The term peneconcordant is proposed to describe the form of the numerous and highly productive U deposits in sedimentary rocks of the Colorado Plateau, Wyoming, the Dakotas, and Texas. Peneconcordant U deposits are tabular, lenticular, or irregularly-shaped masses of widely differing size that are, in general, concordant to the gross sedimentary structures of the enclosing rock but that in detail cut across sedimentary structures. They differ from vein deposits in that they do not occupy or follow fractures or shear zones in the sedimentary rocks. Furthermore, they are not coextensive with a specific lithologic unit, such as U-bearing marine black shale. The many terms now used to describe these U deposits are based variously on petrographic, mineralogic, geographic, and genetic considerations; none is satisfactory for describing all these deposits having a common and distinctive form. Furthermore, some of the terms are ambiguous and have been commonly used to group together deposits having distinctly different forms. The term peneconcordant U deposit clearly sets apart a widespread and important type of U deposit that has a common and distinctive form, namely, nearly concordant to the bedding of the host rock. The meaning of the term is self-evident, and the term is readily applicable to field usage.

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