New Mexico Geological Society Annual Spring Meeting — Abstracts

In north-central New Mexico (Santa Fe and Sandoval counties), the Upper Cretaceous Blue Hill Member of the Carlile Shale is up to 33 m thick and consists of olive gray shale with numerous limestone concretions and septarian nodules, intercalated locally with the offshore bar deposits of the Semilla Sandstone Member. The Blue Hill Member yields middle Turonian ammonoid assemblages of the (ascending order) Collignoniceras praecox and Prionocyclus hyatti zones. Extensive collections of ammonoids from the Blue Hill Member in north-central New Mexico reveal striking differences in the composition and relative abundances of key ammonoid taxa at different localities. Thus, at Galisteo Dam (T14N, R7E), the assemblages are dominated by P. hyatti with few to no specimens of Spathites puercoensis and Coilopoceras springeri. Along the Rio Puerco (T14N, R3W), however, P. hyatti is not common, but S. puercoensis and C. springeri are abundant. In contrast, at Marquez Wash (T15N, R1W) and on the Ojo del Espiritu Santo Land Grant (T16N, R1W), S. puercoensis is common but decreases in abundance northward, C. springeri is uncommon, and P. hyatti is common, but progressively dominated by larger individuals northward. Furthermore, at Marquez Wash and northward on the Ojo del Espiritu Santo Land Grant, a turritellid limestone, indicative of relatively shallow water, is present where Spathites is most abundant. The differences in the ammonoid assemblages also correlate with the presence/absence of the Semilla Sandstone Member, which is thick and present at Marquez Wash and along the Rio Puerco. We hypothesize that the more heavily ornamented P. hyatti , with its strong ribs and massive ventolateral horns, is a deeper water ammonoid than the nearly smooth, unornamented S. puercoensis and C. springeri, and that Blue Hill Member outcrops where the Semilla Sandstone is present and relatively thick represent shallower water than the outcrops where the Semilla is extremely thin or absent. Those inferences support the conclusion that the differences among the age-equivalent ammonoid assemblages of the Blue Hill Member in north-central New Mexico reflect differences in water depth, with distinct deeper water (P. hyatti-dominated) and shallower water (S. puercoensis- and C. springeri-dominated) ammonoid assemblages