Description
Laguna Mormona is a coastal evaporite complex located approximately 350 kilometers south of the International Border in Baja California, Mexico. The entire complex encompasses approximately 35 square kilometers, with the important components being an evaporite flat with a thick cyanobacterial mat, hypersaline Laguna Grande, and a barrier dune system that separates the entire complex from the Pacific Ocean. The mineral assemblage includes: 1) the carbonates aragonite, high Mg calcite, and low Mg calcite which account for 10 to 70 weight percent of a given sample; 2) gypsum varying between 10 and 40 weight percent; and 3) a detrital suite of quartz, feldspar, and mica ranging between 0 and 75 weight percent. Weight percent organic matter varies between 2 and 20 percent. Several factors interact to determine which carbonate phase will form. Aragonite is the primary carbonate mineral in areas undergoing active recharge as well as in areas in which there is prolific growth of the algal mats. High Mg calcite forms in areas where the interstital waters are more saline, with high molar Mg : Ca ratios and low alkalinities. Low Mg calcite forms either at the expense of gypsum in reducing zones or in areas of active recharge. It is usually associated with an unidentified Mg silicate mineral. In areas undergoing recharge, grains were recrystallized to euhedral low Mg calcite at depths greater than 15 cm. In the more saline waters, recrystallization is not as extensive; in no instances were euhedral grains noted. Elemental analysis revealed no trends that can be attributed to diagenetic reactions. The maximum rate of sediment accumulation was determined through the use of Pb 210 to be on the order of 2.5 mm per year. This is in good agreement with previously determined rates based on the growth of the algal mats. A minimum sedimentation rate, related to stabilization of the Holocene sea level, is on the order of 0.5 mm per year.
