Chronostratigraphic Evaluation of the Late Cretaceous-Recent Sediments in Parts of Calabar Flank

Deep beneath the surface of the Earth, layers of rock and sediment act as a vast, silent library. Each layer tells a story of a specific moment in time, preserving the remains of ancient life and the conditions under which they lived. By reading these layers, geologists can reconstruct the history of our planet, understanding how continents drifted, how oceans rose and fell, and how life evolved over millions of years. This process, known as stratigraphy, relies heavily on finding specific fossils—remains of organisms that lived for only a short, well-defined period. These fossils serve as precise time markers, allowing scientists to date rock formations with great accuracy. In the southeastern part of Nigeria, a region known as the Calabar Flank, this library is exposed at the surface, offering a rare and direct look at sedimentary rocks deposited from the Late Cretaceous period to the present day. Understanding the timeline of these rocks is crucial not only for piecing together the geological history of Africa but also for locating potential energy resources hidden within them.

A team of researchers from the Federal University of Technology Owerri recently turned their attention to this specific region to create a high-resolution timeline of its geological history. Their goal was to move beyond general estimates and pinpoint the exact ages of the different rock layers found there. They focused on the Calabar Flank, a sedimentary basin that sits at a right angle to the major fault lines of the nearby Benue Trough. This area has long been a subject of debate among geologists, with conflicting names and ages assigned to the same rock layers. To resolve these uncertainties, the team traveled to various outcrops along major highways, including the Calabar-Ikom and Calabar-Itu routes, collecting fresh samples of rock and loose sediment. They examined these samples under microscopes, looking for two distinct types of microscopic fossils: tiny shelled organisms called foraminifera and microscopic plant spores and pollen grains known as palynomorphs. By identifying which species were present in each layer, the researchers could assign a specific geological age to the rocks, effectively calibrating the local timeline against global standards.

The study revealed a clear and detailed sequence of events stretching back over millions of years. The oldest rocks they examined belong to the Awi Formation, a layer of sandstone and conglomerate that sits directly on top of the ancient basement rock of the region. The researchers found that this formation, which was deposited in a river and shallow coastal environment, dates back to a time between the Aptian and Albian stages, roughly 125 to 100 million years ago. Moving up the timeline, they encountered the Mfamosing Limestone, a thick and cavernous rock formation that formed in a shallow sea. The presence of specific microscopic fossils in this limestone confirmed it was laid down during the Middle Albian stage, about 103 to 96 million years ago. This limestone is notable for its extensive cave systems, which were formed later by the dissolving action of water on the rock.

As the researchers moved to younger layers, they analyzed the Ekenkpon Shale, a dark, muddy rock unit that formed in a deeper marine environment. The fossils found here, including a mix of floating and bottom-dwelling microscopic organisms, pointed to a time between the Cenomanian and Early Turonian stages, approximately 100 to 90 million years ago. Above this lies the New Netim Marl, another marine deposit that the team dated to the Late Turonian and Coniacian stages. The next major layer, the Nkporo Shale, represents a return to deeper ocean conditions during the Late Campanian and Maastrichtian stages, roughly 83 to 66 million years ago. The fossils in this layer, particularly a specific type of planktonic shell, helped the researchers confirm that these sediments were laid down just before the mass extinction event that ended the age of the dinosaurs. Finally, the team examined the youngest rocks in the sequence, the Benin Formation. These are the Coastal Plain Sands, which sit on top of all the older layers. Unlike the deep-sea deposits below, these sands were formed in a non-marine environment, and the plant fossils found within them indicated an age ranging from the Late Miocene to the Early Pliocene, a much more recent period spanning roughly 11 to 3 million years ago.

The significance of this work lies in its ability to bring order to a complex geological puzzle. By using a combination of microscopic animal and plant fossils, the researchers were able to assign precise ages to rock formations that had previously been the subject of confusion and debate. They confirmed that the rock layers in the Calabar Flank follow a logical progression from ancient river systems to shallow seas, then to deep oceans, and finally back to land-based environments. This detailed timeline not only clarifies the geological history of southeastern Nigeria but also provides a reliable framework for future exploration. With a clear understanding of when and how these rocks were formed, scientists and resource managers can better predict where valuable resources, such as oil and gas, might be found within the basin. The study demonstrates that even in areas where the rock record is fragmented, careful analysis of microscopic life can reveal the grand narrative of Earth's past, turning scattered stones into a coherent history of our planet.