TURONIAN AMMONITES FROM THE UPPER CRETACEOUS CARLILE AND SEMILLA SANDSTONE MEMBERS AND REFERENCE AREA OF THE JUANA LOPEZ MEMBER OF THE MANCOS SHALE, EASTERN SIDE OF THE SAN JUAN BASIN, LA VENTANA, SANDOVAL COUNTY, NEW MEXICO

Abstract

At the reference section of the Juana Lopez Member of the Mancos Shale near La Ventana in central New Mexico, the unit is thick and well exposed. In 1966, Dane and others reported a reference section of 107 ft thick, the same thickness as the type section at Galisteo Dam. The Juana Lopez consists of three lithic intervals--lower calcarenites, middle shale and upper calcarenites, and is underlain by the Carlile Member of the Mancos Shale. Only the upper part of the Carlile Member is exposed at La Ventana, and the Semilla Sandstone Member is very thin. The ammonite fauna from the Carlile and Semilla includes Prionocyclus hyatti (Stanton), Coilopoceras springeri Hyatt, Romaniceras (Romaniceras) mexicanum Jones and Placenticeras cumminsi Cragin. The ammonite fauna from the Juana Lopez includes Scaphites whitfieldi Cobban, Prionocyclus novimexicanus (Marcou), Prionocyclus macombi Meek, Coilopoceras colleti Hyatt and Baculites sp. Ammonite diversity in the upper calcarenite interval of the Juana Lopez is lower than at the type section. It includes primarily P. novimexicanus , secondarily S. whitfieldi , and rarely Baculites sp. The middle shale interval, as at the type section, has a low diversity ammonite fauna dominated by P. macombi with an occasional C. colleti. The P. hyatti Zone, present in the Carlile Member at La Ventana, also occurs in other places in New Mexico, especially in the Carlile Member at Galisteo Dam. The P. macombi and P. novimexicanus zones are present in the Juana Lopez Member