New Mexico Geological Society Annual Spring Meeting — Abstracts

Twenty-four Upper Cretaceous ammonite zones have been identified in the southeastern San Juan Basin. In ascending order, the zones are: Conlinoceras tarrantense in the Oak Canyon Member, Acanthoceras bellense in the Cubero Sandstone, Acanthoceras amphibolum in the Clay Mesa Member and Paguate Sandstone, Calycoceras canitaurinum in the Two Wells Sandstone, Mammites nodosoides in the upper Greenhorn Limestone, Collignoniceras woollgari and Prionocyclus hyatti in the Carlile Member, Prionocyclus hyatti in the Semilla Sandstone Member, Prionocyclus macombi, Scaphites warreni, Scaphites ferronensis and Scaphites whitfieldi in the Juana Lopez Member, Scaphites preventricosus in the Montezuma Valley Member, Gallup Sandstone, Tocito Sandstone Lentil and Mulatto Tongue, Scaphites ventricosus in the upper Mulatto Tongue, Scaphites depressus in the El Vado Sandstone, Clioscaphites vermiformis, Clioscaphites choteauensis, Desmoscaphites erdmanni and Desmoscaphites bassleri from the Satan Tongue, Scaphites leei III from the Satan Tongue and lower Point Lookout Sandstone, Baculites asperiformis from the La Ventana Tongue of the Cliff House Sandstone, Baculites perplexus, Baculites gregoryensis and Baculites scotti from the Lewis Shale and Didymoceras stevensoni from the Pictured Cliffs Sandstone. The C. tarrantense Zone is indicated by a molluscan fauna consisting of Turrilites acutus, Arrhoges modesta, Exogyra aquillana and Pinna petrina. The M. nodosoides Zone is indicated by the Mytiloides mytiloides inoceramid zone. The S. preventricosus Zone is identified in the Montezuma Valley Member, Gallup Sandstone and Tocito by the Cremnoceramus deformis erectus inoceramid zone and in the Mulatto Tongue by the Cremnoceramus crassus crassus inoceramid zone. The S. ventricosus Zone is indicated by the Volviceramus involutus inoceramid zone in the upper Mulatto.

There is a large hiatus in the ammonoid record of the southeastern San Juan Basin that represents most of late Cenomanian and early Turonian time that includes the majority of the Dunveganoceras problematicum through Vascoceras birchbyi zones. Ammonites diagnostic of the upper Cenomanian zones of Vascoceras diartianum, Euomphaloceras septemseriatum, Burroceras clydense, Neocardioceras juddii, and Nigericeras scotti have not been collected from this area. However, Sciponoceras gracile occurs in the lower Greenhorn Limestone and because it ranges from the E. septemseriatum through the N. juddii zones, it indicates at least one of these zones. If there was no hiatus, the D. problematicum through V. birchbyi zones would have to occur in the interval occupied by strata equivalent to the Graneros and Greenhorn members of the Mancos Shale. However, these 11 zones could not possibly fit within so thin of a stratigraphic interval. Also, these members are mostly poorly exposed in the southeastern San Juan Basin, and few fossils have been found in them. Another large hiatus in the ammonoid record from the southeastern San Juan Basin includes the lower Campanian zones of Scaphites hippocrepis I, II and III, Baculites sp. (smooth) and Baculites sp. (weak flank ribs), and the lower middle Campanian zones of Baculites obtusus and Baculites maclearni. These missing zones, the zones of S. hippocrepis I through B. maclearni, should occur in the interval occupied by the middle and upper parts of the Point Lookout Sandstone and the overlying, terrestrial Menefee Formation.