-arm structure of the ridge

- HUBBARD, F. H. 1975. Precambrian crustal development in western Nigeria: Indications from Iwo Region. Bulletin of Geological Society of America, 86, pp. 548–554.

- TURNER, D. C. 1983. Upper Proterozoic schist belts in the Nigerian sector of the Pan African province of West Africa. Precambrian Research, 21, pp. 55-79.

1 comment:

  1. Interesting! Never knew geological/geomorphological concepts could be explained in such a simple and fun way.


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