Turonian-Coniacian boundary; definition, recognition and stratotype problems

Category Paleontology and Stratigraphy
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Walaszczyk, Ireneusz۱; Wood, Christopher J.۲
Holding Date 29 September 2008

Since the initial modern work on the Turonian/Coniacian boundary by Wood et al. (1984), the recognition of the bioevents and understanding of inoceramid evolution (the critical biostratigraphical group for the interval) has improved considerably (see Kauffman et al. 1996, Walaszczyk, Wood 1999, Walaszczyk, Cobban 2000, Wood et al. 2004). The boundary definition, as accepted in Brussels 1995 (Kauffman et al. 1996), i.e. at the first appearance (FAD) of C. deformis erectus (Meek) [C. rotundatus (sensu Trِger non Fiege) at that time] appeared then to be the best choice available. It later became clear (Walaszczyk, Wood 1999) that the appearance of this species marked a well recorded cladogenetic speciation event.
The FAD of C. deformis erectus is clearly recognizable in the whole Euramerican biogeographic region, and in the Tethyan Realm; and it appears to be easily correlatable outside these areas. This Euramerican biomarker is therefore either valid on its own for recognizing the base of the Coniacian Stage directly, or it enables indirect correlation with most of the coeval marine successions elsewhere in the world.
The inoceramid-based lower Coniacian boundary slightly post-dates the traditional ammonite (FAD of Forresteria petrocoriensis) position of the boundary (see Walaszczyk, Cobban 2000, Kennedy, Walaszczyk 2004).
Of the two stratotype candidates presented in Brussels 1995 (Kauffman et al. 1996), neither could have been formally proposed. The Wagon Mound section (US Western Interior) subsequently appeared to be entirely Turonian (Walaszczyk, Cobban 1999), while the Salzgitter-Salder section (northern Germany), was shown to contain a gap just at the accepted boundary level (Wood et al. 2004). The two other sections listed by Kauffman et al. (1996), i.e. the Slupia Nadbrze؟na (central Poland) and the Pueblo sections (US Western Interior) are also unsuitable. The former, with perhaps the most complete succession across the boundary known to date, is unfortunately too poorly exposed to be considered. The latter, well exposed with a very good inoceramid and ammonite record, appears to be incomplete a small distance above the boundary, as suggested by calibration against recent as yet unpublished studies of the Seebe Dam section, in Alberta, Canada (investigations led by Guy Plint, Ontario).
The latest stratotype proposal, suggested recently by Dee Ann Cooper and Roger Cooper, Texas, is the Hot Springs Trail section, Big Bend National Park, Texas. This fossiliferous section still needs further investigation, but the macropalaeontological documention made so far (Bill Cobban, Denver) proves an expanded and complete upper Upper Turonian and Lower Coniacian succession, although its Coniacian part still needs bed-by-bed studies. If supported by further results, the section offers almost unlimited availability, inoceramid/ammonite documentation, and preservation of the site guaranteed by its location within the national park area.