DINOSAUR TRACKS CONFIRM LATE CRETACEOUS AGE FOR THE LOWER GRAND CASTLE FORMATION, IRON COUNTY, UTAH: STRATIGRAPHIC AND TECTONIC IMPLICATIONS
The occurrence of a theropod dinosaur track from the middle white sandstone now confirms at least the two lower members of the Grand Castle are Upper Cretaceous. Recent geological mapping suggests that the two lower members of the Grand Castle may be equivalent to upper Straight Cliffs Drip Tank Member (uppermost Santonian/llower Campanian) and/or Wahweap(?) Formations in the Markagunt and possibly the Paunsaugunt Plateaus.
We suggest that the low Grand Castle was deposited in a proximal Sevier flexural basin followed by deposition of the middle member as a result of late isostatic adjustment in the thrust belt, which ultimately resulted in the progradation of pure quartzose sandstone into the distal reaches of the foreland basin (capping sandstone member of Wahweap?)
In Parowan Gap west of the Markagunt Plateau, the upper Grand Castle conglomerate unconformably overlies the Iron Springs Formation, commonly at high angle. This relationship is consistent with deposition on a wedge-top depozone and may record the onset in this region of Laramide orogenesis.
A further result of this study is that the unconformity below the Grand Castle is of much shorter duration than previously suspected. We are uncertain of the age and correlation of the upper Grand Castle conglomerate; it may correlate with the Canaan Peak Formation (Paleocene) of the High Plateaus of southern Utah.