Physical and chemical controls on microfabric of recent ooids from the Great Salt Lake, Utah

Great Salt Lake ooids were sampled from four localities and from subaqueous, beach, berm, and backshore facies. Significant differences in cortex microfabric occur between high-energy rocky headland and cove settings, and low-energy sandflats. Highenergy environments are dominated by clear, coarsely crystalline radial ooids with little micritic material between radial crystals and between growth layers. Low-energy environments are dominated by dark, finely crystalline radial-concentric and concentric ooids with large amounts of micrite between radial crystals and between growth layers. Therefore, percentage of micrite in ooid cortices, either in concentric growth bands or in inter-ray micrite, is a key indicator of energy in the environment of formation. Measurement of Ca, Na, Sr, K, Mn, Fe, B, S, Ctot, Corg, Stat, Sai, and insoluble residue in ooids shows no significant variations either in different facies or in different localities. These similarities suggest that water chemistry is similar at all localities; and, therefore, that differences in water chemistry are not the cause of differences in cortex fabric in different localities. Carbon and oxygen isotopes tightly cluster around an average 613C = +3.8 o/oo and 15180 = -5.0 0/00, indicating that lake water was well mixed isotopically during the time of ooid formation. The measured 151so value of the ooids suggests that most of the aragonite in the ooids formed under conditions similar to those experienced today during the late spring/early summer, when water temperature is about 25�C and the 15180 of the water is about -3 o/oo (SMOW). Mg abundance is greater in micrite than in coarse crystalline aragonite in the Great Salt Lake ooids. Mg in inter-ray micrite and concentric growth bands is present mostly as high-Mg calcite. Mg in ooid peloid nuclei is present as both high-Mg calcite and Mg-rich clay minerals. Precipitation of small amounts of high-Mg calcite in the Great Salt Lake ooids occurs under low-energy conditions when degassing and fluid shear remain low for a long enough period to allow high-Mg calcite to form. Microbial activity also may be important in forming concentric bands that mark the boundaries between growth layers in the ooid cortices. These bands are more numerous in low-energy environments, which suggests that in these environments there are longer hiatuses between addition of radial cortex laminae and/or less abrasion that would remove nannobacteria from the ooid surface. The presence of abundant micrite, numerous concentric growth bands, and elevated Mg abundance in cortices of radial ooids formed in the Great Salt Lake suggest overall low-energy conditions. In contrast, little micrite, few concentric growth bands, low Mg abundance, and large, clear radial crystals in cortices suggest overall high-energy conditions for these ooids. These observations may be useful in evaluating conditions of formation of aragonite ooids formed in other lacustrine settings.