Soils, Slopes and Source Rocks: The Roles of Soil Chemistry and Nutrient Delivery to Source Rock Deposition in Rift Lakes

Nicholas B. Harris¹ and Greg Tucker^2
1Geology and Geological Engineering, Colorado School of Mines, Golden, CO
²Department of Geological Sciences, University of Colorado, Boulder, CO

Lacustrine source rocks in rift basins occupy a distinct position in the stratigraphy of the rift fill, typically occurring in the late rift sequence. Examples include the Early Cretaceous rifts of the South Atlantic margin and the Late Cretaceous rifts of north-central and eastern Africa. Lacustrine source rocks also exhibit strong geographic differences in quality, in equatorial west Africa, for example, changing in both TOC content and organic matter type with respect to the paleo-equator. Studies of the Lower Cretaceous lacustrine shales on the South Atlantic suggest that organic productivity triggered by highly elevated nutrient fluxes led to deposition of the richest source rock intervals.

We hypothesize that soils in drainage basins surrounding the rift lakes are the critical link between climate, rift tectonics and topography and the deposition of rich source rocks. We test this hypothesis through the application of computer program, CENTURY, which simulates soil chemistry. Our results show that the fluxes to groundwater of key organic nutrients, C, P, N and S are highly sensitive to the slope gradient in drainage basins surrounding a rift lake, increasing as the topography degrades during late stages of rifting. Fluxes are also sensitive to rainfall levels, decreasing substantially as precipitation decreases. These results suggest that nutrient flux to rift lakes and, therefore, organic productivity increase: (1) as topographic relief declines; and (2) in wetter climates.

We apply these models to the topography of the Rio Grande rift system, where relief decreases from north to south along the rift axis. Even where precipitation is held constant, nutrient flux to groundwater increases as much as tenfold from north to south. Changing precipitation can accentuate these differences.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas