New Mexico Geological Society Annual Spring Meeting — Abstracts

In 1904, Clarence Luther Herrick (1858-1904) published an article titled "A coal-measure forest near Socorro, New Mexico" in the Journal of Geology. In it, Herrick described a lycopsid flora (including three new species of Lepidodendron) from "fire clay" of Pennsylvanian age being mined for brick manufacturing in Socono. Herrick's description of the locality was vague, and it has not been revisited in nearly a century. In 2002, we relocated Herrick's locality, which is east of Socorro near Arroyo de la Presilla in sec. 11, T3S, RIB (NMMNH locality 5312). The "fire clay" is a refractory dark gray to black shale in the lower part of the Middle Pennsylvanian (Atokan) Sandia FOlmation that can be followed on strike for more than 2 km, and that is fault repeated often along this transect. The Sandia Formation section at the lycopsid locality begins with ~4 m of trough-crossbedded quartzose sandstone and quartzite-pebble conglomerate scoured into the underlying Proterozoic granite. These coarse clastics are sharply overlain by ~2.5 m of gray and yellow, fine-grained, massive to thinly laminated sandstone. The lycopsid bark is concentrated in the top of this fine sandstone interval, which is overlain by the "fire clay" interval, ~4 m of gray and black shale, siltstone and fine sandstone, which in the lower part contains localized lenses of coal, few Lingula and an extensive flora. We suggest this succession represents fan-delta (coarsening-up basal clastics) directly overlain by an estuarine deposit (lycopsid beds and fire clay). Our collections of fossil plants from the lycopsid bed include Lepidodendron cf. L. aculeatum, L. cf. L. mannabachense and L. cf. L. jaraczewskii, Lepidostrobus, Synchysidendron, Sphenophyllum, sligmarian roots and strap like lcaves of the lcpidodcndrids. The type specimens of the species of Lepidodendron Herrick named were destroyed in a fire in 1910, so newly collected lycopsid specimens are topotypes of the species. However, Herrick's species appear to be within the range of variability known from single trunk specimens, or to be sufficiently similar to previously described species to be problematic. This lycopsid locality in the Sandia Formation is significant because it indicates that a
typical coal swamp flora existed in New Mexico during early tectonism of the Ancestral Rocky Mountain orogeny.