the florida state university college of arts and sciences evolution of ...

Skewness can be used to identify the source of the material making up the beach ridges
on the island. Positive skewness values indicate the presence of excessive fine grains, while
negative skewness is indicative of excessive coarse grains (Leeder, 1982). Skewness is sensitive
to depositional environment. River sands generally show a positive skewness because fine
grains are selectively winnowed by constant wave action, leaving a tail of coarser grains. Wind
blown sands tend to be positively skewed because of the low efficiency of wind in moving
coarse particles, which end up being left behind as a lag deposit. The scatterplot of suite mean
versus suite skewness for the St. Vincent Island beach ridge samples (Figure 4.32) does not show
a correlation between mean grain size and skewness. This would suggest that skewness and
grain size are not related for these samples. The skewness results, which range from –0.003 to
1.3 (Table 4.4), indicate that the sand that makes up the majority of beach ridges on the island
has a beach or riverine source. There also appears to be a relationship between mean grain size,
skewness, standard deviation, kurtosis and location relative to the Apalachicola River mouth.
The coarsest material is deposited near the river mouth while the finer material is deposited on
the barrier islands to the east and west of the river (Figure 4.37). Figure 4.38 shows that the
sediment deposited on the barriers near the river mouth is poorly sorted. Skewness is lowest in
close proximity to the river mouth (Figure 4.39), suggesting that the islands close to the river
mouth have an excess of coarse-grained material. Kurtosis appears to increase from east to west
and doesn’t appear to be related to the location of the Apalachicola River (Figure 4.40).
The samples that showed an eolian signature after being subjected to Tanner’s method of
SELF determination (Tanner, 1986, 1991) suggest eolian decoration at the top of the beach
ridges. The three anomalous samples that had an eolian signature deep within the ridge could be
a result of bioturbation that mixed sediment from the top of the ridge with sediment towards the
base of the ridge or could represent a period of eolian deposition preceding ridge formation.
The results of the granulometric analyses were further analyzed using two of the plots
developed by Tanner (1991). Figure 4.34 is a tail of fines plot. This plot is dependent on the
fluid dynamics of the environments of deposition. The suite mean separates a large new
sediment supply (river or closed basin sediments) from winnowing or sorting products (beach
and dune sands). The suite standard deviation separates mature sediments and large mass
density differences from settling and winnowing products. The downfall of this plot is that it
doesn’t always provide the final agent of transport and deposition but may provide the previous
120