The Transfer of River Load to Deep-Sea Fans: A Quantitative Approach

Abstract

The size of a river-fed deep-sea fan is controlled in the long term mainly by the amount of sediment available from a terrestrial source, whereas sea level fluctuations only trigger mass transfer to the deep sea. The deposition rate (i.e., the sediment volume deposited per unit time) and fan length correlate for most fans formed on abyssal plains. Fan size is independent of depositional environment (lake or sea), time span, or geological period, which may be characterized by different amplitudes and frequencies of sea level fluctuations. In climatically stable regions such as the tropics about 25 ±10% of the suspended river load reaching the river mouth is transported to the deep sea over the long term. The type of river mouth, however, affects the amount of material transported to the deep sea: estuaries with deeply incised canyons may transfer 6-8 times more material than fluvial-dominated and lobate deltas, provided the suspended river load is equal in both cases. However, the exact difference in the transfer ratio can only be derived indirectly because the available database is too small. For most river-fed deep-sea fans, a well-defined geometry develops on unconfined abyssal plains. The width/length ratio is about 0.2 at the base of the slope, and reaches a maximum of 0.5 farther downward. This is in good agreement with flume experiments. The volume of such fans resting on a planar base is roughly 0.35 × area × maximum thickness. The quantitative relationships of fans with respect to geometry, deposition rate, and river suspended discharge may provide some basic data for basin modeling and calculation of the sediment budget of erosional-depositional systems.