The Role of Sulphate-Reducing Bacteria and Unconsolidated Organic Matter in the Formation of Primary Dolomite

When overlain by modern Bahamian ooids and bathed in a modified seawater solution for several weeks, micron-sized, sub-spherical dolomite precipitates were observed in and/or on particulate organic matter within tidal flat silts and clays gathered from Christmas Bay, Texas. These experiments were conducted upon tidal flat muds to test whether dolomite precipitation could be instigated within entirely siliciclastic sediments with relatively minor changes to their physical and chemical surroundings. The presence of Bahamian ooids and the chemistry of the modified seawater solution were varied systematically between trials. Sulphate-reducing bacteria (SRB), almost ubiquitous within near-surface, anoxic sediments with an abundance of decaying organic matter, facilitated the process of primary dolomitization within the tidal flat muds by chemically altering their micro-environment such that dolomite became super-saturated immediately around them. This process only occurred in or on decaying particulate organic matter within the tidal flat muds because this is where SRB tend to accumulate. Although calcite precipitates were observed in almost every trial, dolomite precipitates were strictly observed within siliciclastic muds that were immediately overlain by modern Bahamian ooids. Ooids increased the speed and likelihood of dolomitization by contributing Ca2+ and CO32- ions to the system through aragonite dissolution. It is suggested that the reason why primary dolomite is not found naturally within the Christmas Bay sediments, and perhaps similar locales around the world, is because the process of primary dolomite precipitation requires the presence of a plentiful carbonate source.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California