Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/51

What is the Niger Delta? 4.2 RELIEF

Map 3A shows the geological setting of the Niger Delta. As with any landscape, it has undergone vast changes and continues to develop over time.

It is important to understand that there have been two distinct Niger Deltas over recent geological time. In Tertiary times, the sea level was at least 30m higher than it is today. Therefore the second delta, laid down in Quarternary times, does not simply lie on top of the Tertiary deposits that formed the first Delta. Rather, the Niger river cut a wide flood plain between Yenegoa and Onitsha on its way to the more distant sea. This sudden drop in level is still very evident today, particularly in Port Harcourt. Here, a cliff rises straight up for 10 metres above the 20-metre deep Bonny River, making an ideal site for the busy port.

During the Tertiary period, a number of basins on the perimeters of the continents filled up with marine sediments; over immense periods of time these have produced rich deposits of oil. One such basin, known to geologists as the Benue Basin, formed much of what are now the Niger and Benue river basins. It extends inland from the Niger Delta and from Mount Cameroon as far West as present-day Cotonou.

The more recent Quarternary deposits form a dynamic landscape. The rivers meander to form oxbow lakes or to 'capture' the flow of other rivers, which may then dry up; they may form levees, or over geological time they may swell and erode these away again.

Rivers to the East and West of the Niger do contribute to the Delta, but they are relatively short. By far the majority of the Quarternary deposits originate from the Niger/Benue system itself. However at various times, the river system has shifted and changed and different branches have carried more or less water. Today, the Forcados and the Nun are the largest, but within the past 75,000 years the Brass River has been more important than it is now. At one time the Sombreiro/New Calabar/Bonny systems will have been more major branches of the Niger. Features in the Anyama district illustrate past changes in river activity (see Map 8); at one time the Kolo Creek carried most water. Then local uplift, caused by deeper earth movements, allowed the Ekole Creek to take over: in turn its own head waters were captured by the Nun.

Such localised uplift is in tension with the depression caused by the weight of deposition. This interrelationship complicates the local geology as explained in Chapter 14. Another example is the Sombreiro-Ogoni terrace plain, which is several metres higher than the rest of the old coastal terrace (the Sombreiro-Warri delta plain). Again, 49