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Rocky Mountain (63rd Annual) and Cordilleran (107th Annual) Joint Meeting (18–20 May 2011)

Paper No. 6
Presentation Time: 3:30 PM

DINOSAUR TRACKS CONFIRM LATE CRETACEOUS AGE FOR THE LOWER GRAND CASTLE FORMATION, IRON COUNTY, UTAH: STRATIGRAPHIC AND TECTONIC IMPLICATIONS


HUNT, Gary J.1, BIEK, Robert F.1, DEBLIEUX, Donald D.1, MADSEN, Scott K.1, MILNER, Andrew R.C.2, ROBERTS, Eric M.3 and EATON, Jeffery G.4, (1)Utah Geological Survey, PO Box 146100, Salt Lake City, UT 84114-6100, (2)St. George Dinosaur Discovery Sight at Johnson Farm, 2180 East Riverside Dr, St. George, UT 84790, (3)Earth and Environmental Sciences, James Cook University, Townsville, 4810, Australia, (4)Department of Geosciences, Weber State University, Department Of Geosciences, Ogden, UT 84408-2507, garyhunt@utah.gov

The Grand Castle Formation is divided into 3 informal members consisting of lower and upper alluvial/fluvial conglomerates and a middle fluvial, nearly pure quartz sandstone. Outcrops are confined to the High Plateaus of southern Utah, including the type area on the western Markagunt Plateau, and isolated locations west in the Basin and Range province of Iron County, Utah. The age of the Grand was regarded as Paleogene, although uppermost Santonian/basal Campanian Iron Springs Formation below constrains its maximum age..

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.

Session No. 9

Paleontology and Paleoclimatology
Wednesday, 18 May 2011: 1:30 PM-5:00 PM
Cottonwood (Riverwoods Conference Center)
Geological Society of America Abstracts with Programs. Vol. 43, No. 4, p.16
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