The Inyan Kara is a diversified group of sandstone, shale, conglomerate, variegated siltstone, claystone, and some lignite at the base of the Cretaceous in the Black Hills and surrounding subsurface area. Its unconformable contact with underlying formations reflects epeirogenic uplift and gentle folding in very Late Jurassic to very Early Cretaceous time. Thickness ranges from 22 ft. or less in central South Dakota to about 700 ft. in Black Hills outcrops.

Two dominantly sandy formations, the Lakota and the overlying Fall River, comprise the Inyan Kara Group. They are separated by a regional disconformity. The Lakota is a continental deposit with conglomeratic material, claystone, and variegated beds. The Fall River, which has greater regularity and bed continuity, consists of offshore shale, neritic to littoral sandstone, and deltaic and other marginal marine deposits of the first major Cretaceous marine transgression. The Fall River intertongues northwestward with the overlying marine lower Thermopolis Shale.

Persistent shale breaks divide the Fall River into three members (ascending): Liscom Creek, Morton, and Coyote Creek. Gross arrangement of members is shingle-like, for where one is thick the others tend to be thin or absent.

Most of the Inyan Kara sediments were transported seaward by streams originating on the Sioux uplift. During Lakota deposition, a major northwest-flowing river developed along the regional syncline which lay east of the Chadron arch and extended through the site of the Black Hills into Montana. Southward encroachment of the sea and shifting of deltas explain the thickness and facies relationships of the members of the Fall River Formation.

Many oil fields on the eastern flank of the Powder River basin in Wyoming have producing sandstones up to 80 ft. thick. Most of the oil is produced from channel sandstones in the Coyote Creek Member of the Fall River. Some important fields produce from Lakota channel sandstones. The oil is trapped behind convex updip permeability barriers at the margins of sandstones deposited in meandering channels which are approximately parallel to structural contours. Favorable stratigraphic and structural conditions for petroleum accumulation also exist in parts of southeastern Montana and western South Dakota.

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