Gravity Field and Structure of the Niger Delta, Nigeria, West Africa
Data are given for 73 gravity stations situated on land in the general area of the Niger delta, Nigeria, West Africa. This information is presented in the form of maps showing the location and height of the gravity stations, Bouguer anomalies, and isostatic anomalies for a normal crustal thickness of 30 km. The bathymetric map of the Gulf of Guinea (adjacent to the Niger delta) shows a bulge in the isobaths opposite the Niger delta. This bulge is interpreted as an indication that the delta occupies an area once occupied by sea water reaching oceanic depths. This interpretation allows one to estimate the extra load of sediment on the crust of the earth which the Niger delta would represent if there had been no subsidence under this load. In the extreme southwestern part of the subaerial Niger delta the sediments constituting this hypothetical extra load are about 3000 m thick. In a comparison between observed gravity anomalies and anomalies computed on the assumption that no subsidence under load has occurred, the observed gravity field shows negative values of low magnitude, whereas the computed anomalies are positive and large. Thus there must have been considerable subsidence; the Niger delta must in fact be nearly in isostatic equilibrium. Following earlier work by Lawson, the relationships between load and subsidence and the resulting total thickness of deposited sediment are analyzed quantitatively. The resulting total thickness of deposited sediment is proportional to the difference between former and present water depth; the constant of proportionality can be calculated from the densities of sea water, sediments, and the substratum on which the crust is assumed to float. The results of this analysis allow one to construct a hypothetical section of the Niger delta. Assuming a value of 2.40 g/cm 3 for the average density of the sediments of the delta, the maximum thickness of these sediments appears to be about 8000 m, and the total volume, about 500,000 km 3 . The latter result is considered in terms of the age of, and present sediment supply to, the Niger delta. The “Niger delta Minimum,” a gentle gravity minimum reaching –40 mgal that covers most of the subaerial part of the Niger delta, may be a result of an uncompensated downwarp of the crust, density contrasts within the basement, imperfect corrections for the presence of light sediments, or a combination of any or all these factors.