Constraints on the formation of depositional placer accumulations in coarse alluvial braided river systems / by John Peter Burton
Burton, John Peter
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Placer accumulations are formed by the preferential sedimentation of heavy minerals from the general population of detritus being transported by a fluid. The depth, velocity, and grain size conditions under which placers form on beaches, sand-dominated meandering, and braided fluvial systems is at present only partially understood. Our knowledge of the controls on alluvial placer formation in gravel-dominated longitudinal bars of braided rivers is even more poorly developed despite their obvious economic importance. The accumulation of heavy minerals in coarse-grained longitudinal gravel bars was studied by examining and sampling surficial and matrix sediments from modem, naturally occurring bars, and by simulating these bars under a variety of controlled flow conditions in a sediment-water recirculating flume. Two processes dominated the deposition of sediments in both the natural and artificial systems studied: 1) suspension rain out; and 2) avalanche face progradadon. Sediments which were deposited as a result of avalanche face progradation were found to contain significantly higher concentrations of heavy minerals in both the naturally occurring and experimental longitudinal gravel bars. Data also indicate that the difference in heavy mineral content amongst sediments deposited as a result of these two processes will increase substantially with increasing density of the detrital minerals present. This suggests that denser heavy minerals are more likely to be deposited amongst less dense surficial sediments whereas less dense heavy minerals are more likely to be vertically distributed throughout the bar sequence. In the natural systems studied, heavy mineral content was found to be much higher in poorly sorted, coarse-grained sediments deposited amongst pebble sized clasts. Flume tank experimentation similarly revealed that detrital lead content was highest amongst pebble sized clasts during the fastest velocity runs. In addition, an increase in clast size resulted in a decrease in the amount of heavy minerals accumulating in surficial sediments. This study has also highlighted two processes which result in the formation of alluvial depositional placer accumulations in coarse-grained braided river systems. The first process occurs as a result of heavy minerals in channel bottom sediments becoming progressively enriched through the winnowing of less dense sediments, resulting in the formation of an erosional placer deposit. Flume experimentation revealed that when high concentrations of heavy minerals armouring the stream-bed were reached, this often resulted in the initiation of their movement downstream. This process can also be triggered by catastrophic events such as large floods or regional tectonic uplift. A sudden increase in energy typically associated with such events results in the flushing of erosional placers and their eventual deposition in areas of higher preservation potential. Therefore, a catastrophic adjustment helps to flush out erosional placer deposits into the basin to form a depositional placer accumulation. The second process of depositional placer formation results from heavy minerals travelling in bed load transport, while less dense sediments are kept mostly in suspension. With a decrease in velocity, heavy minerals are sedimented with hydraulically equivalent sized, less dense sediments in open framework gravels.