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Lewis Shale
Stratigraphic range: Campanian to Maastrichtian
Lewis Shale.jpg
Hills underlain by Lewis Shale near Cuba, New Mexico.
Type Formation
Sub-units Dad Sandstone Member, Huerfanito Bentonite Bed, Teckla Sandstone Member.
Underlies Pictured Cliffs Formation
Overlies Mesaverde Group
Thickness 600 m
Primary Shale
Other Sandstone
Coordinates 37°12′55″N 108°00′23″W / 37.2153544°N 108.0062913°W / 37.2153544; -108.0062913
Region Mountain states, United States
Country  USA
Type section
Named for Fort Lewis (former Army installation in La Plata County, Colorado)
Named by Charles Whitman Cross, A.C. Spencer, and C.W. Purington

The Lewis Shale is a geologic formation in the Western United States. It preserves fossils dating back to the Campanian to Maastrichtian stages of the late Cretaceous period.


San Juan Basin Upper Cretaceous stratigraphy PeerJ e5435 fig 1
San Juan Basin Upper Cretaceous stratigraphy

The Lewis Shale is an olive-gray marine shale with some thin beds of claystone, siltstone, sandstone, and limestone. It was deposited in the Western Interior Seaway in the late Cretaceous. The formation crops out in the Bighorn Basin, Green River Basin, Powder River Basin, San Juan Basin, and Wind River Basin. In the San Juan Basin, the formation has a maximum thickness of over 600 meters in the northern part of the basin, but pinches out in the southern part of the basin. It is the stratigraphically highest marine shale in the basin.

In the Green River Basin, the formation is also up to 600 meters thick and is divided into upper and lower shale members by the Dad Sandstone Member.


The formation is relatively poor in fossils. However, the formation contains fossils of the ammonites Baculites clinolobatus, Baculites asper, and Rhaeboceras.

Economic geology

The Lewis Shale in the San Juan Basin has historically been a minor source of natural gas. However, there is growing interest in increasing the production of this formation via hydraulic fracturing. While only sixteen wells were completed into the Lewis Shale before 1990, there were over 101 well completions by 1997.

History of investigation

The formation was named by Charles Whitman Cross and A.C. Spencer in 1899 for outcrops near Fort Lewis, Colorado.

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