Morrison Formation of southeastern San Juan Basin, New Mexico

Abstract

Exposures of the Morrison Formation (Jurassic) between Cuchillo Arroyo and Cuba, New Mexico include all the strata above the Todilto Formation (Jurassic) and below the Dakota Formation (Cretaceous). Four members recognized in the southern part of the area are, in ascending order: a lower member composed of reddish-brown and maroon-brown mudstone and gray, very fine-grained sandstone; the Westwater Canyon Member composed of cliff-forming, feldspathic sandstone; the Brushy Basin Member composed of red and green mudstone and sandstone interbeds; and, the upper member composed of whitish, kaolinitic sandstone, and minor amounts of green mudstone. In the northern part of the area only three members are present: the lower member, the Brushy Basin Member, and the upper member.The lower member contains beds correlative with the Summerville Formation and the Recapture Member of the Morrison Formation of the Laguna area. The Westwater Canyon Member is sandstone in the south but grades northward into mudstone similar to that in the Brushy Basin Member. The upper member correlates with the Jackpile sandstone (Jackpile orebearing bed) of the Laguna area on the basis of lithologic similarity and stratigraphic position.