Basin dynamics, channel processes, and placer formation; a model study

Abstract

The role of basin dynamics, tectonics, and channel processes in the formation of placers has not been well understood. In this study, these problems were investigated in a miniature drainage basin subjected to rejuvenation. Sediment samples collected during the experiment show the response of total sediment discharge and heavy mineral discharge to rejuvenation. Total sediment yield peaked shortly after rejuvenation began and then decayed exponentially. This decay does not characterize magnetite yield which was produced in discrete events separated by periods of little output.Samples within the source area indicated that magnetite was concentrated and stored on uplands and hillslopes as well as in the floodplain. Valley fill contained horizontal layers deposited as the channel shifted laterally and trough-shaped scour and fill deposits, which marked former channel courses. Within these deposits magnetite was concentrated on bedrock channel bottoms, on alluvial channel bottoms (false bottoms), and in similar positions within terrace deposits.Complex response dominates basin rejuvenation. Initially, placers are destroyed and transported from the basin. Aggradation then overloads channels and heavy minerals are stored. This is succeeded by degradation during which heavy minerals are concentrated and flushed from the basin. Superimposed upon this complex response are shorter periods of heavy mineral transport and storage controlled by internal geomorphic thresholds. In both cases, heavy mineral transport is directly controlled by channel activity. Storage occurs when the channel aggrades, and when the channel degrades heavy minerals are concentrated in the channels and transported from the basin.Although basin rejuvenation destroys existing placers, it also creates an environment for the formation of new placers. In addition, the complex response of the source basin to a single uplift will cause the multiple reworking of alluvium necessary for placer formation.