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Detailed stratigraphy, a prerequisite for useful interpretation of precise geochronologic data from Quaternary coral reefs

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H. Allen Curran1, Mark A. Wilson2, William G. Thompson3, and Brian White1

1Department of Geosciences, Smith College, Northampton, MA 01063, USA

2Department of Geology, The College of Wooster, Wooster, OH 44691, USA

3Department of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

acurran@smith.edu, mwilson@wooster.edu, wthompson@whoi.edu, bwhite@smith.edu

 

            As instrumentation and methodologies for increasingly accurate U-series measurements of fossil coral ages continue to improve, the reasons for establishing clear stratigraphic frameworks and documenting field sampling for fossil reefs become even more important.  The interpretative scientific value of a set of coral ages is only as good as the documentation of stratigraphic context.  Any large fossil coral reef will manifest stratigraphic complexity, likely with abrupt lateral and vertical facies changes.  Detailed, carefully constructed transects of vertical sections are required.  Transects should have a legend for all facies and contacts represented, clearly show sample locations, and be as accurately referenced to geographic location and sea-level position as possible.  Mapping of reef exposures, as with the Eemian Cockburn Town and Sue Point fossil reefs on San Salvador, Bahamas, also can be highly useful, although mapping of larger fossil reefs may not be feasible.  Sets of many dated corals from key reef facies are the ideal to enable testing of coral ages within their stratigraphic context.

            Detailed transects of fossil reef sequences permit recognition of disconformities that may record important, short-term events of sea-level change, as documented for the Cockburn Town reef on San Salvador and the Devil’s Point reef on Great Inagua, Bahamas, and, more recently, at the Xcaret reef of the Yucatán peninsula, Mexico (Blanchon et al., 2009).  Such disconformity surfaces are best recognized in a three-dimensional context and can manifest in a variety of ways, including as wave-cut platforms with planed off, large and in situ Montastraea annularis colonies, as formed with short-term regression at Devil’s Point and, to a lesser extent, at Cockburn Town.  With subaerial exposure, reddish, thin caliche (calcrete) layers likely will form, along with rhizomorphs created by plant roots.  Subsequent sea transgression over the reefal substrate can result in surficial borings by sponges and bivalves, encrustations of serpulid worm and vermicularid gastropod tubes, and crustacean burrows that branch along the disconformity surface. Recolonization by corals, with new coral colonies growing on an older, eroded coral substrate or a surface of previous subaerial exposure also can be a prominent marker of important disconformity surfaces.

Last updated: September 9, 2009
 


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