Upper Cretaceous rocks and ammonite faunas of southwestern New Mexico
Authors: Cobban W. A. , Hook S. C. , Kennedy W. J. 
Published: March 2023

Upper Cretaceous rocks in southwestern New Mexico crop out in the Cooke's Range, Silver City-Santa Rita area, Little Burro Mountains, Big Burro Mountains, and Virden area. In these areas, marine rocks of late Cenomanian through early Turonian age are included in the Colorado Formation. In the Cooke's Range, the Colorado Formation rests disconformably on the upper Albian and lower Cenomanian Sarten Sandstone. In the rest of the area, the Colorado Formation rests on the Beartooth Quartzite, a resistant unit that has not been dated, but is assumed to be Late Cretaceous.

The lower part of the Colorado Formation consists of shale, limestone, siltstone, and sandstone of marine origin, whereas the upper part is chiefly mudrocks and sandstone of nonmarine origin. This report concerns only the marine part and its ammonite faunas. These marine rocks are the age equivalents of most of the Greenhorn Limestone of the Great Plains region. In the Cookes Range, the Colorado Formation is divided into the following units from oldest to youngest: flag member, lower shale member, Bridge Creek Limestone Member, upper shale member, and sandstone and shale member. In the rest of the area, the Colorado is divided into a shale member overlain by a sandstone member. Eight ammonoid zones are recognized in the Colorado Formation. The lower five, of late Cenomanian age, are separated from the upper three, of early Turonian age, by a hiatus that probably represents latest Cenomanian time.

The oldest zone, that of Calycoceras canitaurinum, is most fossiliferous in the Cookes Range where it contains 16 kinds of ammonites representing the following nine genera: Borissiakoceras, Moremanoceras, Cunningtoniceras, Calycoceras, Tarrantoceras, Metoicoceras, Hamites, Turrilites, and Neostlingoceras. Moremanoceras costatum and Neostlingoceras bayardense are new species. This zone is found in the flag member in the Cookes Range and in the basal part of the shale member in the Silver City-Santa Rita area, but it is not known farther west.

The second zone, that of Metoicoceras mosbyense, is more widely distributed. Fossils of this zone have been found in limestone concretions in the basal part of the Bridge Creek Limestone Member in the Cookes Range and in concretions in the equivalent part of the shale member of the Colorado Formation as far west as the Virden area. Twenty-two species of ammonites are present in this zone and represent the genera Moremanoceras, Placenticeras, Forbesiceras, Cunningtoniceras, Calycoceras, Eucalycoceras, Euomphaloceras, Metoicoceras, Nannometoicoceras, Vascoceras, Hamites, Metaptychoceras, and Neostlingoceras. New species named are Cunnintoniceras novimexicanum, C. cookense, Calycoceras inflatum, Euomphaloceras merewetheri, Hamites salebrosus, Metaptychoceras hidalgoense, Neostlingoceras procerum, and N. virdenense.

The third zone is that of Sciponoceras gracile, which lies in the upper part of the Bridge Creek Limestone Member in the Cookes Range and in the equivalent part of the shale member of the Colorado Formation as far west as the Virden area. Ammonoid genera include: Moremanoceras, Placenticeras, Calycoceras, Pseudocalycoceras, Sumitomoceras, Euomphaloceras, Metoicoceras, Vascoceras, Allocrioceras, Neostlingoceras, Sciponoceras, and Worthoceras. Thirteen species are recognized, of which only Neostlingoceras apiculatum is new.

The fourth zone, that of Burroceras clydense, represents a very thin zone that is known with certainty only in the Big Burro Mountains, where it contains 10 species of the genera Placenticeras, Burroceras (new), Paraburroceras (new), Vascoceras,Hamites, Scipsnoceras, and Worthoceras. New species or subspecies named are Burroceras clydense, Paraburroceras minutum, Vascoceras baroicense exile, and Hamites pygmaeus. The next zone of Neocardioceras juddii is the highest Cenomanian zone widely recognized in southwestern New Mexico. Fossils of this zone occur at a hiatus horizon at the top of the Bridge Creek Limestone Member in the Cookes Range and in the equivalent part of the shale member of the Colorado Formation farther west to Virden. The extensive ammonoid fauna consists of 23 species of the genera Placenticeras, Neocardioceras, Watinoceras, Euomphaloceras, Burroceras, Pseudaspidoceras, Vascoceras, Fagesia, Rubroceras, Hamites, Anisoceras, Sciponoceras, and Worthoceras. New species named are Neocardioceras woodwardi, Watinoceras odonnelli, Burroceras irregulare, B. transitorium, Rubroceras alatum, R. burroense, R. rotundum, and Anisoceras coloradoense. Hiatuses at the top of the Bridge Creek Limestone Member and in the base of the overlying upper shale member of the Colorado Formation in the Cookes Range and in the equivalent parts of the shale member of the Colorado farther west probably represent latest Cenomanian time-probably the time span of the zone of Nigericeras scotti of southeastern Colorado and northeastern New Mexico.

The lowest Turonian zone in southwestern New Mexico is that of Pseudapidoceras flexuosum. Fossils, which are scarce and poorly preserved, are found in thin sandy beds in the lower part of the upper shale member of the Colorado Formation in the Cookes range and in the equivalent parts of the shale member farther west. The few ammonites identified include Watinoceras sp., Quimaniceras reaseri, Pseudaspidoceras flexuosum Powell, Vascoceras sp., and Fagesia catinus. A possible Kamerunoceras was also collected.

The second lower Turonian zone is that of Vascoceras birchbyi, which is found in the lower part of the sandstone and shale member of the Colorado Formation in the Cookes Range and in the basal part of the sandstone member farther west. Nine species of ammonites of the genera Watinoceras, Nigericeras, Vascoceras, Fagesia, Neoptychites, and Thomasites are known. No new forms are named.

The highest ammonite zone in southwestern New Mexico is that of Mammites nodosoides of latest Turonian age. This fauna, which is known only from the east side of the Cookes range, occurs in the youngest sandstone bed of the sandstone and shale member of the Colorado Formation. The sparse ammonoid fauna consists of Watinoceras sp., Mammites nodosoides, Infabricaticeras lunaense gen. et sp. nov., and Thomosites? sp.