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 Allbian-Cenomanian fluvial-estuarine succession (Dakota Formation) at Rose Creek Pit (RCP), Nebraska, reveals a negative excursion of similar to 3 parts per thousand in late Albian strata. Overlying Cenomanian strata have delta C-13 values of -24 parts per thousand to -23 parts per thousand that are similar to pre-excursion values. The absence of an intervening positive excursion (as exists in marine records of the Allbian-Cenomanian boundary) likely results from a depositional hiatus. The corresponding positive delta C-13 event and proposed depositional hiatus are concordant with a regionally identified sequence boundary in the Dakota Formation (D-2), as well as a major regressive phase throughout the globe at the Allbian-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-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 C-13 records may be useful in recognizing and constraining sea-level changes in the geologic record.