Wellington Formation, Geological formation in Kansas, United States.

The Wellington Formation is an Early Permian sequence 150–210 m thick in Kansas, northwest Oklahoma and the northeast Texas Panhandle. It appears mostly as dark gray, thinly bedded soft rock, much of it terrestrial, with a central salt body (the Hutchinson Salt Member) and beds of anhydrite and gypsum. Localized red beds and green shales mark exposed soils.

The Wellington Formation was first described as marine shales, but most of its 150–210 m of dark gray, thinly bedded rock is terrestrial, laid down in fresh-water ponds and salt lakes. Its central Hutchinson Salt Member, up to 91 m of repeating salt, anhydrite and gray shale, is mined at Hutchinson, Lyons, Kingman and Kanopolis. Marine fossils are confined to the lowest beds.

The Hutchinson Salt Member is better known locally than the formation itself, and salt is still mined at Hutchinson, Lyons, Kingman and Kanopolis. The shales yield tens of thousands of insect fossils, held at Harvard's Museum of Comparative Zoology and the Yale Peabody Museum. Beds of clam shrimp record measurable seasonal cycles.

Surface outcrops run along or between U.S. highways 81 and 77 in western Kansas, northwest Oklahoma and the northeast Texas Panhandle; the unit is absent in Nebraska. South of Salina the Hutchinson Salt has dissolved, letting the upper Wellington collapse into lowlands such as the McPherson Valley Wetlands. The salt is worked by shaft and dissolution mining around Hutchinson, Lyons, Kingman and Kanopolis.

The formation's insect fossils are so abundant that tens of thousands of specimens from the Wellington shales are held at the Harvard Museum of Comparative Zoology and the Yale Peabody Museum of Natural History. The layers of windblown dust also let researchers measure the passage of seasons in the Permian.