--> Abstract: Origin of Diagenetic Dolomite in a Non-Marine, Coastal Evaporitic Basin, Lake Hayward, Western Australia: Implications for Interpreting Conflicting Chemical Data in Ancient Coastal Carbonate Sequences, by M. R. Rosen, L. Coshell, and J. V. Turner; #90987 (1993).

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ROSEN, MICHAEL ROBERT, Limnological Research Center, Univ. Minnesota, Minneapolis, MN; L. COSHELL, School of Applied Geology, Curtin University of Technology, Perth, Western Australia; and J. V. TURNER, Division of Water Resources, CSIRO, Perth, Western Australia

ABSTRACT: Origin of Diagenetic Dolomite in a Non-Marine, Coastal Evaporitic Basin, Lake Hayward, Western Australia: Implications for Interpreting Conflicting Chemical Data in Ancient Coastal Carbonate Sequences

Lake Hayward is one of 11 small saline lakes (Na-Cl-SO<4>) located near the coast of southwestern Australia in the Clifton-Preston lakeland system. Although only 2.75 m deep, it is seasonally stratified with sharp temperature and salinity gradients of 20 to 37 degrees C, and 77 to 208 g/L, across the thermocline and chemocline respectively.

Significant occurrences of diagenetic dolomite have been found in lake-center and lake-margin cores. The amount of dolomite decreases and the amount of aragonite increases towards the top of the cores. The Mg/Ca ratio in the sediment pore water is highest in the zone of dolomite precipitation. Molar Mg/Ca ratios in the lake and sediment pore waters

(8.6-12) are higher than sea water. Less saline groundwater entering the lake is lower (1.5). The regional groundwater has Mg/Ca ratios less than 1. The high Mg/Ca ratios are due to precipitation of calcium carbonate as crusts around the margins of the lake. The elevated Mg/Ca ratios of lake and sediment pore waters in conjunction with microbial activity and elevated pore water temperatures enhances the conditions for diagenetic dolomite precipitation.

The isotope{18}O composition of lake water (+5 to +8 o/oo SMOW), sediment pore waters (+4 to +6 o/oo SMOW), and dolomite (+3 to +6 o/oo PDB) are enriched by evaporation. The isotope{18}O composition of the dolomite indicates that it is forming in isotopic equilibrium with the enriched pore waters. The values are similar to dolomite formed from evaporated marine brines. The large variation in isotope{13}C of the dolomite (+2 to -6 o/oo PDB) suggests enrichment of the pore water isotope{13}C<DIC> by methanogenesis although sulfate reduction is also an active process in the lake sediments.

Although the brine is derived from evaporation of inflowing nonmarine groundwater, the molar Na/Cl and Cl/Br ratios of the lake brine are similar to sea water. In addition, the carbonate in Lake Hayward has marine-like isotopic values and cement fabrics which could be confused with fabrics formed in a normal-marine environment. Ancient carbonate platform sequences, such as the Permian carbonate complexes of West Texas, which exhibit seemingly contradictory marine and nonmarine isotopic, petrographic, and chemical trends may be explained by the Lake Hayward type of situation. These results suggest that care must be taken when interpreting the origin and diagenetic trends of ancient nearshore carbonate sequences.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.