Jurassic stratigraphy in the Chama Basin, northern New Mexico
— Spencer G. Lucas, Adrian P. Hunt, and Justin A. Spielmann

Jurassic strata have an extensive outcrop belt in the Chama Basin of Rio Arriba County, New Mexico, and are assigned to the Entrada, Todilto, Summerville and Morrison formations. Previous workers have identified the Morrison Formation in the Chama Basin as the strata between the Jurassic Todilto Formation and the Cretaceous Burro Canyon Formation, or they have identified a “Wanakah Formation” between the Todilto and Morrison base. However, recent restudy of the Jurassic section in the Chama Basin (especially at Ghost Ranch and at Mesa Alta) indicate the Jurassic section is: (1) Slick Rock Member of the Entrada Sandstone, 60 to 76 m of trough crossbedded and ripple laminated sandstone; (2) Todilto Formation, consisting of a lower limestone-dominated Luciano Mesa Member (2-8 m thick) and an upper, gypsum dominated Tonque Arroyo Member (0-30 m thick); (3) Summerville Formation, 74 to 111 m of thinly and cyclically-bedded, grayish red and yellowish gray siltstone, sandy siltstone, fine gypsiferous sandstone and mudstone; (4) Bluff Sandstone (Junction Creek Member), 30 to 43 m of light gray, very fine grained, well sorted sandstone with crossbeds in thick sets; (5) Recapture Member of Bluff Sandstone, 6 to 14 m of grayish red gypsiferous siltstone, fine sandstone and minor mudstone; and (6) Brushy Basin Member of Morrison Formation, 41 to 68 m of variegated pale greenish gray and reddish brown, bentonitic mudstone and a few beds of trough-crossbedded, pebbly sandstone. This revised lithostratigraphy is consistent with regional lithostratigraphy that recognizes the Summerville Formation across northern New Mexico, and the Bluff Sandstone in the eastern San Juan Basin, adjacent to the Chama Basin. Only the Slick Rock Member of the Entrada Sandstone is present, and this suggests a depositional high in the Chama Basin area during the time of the Carmel transgression. The absence of a basal Salt Wash Member of the Morrison Formation in the Chama Basin is a striking departure from the regional stratigraphy. We interpret this absence as direct evidence of the J-5 unconformity, which separates the base of the Morrison (usually the Salt Wash Member, but in the Chama Basin the Brushy Basin Member) from underlying San Rafael Group strata.