Abstract

JOHNSON, ME; LOPEZ-PEREZ, RA; RANSOM, CR  and  LEDESMA-VAZQUEZ, J. Late Pleistocene coral-reef development on Isla Coronados, Gulf of California. Cienc. mar [online]. 2007, vol.33, n.2, pp.105-120. ISSN 0185-3880.

Part of an extensive fossil reef composed of corals attributed to Porites panamensis is exposed in cross section along the walls of Cañada Coronados on the south side of Isla Cononados in the Gulf of California, near Loreto, Baja California Sur (Mexico). Based on laboratory analysis of uranium/thorium isotope ratios derived from a coral sample, the reef developed sometime between 121,000 and 127,000 years ago during a highstand in sea level correlative with marine isotope substage 5e. Densely packed bouquet-shaped colonies with multiple branches reach a maximum height of 1.1 m and maximum diameter of 1.1 m. Colony size diminishes markedly in the landward direction. About 60% of the corals show growth from an attachment point on andesite cobbles or small boulders. These clasts formed a debris apron on the floor of a large lagoon that effectively stabilized carbonate sand derived from crushed rhodoliths. Associated with the cobbled surface is a transitional intertidal biota that includes clast-encrusting coralline red algae, fixed bivalves such as Pseudochama janus, Modiolus capax, Barbatia reevseana, and Arca pacifica, in addition to gastropods such as Turbo fluctuosus, Acanthina tuberculata, and Nerita bernhardi. The succeeding reef body covered a minimum area of 1.3 ha. Using a conservative estimate of one coral colony per square meter, the reef deposit at Cañada Coronados represents the coalescence of 13,000 colonies. Not strictly coeval in ecological age, additional reef structures may have occupied up to 50 ha around the Cañada Coronados site on Isla Coronados. The southern exposure of fossil reefs on Isla Coronados is a pattern commonly found on other islands and sheltered bays on the peninsular coast in the lower Gulf of California.

Keywords : coral limestone; Pleistocene substage 5e; rocky-intertidal fauna.

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