Recognizing the Albian-Cenomanian (OAE1d) sequence boundary using plant carbon isotopes; Dakota Formation, Western Interior Basin, USA

Analysis of bulk sedimentary organic matter and charcoal from an Albian-Cenomanian fluvial-estuarine succession (Dakota Formation) at Rose Creek Pit (RCP), Nebraska, reveals a negative excursion of approximately 3 ppm in late Albian strata. Overlying Cenomanian strata have delta (super 13) C values of -24 ppm to -23 ppm that are similar to pre-excursion values. The absence of an intervening positive excursion (as exists in marine records of the Albian-Cenomanian boundary) likely results from a depositional hiatus. The corresponding positive delta (super 13) C event and proposed depositional hiatus are concordant with a regionally identified sequence boundary in the Dakota Formation (D (sub 2) ), as well as a major regressive phase throughout the globe at the Albian-Cenomanian boundary. Data from RCP confirm suggestions that some positive carbon-isotope excursions in the geologic record are coincident with regressive sea-level phases. We estimate using isotopic correlation that the D (sub 2) sequence boundary at RCP was on the order of 0.5 m.y. in duration. Therefore, interpretations of isotopic events and associated environmental phenomena, such as oceanic anoxic events, in the shallow-marine and terrestrial record may be influenced by stratigraphic incompleteness. Further investigation of terrestrial delta (super 13) C records may be useful in recognizing and constraining sea-level changes in the geologic record.