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Sediment Analyses
In addition to samples collected for OSL dating, samples were collected for
granulometric analysis. Samples were collected by pushing small, clear plastic sample vials
horizontally into the trench face or by subsampling vibracores. Samples were brought back to
the lab, weighed and dried to provide an estimate of modern moisture content. The samples were
then mechanically separated on a Ro-tap using 8-inch diameter screens stacked at ¼ phi intervals
between (and including) 4.0 phi and –1.0 phi (1/16 mm to 2 mm). Based on the procedure
outlined in Socci and Tanner (1980), each sample was shaken for 30 minutes. Each grain size
fraction was weighed and results were analyzed using the GRANPLOTS program (Balsillie et
al., 2002).
Two additional sets of data were analyzed. Tanner (1992 and unpublished data) collected
and analyzed a series of grab samples from several transects across St. Vincent Island in 1984
and 1985. Samples were collected from the seaward faces of successive ridges from a depth of
30 cm. Figure 3.7 shows the locations of the samples that were collected by Tanner.
The GRANPLOTS program provides complete granulometric analyses including data
listing, moment measure calculations and frequency and cumulative plots. Probability plots were
interpreted using Tanner’s (1986, 1991) SELF (Settling-Eolian-Littoral-Fluvial) method. In this
method, basic line segment geometries are used to identify transpo-depositional signatures. The
line geometries were determined by Tanner (1986, 1991) as a result of the analysis of over
11,000 sediment samples from known environments. The basis of this method is shown in
Figure 3.8. Line AEF represents the Gaussian distribution. Line segment B indicates that the
operating transpo-depositional element is wave activity. The point relative to segment E is the
so-called surf break. This gentle slope indicates sand deposition on a beach. The higher the
slope, the greater the wave energy. Segment C indicates eolian processes. The point relative to
segment B is the so-called eolian hump. Segment G represents the low energy tail termed the
settling tail and may indicate a lowering of the depositional energy for the total distribution or for
distribution segments containing coarser sediment. It signifies settling of sediment grains
suspended in water. The data collected during the grain size analyses is presented in Chapter 4.
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