New Mexico Geological Society Annual Spring Meeting — Abstracts

The BLM Ojito Wilderness Area in southern Sandoval County, New Mexico, is located in the southeastern San Juan Basin. Most of the strata exposed in the Ojito are of Jurassic age, but its western third exposes outcrops of the intertongued Dakota-Mancos succession of early Late Cretaceous (Cenomanian) age. At the top of this succession is the Twowells Member of the Dakota Sandstone, a thin (1-4 m thick) unit of laminar or bioturbated sandstone that yields ammonites of the upper Cenomanian Calycoceras canitaurinum Zone.

Since W.A. Cobban’s 1977 work on the Dakota-Mancos molluscan fauna of west-central New Mexico, no new taxa of mollusks have been reported from the Twowells Member. A recent survey of the Cretaceous strata of the BLM Ojito Wilderness Area produced an impressive molluscan assemblage from the middle Cenomanian, most of it from the Paguate Member of the Dakota Sandstone and the Clay Mesa Member of the Mancos Shale. It also yielded ammonites and other fossils (bivalves, shark teeth) from 16 localities in the Twowells Member over a 5 km outcrop belt along the western boundary of the Ojito Wilderness Area (NMMNH [New Mexico Museum of Natural History] localities 12749-12752, 12755-12760, 12762, 12786-12788, 12790, and 12820). Here, the Twowells Member is a persistent, yellowish-gray glauconitic sandstone bed that overlies a narrow band of the Whitewater Arroyo Member of the Mancos Shale. Within the crossbedded, fossiliferous coarse-grained sandstone are lenses of lag with coquina, shark teeth, pebbles, and occasional bone fragments.

The Twowells Member assemblage of ammonites recovered consists of Metoicoceras praecox (Haas), Calycoceras (Proeucalycoceras) canitaurium (Haas), and Cunningtoniceras arizonesis (Kirkland) as well as the second and third specimens of Metongococeras (Hyatt) reported in New Mexico. The Metengonoceras are unusual and rare and consist of: (1) NMMNH P-87911 from locality 12755, a three-dimensional sandstone cast of a fragment of a large adult phragmocone/body chamber; and (2) NMMNH P-86638 from locality 12820, a weathered three-dimensional sandstone cast of a phragmocone fragment. We assign these specimens to Metengonoceras cf. M. dumbli (Cragin). They have only one lateral side well preserved. The shells are compressed, involute and discoidal with nearly flat flanks with faint ribs and a steep umbilical shoulder. Possible primary and secondary ribs are visible, and there are two secondary ribs per primary. The narrow venter become more rounded outward. Under indirect light, faint falcate growth lines are visible. These are the first fully documented Metengonoceras reported from New Mexico. The other report is by Cobban in 1987, who briefly mentions a Metengonoceras from the Mancos Shale of Socorro County. M. dumbli is only known from the middle Cenomanian of the Western Interior, and all Twowells Member ammonites from New Mexico are from the upper Cenomanian. M. acutum is only known from the upper Cenomanian. Due to the rarity of well-preserved specimens, variability of morphology, the historical ambiguity of the nomenclature, and the fact that our specimens exhibit morphological features ascribed to both species, we propose that Cenomanian Metengonoceras is only one species: Metengonoceras dumbli.