High resolution stratigraphic record constrained by volcanic ash beds at the Cenomanian–Turonian boundary in the Western Interior Basin, USA

Abstract Five altered volcanic-ash beds have been correlated in the Cenomanian–Turonian boundary interval through the US Western Interior Basin over tens of thousands of square kilometres. Using these independent chronostratigraphic marker-beds to constrain the other classic stratigraphic records such as limestone bed by bed correlations, peaks within the δ 13 C isotope excursion and a few other key biological events, a regional correlation scheme has been obtained between seven sections from Arizona to Kansas and northwards to South Dakota. On the basis of this high resolution scheme, we show that the last occurrences of the planktonic foraminiferid Rotalipora cushmani as well as the first occurrence of Helvetoglobotruncana helvetica are strongly diachronous, the Whiteinella archaeocretacea Partial Range Zone, in which the Cenomanian–Turonian boundary is located, being reduced to almost nothing, particularly at Pueblo (Colorado). At this type section, the δ 13 C isotope excursion presents strongly negative values in the limestone beds, suggesting that they are of diagenetic origin. In spite of this, the onset and most of the fine-scale features of the carbon-isotope excursion identified at Pueblo are recognised over long distances even in the more coastal sections where bed by bed correlations become more difficult. As suggested by our scheme, differences observed between supposedly continuous sections could be related to hiatuses in the sedimentary record. Such gaps could explain the disappearance of some fine-scale features of the δ 13 C isotope excursion. If this interpretation is correct, one hiatus may have been as long as 850 Kyrs.