Paleoenvironments of vertebrate-bearing strata during the Cretaceous-Paleogene transition, eastern Montana and western North Dakota ( USA).

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Exposures of the Hell Creek and Tullock formations in eastern Montana and the Ludlow Formation in western North Dakota allow detailed reconstruction of the paleoenvironments associated with the Cretaceous-Paleogene (K-P) faunal transition in the Western Interior of North America. Facies associations demonstrate an ancient meandering fluvial environment in which gleization in unstable, poorly drained flood plains modified incipient soils. Sedimentologic and pedologic features indicate that concurrent with the faunal transition that occurred in the region, the amount of standing water increased dramatically, changing the earliest Paleogene soils and landscape. Depositional environment imposes taphonomic constraints on interpretations of K-P faunas and floras. Contrary to recent reports, fossil assemblages in Hell Creek channel deposits are reworked; bone and sediment clasts of the channel fills have been subject to traction transport. Age estimations based on supposedly unreworked fossils in channel deposits are thus unreliable. The chronostratigraphic resolution of the sediments under study is to date simply not comparable to the resolution required by researchers of the K-P boundary. - from Author

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