Aragonite and the Genesis of Carbonates in Mahogany Zone Oil Shales of Colorado's Green River Formation

The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of intermountain lakes in three basins along the present-day Green River in Colorado, Wyoming, and Utah. The sediments are deposited in very fine layers, a dark layer during the growing season and a light-hue inorganic layer in the dry season. Each pair of layers is called a varve and represents one year. The sediments of the Green River Formation present a continuous record of six million years. The mean thickness of a varve here is 0.18 mm, with a minimum thickness of 0.014 mm and maximum of 9.8 mm. Of this formation the Mahogany zone is an area of great interest due to the large quantity of high grade oil it contains. The Mahogany Zone derives its name from the reddish brown color of the rock that is the result of a rich-kerogen content.