Geology of the Winnett-Mosby area, Petroleum, Garfield, Rosebud, and Fergus Counties, Montana
Year: 1964
Type: report
Abstract: The Winnett-Mosby area contains about 833 square miles in central Montana, mostly in Petroleum and Garfield Counties; it also includes about 14 square miles in northern Rosebud County and 5 square miles in eastern Fergus County.The area is 49 miles in length from east to west and ranges in width from 12 miles at the north and south ends to 24 miles near the middle in the vicinity of the Musselshell River.The Musselshell River flows north across the eastern part of the area through a deep, rather narrow, steep-sided valley, which is bordered in part on the west by broad terraces.Across the eastern part of the area, a line of steep ridges makes a narrow, V-shaped band, the apex of which is to the east.North of the ridges, in the Musselshell Valley, the country is rough badlands, and east of the ridges it is an uneven sagebrush-covered plain.Prominent westward-facing cuestas cross the country west of the Musselshell River, and a broad rolling upland underlies the southern part of the area.Several small buttes rise 200 to 300 feet above the surrounding plains southwest of Winnett.Sedimentary rocks exposed in the Winnett-Mosby area are Cretaceous in age and have a thickness of 4,800 feet.A deep well has penetrated about 3, 700 feet of strata below the stratigraphically lowest outcropping beds.The unexposed strata range in age from Cambrian through Jurassic, excluding rocks of Silurian, Permian, and Triassic age which have not been identified in this part of Montana.The exposed sequence consists of eight formations, one of which, the Colorado shale, is divided into nine members.The Carlile member of the Colorado shale is further divided into four mappable lithologic units.Outcropping Lower Cretaceous rocks are represented by four members of the Colorado shale.In ascending order, these are the lower sandstone, Skull Creek, unnamed sandy, and Mowry members.The lower sandstone member, about 70 feet thick, forms prominent dip slopes on the west side of the area.Overlying the lower sandstone member is 175 feet of dark shale and thin yellowish-brown sandstone of the Skull Creek member.The lower sandy part of the Skull Creek in the subsurface is informally called the Dakota silt.The unnamed sandy member is about 415 feet thick and consists principally of gray sandy shale and thin glauconitic and locally ferruginous sandstone.This member may be equivalent to the Newcastle sandstone of the Black Hills.Overlying the unnamed sandy member is 200 feet of the Mowry member, the uppermost unit of Early Cretaceous age.It consists of thin very siliceous sandstone interlaminated with shale and weathers to resistant bare silver-gray outcrops.The remaining part of the Colorado shale includes five members of Late Cretaceous age, which are from oldest to youngest, the Belle Fourche, Mosby sandstone, calcareous shale, Carlile, and Niobrara members.The Belle Fourche member 2 GEOLOGY OF THE WINNETT-MOSBY AREA, MONTANA conformably overlies the Mowry member, is 240 to 310 feet thick, and is made up of dark sandy and, locally, bentonitic shale, thin shaly sandstone, and, in the lower half, thin bentonite beds.Abundant ironstone concretions characterize the lower part of this member.The Mosby sandstone member and overlying calcareous shale member together are equivalent to the Greenhorn formation of the Black Hills.The Mosby is 29 feet thick and consists of noncalcareous sandstone and shale; the calcareous shale member is 23 feet thick and consists of calcareous shale that weathers very light gray.The Carlile member, which is 310 feet thick, is subdivided into four units: the first and oldest unit is dark bluish-gray shale, the second is dark-gray shale containing abundant ironstone concretions, the third is dark-gray sandy shale containing abundant limestone concretions that weather yellowish orange, and the fourth and youngest is olivegray to dark-gray sandy shale containing large septarian limestone concretions that weather light gray.The Niobrara, the uppermost member of the Colorado shale, is a sequence, 380 feet thick, of noncalcareous and calcareous shale and bentonite beds; the Sage Hen limestone member of Lupton and Lee (1921), a thin persistent bed of concretions, is at the base of the Niobrara.Conformably overlying the Colorado shale is 130 to 235 feet of sandy shale and soft sandstone of the Telegraph Creek formation of Late Cretaceous age; next in order is the Eagle sandstone, about 290 feet thick, which forms prominent hogbacks and cuestas.The two formations are undifferentiated over part of the area east of the Musselshell River.Brownish-gray marine shale containing large distinctive concretions and many bentonite beds characterizes the Claggett shale which overlies the Eagle sandstone.The thickness of the Claggett ranges from 345 to 430 feet.The Judith River formation overlies the Claggett and is mostly a nonmarine sequence of sandstone, shale, siltstone, and mudstone about 273 feet thick, which form conspicuous hogbacks in areas of steep dips.Conformably overlying the Judith River formation is 1,165 to 1,318 feet of marine shale of the Bearpaw shale.Many bentonite beds and abundant very fossiliferous limestone concretions are characteristic of this formation.The uppermost marine formation of Cretaceous age is the Fox Hills, which in the mapped area ranges in thickness from 63 to 141 feet.The youngest rocks of Cretaceous age are the nonmarine beds of the Hell Creek formation of which an estimated 300 feet crops out in the area.Igneous rock of ultramafic composition has been intruded as dikes, sills, and plugs in the western part of the mapped area.The mapped area is located on a part of the Big Snowy anticlinorium known as the Cat Creek-Devils Basin uplift.Differential vertical movement along faults that bound a compound horst in the basement rocks under the Cat Creek-Devils Basin uplift probably caused the uplift and the secondary structural features within it.En echelon faults cut the exposed sedimentary rocks in a belt along the crest of the Cat Creek anticline and in another belt in the southern part of the area.Tectonic movements that formed the Big Snowy anticlinorium apparently occurred no earlier than Eocene time and possibly as late as middle Oligocene.Intrusion of igneous rocks in the Winnett-Mosby area probably occurred during a late stage of the tectonic activity, and therefore they are Eocene or slightly younger in age.Terraces at eight levels extend along the Musselshell River, and a complex system of terraces also borders the major creeks.The gravel in all the deposits was derived principally from mountains to the west.Boulders ice rafted into the area during the Illinoian or early Wisconsin stage of glaciation rest on all but the ABSTRACT 3 two lowest terrace levels in the valley of the Musselshell River, but no glacial gravel is incorporated in the terrace deposits.Oil is produced from West, Mosby, and East domes on the Cat Creek anticline; it also was formerly produced from a well in the Rattlesnake Butte area.Since the discovery of oil on Mosby dome in 1920, to January 1, 1959, the Cat Creek field has produced 19.2 million barrels of oil.Commercial production is from four zones: the lower sandstone member of the Colorado shale (First Cat Creek sand of Reeves) and the Second Cat Creek sand in the Kootenai formation, both of Cretaceous age; and sandstone beds in the Morrison and Swift formations of Jurassic age.The crude oil ranges in gravity from 47° to 51° API and has no associated gas.Rocks that also may contain oil include the Piper formation of Jurassic age, Mississippian and Pennsylvanian rocks formerly called the Amsden formation in central Montana, and the nonmarine part of the Heath formation of Late Mississippian age.
DOI: 10.3133/b1149
Authors: William D. Johnson Jr., Howard R. Smith
Language: English
Subfield: Geology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: gold
OpenAlex ID: W48095712