New Mexico Geological Society Annual Spring Meeting — Abstracts

An approximately 335-m thick section of the Mancos Formation (= “Colorado Shale”) is exposed on the southwestern flank of the Big Burro Mountains just north of Redrock in Grant County, New Mexico. Here, the Mancos Formation overlies strata traditionally termed Beartooth Formation with evident, transgressive disconformity. Thus, the topmost bed of the Beartooth Formation is coarse-grained sandstone capped by a dark, hematitic crust that locally contains a matrix-supported conglomerate of mostly siliceous pebbles. The base of the Mancos Formation is very finely laminated shale with some silty/sandy interbeds. In the lower 5 m of the Mancos Formation we have collected a sparse but age-diagnostic assemblage of ammonoids in nodular limestone concretions in shale. Three ammonoid taxa are present: Acanthoceras amphibolum Morrow, Tarrantoceras sellardsi (Adkins) and Desmoceras sp. These ammonoids identify the Acanthoceras amphibolum zone of middle Cenomanian age, so this is (within resolution) the age of the base of the Mancos Formation in this part of the Big Burro Mountains. This is an age consistent with previous ammonoid-based determinations of the age of the base of the Mancos Formation in southwestern New Mexico. A volcanic ash bed in the Big Burro Mountains at the ammonoid localities, ~ 14 m above the base of the Mancos Shale, has a zircon U/Pb age of 96.3 ±1.3 Ma. This numerical age just above middle Cenomanian ammonoids is consistent with current numerical calibration of the Cenomanian Stage.