Willow River Dolomite: Ordovician Analogue of Modern Algal Stromatolite Environments

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Abstract
The Willow River Dolomite Member of the Shakopee Formation (Lower Ordovician) in the Upper Mississippi Valley is composed primarily of dolomite and quartzitic dolomite with minor amounts of quartz sandstone, shale, and chert. Locally abundant algal stromatolites are the only common organic remains. Complete dolomitization has not destroyed the textural character or much of the compositional character of the rocks. As a result, environmental interpretations are possible from data gathered by examining thin sections as well as from field observations. Deposition of Willow River sediments took place in a shallow, epeiric sea consisting of three broad environments: (1) a shallow, open marine area in the northwest represented by grain sparite, abundant oölite beds, and relatively abundant fossils; (2) an intertidal or near-intertidal, high physical-energy zone where stromatolitic algae flourished in the central area represented by abundant algal biolithite with associated intrasparite; and (3) a locally intertidal and hypersaline, low physical-energy zone in the southeast represented by algal mats, micrite, and abundant desiccation features. Terrigenous materials were derived primarily from Precambrian igneous and metamorphic rocks and Cambrian sandstones to the northeast. General environment of deposition during Willow River time was similar to that in which other Lower Ordovician strata were deposited in the Appalachian, Ozark, and central Texas areas.

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