Effects of Grain Size and Flow Rate Variation from Channel Formation

Antonio Guirro
E&P, Petrobras, Santos, Brazil

Open-ended flume experiments with fine- and coarse-rich turbidity currents carrying kaonilite and quartz particles were performed. Influence of low and high flow rates was investigated. The turbidity currents were depletive and presented a bipartite character.

In fine-rich currents, regardless of flow rate, the higher concentration less turbulent basal part of the flow expanded up to a given width and then confined itself along its path, generating a depositional channel. The turbulent cloud expanded all over the area. Depositional channels formation was attributed to lateral changes in flow velocity and in suspended sediment concentration. Along the syn-generated channel trough velocity was higher and only coarser grains and clay particles were deposited.

Intermediate-sized grains were carried downslope. Levee crests were formed laterally to the channels. In the slope-basin transition, current expansion and the development of a terminal deposit occurred. In low-flow rate runs, the path of the current on the terminal deposit has constantly shifted, avoiding topographic highs, abandoning some areas, developing sinuous distributary channels and transfered sediments towards more distal areas. Avulsion and overflow occurred throughout the channel length. Only the turbulent cloud has flown off depositing a very thin, fine-grained layer.

In coarse-rich turbidity current experiments self-confinement was not observed. Low-flow rate currents generated shorter concomitant depositional channels with frontal splay lobes. The creation of these channels was attributed to the existence of depositional protuberances formed during the advance of the current front. Downslope velocity increase after crossing protuberances inhibits deposition and erodes the bottom generating channel gutters. High-flow rate currents created only an extensive fan without channel development.

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