Early Permian Unconformity in Southeastern Wyoming and North-Central Colorado: ABSTRACT

Abstract

An unconformity is present in southeastern Wyoming and north-central Colorado between the Goose Egg Formation, the Owl Canyon Formation or Lyons Sandstone, and the Casper or Ingleside Formation. Evidence for its existence is provided by reworked basal sandy zones and conglomerates, truncation of underlying cross-strata, local relief, possible duricrust or caliche zones in subjacent rocks, and an isopach description of the configuration of the erosion surface. Subjacent strata range from Wolfcampian to pre-Desmoinesian age. The subcrop becomes older from east to west. Superjacent strata belong to 3 units ranging from early to late Leonardian age. In the southern and central Laramie Range and most of the Laramie basin, the Owl Canyon Formation forms the supercrop. It thins to a zero edge northward and westward by depositional onlap and fills in relief on the underlying unconformity. The Opeche Shale Member of the Goose Egg Formation constitutes the supercrop in the Shirley basin, the northern Laramie Range, and probably most of the western part of the study area. It thins northward slightly in the Shirley basin and the northern Laramie Range and fills in relief on the underlying erosion surface. The Lyons Sandstone forms the supercrop on the southwest margin of the Laramie basin. Stratigraphic relations in southeastern Wyoming are in accord with an interpretation of the growth of an extensive land area and its transgression in the Rocky Mountain and western Mid-Continent areas in Early Permian time. End_of_Article - Last_Page 1392------------