Micromorphological and SEM analysis of cretaceous-paleogene petrosols from Eastern Montana and western North Dakota

Microscopy is used to distinguish pedogenic features from those associated with primary sedimentary fabric and non-pedogenic diagenesis in a suite of 65 million year old sediments. Sand-to-clay-sized material was deposited in meandering river systems over a broad coastal alluvial plain, and pedogenesis occurred in all lithologies. A variety of petrosol (lithified soil) horizons which exhibit both syndepositional bedding and pedogenic and non-pedogenic diagenetic features remains today. Features attributable to the in-situ interaction between root and soil provide the most convincing evidence of pedogenesis. In addition, features such as illuvial clay and iron-oxide accumulations around voids, which are commonly but not exclusively associated with soil environments provide accessory evidence of pedogenesis. Stress-related void and ground-mass birefringence fabrics are evident in the petrosols and some purely sedimentary deposits, and thus cannot be used as diagnostic indicators of pedogenesis. In this fluvial setting, erosion and transport of soil material occurred, which is indicated by the presence of pedorelicts in petrosol horizons and purely sedimentary deposits. Detailed study both by petrographic and by scanning electron microscope elucidates pedogenic indicators not visible in hand samples. Such tools are useful in resolving ambiguities between pedogenesis and non-pedogenic diagenesis.