Ammonites of the Upper Cretaceous Twowells Member of the Dakota Sandstone, Ojito Wilderness Area, Sandoval County, New Mexico

Abstract

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