Out and about in Southern Nebraska, collecting joint data

Nebraska is set in the Great Plains region of the United State,s where the topography is almost ‘flat’ and bedding dips are ‘pseudo’ horizontal in most places. It was my first time of collecting field joint data from sedimentary rocks. I had (before now) only done this on basement rocks… and the brittleness of hard rocks makes joint observation on them pretty interesting. Also, my experience in joint analysis on sedimentary rocks had been limited to seismic data and satellite remote sensing data; so, I had a great experience driving around southern Nebraska and northern Kansas, collecting and interpreting joint data on different lithologies.

Some of the outcrops I visited were deeply weathered, while some had paleosols- both of which inhibited detailed observation of joint on the rocks. However, a lot of the outcrops had interesting structural information. See photos below:

- Open joint on a siltstone outcrop.

- Open joint on cross-bedded sandstone. The joint appears refract as it propagates into the bottom layer of relatively different mechanical property (layer with lots of cross bedding).

- Joints in interbedded shale and limestone. Because of the low competence of shale, it deforms less ‘brittly’ and more ‘ductly’, thereby making it difficult to find open joints on them. On the other hand, limestone is a more competent material than shale, therefore and it is more likely to have a higher density of joints,

- Clay-filled joint plane in shale (with thin bentonite interbedding). The vertical joint also appears to have been offset along the lower bentonite bed plane.

- Road-cut covered with paleosol. The paleosol has mud cracks which exhibit popcorn texture. It is so difficult to observe any structural discontinuity on the underlying lithology.

- Observing naturally-emplaced joints on limestone with interbedded chert. The road cut was made with explosives, which introduced many un-natural joints in the rock (which can also have their own systematic orientations). So, picking and measuring a joint on this type of exposure demands some care and thinking.

A threatening earthquake in SW Nigeria?

The increase in reports of tremors in the south-west Nigerian sub-region within the past two-three decades has become disturbing; and several research works (Elueze, 2003, Odeyemi, 2006, Anifowose et al., 2006, Kolawole & Anifowose, 2011) had referred to the Ifewara-Zungeru mega-structure and its connecting Southern-Atlantic transform faults as the major sources.

Below is a historical record of earth tremors in Nigeria (Akpan & Yakubu, 2010):

 

However, these questions may arise:

Considering the spatial coverage of the effects of the tremors, is it possible that these tremors were produced by more than a single fault (Ifewara Fault)? or are there possibly more active faults in the region?

 

I will like to know your thoughts on this issue. Kindly drop your opinions in the comment box.

   

References:

1. Akpan, O. U. and Yakubu, T. A. (2010). A review of earthquake occurrences and observations in Nigeria. Earthq Sci. (2010) 23. Pp. 289-294.

2. Anifowose, A. Y. B., Odeyemi, I. B. and Borode, A. M. (2006). The tectonic significance of the Ifewara-Zungeru megastructure in Nigeria. In: Teme S C and Ezeigbo C U. Proceedings of the 1st International Workshop on Geodesy and Geodynamics. Centre for Geodesy and Geodynamics, Toro, Nigeria. Pp. 17–28.

3. Elueze, A. A. (2003). Evaluation of the 7 March 2000 earth tremor in Ibadan area, southwestern Nigeria. Jour Min Geol 39(2). Pp. 79–83.

4. Kolawole, F. and Anifowose, A.Y.B. (2011). Remote sensing analysis of a dextral discontinuity along Ifewara-Zungeru area, Nigeria, West Africa. Ind. Jour. of Sci. & Tech. Vol. 4 No. 1. Pp. 46-51.

5. Odeyemi, I. B. (2006). The Ifewara fault in southwestern Nigeria: Its relationship with fracture zones along the Nigerian coast. Centre for Geodesy and Geodynamics, Toro, Bauchi State. Pp. 1–13.

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

References:

- 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.