FIG. 25 in Allostratigraphy And Biostratigraphy Of The Upper Cretaceous (Coniacian-Santonian) Western Canada Foreland Basin

FIG. 25. Comparison of the organic carbon isotope stratigraphy at Cutpick Creek with the carbonate carbon isotope reference curve for the English Chalk succession (after Jarvis et al., 2006, recalibrated to GTS2012 after Laurin et al., 2015). The Cutpick section, and corresponding carbon-isotope curve, has been "expanded" to show the location of hiatuses determined on the basis of regional subsurface correlation (i.e., fig. 4). Biostratigraphic collections made in all the Coniacian sections included in this study (fig. 24), allow three tiepoints to the UK Chalk to be established at the lowest occurrence (LO) of Cremnoceramus crassus crassus, and the lowest occurrence of Volviceramus. The base of the Santonian is defined at the co-appearance level of Clioscaphites saxitonianus and Sphenoceramus ex gr. pachti, which corresponds to surface SS0 of this study. The lowest occurrence of C . crassus, Volviceramus and Scaphites (S .) depressus allows tentative correlation to the Portland core, which does not extend as high as the Coniacian–Santonian boundary (Joo and Sageman, 2014). The highest local occurrence of Inoceramus gibbosus, which is widely distributed in allomembers CA3 and CA4 in the western Alberta foredeep, is marked by the lag-strewn erosion surface CS4, suggestive of significant sea-level fall and subsequent transgression. The absence of the I . ex gr. gibbosus interval in most sections of topmost lower Coniacian strata in other parts of the world suggests that a significant hiatus exists at the base of the overlying Volviceramus Zone.