A POSSIBLE AGE CONSTRAINT ON UNDIFFERENTIATED LOWER ORDOVICIAN CARBONATES NEAR CEDAR HILL, MISSOURI

Lower Ordovician carbonates near Cedar Hill in northeastern Jefferson County, Missouri, have been previously mapped as undifferentiated Jefferson City Formation. We proposed to use recently determined carbon isotope profiles from the Roubidoux and Jefferson City formations near Westphalia, Missouri, and from the Cotter Dolomite near Branson, Missouri, to see if additional age constraints could be placed on the outcrop near Cedar Hill.

The Cedar Hill roadcuts expose a succession of shallow-water carbonates that are punctuated by at least two, and possibly several episodes of exposure, and a prominent 20-30cm sandstone. The upper part of the succession contains a karst that appears to be associated with small stromatolite mounds, overlain by a fine-grained, well-bedded ledge-forming dolomite. Several thin green shale units are also found within the lower section.

δ¹³C results from the section were generally negative, with a strongly negative excursion near -6‰ V-PDB in the lower part of the section. A similar excursion was previously near the base of the Jefferson City Formation, and within the Cotter Formation near Branson. Although additional δ¹³C variation has been found higher in the section, we have not yet found an unambiguous correlation to previous δ¹³C profiles from Missouri or elsewhere. At present, we are planning to extend our profile through a younger section to the east, to determine whether a more extensive δ¹³C profile will help resolve this ambiguity.