New Mexico Geological Society Annual Spring Meeting — Abstracts

Hourcquia mirabilis is a very rare ammonite in the Upper Cretaceous of the Western Interior. We report four additional specimens of H. mirabilis from the middle shale interval of the Juana Lopez Member in Sandoval and Santa Fe counties, NM. Three localities are east of Mesa Prieta in Sandoval County, and one is from the type area of the Juana Lopez Member at Galisteo Dam in Santa Fe County. They occur in the middle Turonian Coilopoceras inflatum Subzone in the upper part of the Prionocyclus macombi Zone. Distinguishing features of the newly discovered specimens include an involute shell with a trapezoidal whorl section, a fastigiate venter with a prominent siphonal keel and strong, straight, rounded, prorsiradiate primary ribs that arise from strong umbilical bullae and terminate in strong, blunt ventrolateral nodes, as has been previously described in H. mirabilis.

Hourcquia mirabilis has also been reported from the Prionocyclus macombi Zone in association with Coilopoceras inflatum in the basal 1.5 m of the D-Cross Tongue at Mescal Canyon in Sierra County and in the basal sandstone of the Juana Lopez at Bull Gap Canyon in Lincoln County, where it is also associated with C. inflatum (Hook and Cobban, 2013). It has also been reported near the base of the Juana Lopez Member at Taylor Springs in Colfax County and near the base of the Juana Lopez at the type locality (Kennedy et al., 1988). Hourcquia is a very rare ammonite, known in the USA from the middle Turonian zones of P. macombi in New Mexico and (questionably) P. wyomingensis in Trans-Pecos Texas (Kennedy et al., 1989). Leckie et al. (1997) also reported H. mirabilis from the Juana Lopez Member near Mesa Verde, Colorado. Hourcquia has also been described from Madagascar, Japan, Sakhalin (Russia) and Venezuela (Kennedy et al., 1988). This is the first report of H. mirabilis from Sandoval County.

References:

1. Hook, S. C. and Cobban, W. A., 2013, The Upper Cretaceous (Turonian) Juana Lopez beds of the D-Cross Tongue of the Mancos Shale in central New Mexico and their relationship to the Juana Lopez Member of the Mancos Shale in the San Juan Basin: New Mexico Geology, vol. 35, no. 3, p. 59-81.
2. Kennedy, W. J., Cobban, W. A. and Hook, S. C., 1988, Hourcquia Collignon, 1965 (Cretaceous Ammonoidea) from the upper Turonian of the southern United States: Paläontologische Zeitschrift, v. 62, issue 1-2, p. 87-93.
3. Kennedy, W. J., Cobban, W. A., Hancock, J. M. and Hook, S. C., 1989, Biostratigraphy of the Chispa Summit Formation at its type locality; a Cenomanian-Turonian reference section for Trans-Pecos Texas: Geological Institutions of the University of Uppsala, Bulletin, n. ser., v. 15, p. 39–119.
4. Leckie, R. M., Kirkland, J. I., and Elder, W. P., 1997, Stratigraphic framework and correlation of a principal reference section of the Mancos Shale (Upper Cretaceous), Mesa Verde, Colorado: New Mexico Geological Society, Guidebook 48, p. 163-216.