THE ORGANIC CARBON RECORD OF LOWER CRETACEOUS RIFT LAKES, CONGO BASIN, AFRICA: A RECORD OF RIFT TOPOGRAPHY, SOIL DEVELOPMENT AND TOPOGRAPHY

The organic carbon record of Lower Cretaceous synrift lacustrine shales from the Congo Basin, west Africa, is an expression of the interplay between rift topography, soil development, nutrient delivery and bioproductivity, not a simple function of water depth and resulting anoxia. The sequence includes an active rift section, deposited in relatively deep water during active faulting and subsidence, overlain by a late rift or sag phase section, deposited in shallower water during reduced faulting and subsidence. Total organic carbon (TOC) averages 2–3 wt.% throughout the active rift section and consists of Types I and III kerogen. TOC averages 6% in the late rift section, consisting of Type I algal and bacterial kerogen. The lake was relatively reducing throughout deposition of the rift section, so enhanced anoxia did not trigger deposition of the richest source rocks. Rather, they are associated with high organic productivity, so nutrient flux was critical. We propose that reduced topography in the late rift stage led to efficient cycling of plant-derived carbon into soils, enhancing chemical weathering and nutrient flux to the rift lake. Decreased topographic relief had several effects on soils: soils became thicker and finer, erosion of surface and soil organic matter decreased, and fractionation of precipitation into infiltration increased.

This hypothesis is tested by application of CENTURY, a box model that simulates transfer of nutrients within soil pools. The model is first applied to a rainforest soil, with several parameters individually varied. Experiments show that the concentrations of the nutrients C, N and P in groundwater decrease rapidly as infiltration decreases, due to increased slope or decreased precipitation. Erosion of plant litter and topsoil substantially decreases nutrient concentrations in groundwater. Increased sand in soil increases in nutrient concentration. A model integrating these effects shows that C and P concentrations in groundwater increase substantially as slope gradient decreases. Thus, evolving topography during rift development can significantly influence nutrient concentrations in groundwater and, if these nutrients flow into rift lakes and stimulate organic productivity, account for the deposition of rich oil-prone source rocks in late rift stages.