--> The Influence of Clay Type and Grain Size on the Bedload Deposition of Muds - An Experimental Perspective

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The Influence of Clay Type and Grain Size on the Bedload Deposition of Muds - An Experimental Perspective

Abstract

Fine grained (<63 µm) particles may reach the seafloor via settling from surface waters, but bottom currents typically will redistribute them laterally. Mechanisms include liquid muds, bedload transport of water-rich rip-up clasts, or swift moving suspensions (the focus of this presentation). Muddy suspensions that move at velocities in the 15-30 cm/sec range (measured 5 cm above the bed and sufficient to move medium sand) can be shown to generate floccules that travel as bedload ripples. Uncertainty is introduced because flume experiments on mud behavior rely on commercially available kaolinite, whereas ancient mudstones typically consist of illite and/or smectite. To better understand the influence of clay type and water chemistry on flocculation and the critical velocity of sedimentation (CVS), a series of experiments was conducted, wherein the flocculation, transport, and deposition characteristics of smectite, illite, and kaolinite were examined in relationship to suspended sediment concentration (SSC) and salinity. Results: (1) CVS values are comparatively insensitive to suspended sediment concentration, although they do increase somewhat when SSC is high (several g/l); (2) CVS values for these clays, as well as carbonate mud, fall into the 20 to 30 cm/sec velocity range; (3) salinity enhances flocculation efficiency, but does not significantly change CVS values seen in freshwater. The grain size distribution of a mud, though difficult to assess in consolidated material, affects CVS values for a given material. Commercial clays are made by grinding rocks into powders, and invariably contain rock fragments of variable size that do not (or not fully) participate in flocculation. Using a sonic probe, clay powders were disintegrated over increasingly longer time periods. The increase of the finer than 5 µm fraction strongly affected the settling behavior of suspensions (slower). In flume experiments more “fines” led to generation of deposition prone floccules at higher CVS thresholds. Preliminary results indicate as much as 10 cm/sec difference between a “coarse” (abundant 30-60 µm particles) and a “fine” (abundant <10 µm particles) version of the same source clay. It appears therefore that just like in sandstones, where fine, medium, and coarse imply different thresholds for transport and deposition, analogous distinctions may be possible in mudstones and allow for more sophisticated assessment of physical conditions of sedimentation.