Subsidence and Gravel Transport in Alluvial Basins

Abstract

In recent years the mechanics of basin subsidence have become much better understood, but exploration of the effects of subsidence on basin filling is just beginning. The primary effect of subsidence is to induce deposition. In alluvial systems transporting coarse, mixed-size sediments, deposition selectively removes the coarsest material from the flow, leading to a fundamental connection between subsidence and downstream fining. In this paper I develop three simple mass-balance models that relate subsidence to sediment distribution. The first model, which considers only selective deposition in bimodal gravel-sand mixtures, shows why slow, uniform subsidence tends to produce thin sheet gravels while rapid, asymmetric subsidence produces thick, areally restricted gravels. This result suggests that areally extensive gravel units may signify periods of reduced subsidence rate rather than source-area tectonism. The second model incorporates clast abrasion as an additional cause of downstream fining and shows that small basins with rapid proximal subsidence tend to sort clasts by size while large, slowly subsiding basins tend to sort clasts by durability. The third model presents a partial solution to the complex problem of detailed quantitative modelling of downstream fining. A simple mathematical transformation is proposed that relates longitudinal grain-size profiles in nonuniformly subsiding basins to a reference profile for uniform subsidence, which is assumed to be exponential. The transformation distorts this uniform-subsidence profile so that most of the fining takes place in areas where subsidence rate is highest. Quantitatively, the third model compares well with data for two available test cases. All three models indicate the need for, and might be useful in planning, further basic studies of grain size, gravel content, dispersal pattern, and subsidence rate in alluvial basins.