WSP00003

• File Number: 8200.760
• Description: Yampa River General
• State: CO
• Basin: Yampa/White
• Water Division: 6
• Date: 4/1/1969
• Author: CWCB - USDA
• Title: Water and Related Land Resources - Yampa River Basin - Colorado and Wyoming
• Water Supply Pro - Doc Type: Report/Study

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Mesozoic rocks crop out mainly around the eastern and southern margins of
the Washakie Basin and include several thousand feet of alternating beds
of shale, siltstone, and sandstone, This sequence of rock includes: the
Moenkopi and Chinle Formation of Triassic age; the Navajo Sandstone of
Triassic and Jurassic age; the Carmel Formation, Entrada Sandstone,
Curtis Formation, and Morrison Formation of Jurassic age; the Dakota
Sandstone and Mowry Shale of early Cretaceous age; and the Frontier
Formation, Mancos Shale, Mesaverde Group, Lewis Shale, and Lance
Formation of late Cretaceous age.

The most extensive of the Mesozoic rocks are the Mancos Shale and the
Mesaverde Group. The Mancos Shale has a thickness of more than 5,000
feet. It consists essentially of dark gray calcareous marine shale and
often underlies broad open valleys or basins. The Mesaverde Group con-
sists mainly of sandstone with interbedded shale and several coal beds.
These rocks are usually resistant to erosion and often form prominent
mesas, ridges, and cliffs.

Rocks of early Tertiary age include the Fort Union Formation, Wasatch
Formation, Green River Formation, and Bridger Formation. The Fort Union
Formation crops out along the eastern side of the Washakie Basin and
consists of interbedded sandstone, shale, and coal beds. The Wasatch
Formation occurs extensively around the Washakie Basin and consists
mainly of red, purple, and gray shale and clay with some sandstone
layers. The Green River Formation crops out in the central portions of
the Washakie Basin and is composed mainly of light gray to light brown
shale and marlstone with some sandstone, limestone, and oil shale. The
Bridger Formation occupies large areas in the central parts of the
Washakie Basin where it forms extensive badland areas. It consists
mainly of gray to greenish gray sandy tuffaceous mudstone.

The Browns Park Formation of late Tertiary age, whose type locality is
Browns Park north of the Uinta Mountains, consists predominately of
chalky white to light gray soft crossbedded tuffaceous sandstone with
thin layers of greenish gray sandy mudstone. This formation extends as
an irregular band from the type locality eastward along the southern
flank of the Washakie Basin to Cedar Mountain northwest of Craig,
colorado. Other prominent exposures occur along the east side of the
Yampa River Basin at the foot of the Park Range, along the north flank of
the White River uplift, and throughout the Elkhead Mountains.

Volcanic rocks of late Tertiary age occur in the Elkhead Mountains and
adjacent areas and in the White River Plateau area. The volcanic rocks
of the Elkhead Mountains consist of a number of intrusive dikes, sills,
stocks, and plugs as well as thick flows of basaltic lava. The rocks of
the White River Plateau consist of basaltic lava flows several hundred
feet in depth.

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