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Publication Type
Conference Proceedings
UWI Author(s)
Author, Analytic
Khan, Shakira A.; Mitchell, Simon F.
Author Role
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Author Affiliation
Department of Geography and Geology
Paper/Section Title
Possible source of a major sediment producer on tropical carbonate beaches - Amorphous grains
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Proceedings Title
Proceedings of the Sixth Conference, Faculty of Pure and Applied Sciences edited by Daniel Coore and Robert J.Lancashire.
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Place of Meeting
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Place of Publication
Kingston, Jamaica
Publisher Name
Faculty of Pure and Applied Sciences, University of the West Indies
Date of Publication
2003
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Volume ID
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Location in Work
34
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Abstract
Jamaica's carbonate sand beaches consist of skeletal and non-skeletal grains. The skeletal fraction of the sediment includes fragments of marine organisms, such as foraminifers, red algae, green algae, echinoide, molluscs and corals. The non-skeletal fraction consists of crystalline, amorphous and clastic grains. Constituent analysis, of samples collected from the island as well as Lime and Morant Cays, show that these non-skeletal grains account for more than 50% of the total sample at many sites. Because of tehir importance to the beach sediment, it is critical to understand the origin of these non-skeletal grains. This study focuses on identifying the origin of amorphous grains. Individual grains were fractured and mounted on stubs for analysis under the scanning electron microscope (SEM). The amorphous grains consist of a dense carbonate matrix and less dense crystalline fabric. The crystalline fabric included needle-like aragonite and blocky calcite spar. These are interpreted as cements growing within cavities in the grain. The cavities have the morphological attributes of utricles within Halimeda grains. The matrix consists either of fine interlocking crystals of aragonite or blocky calcite. The needle-like aragonite fabric is reminiscent of the fabric seen in recent Halimeda. We interpret the blocky calcite as a replacement texture of the original aragonitic fabric. The SEM results are consistent with thin-section analysis of Halimeda grains from Lime Cay. At Lime CAy, a progressive alteration of Halimeda plates was suggested based on the gradual in filling of utricles and pits around the outer margin of the grain which led to a complete obliteration of the internal features. We therefore, suggest that the amorphous grains identified in the carbonate sands of Jamaica are primarily produced by the alteration of Halimeda plates.....
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