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

CHAPTER 5
DISCUSSION
The beach ridges making up the St.Vincent Island beach ridge plain have been
investigated using a variety of techniques. The surface and subsurface morphology of the beach
ridge plain was studied by using GPR and high-precision topographic surveys. Borehole records
from previous studies were accessed to gain additional subsurface information. The
granulometric characteristics of the beach ridge plain were quantified based on the analysis of
samples that were collected by vibracoring and trenching. The sediment analyses involved
determining the grain size characteristics of each sample by sieving and using the GRANPLOTS
sediment statistical program (Balsillie et al., 2002) and by employing a series of probability plots
developed by Tanner (1986, 1991). Selected beach ridge sediment samples were dated using
OSL techniques. These several techniques can be synthesized in order to reconstruct the
depositional history of the island. Additionally, a comparison of the island’s growth history
with the northern Gulf of Mexico sea-level history enables a better understanding of the
relationship between sea-level change and barrier evolution, as will be discussed below.
Surface and Subsurface Morphology of the Beach Ridge Plain
GPR has proven to be a useful tool for analyzing the internal structure of the St. Vincent
Island strandplain. Previous studies conducted on the island have provided estimates of the
depth of the pre-barrier surface beneath the island (Figures 4.21 and 4.22). This boundary is a
sharp contact between brightly colored (yellow and orange mottled) Pleistocene sand and
overlying dark gray Holocene muddy sand (Figure 4.21). Oxidation, the process that has given
the Pleistocene sands their orange color, tends to occur above the water table where atmospheric
oxygen is readily available (Ritter, 1986). The Pleistocene period was a time of widespread
glaciation. With water locked up in the ice sheets, the climate was drier than present. Sea level,
as the Wisconsinan glaciers began to retreat, was much lower than present (Figure 1.5, lower
sea-level curve). Towards the end of the final Pleistocene ice advance, about 20,000 years ago,
sea level began to fluctuate and rise (Figure 1.5). By the mid-Holocene, the northern hemisphere
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