New Mexico Geological Society Annual Spring Meeting — Abstracts

The "basal white sandstone" of the Cretaceous (Albian) Mesa Rica Sandstone in the Tucumcari basin is now recognized as a series of localized fluvial deposits properly belonging to the underlying Jurassic Morrison Formation. previous misinterpretation of this relationship has resulted in incorrect placement of the J-K boundary throughout northern exposures in the Tucumcari basin. Locally, the kaolinitic and feldspathic "basal white sandstone" can be directly correlated to Morrison sandstones and shales that underlie the Albian Tucumcari Shale.
In a landward direction, as the Tucumcari pinches out and the Mesa Rica becomes fluvial , distinction of Jurassic and Cretaceous sandstones is more problematic.

The story of the J-K boundary is further complicated by the presence ofa previously unrecognized, genetically distinct sandstone that occurs locally between the Morrison ("basal white sandstone") and either the Tucumcari or Mesa Rica formation. This clean, locally bioturbated, quartz arenite was deposited in fluvial and marginal marine (estuarine) environments that backfilled topographic lows on the post-Morrison surface during base-level rise associated with the encroaching Tucumcari sea. As yet unnamed, we tentatively place this unit as a basal member of the Tucumcari Shale. A possible stratigraphic equivalent in northeastern New Mexico and southeastern Colorado is the basal sandstone member of the Glencairn Shale.