DETRITAL ZIRCONS FROM THE LATE CRETACEOUS MIDDLE MEMBER GRAND CASTLE FORMATION, SOUTH-CENTRAL UTAH: RECYCLED MESOZOIC AEOLIANITES

Three samples of the Grand Castle Formation were examined by U-Pb LA-ICP-MS at the University of Arizona to determine their provenance. The Grand Castle contains upper and lower cliff-forming conglomerate members and a middle member of quartzose sandstone. Previous workers assigned a Paleocene age to the formation, but the discovery in 2010 of a theropod track and Santonian to Campanian pollen in the middle member constrain the lower two members to the Late Cretaceous; the age of the upper conglomerate member is presently unresolved. The Grand Castle represents braided fluvial deposition with paleoflow to the southeast, with source areas in the Wah Wah, Blue Mountain and Iron Springs thrust sheets of the central Utah thrust belt.

In a sample from the Paunsaugunt Plateau, about 20% of the grains are Paleozoic age (362 to 484 Ma), 15% are Cambrian and Ediacaran (520 to 660 Ma), 50% of the grains are broadly Grenvillean (883 to 1412), and 10% are Paleoproterozoic (1610 to 1893 Ma). The youngest grain is 96 Ma.

In a sample from Webster Flat on the Markagunt Plateau 15% of the grains are Paleozoic (356 to 457 Ma), 15% are Cambrian and Neoproterozoic (532 to 637 Ma), 50% are Grenvillean (900 to 1200 Ma). Paleoproterozoic and Archean grains make up <5% populations. The youngest grain is 163 Ma.

In a sample from southwest of Parowan, Utah, also on the Markagunt Plateau, 24% of the zircons are Paleozoic (371-494 Ma), 15% are Cambrian and Neoproterozoic (509 to 674 Ma), 49% are Grenvillean (1117 to 1235 Ma), and 15% are Paleoproterozoic (1619 to 1789 Ma). The youngest grain is 100 Ma.

The three detrital zircon populations are very similar suggesting the middle member contains sediments of one braided stream system. Of 248 total grains Paleozoic and Neoproterozoic zircons (27% of total) were likely derived ultimately from the Appalachians and recycled through Mesozoic aeolianite sandstones. About 60% of the grains are broadly Grenvillean age. There are small populations (<5%) of Mesoproterozoic, Paleoproterozoic, and Archean grains.

As these quartz arenites were derived from recycled Mesozoic sandstones (as suggested by Goldstrand and Mullett, 1997), rather than from the magmatic arc to the southwest, Cretaceous volcanic grains are very sparse.