Paleohydraulic reconstruction of a late cretaceous river in the middle Benue trough (Nigeria) and its limitations

Abstract

Several fining-upward cycles and epsilon cross-bedded units have been recognized in the fluvial Makurdi Formation (Turonian) of the Benue trough of Nigeria. The maximum thickness of a coarse-member unit of a fining-upward cycle served as a measure of the bankfull depth ( D) of the ancient Makurdi river. The width ( W) of the river has been estimated from Cotter's regression equation which related width of a stream to its depth. After determining percentages of silt-clay content in channel bed ( Sc) and channel bank ( Sb), the basic data have been used in several empirical equations and graphs derived by Schumm and others to estimate morphology and flow characteristics of the Makurdi river. The results indicate that the ancient river in the study area was about 227 m wide and 10 m deep; it had a 900 km long, meandering channel; the meander wave length averaged 2580 m with an amplitude of about 1186 m and a mean radius of curvature of about 640 m. The river drained an area of about 57 000 km 2; it had a mean annual discharge of about 640 m 3 sec −1 and a mean annual flood of 2300 m 3 sec −1; the discharge at bankfull stage was 2300–2600 m 3 sec −1. It was a mixed-load river with a bankfull flow velocity of 219 cm sec −1 and a moderate gradient of about 0.14 m km −1. There are several limitations in the paleohydraulic methodology. The most serious of these is the likelihood of getting widely divergent results from using several of the empirical equations. The majority of the existing empirical relations and graphs have been derived from studies of stable channels in temperate regions; their validity for larger rivers in tropical setting, which have great preservation potential in the rock record, has not been tested. The results may also suffer from use of only a few measured variables to calculate other variables and parameters of a paleochannel so that errors are compounded. Nevertheless paleohydraulic measures provide an approximation of the morphology and flow characteristics of an ancient channel.