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

not varied much outside of 15-25% since ridge deposition. Therefore, all sample ages shown in
Table 4.6 were calculated using an assumed water content of 20±5%. OSL ages were also
calculated using an assumed moisture content of 5±2% to determine the sensitivity of the age
calculations to water content (Table 4.7). Water content variation represents approximately 3-
12% of the calculated sample ages.
Mean progradation rates for the island and their associated sea level status were
determined based on the dated sites. Table 4.8 shows the calculated rates and sea level status.
The calculated rates were then plotted against age and superimposed on the northern Gulf of
Mexico sea level curve of Balsillie and Donoghue (2004) (Figure 4.42). The progradation rate of
the beach ridge plain has varied over the island’s 4000+year history.
The distances between the dated sites were determined using one of two methods. For
dated sites that were located along the same transect, a straight-line distance was measured. For
those dated sites that were not on the same transect, an orthogonal distance was measured based
on the measured straight line distance between the sites and angular difference between the
straight line drawn between the sites and the angle of strike of the ridges at the sites. The
distances were then used to calculate the progradation rates. Two of the sites (SVI 003 and SVI
004) had identical ages. To determine the progradation rate between these two sites, the
analytical error of +/-300 years between was used to represent the estimated maximum time
interval between the two dated ridges. The error associated with each calculated rate is based on
the OSL age errors shown in Table 4.6. The errors associated with the horizontal position of
each site are on the order of 1-4 cm. These errors are negligible and were not taken into account
in the progradation rate calculations.
Mean progradation rates ranged from 0.7±0.3 to 4.3±3.0 m/yr. The distance of 2,550 m
between sites SVI 015 and SVI002 equates to a progradation rate of 4.3±3.0 m/yr. There is a
distance of 682 m between sites SVI 002 and SVI 003, which equates to a progradation rate of
0.7±0.3 m/yr. The spacing between sites SVI 003 and SVI 004 is 779 m, which provides a rate
of progradation of 2.6±0 m/yr. There is 1,350 m between SVI 004 and SVI 023, which works
out to a rate of 1.0±0.2 m/yr. Between sites SVI 024 and SVI 025 there is a distance of 816 m,
which equates to a progradation rate of 2.0±0.5 m/yr. There is a distance of 775 m between the
two southernmost dated sites, which represents a progradation rate of 1.9±0.5 m/yr. The lowest
rate of progradation was between 3,500 and 2,500 years ago and the highest rate was between
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