New Mexico Geological Society Annual Spring Meeting — Abstracts

The rare ammonite Forresteria peruana as well as Scaphites preventricosus, Baculites yokoyamai and Cremnoceramus deformis erectus were collected from the Mancos Shale just below the Tocito Sandstone Lentil on the northeastern side of Mesa Prieta, Sandoval County, New Mexico. Also, Inoceramus erectus (C. deformis erectus) and Forresteria hobsoni have been reported from the Tocito Sandstone Lentil a short distance north of our localities. There are conflicting reports as to whether F. peruana extends from the upper Turonian into the lower Coniacian or is restricted to the upper Turonian. In 1991, Kennedy and Cobban listed a locality with F. peruana occurring with Inoceramus (Cremnoceramus) erectus (C. deformis erectus) just below the Tocito Sandstone Lentil in Sandoval County, New Mexico (same as our locality). C. deformis erectus is the index taxon of the lowermost, lower Coniacian inoceramid zone. Another report (Cobban et al., 2008) shows F. peruana occurring in part in the lower Coniacian, in the C. deformis erectus Zone and mentions that F. peruana straddles the Turonian-Coniacian boundary. But, other reports (Walaszczyk and Cobban, 2000; Cobban et al., 2006, USGS 2006-1250; Walaszczyk et al., 2010) show F. peruana only in the upper Turonian in the Mytiloides scupini inoceramid Zone. In two of these reports, F. hobsoni is reported to extend from the upper Turonian into the lower Coniacian. Prionocyclus germari and M. scupini (Inoceramus frechi) occur together in the Montezuma Valley Member of the Mancos Shale and, a little higher stratigraphically, F. peruana and M. scupini do occur together at the base of the Gallup Sandstone west of Albuquerque in Bernalillo County. But, our collections of ammonites and inoceramids from the Mancos Shale just below the Tocito Sandstone Lentil show that C. deformis erectus and F. peruana do indeed occur together along with S. preventricosus. Furthermore, the S. preventricosus ammonite Zone and C. deformis erectus inoceramid Zone are both of early Coniacian age. Therefore, Coniacian ammonites and inoceramids from the Mancos Shale just below the Tocito Sandstone Lentil on the northeastern side of Mesa Prieta demonstrate that F. peruana extends from the upper Turonian into the lower Coniacian.