STRATIGRAPHY OF THE UPPER JURASSIC MORRISON FORMATION, CHAMA BASIN, NORTHERN NEW MEXICO

The Upper Jurassic Morrison Formation has an extensive outcrop belt in the Chama Basin of Rio Arriba County, New Mexico, but is relatively unstudied because of the rugged terrain and forest/colluvium cover that hinder access to good exposures. 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 post-Todilto Jurassic section is: (1) Summerville Formation, ~ 70-80 m of thinly and cyclically-bedded, maroon and grayish red siltstone, sandy siltstone, fine sandstone, mudstone and gypsum; (2) Bluff Sandstone (main body), 30-90 m of light gray, very fine grained, well sorted sandstone with crossbeds in thick sets; (3) Recapture Member of Bluff Sandstone, 8-14 m of grayish red gypsiferous siltstone, fine sandstone and mudstone; and (4) Brushy Basin Member of Morrison Formation, 40-60 m of variegated pale green and orange, 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. 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. The J-5 unconformity is a tectonosequence boundary that corresponds to a significant reorganization of the Jurassic depositional system from the dry lithofacies of the San Rafael Group, which had a generally down-to-the-west paleoslope to the wet lithofacies of the Morrison Formation, which had a generally down-to-the-east paleoslope.