Lagoon Microbialites on Isla Angel De La Guarda, Gulf of California, Mexico

Examples of two closed lagoons with extensive growth of Recent microbialites showing variable surface morphology are found on Isla Angel de la Guarda in the Gulf of California (Mexico). This discovery records the first verified living microbiolites in the Gulf of California. The perimeter of both lagoons features structures indicative of thrombolites with a crusted surface morphology from 2 cm to 3 cm in relief and internal clotting without any sign of laminations. An inner zone is dominated by microbial mats that are exposed in profile along the edges of desiccation polygons. Under subaerial conditions, both zones exhibit a white calcified crust below which a soft substratum of dark organic material 4 cm to 6 cm in thickness is protected. Laboratory cultures from lagoon samples yielded the solitary cyanobacterium Chroococcidiopsis as the predominant taxon interspersed with filamentous forms. The outer zone varies from 2 m to 3 m in width. The inner mat zone has the same width but extends into hypersaline water, where the crusted surface is pink. The larger lagoon has an east-west azimuth and covers 22.5 hectares, while the smaller lagoon has a roughly north-south axis and covers 2.25 hectares. Pliocene strata along the edge of the small lagoon include intervals with laminated siltstone bearing calcified platelets suggestive of a microbial origin. Dry lagoons abandoned during later Pleistocene time occur inland at higher elevations, but retain no fossils except for white crusts cemented on cobbles around distinct margins. These geomorphologic features suggest that Isla Angel de la Guarda experienced Quaternary uplift similar in scale to other gulf islands on which marine terraces are preserved.