Jurassic stratigraphy in the Chama Basin, northern New Mexico

Abstract

—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. FIGURE 1. Distribution of Jurassic strata in the Chama Basin showing location of measured sections at Mesa Alta and Ghost Ranch and of Echo Amphitheater. 183 JURASSIC STRATIGRAPHY IN THE CHAMA BASIN lower member (limestone) and an upper member (gypsum). They also divided the Morrison Formation into a lower member (up to 400 ft thick), “an alternating sequence of pale brown, chocolate, or deep purple mudstones and white to pale gray siltstones” (p. 13), overlain by the Brushy Basin Member. Dane and Bachman (1965) similarly assigned Jurassic strata in the Chama Basin to the San Rafael Group (Entrada and Todilto formations) and Morrison Formation. Ridgley (1977, 1986, 1989) revised the Jurassic stratigraphic nomenclature in the Chama Basin to meet then-current regional Jurassic nomenclature used by the U.S. Geological Survey (e.g., Condon and Peterson, 1986). Thus, she assigned the Jurassic strata to the Entrada, Wanakah, and Morrison formations (Fig. 2). The Wanakah was divided into a lower, Todilto Limestone Member and an upper clastic member. Ridgley also divided the Morrison Formation into a lower member, middle sandstone member and Brushy Basin Shale Member. The preoccupied term Wanakah should be abandoned as a stratigraphic name for Jurassic rocks in the Southwest (e.g., Anderson and Lucas, 1992; Lucas and Anderson, 1997, 1998). Todilto has long been, and continues to be mapped as a formation rank unit, with two distinct members (Lucas et al., 1995; Anderson and Lucas, 1996; Lucas and Anderson, 1997, 1998). Furthermore, as discussed below (also see Anderson and Lucas, 1996), strata assigned by Ridgley (1977, 1989) and earlier workers to the lower part of the Morrison Formation are lithologically the same as strata of the Summerville Formation and Bluff Sandstone elsewhere. Therefore, Lucas and Anderson (1998; also see Anderson and Lucas, 1996) revised the Jurassic stratigraphic nomenclature in the Chama Basin, and that nomenclature is used here (Fig. 2). The base of the Jurassic section is Entrada Sandstone (Slick Rock Member, see Lucas and Heckert, 2003). The overlying Todilto Formation can be divided into a lower, limestone-dominated Luciano Mesa Member and an upper, gypsum-dominated Tonque Arroyo Member. Summerville strata above the Todilto consist of lower and upper members. The Bluff Sandstone above that is mostly eolian sandstone of the Junction Creek Member overlain by thin gypsiferous sandstones and siltstones of the Recapture Member. The overlying Morrison Formation strata are assigned to the Brushy Basin Member.