|Fowler, 2009: NAPC abstract|
FOWLER, D.W. (2009), A sequence stratigraphic subdivision of the Hell Creek Formation: beginnings of a high-resolution regional chronostratigraphic framework for the terminal Cretaceous, North American Paleontological Convention, abstracts volume: 120
The late Maastrichtian fluvial Hell Creek ( Montana , North Dakota , South Dakota ) and Lance Fms ( Wyoming ) are notable for their vertebrate fossils and for the KT mass extinction at or near their upper boundaries. Despite the units' similar age, past efforts at accurate correlation have met with very limited success, mainly due to problems with lithostratigraphic methodology, discontinuous outcrop, lateral discontinuity of facies, poorly constrained biostratigraphy, lack of radiometric dates, and magnetostratigraphic data of limited use. This has hindered our ability to study this important interval in earth's history.
By use of terrestrial sequence stratigraphic methods, this study has subdivided the Montanan Hell Creek Fm into three, possibly four sequences. These reflect 4 th order base-level cycles superimposed on the 3 rd order base-level rise under which the formation was deposited. These previously unrecognized sequence boundaries are defined by three, possibly four, laterally continuous disconformities within the Hell Creek Fm of Fort Peck, Montana. The disconformities are overlain by amalgamated channel complexes, or less commonly, correlative interfluve paleosols. Disconformities were formed by pauses in the creation of accommodation space, associated with base-level stabilization or fall during 4 th order cyclicity.
Amalgamated channel deposits at the base of the new sequences occur in the same stratigraphic positions within the Montanan Hell Creek Fm as brackish units observed in the North Dakotan Hell Creek Fm, which are similarly indicative of 4 th order base-level cyclicity. Magnetostratigraphy and new biostratigraphic data support correlation of the upper Montanan sequence with the North Dakotan Cantapeta tongue (and overlying fines) and Canadian Scollard and Frenchman Fms. Recognition of 4 th order cycles in subsurface data is being used to map a regional high-resolution chronostratigraphic framework for the terminal Cretaceous. This is already being used to study evolutionary trends in vertebrates, leading up to the Cretaceous-Tertiary mass extinction.