LIFE AS A GEOLOGIST, THE HURDLES, ADVENTURE AND DISCOVERIES #1 - ANAMBRA BASIN

Hi steemians!!!!

DID YOU KNOW . . . ?

The earth has been subjected to various changes ranging from physical; chemical to biological and these has altered the natural sequential order of deposition of sediments as opposed to the underlying geologic principles!

Does the above scenario seem familiar? Do you consider it real? If so, take a moment to analyze the situation. What do you think?

I tried really hard to imagine the mystery behind the origin of basins when I was a student geologist; until it came a day I was exposed to the reality of earth as a planet in the field, observing major rock types, structures, textures, topography, geological features and what geologic activities have occurred. It sounds awful always reading from books without seeing with the eyes. !! A day never to be forgotten!!

Guess what? “A Trip to the Anambra Basin”, Nigeria

This basin is a sedimentary succession that is located at the southern end of the Benue trough which is accumulated by Nkporo shale and younger sediments that extends southwards to the Niger Delta basin. The basin is bounded to the west by the Okitipuppa ridge. Northwesterly, it overlies the basement complex between Auchi and Igarra (Northern Edo state) and the sediment basement contact between Itobe and Anyingba (Kogi state). It became an important depo-center harboring sedimentary successions of oil sands, Nkporo shales, Mamu formation, Ajali sandstones and the Nsukka formation from beneath upwards (Murat, 1972).

Never in my life did I believe I could sojourn through some part of this basin like the Igbo Etiti, Akpugo Ezedike, Ukpata, Nkpolgu, Udueme , Aku and part of Ihe-Nze areas of Enugu state in Nigeria. During the trip, I went along side with my colleagues; strong and energetic as they was, I never felt reluctant! It was a journey with a whole basket of fun…!

What do you think would have led to the tilting and faulting of sediments in this basin?

A question I asked myself! Hardly did I know there was an over-ridding effect of tectonism on the various outcrops. Tracing through the exposed outcrops, taking stratigraphic sequence and thicknesses with lithologic similarities; I discovered that the area was dominated by major sediments of two well-defined lithologic stack where the individual host bed differ from a succeeding bed in-terms of the grain sizes, geometry and color.

During the course of the journey; lo and Behold, the great “Ohene Hill” at Akpugo Ezedike, a very massive exposure of different rock lithologies comprising a basal grey-brown claystone unit of 33 m thick and lying beneath it was a ferrugenized sandstone unit of similar thickness hosting a dark thick shale estimated to be above 3m and dipping 08 Northwesterly and 188 degrees strike.

Was that really stressful?

No! to me; it was a lesson, an adventure and a good and wonderful experience to explore and discover. No more doubts on what I read from books!

Field Measurements

That was a hobby for me! Imagine facing the hurdles under the hot sun; I felt nature and creation!!! I felt life in a misery!!! Not just defeating the struggles; I went back home with a basket filled with beautiful and interesting gift of nature.

What will that be? “The rock and sediment samples”

These…! A blue print to the origin, formation and depositional environment of the basin.
With concise laboratory studies of the rock and sediment samples; I discovered the sedimentary structures of Ajali sandstone is profusely cross bedded, characterized by herringbone structures and Ophiomorpha burrows which indicates a tidal environment with high energy (Tucker, 1996) with a clue that the ancient rivers responsible for sediment transport might have left evidences of their flow directions in the forms of planar and cross stratification.

Conducting a sieve analysis on the samples, it showed that Ajali sandstone is coarse, medium to fine grain poorly sorted sandstones suggesting fluctuation in energy of deposition with its coarsening upward nature also suggesting that the basin was shallowing upwards (Dapple, 1974 and Tucker, 1996) which may have been due to fluvial interference with the marine environment.

Laboratory Measurements

An adventure not wasted, I smiled with a deep sense of proofs!!
From my studies, the fissile fine-grain nature of the Enugu shale as indicated in the field data collected suggest that the shale was deposited below the wave base accumulated in relatively low energy environment and also its light grey color showing that it was deposited on the surface of the basin where oxidation could take place (Dapple, 1974). With the presence of extra-formational clast within the shale, I discovered there was a fluvial influence during the period of deposition of this shale within the Anambra basin (Tucker, 1996).
Wow!!!! I never rested until my proofs where complete and genuine.

Final Words

To the conclusion, the deposition of sediments in the basin is compatible with the prevailing geologic model of Northeast-Southwest (NE-SW) trend of the Pan African Orogeny that controls every events in Nigeria causing the dipping of every structures between zero and ten degrees except for fore-set beds in the entire basin. This has always been my inspiration in digging dip about the mysterious earth as a planet.

References

• Murat, R. C. (1972). Stratigraphy and Paleogeography of the Cretaceous and lower Tertiary of Southern Nigeria. In T. F. J. Dessauvagie and Whiteman (Eds), African Geology, Ibadan University Press, Nigeria 251 – 266.
• Tucker, M.E. (1996). Sedimentary Rocks in the Field. John Weley and Sons, Chichester, New York, 150 p
• Dapple, E.C. (1974). Sandstone types and their Associated Depositional Environments. Journal of Sedimentary Petrology. Vol. 4, pp. 695-707.

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It is a great pleasure sharing this report of a State of the Art Geologic Adventure to Anambra Basin in Nigeria. I hope you enjoyed your read?

All Images used in the blog are original captured by my cam during the exercise.