U/PB DETRITAL ZIRCON GEOCHRONOLOGY AND PROVENANCE OF THE MIDDLE-ORDOVICIAN ST. PETER SANDSTONE AND STARVED ROCK FORMATION, IOWA

Detrital zircon ages from 13 samples of the Middle-Ordovician St. Peter Sandstone and Starved Rock Formation of the Iowa Basin provide new constraints on the provenance of Paleozoic sedimentary successions deposited on the southeastern flank of the Transcontinental Arch of Laurentia. The new analyses define major U/Pb age populations at 1050-1200 Ma, 1300-1500 Ma, and 2600-2800 Ma and subordinate age clusters scattered from 1600-2000 Ma and 2800-3300 Ma. Features of individual spectra include a broad 1000-1500 peak, a large peak at 1050-1100 Ma, and/or a prominent peak centered on 2700 Ma. The detrital zircon signatures record either first cycle derivation dominantly from the 1000-1300 Ma Grenville orogen, 1050-1120 Ma Midcontinent Rift, and Archean Superior Province with minor input from the 1300-1500 Ma Granite-Rhyolite Province or recycling from Paleoproterozoic and Mesoproterozoic basins (e.g., Huron, Mid-Continent Rift). The paucity of 1600-1750 Ma grains in all samples makes a first cycle origin from the underlying Yavapai and Mazatzal provinces unlikely. Similarly, the general lack of 1800-1900 Ma grains argues against derivation from Trans-Hudson or Penokean orogen basement north and south of the Transcontinental Arch, respectively, or through recycling of Mid-Continent Rift basin sediments. Konstantinou et al. (2014, J. Geol.) analysed detrital zircon from Cambrian and Ordovician rocks to the north and east and documented a general shift from spectra dominated by 2600-2800 Ma grains in Cambrian strata to spectra dominated by 1000-1300 Mid-Continent Rift and Grenville orogen ages in Ordovician rocks. The combined data sets demonstrate that the shift between dominant peaks at 1000-1300 and 2600-2800 Ma is repeated several times within the Ordovician section. Samples dominated by the 2600-2800 Ma signature occur directly above fine grained layers associated with parasequence boundaries, suggesting that the changes in provenance is largely controlled by the effects of long-term sea level fluctuation.