δ13C CONSTRAINTS ON THE MIDDLE LOWER ORDOVICIAN OF CENTRAL MISSOURI

The correlation of the classic Ibexian Series of the Ordovician System from western North America to age-equivalent successions in the Midcontinent region is hampered by the paucity of diagnostic calcareous macrofossils in many Midcontinent sections. This has complicated high-resolution chronostratigraphic correlation of the Stairsian Stage, the second stage of the Ibexian, to lower Ordovician units in Missouri. The difficulty is further compounded by recognition of only two or three long-ranging conodont assemblage zones in the Stairsian, compared to the nine conodont assemblage zones in underlying Skullrockian Stage.

Previous carbon isotopic ratio (δ¹³C) results of Stairsian sections in New Mexico and Utah indicate a shift in marine carbon cycle behavior. The shift begins with a sharp positive excursion at the base of the Stairsian, followed by an overall general negative trend through the entirety of the Stairsian Stage. This negative trend ends with a sharp drop in δ¹³C values, to -2.5‰ V-PDB in Utah and -4.0‰ V-PDB in New Mexico, that has previously been referred to as the ‘Jose Event’. In both regions the minimum δ¹³C values were found in conjunction with significant lithologic change that may represent a rapid deepening followed by a return to shallower conditions.

Although general Lower Paleozoic chronostratigraphic relationships are well-known in Missouri, detailed correlations to western sections have proven to be elusive. Based on conodonts, the Stairsian Stage is known to span the heterolithic Roubidoux Formation and to end within the basal units of the Jefferson City Formation. A high resolution δ¹³C profile was determined from a section near Westphalia, in central Missouri, that spans this lithologic boundary. The Westphalia profile was compared to the profiles from New Mexico and Utah and major features of the δ¹³C profile can be correlated with confidence, including the negative trend and a sharp drop to δ ¹³C values near -6.0‰ VPDB. This correspondence provides strong support for detailed lithostratigraphic correlations between the Midcontinent region, the El Paso Group in western Texas, and the Fillmore Formation in western Utah. Additional work is ongoing to extend these correlations through younger Lower Ordovician successions in central and eastern Missouri.