A SEQUENCE STRATIGRAPHY APPROACH TO UTILIZING DETRITAL ZIRCONS TO UNDERSTANDING THE OZARK PLATEAU

The Salem Plateau of south-central Missouri is underlain by Ordovician to Pennsylvanian sedimentary strata. The origins of remnant conglomerates outcropping on isolated highs on the Salem Plateau and of pervasive filled sinks associated with the contracted erosional history of the Salem Plateau are poorly understood. In this study, four samples from the Graydon Formation and the “Cave Rock” exposure off U.S. Highway 50 are analyzed for detrital zircon U-Pb ages for comparison with the age of a Salem Plateau paleokarst filled sink sample from previous study and the ages of several North American geologic provinces.

Three zircon age populations were defined from four samples. The populations are (1) grains from the Superior Province (>2500 Ma, with peaks near 2750 Ma), (2) grains from the Grenville Province (1300-900 Ma, with peaks near 1050 Ma), and (3) grains from the Appalachian Mountains (360-760 Ma, with peaks near 450 Ma). The data indicates two major source changes on the Salem Plateau: (1) Grenville rocks became a major sediment source in the Middle Ordovician, (2) the source shifted from Superior and Grenville regions to the Appalachians in the Carboniferous Period. The paleokarst filled sink has a high probability to be coeval with the “Cave Rock” exposure due to the close similarity of the normalized probability zircon U-Pb age curves of these two samples. This interpretation is quite different from the contemporary hypothesis that paleokarst filled sinks of the Salem Plateau formed in the Pennsylvanian Period. Based on the conglomerates from the Early Pennsylvanian Graydon Formation and the transgression in the Early Pennsylvanian in the Salem Plateau, the conglomerates are likely to record a younger erosional history of the Salem Plateau.

Additional sedimentological analyses to identify mineralogy, texture, and depositional environments are needed in combination with the U-Pb analyses to further constrain the erosional history of the Salem Plateau.