A δ13C STRATIGRAPHIC PROFILE FROM THE ROUBIDOUX AND JEFFERSON CITY FORMATIONS, CENTRAL MISSOURI

High-resolution chronostratigraphic correlation of the classic Ibexian Series in western North America to age-equivalent successions in the Midcontinent region is limited by the relative paucity of diagnostic fossils in many sections. Correlation of the Stairsian Stage (the second stage of the Ibexian) away from the type region is further complicated by a lack of detailed faunal zonation; there are only two (or three) generally recognized Stairsian conodont assemblage zones, as opposed to as many as nine conodont assemblage zones in the underlying Skullrockian Stage.

Previous carbon isotopic ratio (d¹³C) results from Stairsian-aged sections in New Mexico and western Utah indicate a major shift in the behavior of the marine carbon cycle that spans virtually all of the Stairsian, ending in a sharp drop to d¹³C values near -4‰ (V-PDB). In both regions, minimum d¹³Cvalues were found in conjunction with significant lithologic change that may represent a rapid deepening, followed by a return to shallower conditions. This conjunction of a Stairsian-aged d¹³Cminimum and sea-level change has also been recognized in the Argentina Precordillera region.

Although general Lower Paleozoic chronostratigraphic relationships are well-known in Missouri, detailed correlations to deeper water sections have proven to be elusive. Based on conodonts, the Stairsian Stage is known to span from the upper Gasconade Formation, through the heterolithic Roubidoux Formation, and end within the lower Jefferson City Formation. High resolution d¹³C profiles from two sections of the Roubidoux and Jefferson City formations in Missouri were developed, and correlated to previous d¹³Cresults from New Mexico and Utah. Major features of the d¹³Cprofile can be correlated with confidence, but there are important differences that remain the focus of ongoing study.