Abstract

Rhodoliths from the Late Pleistocene Ironshore Formation in the Crystal Harbour area on the western part of Grand Cayman, which are up to 14 cm long, are characterized by thick cortices that were constructed around nucleii formed primarily of coral or bivalve fragments. Two different communities of encrusting organisms constructed the cortices. The thin inner zone (average ∼2 mm thick) was formed by the Coralline Community which was dominated by Lithoporella, Lithophyllum and Neogoniolithon, along with fewer Peyssonnelia rubra, Hydrolithon reinholdi, Lithothamnium, and Porolithon. The thick outer zone (up to 60 mm thick) is dominated by Peyssonnelia rubra, along with scattered coralline algae, serpulid worms, foraminifera, and bivalves.

The Cayman rhodoliths grew in narrow zones surrounding large patch reefs that developed in water less than 14 m deep. The sciaphyllic Peyssonnelia rubra may have grown beneath a non-calcareous fleshy alga that covered the encrusting organisms. The change from the Coralline Community to the Peyssonnelid Community, evident in every rhodolith, probably represents a developmental succession in the encrusting communities.

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