limestone - very light tan lacustrine limestone from the Eocene Green River Formation - teaching hand specimen
This lacustrine tan to white limestone is from the middle member of the Eocene Green River Formation. It was collected at an elevation of 8,900 feet at Indian Creek Pass along US Highway 191, between Helper and Duchesne in northeastern Utah, on the West Tavaputs Plateau of the Uinta Basin.
A series of extensive inland lakes covered the region during the Eocene. The Laramide Orogeny 88-40 million years ago, created the uplifts that surround the Uinta Basin. The Green River Formation in this basin is a sequence of carbonate and organic-rich mudrocks of lacustrine origin and fluvially-derived sandstones. Famous for its fossil flora and fauna, the Green River contains huge reserves of oil shale.
Limestones are primarily biogenic in origin, being composed of the protective shells of small organisms that extract calcium carbonate from seawater to build their shells. There is a constant rain of microscopic shells and other calcareous parts onto the sea or lake floor, which when accumulated and consolidated becomes limestone. Chalk is a form of limestone composed entirely of the minute calcium carbonate shells of coccolthophores, a unicellular phytoplankton. Coquina is a form composed of shells and shell fragments, only partly consolidated. When a limestone contains clay, it grades towards shale and is called an argillaceous limestone. Because the composition of limestone is calcium carbonate, a limestone will strongly effervesce with dilute hydrochloric acid.
This is a particularly attractive limestone, white to the eye in the field, but actually a light tan. The field photo shows interbedded shales and limestones in the Green River Formation just south of the summit of Indian Creek Pass.
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