The Washakie and Sand Wash basins, southeast segments of the overall Green River basin, are located in central-southern Wyoming and northwestern Colorado. The two basins are bounded by and contain structural elements of varying age. These are: Rock Springs uplift and Wamsutter arch (Late Eocene-Oligocene) to west and north, Sierra Madre and Park Range uplifts (Late Cretaceous-Paleocene) to east, and the White River, Axial and Uinta Mountain uplifts (Paleocene-Eocene) from southeast through southwest. The basins are separated by a low arch and fault zone of late Tertiary age, which parallels the Wyoming-Colorado boundary.

The present obvious structural trends are the composite of many less obvious structural episodes of Late-Cretaceous-Paleocene, early Late Cretaceous, Early Cretaceous, Permian-Pennsylvanian and early Paleozoic time. Structural elements related to these older orogenic episodes are now mostly concealed beneath younger sediments and basin downwarping. Evidence for many structural episodes, particularly those of the early Paleozoic, have been all but effaced by erosion related to younger orogeny. Maximum structural activity occurred from latest Cretaceous through Mid-Eocene time.

The full sedimentary column (Eocene and older) in the Sand Wash basin exceeds 32,000 feet apportioned approximately as follows:

pre-Pennsylvanian                 3%
Pennsylvanian-Early Cretaceous   15%
Late Cretaceous                  46%
Paleocene-Eocene                 36%

The maximum sedimentary column in the Washakie basin may exceed 36,000 feet in the central and northern parts of the basin. Several thousands of feet of late Tertiary sediments and igneous extrusives occur in limited areas of both basins.

Precambrian structural trends are imperfectly known in the bounding Uinta, Park Range and Sierra Madre uplifts, but have had obvious, important influence on subsequent structural trends and movements to the present.

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