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The usefulness of supratidal-flat sediments in paleogeographic reconstructions is decreased by the difficulty in distinguishing between island and mainland deposits. Holocene carbonate sediments in British Honduras form supratidal-flat deposits on shelf-margin islands and on the mainland. The vertical sequence and sedimentary structures of deposits in both areas are similar, reflecting similar modes of formation and physical settings, respectively.
Constituent compositions and the nature of nearby deposits differentiate these island and mainland supratidal carbonates. Shelf-margin islands are associated with backreef coralgal sand; fragments of corals and coralline algae are included in some island deposits. Mainland deposits lack these indicators of a shelf-edge environment but contain brackish-water organisms and terrigenous material, such as quartz and clay minerals. Furthermore early diagenetic dolomite occurs only in island sediments. These distinguishing compositional differences, though diagnostic for northern British Honduras, are not absolute. Rather, they illustrate types of characteristics which may be useful environmental indicators in other areas.
In the absence of compositional differences, lateral facies relations may distinguish island and mainland deposits if sufficient stratigraphic control is available. An island interpretation is favored where submarine shelf-lagoon deposits occur between discontinuous supratidal deposits which are separated by several miles,
especially within a marine-to-terrestrial facies transition. Otherwise, an interpretation of frequently shifting sites of supratidal sedimentation along a mainland is favored.
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