Geology of the Fort Laramie '.* Area, Platte and Goshen >. Counties, Wyoming > By LAURA W. McGREW ONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY BULLETIN 1141-F *repared as part of a program of the ^Department of the Interior for develop­ ment of the Missouri River basin, with the cooperation of the Geological Survey of Wyoming and the Department of Geology, University of Wyoming JNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington D.C. 20407 CONTENTS Page Abstract__.._..._....._..._.____.__.......__-_.-..__-.__.____---- Fl Introduction.____________________________i..___^_________________- 2 Stratigraphy._____________________________________________________ 4 Precambrian rocks__._--______._____-_-___-___-__--_--______--- 5 Cambrian(?) system.__________________________________________ 5 Devonian and Mississippian systems.____________________________ 6 Guernsey formation______________________________________ 6 . Mississippian(?), Pennsylvanian, and Permian systems..____.______ 7 Hartville formation. ...............^........................ 7 Permian system.___---_--_______-______---_-__-_--_-_---_-_--- 10 Opeche shale___.__________________________________________ 10 Minnekahta limestone___-___________---_--_-_-____-__--_-_- 10 Gypsum and red shale sequence._______-_--_-_--____-____-__ 11 Triassic system..._____________________________________________ 11 Chugwater formation______________________________________ 11 Jurassic system______________________________________________ 11 Sundance formation______________________________________ 11 Morrison formation._______________________________________ 12 Cretaceous system_____________________________________________ 13 Lower Cretaceous rocks.___-__-_____-__-_-_-_-__-_______--- 13 Cloverly formation. ___________________________________ 13 Thermopolis shale.-_______--_-_--_-__---_-_________--- 13 Mowry shale_____________________-__------_______.__-_ 14 Upper Cretaceous rocks.;____-______-___--___--____________ 14 Frontier formation____________________________________ 14 Niobrara formation.___________________________________ 14 Pierre shale---__--_____-____.________-_-_-__________-_ 14 Fox Hills sandstone_____________________________________ 15 Lance formation...-____________ ....................... 15 Tertiary system-____---___.___________-___-_-_----_____________ 15 Oligocene series___________________________________________ 17 White River formation.__________________________'______ 17 Oligocene and Miocene series, undivided____________________ 21 Unnamed conglomerate sequence. _______________________ 21 Miocene series._______________________________________ 22 Arikaree formation.___________________________________ 22 Deposits of post-early Miocene and pre-Pliocene age __.__-_ 25 Pliocene(?)__-_.--._.-.___.....___._________._______.....- 25 Quaternary system. ___________________________________________ 25 High-level gravel-_________________________________________ 25 Terrace deposits--_________________________________________ 26 Sand and loess deposits.J__._________________________________ 26 Tributary valley alluvium. -__.______.___-_-_---__________-- 26 Flood-plain deposits.______________________________________ 28 in IV CONTENTS Page Structure._ __________________________: F28 Faults__________________________ 30 Folds...._._.___._________..____. 32 Tertiary and Quaternary geologic history- 32 Tertiary history___________________ 32 Quaternary history_^_.____________ 34 Economic possibilities._________________ 34 Oil and ga£__-________-__--___--_- 34 Sand and gravel-__________________ 36 Uranium _ ________________________ 36 References. ___________________________ 37 ILLUSTRATIONS [Plates are in pocket] Page PLATE 1. Geologic map and section of the Fort Laramie area, Platte and Goshen Counties, Wyo. 2. Correlation of formations in and near the Fort Laramie area. 3. Composite stratigraphic section of rocks in the Fort Laramie area. FIGURE 1. Index map of Wyoming showing location of the Fort Laramie area._________________-_-____--_--_-___-_---___---___ F3 2. Correlation chart showing age relations of Tertiary rocks_--_- 16 3. Valley fill, Arikaree formation, and terrace gravel in railroad cut________________________________________________ 27 4. Map of southeastern Wyoming showing structural setting of the Fort Laramie area________________________________ 29 5. Whalen fault in North Platte River escarpment. __ __________ 31 TABLES Page TABLE 1. List of test wells for oil and gas____________________________ F6 2. Heavy-mineral distribution in eight dated samples of Tertiary rocks __________-____------__-__-----_-_--_----------_- 18 3. Vertebrate fossils from the Fort Laramie and adjacent areas.__ 19 4. Uranium analyses of water samples_.____-_-___-___-___-_-_- 37 CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGY OF THE FORT LARAMIE AREA, PLATTE AND GOSHEN COUNTIES, WYOMING By LAURA W. McGREW ABSTRACT The Fort Laramie area comprises about 200 square miles along the northwest margin of the Julesburg basin in southeastern Wyoming. Bocks exposed are the White River formation of Oligocene age, an unamed conglomerate sequence of late Oligocene and earliest Miocene' age, the Arikaree formation of early and early middle Miocene age, lag boulders from deposits of post-early Miocene and pre-Pliocene age, and loess, silt, sand, and gravel depo­ sits of Pleistocene and Recent age. Pre-Oligocene rocks in the subsurface in­ clude igneous and metamorphic rocks of Precambrian age; the Guernsey forma­ tion of Devonian and Mississippian age; the Hartville formation of Mississip- pian(?), Pennsylvanian, and Permian age; the Opeche shale, Minnekahta limestone, and a gypsum and red shale sequence of Permian age; the Chugwater formation of Triassic age; the Sundance 'and Morrison formations of Jurassic age; and the Cleverly, Tnermopolis, Mowry, Frontier, Niobrara, and Pierre formations of Cretaceous age. The Fox Hills sandstone and Lance formation of Late Cretaceous age were penetrated in one well about 11 miles southeast of the Fort Laramie area. The White River formation is exposed in Goshen Hole in the southeastern part of the map area and in the North Platte and Laramie River valleys in the north-central part of the map area. It consists of about 80 feet of variegated bentonitic claystone containing gray conglomeratic, crossbedded channel sand­ stone, overlain by 290 to 570 feet of orange-gray tuffaceous siltstone and silty claystone also containing gray conglomeratic, crossbedded channel sandstone beds. Vertebrate fossils from many horizons in the formation indicate an early through late Oligocene age. Overlying the White River formation in the northwestern part of the map area and around the northwestern part of Goshen Hole is a conglomerate se­ quence 50 to 100 feet thick. Constituents of the conglomerate range in size from sand to boulders, which are derived in part from rocks of Precambrian age and in part from the Guernsey and Hartville formations. Vertebrate fossils in­ dicate that the conglomerate is of late Oligocene and earliest Miocene age. The Arikaree formation rests on the conglomerate sequence where that unit is present and directly on the White River formation elsewhere. It is as much as 700 feet thick and is divisible into three units: a lower 200- to 300-foot se­ quence of soft massive orange-gray tuffaceous sandstone of early and middle early Miocene age; a middle 200-foot sequence of gray tuffaceous sandstone Fl F2 CONTRIBUTIONS TO GENERAL GEOLOGY of late early Miocene age containing many limy sandstone and fresh-water limestone beds; and an upper 200-foot sequence of soft massive orange-gray tuffaceous sandstone of early middle Miocene age. Limy sandstone concretions and siliceous root casts are abundant throughout the Arikaree formation. Lag boulders 3 to 4 feet in diameter rest on the Arikaree formation in the northwestern part of the map area. These are believed to have weathered out of a post-early Miocene and pre-Pliocene deposit. Deposits of Quaternary age cover a large part of the Fort Laramie area. These deposits range in thickness from 0 to 80 feet. Units mapped include terrace deposits of sand, silt, and gravel; sand and loess deposits forming up­ land soils; tributary valley alluvium; and flood-plain deposits. The so-called Larainide orogeny that produced the Laramie Mountains, Hart- ville uplift, Richeau Hills, and Moonshine Hills in eastern Wyoming probably reached its climax in the early Eocene. Following the orogeny, but prior to deposition of rocks of Oligocene age, the Larainide folds were extensively eroded and in many places rocks of Precambrian age were exposed. On the surface thus formed, sediments of Oligocene and younger age were deposited. Oligocene and Miocene sediments were deposited on broad flood plains with intermittent channel cutting and filling. Northwestern Wyoming is suggested as a possible source for the volcanic ash, an important constituent of the Oligocene and Miocene deposits. Following the deposition of rocks of early middle Miocene age in eastern Wyoming, an orogenic disturbance with extensive block faulting resulted in a conspicuous horst block bounded by the Wheatland fault system on the north­ west and the Whalen fault system on the southeast. Associated regional fold­ ing resulted in a broad donial uplift now breached to form Goshen Hole,