Distinction Between Sortable Silts and Flocculated Particles in Muddy Intertidal Sediments: The Role of Flocs and Aggregates in Muddy Sediment Turnover

Tae Soo Chang, Olaf Joerdel, Burg Flemming, and Alexander Bartholomae
Senckenberg Institute, Wilhlemshaven, Germany

In muddy sediments, the distinction between non-cohesive mineralic silt particles and cohesive silt-clay aggregates and flocs provides valuable insights into fine particle dynamics. This is highlighted by a comparison of particle size distributions of deposited muds with those of suspended matter in a back-barrier tidal basin of the Wadden Sea (southern North Sea). For this purpose, the dispersed fine fractions of samples from muddy intertidal sand flats were analysed by Sedigraph in the laboratory, whereas particle sizes of suspended matter over the same tidal flat were analysed in-situ by a laser particle sizer (LISST). The size-frequency distributions of deposited muds comprise two distinct subpopulations, a well-sorted coarser silt (so-called sortable silt) and an unsorted (or poorly sorted) silty clay. The two subpopulations are separated by a narrow size window located around 8 μm (7 phi). The population composed of particles <8 μm is postulated to have been deposited in the form of flocs and aggregates which are considered to be hydraulically equivalent to the local sands and coarser silts. This was verified by comparing the size distributions of dispersed pump samples generated by Sedigraph with those of the in-situ particle sizer. It was found that 85% of the single grains in the dispersed pump samples were <63 μm in size, whereas 75% of the in situ particles were >63 μm in size. The distinction between aggregated and non-aggregated particle groups should thus be given more attention when interpreting the depositional environment of muddy intertidal sediments in the rock record.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005