EVIDENCE FOR LONG DISTANCE EOLIAN TRANSPORT OF 1460 MA ZIRCONS IN THE BORDEN SILTSTONE, ILLINOIS BASIN, USA

The Kinderhookian Borden Formation is an extensive clastic body that occurs at the base of the Mississippian section in the Illinois Basin. It consists primarily of siltstone with lesser amounts of shale, sandstone and limestone. Elemental analysis (pXRF) of a new core from Morgan County, Indiana revealed zirconium peaks associated with sandy intervals. Detrital zircons (n=82; fine sand and coarse silt size) were separated from this interval and analyzed by LA-ICP-MS at the University of Arizona LaserChron Center. Calculated ages from U-Pb results range 470-2710 Ma and have a primary age peak of 1460 Ma (68% of grains), implicating the Midcontinent Granite-Rhyolite Province as the primary source area. Grenville-age grains comprise about 20% of the age spectrum. The Borden age spectrum is distinct from underlying Devonian and Ordovician sandstones in the Illinois Basin, which are comprised mostly of Archean and Grenville age grains. It is also distinct from the overlying Carboniferous sandstones, which are dominated by first cycle sediment derived from the Alleghenian orogeny. Previous interpretations characterize the Borden Formation as a distal Acadian submarine clastic wedge derived from the east. Our new data refute this interpretation. Based on grain size, grain texture, and U-Pb age, we interpret the Borden zircons in central Indiana as derived primarily from a northern rather than eastern source. It follows that during early Mississippian time, the ~1460 Ma Wolf River batholith was exposed, and that some zircons were transported several 100 km across the Illinois Basin via eolian processes.