Mesozoic Stratigraphy in the Thermopolis Area


Formations of the Early Triassic ("Embar") apparently do not outcrop anywhere within the Warm Springs Ranch, so discussion of the pertinent stratigraphy to the Warm Springs Ranch will begin with the Chugwater Formation.

Chugwater Formation

As a whole, the Chugwater Formation thins across the Bighorn Basin in a north-northeasterly direction. South of Thermopolis is the maximum development of the formation where three members are present. The lowest member is known as the Red Peak (Love, 1939). The two upper members, known as the Popo Agie and the Alcova (Lee, 1927), pinch out a few miles north of Thermopolis.

Red Peak Member -

The Red Peak Member of the Chugwater Formation thins from about 1000 feet to 500 feet between Thermopolis and the Montana-Wyoming border. In its lithology it is characteristically red in color -"red-bed"- and predominantly siltstone. Locally the silts grade into silty sandstones. The Red Peak Member's stratigraphic relationship at its base shows no observable discordance. The contact between the Red Peak and the underlying Dinwoody is based upon gradual lithologic change from dolomitic, sandy siltstone below to the silty shales of the Red Peak. It should be noted that the color change from the greenish grays and yellows of the Dinwoody to the red of the Red Peak is not uniform enough to serve as a criterion for separating the two units. The Red Peak Member of the Chugwater Formation is not fossiliferous.

Alcova Limestone Member and Popo Agie Member -

The Alcova Limestone Member of the Chugwater Formation is thin, rarely exceeding 5 to 10 feet, and is generally about 3 feet thick throughout the Bighorn Basin. It consists of a thin, hard, fine-bedded, pinkish to light-gray limestone. This member conformably overlies the Red Peak Member and disconformably underlies the Popo Agie Member, with reworked fragments of Alcova in the lower few inches of the Popo Agie. Fossils from the Alcova place it in Middle or Upper Triassic. The Popo Agie member of the Chugwater Formation is 200 feet thick 6 miles south of Thermopolis, but thins rapidly and pinches out a few miles north of Thermopolis. The lower part resembles the Red Peak member. The top consists of an earthy, powdery, red, yellow, or brown claystone overlying purple to red silty shales and limestone pellet conglomerates. This member does contain fossils.


Gypsum Spring Formation

The Gypsum Spring Formation maintains a fairly consistent thickness of about 200 feet throughout the Bighorn Basin. This is a unit of salmon-pink and light-red siltstones with interbedded white gypsum and some thin, platy dolomites and limestones. Deposited in shallow marine waters intermittently cut off from the main seaway, the Gypsum Spring Formation contains abundant fossils.

Sundance Formation
Lithology and Stratigraphic Relationships

1) The basal Sundance unit, which can be correlated throughout the Bighorn Basin, is represented predominantly by gray and greenish-gray calcareous shales with interbedded limestones. The limestones are fossiliferous and characteristically oolitic. The thickness of the unit varies from 100 to 175 feet. This lower unit is interpreted as having originated in an epicontinental sea that moved eastward from the Cordilleran geosyncline. It is correlative with the "Lower Sundance" of the Wind River Basin, the Upper Twin Creek of southeastern Idaho, and the Rierdon Formation of the Ellis Group of southern Montana. 2) A red-bed unit composed of from 5 to 25 feet of red shale, silts, and some anhydrite, which is conspicuously developed only in the northeastern part of the Bighorn Basin, lies unconformably upon the lower shale. It is thought to represent near-shore sedimentation during a temporary restriction in the extent of the Sundance sea. It is not reported in the western part of the Bighorn Basin, where it is probable that marine conditions prevailed without interruption throughout most of Sundance time. This red unit is correlated with the thin red unit at the top of the "Lower Sundance" of the Wind River Basin and with the Lak member of the Black Hills area.
3) The upper Sundance unit (150-200 feet thick) is predominantly greenish-gray and gray to buff glauconitic sandstone, locally calcareous, and cross-bedded. Interbedded greenish shales and limestones are of minor development. This unit is correlative with the "Upper Sundance" of the Wind River Basin and Stump sandstone of southeastern Idaho. It is believed to be the equivalent of the Swift formation of the Ellis group of southwestern Montana. The withdrawal of the Sundance sea to the west following deposition of the glauconitic sandstone closed a long period of marine sedimentation in the Bighorn Basin.

Morrison Formation

As the Sundance sea began its withdrawal, other significant geologic events were occurring simultaneously in the western region that helped to form the Upper Jurassic Morrison Formation. Mountain building in Utah, Idaho, and areas to the north and west of the Bighorn Basin restricted the extent of the Cordilleran geosyncline. The marine trough became an area in which the eroding clastic material was deposited. In flood-plain, fluviatile, and lacustrine conditions the Morrison Formation was deposited. This formation consists of variegated shale and claystone, predominantly green and greenish-gray. It also contains lenticular, silty sandstones and occasional conglomerates, thin carbonaceous beds, fresh-water marls, and limestone lenses that are characteristic of flood-plain and lake deposits of aggradation. Overlying the Morrison is the Cloverly Formation that is Lower Cretaceous in age. The contact between the two formations is not always definable. Coarse clastics sometimes appear at the contact. Above the zone of conglomerates and conglomeratic sandstones marking the base of the Lower Cretaceous, the shales and sandstones are buff and gray with some purple or maroon and red shales in the middle part.

Lower Cretaceous
Cloverly Formation

Because climate did not change dramatically from the Morrison to the Cretaceous time, flood-plain and playa-lake conditions continued. The Cloverly Formation is, on average, 150 feet thick. The lower portion is predominantly buff to gray sandstones and shaly sandstones with local conglomerates. The middle part consists of gray to pink, maroon, or reddish clays and shales, and is locally sandy. The upper part of the formation is a light-buff sandstone (Greybull Member) which grades upward into the "rusty beds" of the overlying Thermopolis Shale. Each of the three members varies in thickness and lithology and locally may be absent. The Cloverly has the characteristics of the Lakota, Fuson, and Dakota sandstones of the Black Hills region as described by Darton (1906).

Upper Cretaceous

The Upper Cretaceous, as any worker familiar with the geologic history of western North America knows, was dominated by vast interior seas extending from New Mexico to the Arctic and from Idaho to Iowa. The topographically high areas west of Wyoming supplied quantities of clastic material that were transported eastward and deposited as tongues of sandstones with some conglomerates and carboniferous beds. The Bighorn Basin lies intermediate between the western area of thick clastics and the eastern region where black marine shales were deposited.

Thermopolis Shale

The Thermopolis Shale is predominantly a black, nonfissile, marine shale with numerous thin bentonites. At the base of the formation lies the "rusty bed" member, and 175 feet above the base the Muddy Sandstone Member is developed. The basal "rusty beds" are thin, limonitic, silty sandstones with interbedded black shale. The "rusty beds" grade downward into the Greybull Sandstone Member of the Cloverly. The Muddy is a white to buff, fine to medium-grained sandstone. It reaches a thickness of 100 feet in the southern part of the basin but thins to the north, and is only about 10 feet thick at the Wyoming - Montana border. Approximately 400 feet of upper Thermopolis black shales lie between the Muddy Sandstone and the Mowry Shale. The Thermopolis - Mowry contact is generally arbitrarily picked at the base of the siliceous shales of the Mowry.