WHEN APPROACHES COLLIDE: RECONCILING SEQUENCE STRATIGRAPHY, CHEMOSTRATIGRAPHY AND BIOSTRATIGRAPHY IN THE CORRELATION OF THE KATIAN (UPPER ORDOVICIAN) REFERENCE SECTION, CENTRAL OKLAHOMA

The reference section of the Katian Stage (Upper Ordovician) is a roadcut through the Viola Formation at HW99 near Fittstown, Oklahoma. Recent work by Bergström and colleagues on condont biostratigraphy and carbon isotope stratigraphy results in the novel interpretation that this section is entirely older (M5 sequence of eastern Laurentia) than the well-known Viola successions of the south flank of the Arbuckle Mountains along I35 (M6 and younger sequences). Studies of the sequence stratigraphy and trilobite faunas of the Viola and the Kimmswick Subgroup in Missouri challenge this view. At the reference section, the Viola is composed mostly of shallow subtidal facies with diverse trilobite faunas. A record of the graptolite Diplacanthograptus spiniferus at 35 m above the base of the Viola indicates that the upper half of the section is no older than M6. This is in accord with the associated trilobites, which include several species recorded from the Kimmswick Subgroup (M6 and younger). Under this interpretation, the carbon isotope excursion reported from the upper half of this section as the GICE may be the same excursion identified as the KOPE near the base of the Viola in the I35 section. This interpretation also implies that the conodont Belodina confluens makes a relatively "late" entry into the succession at the reference section. The base of M6 at the reference section is marked by a TST of coarse, cross-bedded bioclastic grain- to rudstone facies and appears to correlate with the base of Kimmswick in the St. Louis area. Although the I35 section overlaps with the reference section, there remains a larger break beneath the Viola in the former, with cut-out of part of M4 and most, if not all, of M5. The differing interpretations of these sections underscore the need to consider all lines of evidence in determining the age and correlation of isotope excursions, using sequence stratigraphy and biostratigraphy, preferably with multiple faunal groups.