The distribution of theropod-dominated ichnofaunas in the Moab Megatracksite area, Utah: implications for Late Jurassic palaeobiology along an arid coast

ABSTRACT

The Late Jurassic, ‘single surface’ Moab Megatracksite between Green River, Utah and the Colorado-Utah State line is associated with the sharp, colour-enhanced, inter-formational contact between white, reworked dune sandstones from the aeolian Moab Member of the Curtis Formation and red, sabkha facies siltstones and sandstones of the upper tongue of the Summerville Formation. Previous studies described theropod (Megalosauripus and Therangospodus) dominated assemblages from ~25 sites in the megatracksite core area around Arches National Park, and briefly interpreted the transgressive mechanisms of the Sundance sea which helped create and preserve the megatracksite.

Since the 1990s ~20 newly studied tracksites reveal a cumulative total of ~4,300 tracks and three sites are now designated as, federally managed, interpretative tourist destinations. Similar track assemblages, collectively constituting Megalosauripus-Therangospodus ichnocoenoses have now also been found at lower stratigraphical levels associated with J-3 Unconformity contact zones. These new discoveries relate to stratigraphic studies associated with short-term changes in Sundance Sea level affecting coastal dune facies (Slick Rock and Moab members) across low gradient, inter- and sub-tidal sand flats, and sabkha-like deposits under arid conditions. The multiple (>50) theropod dominated ichnofaunas are integral to palaeobiological understanding of an area from which no body fossils are known.

KEYWORDS:

• Megatracksite
• Curtis Formation
• Megalosauripus
• Therangospodus

Acknowledgments

I particularly thank the late Fran and Terby Barnes for showing me sites and sharing locality information, also shared with the Bureau of Land Management (Moab Office). Although Barnes (1997, 2003) illustrated tracks from what he called the Moab-Entrada deposits, he did not give their locations.

The University of Colorado Dinosaur Trackers Research Group conducted a mapping project in 1987-1988 partially funded by the National Science Foundation. Other survey work was facilitated by the BLM and executed under permits UT-S-04-005 and UT-S09-006. Sue Sternberg, of Moab Utah has provided invaluable help by finding many dinosaur tracksites in the Moab area, including two described here (Figure 25 and 26). Josh Smith also provided information on the Cactus Park site. Neffra Matthews, formerly with the National Operations Center (BLM) generously provided the 3D image used in Figure 29E, and Anthony Romilio, University of Queensland, provided the images used in Figure 24C. Nora Noffke, Old Dominium University, Virginia, has recently worked with the author at some of the sites described here and has provided valuable insights into the factors that have influenced track preservation in the Moab Megatracksite area. I particularly thank Doug Sprinkel, Utah Geological Survey, and Valentin Zuchuat, Department of Geosciences, University of Oslo, for reading this manuscript prior to submission and offering very helpful suggestions. I also thank Don DeBlieux, Utah Geological Survey, and a second anonymous reviewer for their helpful comments.

Disclosure statement

No potential conflict of interest was reported by the author(s).