IMPACT OF THE PROPOSED TURONIAN-CONIACIAN BOUNDARY GSSP AND A U.S. ALTERNATIVE
The historical Turonian-Coniacian boundary in North America is a good case in point. Traditionally the boundary is placed at or near a major lithologic break (base of Austin or Niobrara), which is easily definable on outcrops, as well as in the subsurface on geophysical logs by most geoscientists. Recently, a GSSP site for the Turonian-Coniacian boundary has been proposed in Germany. When translated into South Texas, this macrofaunal break occurs tens of feet above the base of the Austin at a lithostratigraphically non-distinct position in a sequence younger than, and genetically unrelated, to the classic hiatal break. Although subtle microfaunal and microflora proxies can be defined for the proposed boundary, presently no clear geochemical or petrophysical proxies are observed, which limits its utility to the larger geologic community.
In South Texas, the Langtry Member of the Upper Eagle Formation locally occurs as an erosional remnant beneath the base of the Austin Chalk. The maximum flooding surface (mfs) within the Langtry is one of the stronger magnitude transgressions of the lower part of the Upper Cretaceous and also coincides with the end of organic-rich, anoxic deposition within the Upper Cretaceous of South Texas. The marker bed has an easily identifiable nannofossil extinction event (Eprolithus moratus), and is further recognized by a multi-species nannoplankton turnover event that occurs in the overlying highstand systems tract. Two foraminiferal events also occur in or near this mfs adding a further proxies to identify the marker bed. The mfs with the Langtry is an ideal marker bed for geoscientists using outcrop, subsurface well logs, and seismic, and as such is a better candidate for a new Turonian-Conacian stage boundary in Texas, if the boundary is to be moved at all.