Paper No. 221-2
Presentation Time: 9:15 AM
NEW INTERPRETATION FOR VERY LARGE DIAMETER BURROWS IN THE PETRIFIED FOREST MEMBER, UPPER TRIASSIC CHINLE FORMATION, NEAR CANYONLANDS NATIONAL PARK, SOUTHEASTERN UTAH: THERAPSID BURROWS?
Large-diameter (~3-14 cm) and very large-diameter (~14-40 cm) burrows from the Upper Triassic Chinle Formation outside of Canyonlands National Park and around the Moab area had previously been interpreted as crayfish and lungfish in origin, respectively. The crayfish burrows have been referred to the ichnogenus Camborygma, which is found in multiple localities in the Chinle Formation throughout the Colorado Plateau, as well as in other Mesozoic and Cenozoic deposits in North America. The lungfish burrows, however, required further study and greater numbers of specimens to ascertain more accurately their overall morphology and confirm their preliminary identification. Recent fieldwork in Indian Creek and Stevens canyons and Harts Point, all southeast of Canyonlands National Park, has produced a great abundance (1000+) of very large-diameter burrows at multiple localities within the Petrified Forest Member (PFM). The architectural morphology of these burrows is complex: Burrow diameters are circular to subcircular in shape. Shafts are subvertical to helical, and can extend to over 1 m long. Tunnels are subhorizontal and often tortuous, originating from shafts and other tunnels and form T and Y branches. Shafts and tunnels sometimes terminate in a chamber 2 to 3 times the diameter of the tunnel or shaft. Chambers also are found between tunnels. Burrow systems measure 1-3+ m in depth and 2-4+ m in area covered. Surficial morphology, when preserved comprises longitudinal ridges subparallel to the burrow walls, or are knobby in texture; most walls are overprinted by rhizoliths. Burrows are in floodplain deposits overprinted by pedogenic carbonate, blocky-subangular blocky structure, and rhizoliths. Preliminary assessment of burrow morphologies and range of diameters indicate that lungfish did not construct these burrows, and that they were likely constructed by a moderate-sized tetrapod. Therapsids are the most probable candidate for the construction of these burrows based on comparisons to burrows with similar morphologies from Permian, Triassic, and Jurassic deposits, as well as the fossil vertebrates for those time periods. Interestingly, these PFM burrows are similar morphologically to those from pedogenically modified carbonate deposits of the Owl Rock Member in the Navajo Nation, Utah.