U-PB DETRITAL ZIRCON GEOCHRONOLOGY OF PALEOZOIC CLASTICS OF THE OZARK PLATEAU: IMPLICATION FOR SEA-LEVEL FLUCTUATIONS AND FAR-FIELD TECTONICS

The Salem Plateau of south-central Missouri is underlain by Ordovician to Pennsylvanian carbonate and clastic strata. The pervasive filled sink structures associated with the contracted erosional history of the Salem Plateau is poorly understood. In this study, two samples from the Pennsylvanian Warner Formation and two samples from “Cave Hill”, a road cut along Hwy-50, are analyzed for detrital zircon U-Pb ages. These U-Pb ages and U-Pb ages from several similar filled sink structures in the vicinity are compared with the ages of several North American geologic provinces.

Kernel Density Estimation (KDE) is used to plot the normalized detrital zircon ages. Three major zircon age populations were defined from four samples. The populations are (1) grains from the Superior Province (>2500 Ma, with peaks near 2700 Ma), (2) grains from the Grenville Province (1350-900 Ma, with peaks near 1050 Ma), and (3) grains from the Appalachian Mountains (250-760 Ma, with peaks near 450 Ma). The data indicates two major source changes on the Salem Plateau: (1) Provenance shifted from local St. Francois Mountain/Granite-Rhyolite province to Superior province with variable contributions from Grenville province in the Late Cambrian-Early Ordovician and remain consistent through Devonian. (2) In the Mississippian through the Pennsylvanian the source shifted from Superior province to the central/southern Appalachians.

The similarity of the KDE of detrital zircon U-Pb age plots, combined with Multi-Dimensional Scaling (MDS) analyses, of samples from “Pennsylvanian filled sink structures” and Ordovician Sandstones indicates that these filled sink structures have a higher probability of being coeval with Ordovician clastic sediments. This is consistent with field relationships where the “Cave Hill” sandstone unconformably overlies the Jefferson City Formation suggesting it is correlative to the Ordovician Everton Formation. We suggest that many of these “sink structures” may have formed during the Sub-Tippecanoe regression and some (e.g., Cave Hill) as incised valleys, and were “filled” with clastics during the Tippecanoe transgression.