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Dolomite Distribution in Modern and Ancient Microbial Stromatolites: Is the Present a Key to the Past?


Recent discovery of authigenic dolomite micro-rhombs in tissues of microbial mats has invited comparisons to ancient dolomite boundstones and invoke microbial metabolism as a major, rock-forming process. Nearly 40 years ago discoveries of dolomite in Middle East sabkhas led to a variety of hydrological and geochemical models for the origins of dolomite. More recent works have attributed dolomite and lithification to metabolic activities of bacteria. This presentation discusses whether obvious similarities in appearance and depositional setting of modern and ancient stromatolites warrant the conclusion that many ancient evaporitic dolomite rocks are related to microbial activity. Microbial mats of Qatar are surface feature, occurring in evaporitic, lagoon, and intertidal settings. Most mats are 5-10 cm thick, a maximum of 30 cm of layered fabric, and reducing dark-grey microbial sediment underneath. The layered mat is almost entirely composed of water, with a small % organic and mineral matter. Most of the mineral matter is sediment. Aragonite and gypsum are the only minerals to partially lithify Holocene mats. Dolomite is minor, usually not detectable with XRD. There are no published data on bulk volume fraction of dolomite or the proportion of mats lacking dolomite. After 5000-6000 years mats preserve little structure and don't show signs of greater lithification. Buried mats are extremely rare, reflecting low preservation potential. Rare Pleistocene mats are lithified, but preserved as limestone. Thus, near-surface processes acting over geological time do not appear to promote complete replacement by dolomite. These observations suggest living microbes produce dolomite, but a lithified rock framework. Many ancient, evaporitic carbonate reservoirs clearly show that dolomite has formed by replacement of precursor lime grainstone fabrics. Dolomite formed during shallow burial, post-dating mineral stabilization, mold formation, and vadose cementation. Thus, the style of dolomitization of many ancient reservoirs does not resemble isolated microbial rhombs. Ancient microbial mats from evaporitic sequences are frequently replaced by 30-50 micron, dolomite. The recurrent association between evaporites, stromatolites, and dolomite suggests that a genetic relationship is present. Clearly some aspect(s) of the depositional setting establish a template for subsequent diagenesis. However, the mechanism of lithification remains to be documented.