Structural Segmentation and Basement Depth Mapping of the Lower Benue Trough Using Aeromagnetic Data

Deep beneath the surface of the Earth, hidden from view by miles of soil, rock, and vegetation, lies a complex landscape of ancient mountains, deep valleys, and fractured blocks of stone. This hidden world, known as the basement, is the foundation upon which all the layers of sedimentary rock above it were deposited. For geologists, understanding the shape and depth of this basement is like reading the blueprint of a building; it reveals how the land was formed, where valuable minerals might be trapped, and where oil or gas could be hiding. One of the most effective ways to see this hidden landscape without digging a single hole is by measuring the Earth's magnetic field. Every rock has a unique magnetic signature, much like a fingerprint, depending on the minerals it contains. Some rocks, rich in iron, are strongly magnetic, while others are not. By flying sensitive instruments over the ground to map these magnetic variations, scientists can infer the shape of the rocks below, distinguishing between deep, quiet basins and shallow, rugged highlands.

In southeastern Nigeria, a vast geological feature known as the Lower Benue Trough stretches across the landscape. This trough is a long, linear depression filled with layers of rock deposited millions of years ago, and it is surrounded by older, harder crystalline rocks. For decades, scientists have known that this area holds significant potential for natural resources, including lead, zinc, and hydrocarbons. However, the true shape of the basement rock beneath these layers has remained a mystery, obscured by thick sediments and difficult terrain. A team of researchers set out to solve this puzzle by creating a detailed three-dimensional map of the basement using aeromagnetic data. Their goal was not just to see where the rocks are, but to understand how the tectonic forces that shaped the region millions of years ago created a complex architecture of uplifts and depressions that still influences the landscape today.

The researchers began their work by gathering magnetic maps covering a vast area that includes towns such as Nsukka, Abakaliki, and Ogoja. These maps, originally created by the Nigerian Geological Survey Agency, contained measurements of the Earth's magnetic field taken from the air. The team used sophisticated computer software to process this data, separating the broad, deep signals from the local, shallow ones. This process allowed them to isolate the magnetic signature of the deep basement rocks from the noise of the shallower features. To get a clearer picture, they drew seven long lines across the magnetic map, cutting through the most important geological trends. Along each of these lines, they built a two-dimensional model, essentially a cross-section of the Earth, to estimate how deep the magnetic basement was and what kind of rock it might be.

The results revealed a landscape far more dramatic and varied than previously imagined. The depth to the top of the basement rock varies wildly across the region. In some places, the basement is right at the surface, exposed as rocky outcrops. In the deepest parts of the trough, the basement plunges down to a depth of 15 kilometers, buried under a massive pile of sediment. The study showed that the basement is not a smooth, flat sheet but a highly segmented system of raised blocks, known as horsts, and sunken valleys, known as grabens. These features were formed when the Earth's crust was stretched and pulled apart during the Cretaceous period, creating a series of faults and fractures. The researchers found that below a depth of 5 kilometers, the basement becomes more continuous, stretching for hundreds of kilometers, but the upper layers are broken into distinct blocks that move independently.

One of the most significant findings was the difference in the type of rock making up these basement blocks. In the area around Abakaliki, the basement is composed of rocks that are weakly magnetic, suggesting they are made of felsic or altered materials. This area is heavily faulted and fragmented, creating a complex network of cracks that the researchers believe act as pathways for mineral-rich fluids. This is consistent with the known presence of lead and zinc deposits in this region, which often form along such fault lines. In contrast, the block of rock stretching from Afikpo to Okigwe and Uzoakali is made of strongly magnetic, iron-rich mafic rock. This block is shallow, rising close to the surface, and acts as a massive, solid foundation that contrasts sharply with the fractured terrain nearby. The study also identified a deep basin in the northeast, near Ogoja, where the basement drops to its lowest point, filled with thick layers of non-magnetic sediment that could potentially hold oil and gas.

The researchers also mapped the edges of these magnetic blocks, which often correspond to major fault lines. These faults are critical because they control where fluids move underground. The study suggests that the sharp boundaries between the different types of basement rock, combined with the deep faults, create ideal conditions for concentrating valuable minerals. The area around the Abakaliki Anticlinorium, a large fold in the rock layers, is highlighted as a prime location for finding lead and zinc, as the faults there have likely channeled mineralizing fluids over millions of years. Similarly, the deep basins in the Anambra and Ogoja areas are identified as promising targets for hydrocarbon exploration, as the thick layers of sediment provide the necessary environment for oil and gas to form and be trapped.

By combining the magnetic data with these detailed cross-sections, the team created a three-dimensional view of the subsurface that connects the dots between the surface geology and the deep structures. This new map shows that the Lower Benue Trough is a dynamic region where ancient rifting and later compression have created a complex mosaic of rock types and depths. The study confirms that the basement is not uniform but is instead a collection of distinct blocks with different magnetic properties and depths. This understanding provides a solid foundation for future exploration, guiding scientists and miners to the most promising locations for resources. The work demonstrates that even without drilling, advanced magnetic modeling can reveal the hidden architecture of the Earth, turning invisible magnetic signals into a clear picture of the geological history and economic potential of the region.