Paleosols formed from weathering of alluvial mudstones in the Late Cretaceous (Maastrichtian) Prince Creek Formation, North Slope Alaska, are dominated by detrital smectite, discrete illite, kaolinite, chlorite, quartz, and pedogenic illite–smectite (I/S) mixed-layer clays. In the fine clay fraction (< 0.2 µm) illite–smectite mixed-layer clay is the main clay mineral and is interpreted as pedogenic in origin, whereby the I/S is a product of illitization of inherited smectite during weathering and pedogenesis. We consider the detrital clay minerals to be derived from pre-existing sediments eroded from the Brooks Range, mixed with reworked volcanic ash-fall-derived bentonites. In the Prince Creek Formation, smectitic parent materials were deposited by epiclastic volcanic ash-rich alluvium that accumulated on imperfectly drained floodplains. Diagenetic transformation of smectite to illite is unlikely in the Prince Creek Formation, in as much as maximum burial temperatures never exceeded ∼ 48° C. The predominance of bentonite-derived smectite (> 80%), low bulk density, phosphorus accumulation, Fe and Al mass-balance trends, and the presence of Fe–Al–humus complexes in one paleosol profile is interpreted as evidence of andic soil properties, and these paleosols are interpreted, therefore, as Andept-like alluvial soils. These results demonstrate that clay mineralogical studies, in conjunction with geochemical data of paleosols, can be used to identify paleo-andic soil properties which have not been widely recognized in the ancient rock record. Alternating wetting and drying conditions, required to form pedogenic I/S in these alluvial paleosols, resulted from a highly seasonal moisture regime in the Late Cretaceous Arctic.

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