A pedotype approach to latest Cretaceous and earliest Tertiary paleosols in eastern Montana

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Abstract
A pedotype approach to the study of paleosols emphasizes individual profiles. It is an alternative approach to the study of geosols, which are laterally extensive suites of paleosols, or pedofacies, which are pedogenically distinctive sedimentary facies. This is the first pedotype study of paleosols across the Cretaceous-Tertiary boundary in eastern Montana. Pedotypes allow assessment of sedimentation and fossilization. The sequence of paleosols across the Cretaceous-Tertiary boundary at Bug Creek is one of high temporal resolution, because most available geological time can be accounted for by differing degree of development inferred for each pedotype. Paleosols at Bug Creek include pedotypes that preserve plant fossils well, but were unfavorable for preservation of fossil vertebrates. Pedotypes also allow reassessment of ecosystem change. Despite indications of catastrophe at the Cretaceous-Tertiary boundary, the array of Paleocene and Cretaceous pedotypes are not strikingly different. The Upper Cretaceous Hell Creek Formation includes pedotypes interpreted as gleyed Alfisols, Inceptisols, and Entisols probably formed under seasonally waterlogged forest and mean annual rainfall of the order of 900-1200 mm. Most paleosols of the lower Paleocene Tullock Formation were Histosols, but some can be interpreted as gleyed Inceptisols and Entisols probably formed under bald cypress swamps in a humid climate with >1200 mm mean annual rainfall. Broadly comparable pedotypes were present before and after the Cretaceous-Tertiary boundary, although Paleocene flood-plain forest lived in paleosols chemically a little more oligotrophic than Cretaceous paleosols. This modest difference supports the idea that change to coaly facies in the earliest Paleocene was a local shift in sedimentary environment. Such local changes do not begin to account for profound disruption in specific composition of plant and animal communities at the Cretaceous-Tertiary boundary in eastern Montana.