A coal-measure forest near Socorro, New Mexico
Spencer G. Lucas, William A. Dimichele, Karl Krainer, Dan S. Chaney, and Justin A. Spielmann
In 1904, Clarence Luther Herrick described a lycopsid flora (including three new species of Lepidodendron) from “fire clay” of Pennsylvanian age being mined for brick manufacturing east of Socorro. 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. The “fire clay” is a refractory gray to black shale in the lower part of the Middle Pennsylvanian (Atokan) Sandia Formation that can be followed on strike through a series of fault blocks for more than 2 km. The Sandia Formation at the lycopsid locality contains basal troughcrossbedded quartzose sandstone and quartzite-pebble conglomerate about 4 m thick, filling channels scoured into Proterozoic granite. These coarse clastics are sharply overlain by ~2.5 m of gray and yellow, fine-grained, massive to thinly laminated sandstone, which has lycopsid bark concentrated near the top. This in turn is overlain by the “fire clay” interval, ~4 m of gray and black shale, siltstone, and fine sandstone, which in the lower part contains lenses of coal, a few Lingula, and a flora. We suggest this succession represents fluvial deposits directly overlain by an estuarine deposit (lycopsid beds and “fire clay”). Our collections of fossil plants from the lycopsid bed include Lepidodendron aculeatum, Lepidostrobus, possibly Synchysidendron, stigmarian roots and strap-like leaves of the lepidodendrids, Sphenophyllum, and neuropterid foliage. Because the type specimens of the species of Lepidodendron Herrick named were destroyed in a fire in 1910, we collected new specimens to serve as “topotypes” of the species. However, most of Herrick’s species appear to be within the range of variability known from the single species Lepidodendron aculeatum. This lycopsid locality in the Sandia Formation is significant because it indicates that a typical wetland swamp flora existed in New Mexico during early tectonism of the Ancestral Rocky Mountain orogeny.
- Lucas, Spencer G.; Dimichele, William A.; Krainer, Karl; Chaney, Dan S.; Spielmann, Justin A., 2009, A coal-measure forest near Socorro, New Mexico, in: Geology of the Chupadera Mesa, Lueth, Virgil W.; Lucas, Spencer G.; Chamberlin, Richard M., New Mexico Geological Society, Guidebook, 60th Field Conference, pp. 235-242.