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Isla Mujeres, off the northeast tip of Yucatan, consists of oolitic dunerock. The Caribbean coast is subjected to heavy waves, and the sediments are coarse, well sorted, highly polished and rounded; coralline algae, carbonate rock fragments, Halimeda and coral are abundant. The lee (western) beach is protected; sands are finer, well sorted, dull, and angular with Halimeda greatly predominant. Straits separating Isla Mujeres from Yucatan are swept by strong northerly currents: sediments are poorly sorted, negative-skewed, and consist of a bimodal mixture of (1) dominant small, polished oolites, and (2) subordinate coarse oolitic rock fragments with some skeletal grains.
Sorting of beach sediments is a sinusoidal function of mean grain size; best sorting is at 2^phgr, -3^phgr, and -8^phgr. Sorting values (^sgr) of 0.3-0.6^phgr are characteristic of all beaches regardless of grain size, from fine sand to coral boulders, regardless of composition and regardless of wave energy of the coast. Protected bay sediments are muddy and poorly sorted (immature); sediments from current-swept straits are winnowed, but poorly sorted (submature); sediments of lee beaches are well sorted but dull and angular (mature); and sediments of beaches exposed to high waves are well sorted, rounded and highly polished (super-mature).
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