RECONSTRUCTING THE AGE AND LITHOLOGY OF ERODED SEDIMENT

Abstract

The reconstruction of ancient sediment fluxes is based on the assumptions that young sediment is generally unconsolidated, has the greatest areal exposure and thus has the greatest probability of being eroded. Young sediment is therefore recycled more rapidly than old sediment which is more consolidated and has a smaller areal exposure. This assumption is the fundamental principle underlying the theory of sediment recycling. We assume that on a global scale the sedimentary system is in steady state and has a mass that remains constant because most younger sediments are derived from cannibalization of older sediments through erosion. This implies that gains of sediment mass from weathering, erosion, and deposition of igneous and crystalline metamorphic rocks are exactly offset by losses to subduction and metamorphism. The general decline of sediment mass with age is approximated by a simple exponential decay y=Ae-bt where y is the remnant of the original sediment flux at time t, that would be observed today after t m.y. of recycling at a constant rate of erosion b (“average recycling proportionality parameter” of Veizer and Jansen, 1985), and a constant depositional rate, A (the rate at which sediment is being deposited at present) . The new total sediment mass for the Phanerozoic is 2082.6×1021g. Based on a least squares fit of an exponential decay curve to the data, we have determined the average zero-age flux rate of Phanerozoic sediment to be 5.756×1021g m.y.-1 and the average rate of sediment recycling to be -2.062×10-3 m.y.-1. New estimates of the mass of Proterozoic and Archaean sediments are also presented; these are 845.5×1021g and 15.0×1021g for the masses of Proterozoic and Archaean sediments respectively. Because neither rocks of a particular lithology or age can be selectively protected from erosion, it is possible to reconstruct mass-age distributions for different lithologies. Detrital rocks dominate the sedimentary system in terms of mass, and can be regionally confined as a closed system. The recycling rate of detrital rocks can be used on both global and regional scales to reconstruct past sediment fluxes of both detrital and chemically or biologically precipitated materials.