THE TRANSFER OF RIVER LOAD TO DEEP-SEA FANS - A QUANTITATIVE APPROACH

The size of a river-fed deep-sea fan is controlled in the long term ma inly by the amount of sediment available from a terrestrial source, wh ereas sea level fluctuations only trigger mass transfer to the deep se a. The deposition rate (i.e., the sediment volume deposited per unit t ime) and fan length correlate for most fans formed on abyssal plains. Fan size is independent of depositional environment (lake or sea), tim e 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 r iver load reaching the river mouth is transported to the deep sea over the long term. The type of river mouth, however, affects the amount o f material transported to the deep sea: estuaries with deeply incised canyons may transfer 6-8 times more material than fluvial-dominated an d lobate deltas, provided the suspended river load is equal in both ca ses. However, the exact difference in the transfer ratio can only be d erived indirectly because the available database is too small. For mos t river-fed deep-sea fans, a well-defined geometry develops on unconfi ned abyssal plains. The width/length ratio is about 0. 2 at the base o f the slope, and reaches a maximum of 0. 5 farther downward. This is i n good agreement with flume experiments. The volume of such fans resti ng on a planar base is roughly 0.35 x area x maximum thickness. The qu antitative relationships of fans with respect to geometry, deposition rate. and river suspended discharge may provide some basic data for ba sin modeling and calculation of the sediment budget of erosional-depos itional systems.