New Mexico Geological Society Annual Spring Meeting — Abstracts

The Turonian ammonite zones of Scaphites ferronensis, Scaphites whitfieldi and Prionocyclus novimexicanus are present in the D-Cross Member of the Mancos Shale near Puertecito, New Mexico. In the study area, the D-Cross Member is approximately 44 meters thick and is underlain by the Fite Ranch Member of the Tres Hermanos Formation and overlain by the Gallup Sandstone. The ammonite fauna includes Scaphites ferronensis Cobban, Scaphites whitfieldi Cobban, Prionocyclus novimexicanus (Marcou) and Baculites sp. Coilopoceras inflatum Cobban and Hook and Prionocyclus wyomingensis Meek have not been recovered from the Puertecito area. S. ferronensis is common in a zone of limestone concretions ~ 3 m above the base of the D-Cross. Most of the ammonites occur in concretions in gray shale in a 21-m-thick interval that begins about 10 m above the D-Cross base. S. whitfieldi is locally abundant in this interval in small, orange limestone concretions. A third ammonite-producing interval yields P. novimexicanus from concretions in a 4-mthick interval of shale that ends about 3 m below the D-Cross top. At Puertecito, P. novimexicanus is common throughout most of the D-Cross section. Baculites occurs in the S. ferronensis Zone and low in the S. whitfieldi Zone. This is the first report of Baculites from the D-Cross Member at Puertecito.

The occurrence of the standard zone of S. ferronensis near Puertecito indicates correlation of the lowest part of the D-Cross there with the lower part of the D-Cross at Cebollita Mesa and D-Cross Mountain. The occurrence of the standard zone of S. whitfieldi in the lower-middle part of the D-Cross at Puertecito indicates correlation with the middle part of the D-Cross at Cebollita Mesa and the very upper part of the Juana Lopez Member at the type and reference sections. The collignoniceratid zone of P. novimexicanus also occurs in New Mexico in the D-Cross Member throughout most of the section at Cebollita Mesa, the lower part of the section at D-Cross Mountain, Carthage, and Riley, and at the top of the Juana Lopez Member at the type and reference sections. The S. ferronensis Zone is of late middle Turonian age, the S. whitfieldi Zone is of early late Turonian age, and the P. novimexicanus Zone is of late Turonian age.