New Mexico Geological Society Annual Spring Meeting — Abstracts

Middle and Upper Jurassic units along the southern and southeastern edges of the San Juan Basin, particularly in and between the Zuni and Acoma basins, are near the southern extent of their areas of deposition. Units along I-40 and for many miles to the north are relatively consistent but change rapidly in lithology and mode of deposition to the south. Most units become coarser southward and develop facies indicating higher-energy depositional environments. In the Acoma Basin, the Entrada Sandstone forms the base of the Middle Jurassic section. The Iyanbito and medial silty Members of the Entrada pinch out 19-20 miles south of I-40 and the Upper Member is truncated by a basal conglomerate of the Beclabito Member of Wanakah Formation, formerly Summerville Formation, about 25 miles south. The limestone facies of the Todolito pinches out 17 miles south of I-40; the gypsum member is completely gone about 6 miles SW of Mesita. The Horse Mesa Member of the Wanakah Formation, formerly Bluff Formation, thins southward and merges imperceptibly into the overlying Zuni Sandstone. The dominantly eolian Zuni Sandstone pinches out northward from Laguna but thickens dramatically to the south and west. It represents the progradation or re-establishment of a sand-sea following the disappearance of the Todilto salina lake. The dominantly fluvial Morrison Formation suggests the onset of more humid conditions in Late Jurassic time. Morrison distribution is controlled in the area by pre-Dakota southward beveling; it is truncated by the Dakota Formation a few miles south of I-40 near Acoma, near Grants, and near Gallup.

At El Morro on the south flank of the Zuni Uplift a poorly exposed section of equivocal Entrada may comprise the basal 30 ft of the Zuni Sandstone. Several miles to the west the evidence for an Entrada presence is lacking. Still further west the Zuni Sandstone and the Upper Member of the Entrada. separated by a distinct but discontinuous "Todilto notch" make up the entire Jurassic section at Dowa Yalanne (Com Mountain) near Zuni Pueblo. This entire section has been designated Zuni Sandstone in the interest of preserving the original usage of the term as well as using a local geographic name. Other Jurassic units coarsen southward, develop conglomeratic facies, and pinch out between Gallup and Zuni.

The Cow Springs Sandstone, extended into the Zuni Pueblo area from Arizona by earlier investigators, has been shown to be the upper part of the Entrada. As such it is a pre-Todilto unit and thus cannot be correlated with the upper part (above the notch) of the Zuni Sandstone which is demonstrably post-Todilto. We thus propose that Cow Springs be abandoned as a stratigraphic name in New Mexico.