DETRITAL ZIRCON GEOCHRONOLOGY OF BASAL CAMBRIAN ARENITES IN KENTUCKY: REWORKED GREAT GRENVILLIAN SEDIMENTATION EPISODE SAND? OR NEW SAND GENERATED DURING CAMBRIAN TRANSGRESSION AT THE GREAT UNCONFORMITY?

Neoproterozoic rifting in eastern Laurentian was followed by Cambrian transgression that spread cratonward, completing formation of the Great Unconformity, which must have followed erosion of sediment produced in the Great Grenvillian Sedimentation Episode (GGSE). Did the Cambrian transgression rework Neoproterozoic Grenvillian sediment of the GGSE, or was new sediment generated from immediately underlying Precambrian basement during Cambrian weathering? Samples of the basal Cambrian between the Appalachians and midcontinent outcrops (Wisconsin dome; St. Francois Mtns.) are available from rare drill cores. Basement tectonic elements in KY include (1) the ~1.15 Ga East Continent Rift (ECR), the southeast arm of the failed Midcontinent Rift System; (2) the ~0.98 Ga Grenville Front Tectonic Zone (GFTZ) that separates Geon 14 Granite-Rhyolite crust on the west from Geon 13-10 Grenvillian orogenic crust on the east, and which overrode the ECR; and (3) the Early Cambrian Rome Trough (RT), a failed rift that cuts the ECR and GFTZ. Recent re-interpretation of the basal Cambrian assigns the deepest units of the RT to the Chilhowee Group, the drift phase of the post-rifting SE Laurentian margin. Detrital zircon (DZ) dates for the deepest RT samples define the Grenvillian Geon 10-11 age doublet consistent with a source on rift flanks comprised of Ottawan (Geon 10) and Shawinigan (Geon 11) crustal components. A ~550 Ma age mode corresponds to rift volcanics. In contrast, upper RT sands and lateral equivalents covering the ECR and GFTZ in central KY are dominated by an EGR (Geon 14)-Ottawan doublet. Notably absent are DZ representing Shawinigan and ECR magmatic intervals. The ages are interpreted to reflect local basement astride the GFTZ. Beneath the Illinois basin, the basal Cambrian is dominated by Southern Granite Rhyolite DZ (Geon 13), likely to be local basement. However, in the northern IL-WI region late Archean DZ (~2.6 Ga) dominates the Cambrian, interpreted to reflect Superior craton or recycled sources and not local basement. The general absence of a GGSE component in Cambrian cratonic sandstones implies the syn-Grenvillian sediment was stripped and likely transported to Neoproterozoic depocenters (e.g., the Ocoee Basin) around Laurentia during Rodinian breakup.