New Mexico Geological Society Annual Spring Meeting — Abstracts

The ammonite Baculites baculus Meek and Hayden, 1861 was recently recovered from the uppermost part of the Pierre Shale (about 15 m below the base of the Trinidad Sandstone) west of Raton in northeastern New Mexico. This ammonite was found in sandy strata of the distal lower shoreface of the Trinidad shoreline. The B. baculus Zone is earliest Maastrichtian in age. The youngest ammonite zone previously reported from northeastern New Mexico was the upper Campanian Baculites reesidei Zone (Cobban, 1976). This zone was inferred by the occurrence of Inoceramus oblongus Meek, 24-35 m below the top of the Pierre Shale. The B. baculus Zone is three ammonite zones higher than the B. reesidei Zone, and B. baculus in the uppermost Pierre Shale demonstrates that the Pierre seaway regressed from northeastern New Mexico later than previously thought. Across northern New Mexico, the western shoreline of the Western Interior Seaway regressed north and east during late Campanian to early Maastrichtian time. The uppermost lower Maastrichtian Baculites clinolobatus Zone occurs in the uppermost part of the Pierre Shale near Trinidad, Colorado (Berry, 2018). The first/last occurrences (FO/LO) of the B. clinolobatus Zone are 69.67/69.28 Ma and the FO/LO of the B. baculus Zone are 72.18/70.62 Ma (Scott, 2014). By these estimates of the LO of B. baculus and the FO of B. clinolobatus, regression of the Pierre seaway from Raton to Trinidad took slightly less than one million years.

The NMMNH specimen of Baculites baculus from locality 12043 is a moderately-preserved, large, incomplete, adult shell in two pieces, slightly flattened on one lateral side. The larger piece, mostly body chamber, has a length of 215 mm and a maximum diameter of 65.3 mm. The cross section is stout and almost quadrate. The flanks bear low, broad, arcuate undulations. The degree of taper is low. The poorly-preserved suture has simple, broad, rectilinear elements. These are characteristics of B. baculus. It is most similar to its smaller, presumed ancestor B. undatus Stephenson, which occurs much lower in the section with a range of 45-61 m below the top of the Pierre Shale west of Raton (Cobban, 1976). B. grandis Hall and Meek, the descendant of B. baculus and the next ammonite zone higher, is larger and has an almost trigonal cross section.

Therefore, the occurrence of the Baculites baculus Zone in the Raton area establishes an earliest Maastrichtian age for the uppermost Pierre Shale there, places the Campanian-Maastrichtian stage boundary at the base of that zone in the uppermost Pierre and demonstrates that the Pierre seaway regressed from northeastern New Mexico during early Maastrichtian time.

References:

1. Berry, K., 2018, Baculites (Ammonoidea) and the age of the Pierre Shale in the eastern Raton Basin, south-central Colorado: New Mexico Geology, 40 (1).
2. Cobban, W. A., 1976, Ammonite record from the Pierre Shale of northeastern New Mexico: New Mexico Geological Society, Guidebook 27, p. 165-169.
3. Scott, R. W., 2014, A Cretaceous chronostratigraphic database: construction and applications: Carnets de Geologie-Notebooks on Geology, v. 14, p. 15-37.