DIAGENETIC HISTORY OF CRYPTALGAL CARBONATES IN THE EARLY CRETACEOUS CEDAR MOUNTAIN FORMATION, EASTERN UTAH

A 40-140 cm thick lacustrine microbialitic limestone unit extends for more than 0.5 km within the upper Yellow Cat Member of the Cedar Mountain Formation about 20 km southeast of Green River, Utah. The lacustrine unit is contained within a largely fluvial sequence and consists of a massive (thrombolitic) lower bed and well-laminated (stromatolitic) upper bed and is capped by a relatively massive fine-grained (palustrine) sandstone which contains numerous crayfish burrows. The diagenesis of the microbialite samples has been examined through petrographic analysis.

The lower bed of the microbialite exhibits extensive dolomitization. Early dolomite is followed by fine-grained calcite that usually exhibits micro-bladed texture. Calcite is partially replaced by a sequence of silica cements, which typically consists of chert, followed by chalcedony, followed by megacrystalline quartz. Late calcite is found in fractures that cross-cut the silica and as large grains as the final fill of void spaces. The upper bed, which is visibly laminated in hand sample, also contains a similar sequence: initial dolomite followed by an early calcite, silicification and late calcite. However, differences include significant late calcite replacement of dolomite (dedolomitization) in the upper bed. In some samples calcite has replaced nearly all of the initial dolomite. In addition, silicification is much more prevalent in the upper than lower bed.

Dolomite probably formed syndepositionally in a hypersaline environment, which allowed microbial communities to thrive without animal grazing. Early calcite and silica cements developed during burial diagenesis, the source for the silica was likely derived from volcanic ash layers in Jurassic and Early Cretaceous sediments. Late calcite cement and dedolomitization are a result of near-surface diagenesis during uplift.

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