Abstract
Laminated algal mats are well developed at Laguna Mormona, a closed hypersaline lagoonal complex on the west coast of Baja California, Mexico. These mats and underlying organic accumulations attain a maximum thickness of 30 cm. Their morphology is controlled mainly by the degree of desiccation: mound-shaped structures develop under conditions of permanent submergence; flat-topped mounds and horizontal mats broken by desiccation cracks occur in settings where the mats have grown up to the water surface; and mats curved into concave-upward saucers develop under conditions of extreme desiccation. The accumulation rate in these mats is several millimeters per year, with one to several laminae forming per year. Filamentous cyanophytes (viz., Microcoleus chthonoplastes and several species of Lyngbya ), a coccoid cyanophyte ( Entophysalis sp.), and diatoms contribute to the primary organic deposit which is subsequently degraded and modified by bacterial activity. Sheaths, rather than trichomes, of the oscillatoriacean cyanophytes occur at depth in these mats: this is due largely to the motile nature of the trichomes which glide upward leaving empty sheaths behind. However, it also appears that the sheaths of M. chthonoplastes and Lyngbya aestuarii are more resistant to degradation than are the trichomes, and sheaths of the colonial coccoid Entophysalis are more resistant to degradation than are their cellular contents. Although these modern mats are not being permineralized by silica, many aspects of their macro-structure, lamina structure, and biology are applicable to the interpretation of silicified Proterozoic mats and their contained microbiotas.
