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The Cenomanian-Turonian boundary sequence at Pueblo, Colorado, is represented by the lower part of the Bridge Creek Member of the Greenhorn Limestone. Upper Cenomanian ammonite zones are, from oldest to youngest, those of Sciponoceras gracile and Neocardioceras juddii, while the position of the base of a succeeding Cenomanian Nigericeras scotti zone is inferred from correlation with sections in southeast Colorado. The base of the Turonian in ammonite terms is drawn at the base of the Watinoceras devonense zone, a new biostratigraphic unit for the Western Interior sequence; this is succeeded by zones of Pseudaspidoceras flexuosum, Vascoceras (Greenhornoceras) birchbyi and Mammites nodosoides; the base of the succeeding Collignoniceras woollgari zone marks the base of the middle Turonian. Four inoceramid zones are recognized; the Inoceramus pictus zone spans the S. gracile to N. scotti zones; the base of the Mytiloides aff. sackensis zone is just below the base of the W. devonense zone; the base of the Mytiloides columbianus zone correponds to the base of the Pseudaspidoceras flexuosum zone; the base of the Mytiloides mytiloides zone lies some way above the base of the Mammites nodosoides zone. Nine horizons suggested as possible boundary markers at the 1984 Copenhagen Symposium on Cretaceous Stage Boundaries can be recognized within this sequence which, together with the presence of bentonites that are amenable to absolute dating, makes the Pueblo sequence a strong candidate for consideration as a boundary stratotype of the Turonian Stage. The Pueblo succession is correlated with that in southern England, where ten horizons recognized in Colorado can also be recognized in the interval from the base of the M etoicoceras geslinianum zone to the base of the Collignoniceras woollgari zone; these are in the same relative position in each region.

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