Enigmatic Cylindrical Structures in the Navajo Sandstone near Moab, Utah
Enigmatic cylindrical structures found in the Lower Jurassic Navajo Sandstone near Moab, Utah, are described as tetrapod burrows based on their architectural and surficial morphologies. Four localities were studied where cylindrical casts, associated trace fossils, and stratigraphic sections were measured and described to interpret the depositional environments and postdepositional histories of those units. Cylindrical casts are preserved in full relief and occur throughout the strata of the eolian cross-bedded Navajo Sandstone. These casts occur in mounded areas as large as 15 m wide, 20 m long, and 1 m high on horizontal weathered surfaces. Individual casts are elliptical in cross section with an average width of 9.3 cm, average height of 4.2 cm, and a width-to-height ratio of 2.2:1. Casts are in filled passively with sand, show no internal structure, and have relatively smooth walls. At least one cylindrical structure preserves important architectural and surficial burrow morphologies as it passes from eolian deposited sandstone into interdune deposited red mudrock. The upper portion of the ~ 9.2 long cylindrical structure is poorly preserved in the sandstone with the general features described earlier. As it passes into the mudrock, the cylindrical structure exhibits a well-preserved bilobate morphology with walls preserving a series of thin, inclined scratch marks from about mid-height of the wall to the base of the wall along the bilobate floor. The width of this structure is ~ 30 cm. In general, the cylindrical casts crosscut bedding, are variably sinuous, and exhibit complex architecture with Y and T branching, spiral or helical ramps, and large chambers. The tetrapod burrows are most similar to burrow systems of extant social mammals, such as prairie dogs and naked mole rats, and of therapsids from the Triassic of South Africa. The complex architecture suggests gregarious to possibly social behavior. Hypotheses of other biotic and abiotic origins (rhizoliths, dewatering pipes, and wind-eroded structures), are rejected based on the contrast between their known architectural and surficial morphologies with those of the Moab cylindrical casts. The Navajo Sandstone was at one time thought to be devoid of life, but these burrow casts and other trace fossils (rhizoliths, rhizohaloes, Naktodemasis, and Termitichnus) suggest that tetrapods were able to live in this giant sand sea during the Jurassic Period, possibly due to pluvial episodes.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009