San Salvador During the Ice Ages: Final

This discussion topic submitted by Scott Wankel (Wankelsd@miamioh.edu) on 7/27/98.

Overview of last 200 Million Years
1. -Opening of North Atlantic
2. -Sedimentation: Biological, Physical, and Chemical. The Bahamas are 100% marine limestone.
3. -Subsidence: About 1 inch/1000yrs.
4. -Gulf Stream is shitfed/formed and the Bahamas are separated from FL and Cuba (80 Million years ago)
5. Pleistocene (2 Million years ago) with Glaciations (4) and Interglacials (3)

Sea Level Changes:
Evidence of Higher Sea Level:
1. -Fossil Corals (Cockburntown Reef)
2. -Coastal erosion features - nothced cliffs (Manhead Cay)
3. -Raised Beaches
4. -Marine LS and Current Bedding

Lower Sea Level:
1. -Wavecut platforms and Marine Terraces
2. -Limestone Crusts and Peat
3. -Blue Holes and Caves
Limestone dissolves when exposed to air (Rainwater is slightly acidic). Seawater precipitates limestone (It is saturated with CaCO3). Lower sea level allows dissolution of exposed limestone. Vast systems of caves and sinkholes formed. Sea level rises and fills these with seawater. These are today's blue holes. Andros Island has 178 on land and 50 underwater (tidally influenced). None is bottomless.


Growth occurs by lateral accretion and is dependent on sea level, platform and shelf margin configuration, and the orientation of sediment transport sources.

"Carbonate islands originate by shoaling during stormy periods when sediments blanket ing the platform first accumulate as shoals, and progessively emerge into beach ridges and dunes. During low stands, dune deposits indurate and form nucleation points or anchors for subsequent deposition during high sea levels. "

Growth is self regulated: lateral growth on the platform means less source area for sediment production. San Salvador appears to have nearly completed its lateral growth and subsequent growth will be vertically, if accommodated by subsidence or a higher sea level in order to oversteep the high ridges surrounding the island.

Four Phases of landform development:

Phase I: (150 - 200 thousand years ago)
Deposits corners (as small ridges) of San Sal (Dixon Hill, Fortune Hill). Other phases tied to/modified by these "anchors." Most significant caves are within these deposits.

Phase II: (132 - 120 thousand years ago)
Early Sangamonian Interglacial (Sea Level Rising)
Caternary Ridges, which are larger and higher farther inland, are deposited between the "anchors." Most of today's lakes are between these ridges. The long smaller ridge on the north and west side. (Sandy point-Cockburntown-Sue point-Reckley point) represents the stable shoreline on the more tranquil side of San Sal.

Phase III: (80 thousand years ago)
Late Sangamonian (Sea Level only slightly below today)
High promontories on the eastern margin of San Salvador. These lie seaward of all other ridges and form the anchors for subsequent caternary development. From 80 to 12 thousand years ago sea level retreats during the Wisconsin glacial period. (Manhead Cay, Crab Cay, Almgreen Cay, The Bluff).

Phase IV: (5 - 3 thousand years ago)
Lower and simpler ridges around nearly the entire circumference of San Sal. Primarily on the eastern edge of San Sal and are caternary on the Phase III and older anchors.


References:
Sealey, Neil E., 1995, Landforms of the Bahamas.

Hearty, PJ and Kindler P, 1993. New perspectives on Bahamian Geology: San Salvador Island, Bahamas. Journal of Coastal Research v. 9 n. 2, pp. 577-594.

Heart, PJ and Kindler P, 1997. The stratigraphy and surficial geology of New Providence Island and surrounding islands, Bahamas. Journal of Coastal Research v. 13 n. 3, pp. 798 - 812.

Garret, P and Gould, SJ, 1984. Geology of New Providence Island, Bahamas. Geological Society of America Bulletin, v.95, pp.209-220.

Mylroie, JE and Carew, JL, 1995. Geology and karst geomorphology of San Salvador Island, Bahamas. Carbonates and Evaporites, v. 10, pp.193-206.


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