San Juan Sag: Cretaceous Rocks in a Volcanic-Covered Basin, South Central Colorado

Abstract

The San Juan Sag is a foreland basin formerly adjacent to and west of the Laramide northern Sangre de Cristo uplift. Latest Laramide wrench faulting and Miocene rifting split this bounding uplift and formed the San Luis basin adjacent to and east of the sag. Concurrent volcanism concealed the San Juan Sag with over 10,000ft of intermediate-type volcanic deposits and its presence remained in doubt until oil occurrences in the volcanic rocks encouraged exploration for the underlying sedimentary rocks. Recent drilling through the volcanic field has revealed the presence of Eocene elastic sediments, the Cretaceous Lewis, Mancos and Dakota formations and Jurassic Morrison, Junction Creek, Entrada, and possibly Wanakah formations. Additionally, oil and gas shows were found and production established as a result of drilling in 1984.