Within the Portland core, the Cenomanian–Turonian Boundary Interval (CTBI) was studied in a 17.7 m-thick section of the Bridge Creek Limestone (∼12 m) and Hartland Shale (∼12.6 m) Members of the Greenhorn Formation (Cobban and Scott, 1972). These units include organic-rich calcareous shales and rhythmically interbedded couplets of shale and fossiliferous biomicritic limestone. The stratigraphy is also characterized by four bentonite units of 1 to 20 cm that have been regionally correlated (Elder, 1988). Recent sanidine 40Ar/39Ar and zircon 206Pb/238U geochronology integrated with astrochronology constrain the CTB at 93.90±0.15 Ma (Meyers et al., 2012a). The CTBI contains a variety of fossil taxa useful for biostratigraphy (e.g., Gale et al., 1993; Kennedy et al., 2000, 2005; Keller and Pardo, 2004; Keller et al., 2004; Cobban et al., 2006) some of which have intercontinental distributions; however, their transcontinental synchronicity is limited. The dominant foraminifera species spanning the CTBI are Rotalipora cushmani, Whiteinella archaeocretacea and Helvetoglobotruncana helvetica (Eicher and Worstell, 1970). The FO (first occurrence) of the ammonite Watinoceras devonense (Fig. 2; Kennedy et al., 2000) marks the basal Turonian, recorded at the base of bed 86 of the Bridge Creek Limestone (Meyers et al., 2001; bed numbers are based on Cobban and Scott, 1972). The FO of W. devonense coincides with the FO of Mytiloides puebloensis (Kennedy et al., 2000), which can be traced through both Tethyan and Boreal regions (Kennedy et al., 2005).
section of the Bridge Creek Limestone (∼12 m) and Hartland Shale (∼12.6 m)
thick
17.7 m
m
Bridge Creek Limestone
∼12 m
m
Hartland Shale
∼12.6 m
m
bentonite units
four
bentonite units
1 to 20 cm
cm
CTB
93.90±0.15 Ma
Ma