coral reef definition

[Gene Shinn]

As a geologist, I can attest that the argument over what is and what is not
a reef is as old as the hills. Alina Szamant's definition is short and
about as good as any. Remember the Exxon Valdize struck Bligh "Reef" even
though we know it was just a mountain top.
   Using the Fagerstrom definition, and many others, about 99.9 percent of
the so-called reefs in the Florida Keys fail the test. Hundreds of
kilimeters of high resolution seismic profiles and numerous core holes have
shown that what is  generally called the outer reef is just Pleistocene
limestone ridge with a thin coating of corals sponges and gorgonians.  The
major reefs like Molasses, Elbow, etc. and some inner patches like Grecian
and Key Largo Dry Rocks are indeed domal structures built up to sealevl by
corals and would be considered coral reefs by Fagerstrom. However, these
"named" reefs cover less less than 1 percent of the Florida reef tract. For
example, a 29-m-deep core drilled in 14 m of water adjacent to the Aquarius
Underwater habitat encountered about 10 to 12 cm of "coral" accumulation.
The rest was Pleistocene limestone. There are scattered 1 m high coral
heads at least 100 years old in this area but between them there has been
essentially no reef accumulation over the past 6,000 years of submergence.
Without drilling who would have guessed that what looks like a typical
coral reef is just a thin coating over preexisting topography? I still like
to call it a reef however, and I think most of us will continue to do so.
   For a detailed set of maps of the area off Key Largo that show, sediment
and reef thickness, underlying bedrock topography and benthic/sediment
habitats contact, blidz at usgs.gov. and ask for Open-File Map 1-2505. Gene


"When you are on the wrong train, every stop is the wrong stop" Dennis Hubbard.
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		                       | For a different track go to
http://coastal.er.usgs.gov/african_dust/
E. A. Shinn
email  eshinn at usgs.gov
USGS Center for Coastal Geology     |
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