New Mexico Geological Society Annual Spring Meeting — Abstracts

The Bliss Formation of southern New Mexico and Trans-Pecos Texas is the oldest Phanerozoic stratigraphic unit in southern New Mexico. It represents shallow marine, siliciclastic deposition during the Sauk transgression. While the formation is generally regarded as fossil-poor, recent fieldwork in “Cable Canyon,” Caballo Mountains, Sierra County, New Mexico, has yielded a significantly more diverse ichnofossil assemblage than previously reported, particularly from the lower Mud Mountain Member at the Sierrite Mine locality (Lucas and Krainer, 2018; Lucas et al., 2023). We document ichnotaxa from this unit and assess their behavioral and paleoenvironmental significance.

Field observation and surface collection of fallen slabs of wavy bedded fine grain sandstone and photographs of in-place outcrop specimens revealed multiple tiers of ichnoguilds within the lower member, as well as a slab from near the stratigraphic level where minute trilobites are reported that indicates the presence of much larger burrowers. The ichnoassemblage includes tubular grazing and burrowing traces: Palaeophycus (Hall); Helminthopsis (Heer); Phycodes (Richter), and Arenicolites (Salter); branching traces: Chondrites (von Sternberg) and Thalassinoides (Ehrenberg), consistent with previously reported fauna from the upper Cantrell Tank Member (Lucas and Krainer, 2018; Lucas et al., 2023). More significantly, the lower member yielded surfaces with numerous examples of trilobite locomotion traces: Cruziana (d’Orbigny); Didymaulichnus (Young); Monomorphichnus (Crimes); trilobite resting and hiding traces: Rusophycus (Hall). A notable specimen preserves a mollusk trail assignable to Climatichnites (Logan) (Hantzschel, 1975). From near the Cambrian–Ordovician boundary, one surface preserves Palaeophycus, sinuous horizontal burrows produced by animals larger in diameter than the largest trilobites reported from the formation (Taylor and Repetski, 1995).

This ichnoassemblage represents a typical Cruziana ichnofacies consistent with other unstable, shallow marine sandy substrate localities at the Cambrian-Ordovician boundary globally. These ichnofossils demonstrate that the Bliss Formation, and especially the lower Mud Mountain Member, supported a more ecologically complex benthic community than previously recognized, with evidence of diverse locomotion, feeding, and resting behaviors.

References:

1. Lucas, S.G., and Krainer, K., 2018, The Bliss Formation (Cambrian–Ordovician) in New Mexico: New Mexico Museum of Natural History and Science Bulletin, v. 79, p. 1–50.
2. Lucas, S.G., et al., 2023, Geology of the Mud Springs Mountains, Sierra County, New Mexico, USA: New Mexico Museum of Natural History and Science Bulletin, v. 92, 92 p.
3. Taylor, J.F., and Repetski, J.E., 1995, High-resolution trilobite and conodont biostratigraphy across the Cambrian-Ordovician boundary in south-central New Mexico; in Cooper, J. D., Droser, M. L. and Finney, S. C., eds., Ordovician Odyssey—7^th International Symposium on the Ordovician System. Pacifica Section: SEPM, Book 77, p. 133–136