the florida state university college of arts and sciences evolution of ...
the florida state university college of arts and sciences evolution of ...
the florida state university college of arts and sciences evolution of ...
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pr<strong>of</strong>ile <strong>and</strong> deposits a ridge. Each ridge represents a paleo-sea-level position. The best estimate<br />
<strong>of</strong> paleo-sea-level position can be assumed to be somewhere between <strong>the</strong> ridge crest <strong>and</strong> <strong>the</strong> toe<br />
<strong>of</strong> <strong>the</strong> shoreface. The boundaries between beach ridge sets may indicate a change in sea-level<br />
position. Where adjacent sets have a measurable elevation difference <strong>the</strong>re has been a change in<br />
sea level (Tanner et al., 1989). Figure 1.7 illustrates Tanner’s <strong>the</strong>ory <strong>of</strong> beach ridge formation.<br />
As an alternative to this <strong>the</strong>ory, Curray (1996) re-proposed that <strong>of</strong>fshore bars are built to sea<br />
level when sufficient s<strong>and</strong> is supplied to <strong>the</strong> coastline. If enough s<strong>and</strong> is available, <strong>the</strong> bar<br />
becomes emergent during periods <strong>of</strong> low surf. With <strong>the</strong> subsequent rise <strong>and</strong> fall <strong>of</strong> tides, <strong>and</strong> if<br />
surf conditions remain low, <strong>the</strong> bar will grow in height. Taylor <strong>and</strong> Stone (1996) proposed that<br />
during storm activity, s<strong>and</strong> from existing ridges is eroded <strong>and</strong> stored in an <strong>of</strong>fshore bar. During<br />
calm wea<strong>the</strong>r conditions, <strong>the</strong> s<strong>and</strong> is transported onshore <strong>and</strong> a ridge is rebuilt.<br />
Each beach ridge plain displays <strong>the</strong> same relative time sequence. The ridge closest to <strong>the</strong><br />
mainl<strong>and</strong> is oldest <strong>and</strong> <strong>the</strong> ridge immediately adjacent to <strong>the</strong> beach is youngest. All o<strong>the</strong>rs fit in<br />
a sequence between <strong>the</strong> two extremes (Tanner, 1988). Since ridges form on <strong>the</strong> beach face, <strong>the</strong>y<br />
are indicators <strong>of</strong> <strong>the</strong> orientation <strong>of</strong> <strong>the</strong> coastline <strong>and</strong> approximate <strong>the</strong> vertical position <strong>of</strong> sea level<br />
at <strong>the</strong> time <strong>of</strong> <strong>the</strong>ir deposition (Donoghue <strong>and</strong> Tanner, 1992; Fernald <strong>and</strong> Purdum, 1992; Tanner,<br />
1988). A beach ridge plain can be divided into sets. The different orientations <strong>of</strong> <strong>the</strong> sets<br />
indicate changes in oceanographic conditions (sea level rise, changes in river mouth position) or<br />
a change in <strong>the</strong> rate <strong>of</strong> sediment supply (Curray et al., 1969; Dominguez et al., 1992).<br />
Many <strong>of</strong> Florida’s barrier isl<strong>and</strong>s are composed <strong>of</strong> beach ridges. The organization <strong>and</strong><br />
<strong>the</strong> physical character <strong>of</strong> <strong>the</strong> beach ridges that make up <strong>the</strong> Holocene beach ridge plains <strong>of</strong> <strong>the</strong><br />
northwest Florida barrier isl<strong>and</strong>s can be employed to study <strong>the</strong> depositional history <strong>of</strong> <strong>the</strong><br />
barriers.<br />
Sea-Level Change<br />
Sea level is never static, but ra<strong>the</strong>r has fluctuated throughout geologic time. Several<br />
factors can cause <strong>the</strong>se fluctuations including changes in <strong>the</strong> volume <strong>of</strong> seawater (i.e. <strong>the</strong>rmal<br />
expansion <strong>and</strong> contraction, or advance <strong>and</strong> retreat <strong>of</strong> ice sheets), rise <strong>and</strong> fall <strong>of</strong> <strong>the</strong> l<strong>and</strong> or ocean<br />
floor (i.e. tectonics, isostatic rebound) <strong>and</strong> variations in <strong>the</strong> earth’s gravitational field (<strong>the</strong><br />
gravitational forces between <strong>the</strong> sun <strong>and</strong> moon will change <strong>the</strong> distribution <strong>of</strong> <strong>the</strong> earth’s water<br />
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