Rifts as Denudation-Accumulation Systems: Concept, Models, and Examples
We present simplistic quantitative models of rift basin filling, applying the concept of denudation-accumulation systems. Initial rifts are commonly oversupplied and develop a fluvial sedimentary environment because of a high morphometric ratio and high denudation rates due to high relief. During an advanced stage of rift evolution, an increasing morphometric ratio and declining denudation rates often lead to undersupplied conditions with a lacustrine and/or marine sedimentary environment. Sedimentation is here very sensitive to climate fluctuations, and prediction of evaporative mineral formation needs to be modeled by a geochemical computer program. The usefulness of our concept is demonstrated by applications to the Qinghai rift basin, China, and the Turkana rift basin, East Africa. Their sediment filling was reconstructed since the Pliocene on the basis rates of extension, subsidence, and denudation derived from field observations. The climate-controlled short-term evolution during the late Quaternary was modeled using a geochemical computer program, and predictions of formation of evaporitic minerals are provided for three different climate scenarios. In an outlook, we emphasize the combination of the concept of denudation-accumulation systems with more sophisticated two-dimensional models based on sequence stratigraphy.
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Continental rift basins have long been of interest to sedimentologists. Of all the terrestrial settings, rift basins typically provide the greatest accommodation space, and consequently have some of the longest records of continental sedimentation. These records are a product of a complex interplay between several factors that include geological structure and tectonic activity, volcanism, climate and its temporal variability, hydrology, biology and time. Sedimentation in Continental Rifts is a timely update on this exciting interdisciplinary field and presents new approaches and insights into tectonic and structural controls of sedimentation. Other topics included are lacustrine and fluviatile depositional environments and some lesser-known settings, such as springs, wetlands, and paleosols. Several papers consider the behavior of silica in rift lakes, particularly the roles of microorganisms in silica precipitation, whereas others examine the paleoenvironmental importance of freshwater carbonates. The contents of the volume show that sedimentological research in rift basins has progressed beyond basic facies description and general models, and is now focused on understanding the integrative effects of physical, chemical and biological processes in rifts.