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

Abstract

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.