New Mexico Geological Society Annual Spring Meeting — Abstracts

In 1928, N. H. Darton reported tetrapod fossils collected by E. C. Case in the bank of the Arroyo de la Parida east of Socorro in central New Mexico. Although Case had earlier, in 1916, identified these bones as representing taxa of “Permo-Carboniferous” age, Darton concluded that they came from Triassic strata and thus must be reworked. Here, we resolve the evident contradiction—Permian bones in supposed Triassic strata—by demonstrating that Darton erred in his stratigraphy but was correct that the bones were reworked. Darton (1928) provided three kinds of information about the fossil locality that made relocating it possible: (1) he stated that “the bones were mainly in a conglomerate of dark red pebbles, lying on green, blue, and drab shales in the bank of the Arroyo de la Parida at a point not far below the mouth of Canyoncito Colorado….” (2) Darton published a photograph of the fossil site, and, though a century of erosion has modified the landscape, the location can still be recognized; and (3) Darton published a reconnaissance geologic map of part of central New Mexico that shows a Triassic outcrop along the Arroyo de la Parida that encompasses the location described and photographed.

The vertebrate fossils from the site are in the collection of the University of Michigan Museum of Paleontology, where Case was a professor. They include the proximal end of a rib of a diadectid; a complete femur pertaining to the temnospondyl Eryops; and various specimens either pertaining to the eupelycosaur Sphenacodon (a femur; a fragment of jaw), or possibly pertaining to the genus (the distal end of a scapula; the distal end of a large scapula; fragments of a large pelvis). These are characteristic early Permian tetrapod fossils of taxa well known locally from the lower Permian Abo Formation.

Darton’s mapping around the fossil site is very accurate but he mistook as Triassic the siliciclastic red beds of the Eocene Baca Formation in which the bones are reworked. At the fossil site, these Baca strata are scoured into underlying Middle Triassic Moenkopi red beds and comprise a 65-m-thick section of red-bed mudrock and intercalated conglomerates, some of which have limestone boulders up to 1 m in diameter. We conclude that the bones Case collected and described are of early Permian age, and they were reworked as Darton concluded, but reworked into Eocene, not Triassic red beds. What is most unusual is that the reworked Permian bones are not particularly abraded/damaged and that they remained in close proximity to each other while being reworked and redeposited.

References:

1. Case, E. C., 1916, Further evidence bearing on the age of the red beds in the Rio Grande Valley: Science, new series, v. 44, p. 708-709.
2. Darton, N. H., 1928, "Redbeds" and associated formations in New Mexico: U. S. Geological Survey, Bulletin 794, 356 p.