The base of the Turonian Stage is defined as the base of Bed 86 of the Bridge Creek Limestone Member at the western end of the Denver and Rio Grande Western Railroad cut near the north boundary of the Pueblo Reservoir State Recreation Area, west of Pueblo, Colorado, USA. The GSSP coincides with the first occurrence of the ammonite Watinoceras devonense, is in the middle of a global positive excursion in carbon 13 isotopes, and is bracketed by widespread bentonites that have yielded an age of 93 to 93.5 Ma.
The GSSP is located at the western end of the Denver and Rio Grande Western Railroad cut near the north boundary of the Pueblo Reservoir State Recreation Area, west of Pueblo, Colorado, where the Arkansas River cuts through the Rock Canyon anticline at a latitude of 38°16'56"N and 104°43'39"W (the latitude and longitude are from Google Earth, because the Lat/Long values in the published paper are rounded to the nearest degree.)
The Cenomanian - Turonian Boundary lies in the Bridge Creek Limestone Member of the Greenhorn Limestone Formation. The section consists of about 17m of hard, grey, shaly weathering limestones, interbedded with soft calcareous shale and bentonites. The GSSP is located at the base of Bed 86.
The GSSP coincides with the first appearance of the ammonite Watinoceras devonense.
Helvetoglobotruncana helvetica appears for the first time in Bed 89.
The last occurrence of Microstaurus chiastius is between Beds 85 and 88.
The first occurrence of Mytiloides hattini is in the topmost part of Bed 84. The first occurrence of Mytiloides puebloensis coincides with the GSSP in Bed 86.
The δ13Corg (PDB) curve shows that the base of Bed 86 falls immediately below the third positive peak in the curve.
Widespread bentonites from other sections which can be correlated to Pueblo have yielded an age of 93 to 93.5 Ma for the boundary.
Ammonite FAD Watinoceras devonense
Spline fit of Ar-Ar ages and ammonite zones
Kennedy, W. J., Walaszcyk, I., and Cobban, W. A., 2005. The Global Boundary Stratotype Section and Point for the base of the Turonian Stage of the Cretaceous: Pueblo, Colorado, U.S.A. Episodes 28/2, p. 93 - 104.