The Green River Formation (Eocene) in Sanpete Valley, central Utah, contains many thin beds of shiny brown oolites in a very sparse to abundant matrix of calcite and minor quartz. Analytical techniques revealed that the oolites are composed primarily of the smectite stevensite. The brown color of the oolites is a result of organic matter. The stevensite oolites probably were precipitated in place in Lake Uinta.

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