New Mexico Geological Society Annual Spring Meeting — Abstracts

Ammonites and other molluscs were collected from the Pictured Cliffs Sandstone at Mesa Portales, Sandoval County, New Mexico. Here, the Pictured Cliffs Sandstone is ~ 50 m thick and consists largely of friable hematitic and/or clayey sandstone in tabular to massive beds. Contacts with the underlying shale-dominated Lewis Shale and the overlying coaly Fruitland Formation are gradational. Molluscs occur at two stratigraphic levels, a very prolific lower bed ~ 21 m above the base of the Pictured Cliffs Sandstone and a less fossiliferous bed ~ 12 m higher. The moderately diverse fauna (in order of abundance within kind) includes the ammonites Placenticeras intercalare, Didymoceras stevensoni, Oxybeloceras crassum and Baculites crickmayi; the gastropods Lunatia occidentalis, Cantharus (Cantharulus) cf. C. (C.) lemniscatus, an unidentified naticid, Turritella sp., Turcicula imperialis, Acteon sp., Sinum sp., Haminea subcylindrica, and Pyropsis sp.; the bivalves Granocardium whitei, Tellina scitula, Leptosolen biplicatus, Ostrea sp., Cyprimeria sp., Gervillia sp., Corbula sp., an inoceramid, the scaphopod Dentalium sp.; and a solitary coral. This is the first report of these ammonites, and C. (C.) lemniscatus, T. imperialis, Sinum, Pyropsis, L. biplicatus, Cyprimeria, Gervillia and Dentalium from the Pictured Cliffs Sandstone.

Very little has been published on fossil invertebrates from the Pictured Cliffs Sandstone. Reeside (1924, p. 19) listed a small molluscan fauna in San Juan County and near Durango, Colorado. Dane (1936, p. 112) reported the bivalves Cardium (Ethmocardium) whitei, Corbula sp. and the gastropod Buccinum? sp. at Mesa Portales. Fassett (1966) mentioned pelecypods, gastropods and the burrow Ophiomorpha major as being abundant at Mesa Portales. Cobban (1973, p. 150) reported on poorly preserved fragments of Didymoceras from the lower part of the Pictured Cliffs Sandstone near Barker Dome in northern NM, that could be D. cheyennense. Didymoceras stevensoni has also been collected in NM from the Pierre Shale near Cimarron and, together with Baculites crickmayi, from northwest of Vermejo Park. Oxybeloceras crassum is usually associated with D. stevensoni. The fossils from the Pictured Cliffs Sandstone at Mesa Portales represent the early late Campanian D. stevensoni Zone.

References:

1. Cobban, W.A. 1973. Significant ammonite finds in uppermost Mancos Shale and overlying formations between Barker Dome, New Mexico, and Grand Junction, Colorado, p. 148-167. In Fassett, J.E. (ed.), Cretaceous and Tertiary rocks of the southern Colorado Plateau. Four Corners Geol. Society Memoir.
2. Dane, C. H., 1936, Geology and fuel resources of the southern part of the San Juan Basin, New Mexico, pt. 3, The La Ventana-Chacra Mesa coal field: U. S. Geol. Surv., Bull. 860-C, p. 81-166.
3. Fassett, J. E., 1966, Geologic map of the Mesa Portales quadrangle, Sandoval County, New Mexico: U. S. Geol. Surv., Map GQ-590.
4. Reeside, J. B., Jr., 1924, Upper Cretaceous and Tertiary formations of the western part of the San Juan Basin, Colorado and New Mexico: U.S. Geol. Survey Prof. Paper 134, p. 1-70.