Stratigraphy and nomenclature of some Upper Cretaceous and lower Tertiary rocks in south-central Wyoming

Upper Cretaceous rocks described and correlated in this report are the Steele Shale, Mesa verde Group (consisting of the Rock River l!'ormation (new name), Haystack Mountains )formation (new name), Allen Ridge Formation, Pine Ridge Sandstone, and Almond Formation), Lewis Shale, E'ox Hills Formation, Medieine Bow ll..,ormation, and the lower part of the Ferris Formation.'.rhe lower Tertiary rocks described consist of the upper part of the Ferris ll..,ormation of Paleocene age and the overlying Hanna ll..,ormation of late Paleocene and Eocene age.'l'he oldest unit Jnvestigated, the Steele Shale, consists of 2,300-8,800 feet of dark-gray marine shale that contains limestone concretions, beds of bentonite as much as '5 feet thick, and• thin layers of siltstone ami very fine grained sandstone.It becomes sandy upward nnd grades into the overlying Mesaverde Group.'The Mesaverde Group, in the Rawlins-Medicine Bow area, consists of the following formations, from oldest to youngest: Haystack Mountains Formation, Allen Ridge Formation, Pine Ridge Sandstone, and Almond Formation.The Haystack Mountains Formation comprises a sequence of mostly shallow-water marine sandstone and shale that thins southwestwardly across southcentral 'Vyoming from 2,550 to 850 feet.It contains three persistent ridge-forming sandstone members interpreted as beach and barrier-bar deposits.The oldest member, Tapers Ranch Sandstone (new name) , is 270 feet thick •and forms the base of the formation.The second member, O'Brien :Spring Sandstone (new name) is 220 feet thick and lies 1,350 feet below the top of the formation.The Hatfield Sandstone Member, as much as 167 feet thick, lies 450 feet •below the top.Nonmarine sandstone, shale, and carbonaceous beds largely mal{e up the Allen Ridge Formation, but some sandstone and shale of brackish-water and marine origin are included.The Allen Ridge is conformable with the underlying Haystack Mountains Formation.It is as much as 1,500 feet thick.It intertongues with the Rock River Formation, and in the Laramie Basin area, it is replaced by that formation.Dlsconformably overlying the Allen Ridge Formation is the Pine Ridge Sandstone, a white t<l light-gray nonmarine sandstone 60-450 feet thick.The :formation contains carbonaceous shale and impure coal, •and <lrdinarily f<lrms a conspicuous tree-covered ridge.The Almond Formation, 30-575 feet thick, conformably overlies the Pine Ridge Sandstone.It is a sequence of sandstone, shale, and coal, representing fluviatile, brackish-water, and nearshore marine environments of deposition.In the southeastern part of th4~ area (Laramie Basin), the Mesaverde Group consists of the Rock River F<lrmation and Pine Ridge Sandstone.The Rock River is a 1,500-foot-thick sequence of nonresistant sandstone and sandy shale of shallow-water marine origin.It grades eastward into the dominantly nonsandy Pierre Shale.In southeastern Wyoming, the Mesa verde Group is conformably overlain •by the Lewis Shale.The Lewis, 2,200-2,600 feet thick, is chiefly dark-gray marine shale but contains sandy units.In the Hanna Hasin, the Lewis is divided into three parts by the Dad Sandstone l\Iember, which is as much as 1,400 feet thick.•The Fox Hills Formation conformably overlies the Lewis Shale.It is chiefly ridge-forming yeJNowish-gray to light-brown shallow-water marine sandstone and ranges in thickness from less than 200 to more than 700 feet.The Medicine Bow Formation, 3,000-6,500 feet thicl{, rests conformably on the Fox Hills.Lenticular beds of sandstone, siltstone, and shale, •and persistent beds of coal make up the formation.Most <lf the formation is nonmarine, but the lower part contains brackish-water beds.The Ferris Formation, <lf nonmarine origin, conf<lrmably overlies the Medicine B<lw and is divisible into two parts.The lower part, 1,100 feet thick, consists of conglomeratic sandst<lne, sandstone, siltst<lne, •and shale.The congl<lmeratic beds contain small well-rounded pebbles of resistant rocks such as chert and quartzite.The upper part, about 5,400 feet thicl{, is finer grained than the lower part and consists of sandstone, shale, and coal.The Hanna• F<lrmation unconformably overlies the Ferris or older formati<lns.It is as much as 7,000 feet thick and consists of nonmarine congl<lmerate, sandstone, shale, and coal.The conglomerates contain large cobbles of locally derived rocks.The names Foote Creek and Dutt