Tuesday 30 October 2012

Can onion skin form caves?


“..Like onion skins they peeled…and in a short while they form some of the most amazing caves in Naija!”

Exfoliation is a weathering process in hard rocks by which alternate cycles of heating and cooling (expansion and contraction) cause rocks to breakdown mechanically into thin sheets/slabs along their outer surfaces- this is why it is also called “onion-skin weathering”. These sheet/slabs vary in thickness from a few centimetres to few meters.

Ok, let’s go and see some amazing onion-skin weathering sites in south-west Nigeria. We choose Akure, Ondo State capital. The young folks have given the town the street name “Ak-city”. It’s their own ‘small Lagos’ anyways (hahaha… yeye people).

Akure area is characterized by a landscape composed of a basal migmatite-gneiss country rock and granite intrusions, shooting out and outcropping ubiquitously as picturesque inselbergs in different places. These granites (porphyritic) are southern extensions of the Ikere-Ado Batholith.

On these granite inselbergs, we find numerous awe-inspiring rock formations formed by down-slipped exfoliating slabs/sheets. However, only a few of these rock formations qualify as caves and are called Talus Caves. While talus itself are the rock boulders/blocks produced from weathering, talus caves are those cavities and openings formed between the boulders piled up when pilled up.



Now, those talus caves found in Akure area range in form and size. Some appear to have been formed in-situ, while some appear to have been transported to their locations. Also, some differ from others in the type of geometry of the talus/blocks that formed them. These observations have therefore formed the basis of Kolawole & Anifowose (2011)’s review of the pre-existing classification of talus caves by Vidal & Vaqueiro (2007).

Over time, the rock formations of Akure area have been given special attention because of the seemingly precarious but spectacular and interesting poise assumed by the boulders. Iho-Eleeru is located in Isarun village, some 20mins from the outskirts of Akure. It is also known as the “Cave of Ashes” because of the burnt pottery works of the ancient dwellers at the cave. Aba Cave is located on the northern outskirts of Akure, and is given the name because it takes the form of a traditional hut. There is also Kinihun Rock, a massive pile of awe-inspiring rock boulders beautifully set on one another. It takes the form of a lion's skull when viewed from the north-eastern direction, hence it’s name.

These beautiful works of nature on the Nigerian soil proves to possess great and inestimable tourism potentials if properly explored and exploited, as they are not found in every other part of the world due to unsuitability of climatic conditions to facilitate their process of formation. For talus caves to be produced, the tropical climate is the most suitable due its relatively high temperature and humidity conditions.

*In future posts, we shall explore the process of formation of each of these talus caves.

1. Kolawole F. & A. Y. B. Anifowose (2011). Talus Caves: Geotourist Attractions Formed by Spheroidal and Exfoliation Weathering on Akure-Ado Inselbergs, Southwestern Nigeria. Ethiopian Journal of Environmental Studies and Management Vol. 4 No.3 2011. Pp1-6 (www.ajol.info/index.php/ejesm/article/view/71622/60586).

2. Vidal RomanĂ­ J. R. and M. R. Vaqueiro (2007). Types of granite cavities and associated speleothems: genesis and evolution. Nature Conservation 63. Pp41-46.

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.