Algal Mats, Carbonate Laminites, Ooids, Oncolites, Pellets, and Cements in Hypersaline Sea-Marginal Pool, Gulf of Aqaba, Red Sea: ABSTRACT

Abstract

A bar isolates the hypersaline pool from the Gulf of Aqaba. Finely laminated algal mats carpet the shallow shelf of the pool and gypsum floors the slope and bottom. Algae secrete pellets, ooids, oncolites, grapestones, flakes, and carbonate laminites. The ooids have a radial texture; hence, contrary to statements in the literature, ooids with a radial texture are formed in the depositional environment. The carbonate laminites occur between the gray and black algal mats. Although some of them are fibrous, most are cryptocrystalline. Cryptocrystalline laminites which consist of high-Mg calcite mimic the micrite of the geologic rock record; these laminites can preserve the morphology of the mats even after the organic matter has disappeared. Scanning electron micrographs sho the laminites to consist of a mosaic of micron-size rhombohedrons which, during diagenesis, would stabilize to low-Mg calcite. Hence, the origin of some ancient stromatolitic limestones (pelmicrites) may be explained in terms of secretion of cryptocrystalline high-Mg calcite laminites. These laminites are lithified within the algal mats; hence, their origin does not necessitate the introduction of later cement and establishes algal secretion as a potent force in lithification. This inference may supersede the concept that all micrites result from neomorphic replacement of aragonite. The algae create a microenvironment in which Mg becomes enriched in the organic matter, and in which high-Mg calcite with up to 40% molecular MgCO3 is secreted. The total molecular percent of MgCO3 in the Mg-organic complex and high-Mg calcite combined may reach 60. This preferential concentration of Mg may explain the high level of dolomitization of stromatolitic rocks in the geologic record. Amino acids devoid of sulfur, especially aspartic acid, as part of the biologic system may exert considerable influence in the precipitation of the carbonate laminites and particles. End_of_Article - Last_Page 618------------