Physical and Chemical Controls on Microfabric of Recent Ooids from the Great Salt Lake, Utah

Carl Norgauer and Vicki A. Pedone

Significant differences in cortex microfabric of Recent ooids from different shoreline localities in the Great Salt Lake, Utah indicate that wave energy and salinity are the dominant controls on ooid fabric. High-energy environments are dominated by clear, coarsely crystalline radial-concentric ooids with little micritic material between rays and between growth layers. Low-energy environments are dominated by dark, finely crystalline radial-concentric ooids with alternating micritic and clear rays and numerous micritic concentric bands. Electron microprobe analyses of the aragonite ooids show that Mg abundance is consistently higher in dark rays and in micritic concentric laminae (x=880 ppm) than in clear rays (x=136 ppm). The increased Mg content in the micritic areas in icates the presence of small amounts of Mg-bearing clay and/or high-Mg calcite.

Micrite-rich, finely crystalline fabric in low-energy areas suggests that poor circulation promotes increased evaporation and salinity, favoring rapid nucleation and growth of small crystals. In contrast, micrite-poor, coarsely crystalline fabric in high-energy areas suggests that vigorous circulation promotes decreased salinity, favoring slow nucleation and growth of large crystals. In addition, clay is winnowed from the lake margin in high-energy areas, but settles near the shoreline and is incorporated into ooids in low-energy areas, contributing to the volume of fine-grained material and increased Mg content.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California