Paleocene Paleosols of the Petrified Forests of Theodore Roosevelt National Park, North Dakota: A Natural Experiment in Compound Pedogenesis

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The Petrified Forest Plateau of Theodore Roosevelt National Park (TRNP) in western North Dakota provides an excellent setting for exploring the influence of ancient, exhumed land surfaces upon modern ones. Here, a sequence of Paleocene rocks preserves a succession of ancient terrestrial land surfaces characterized by fine-grained, laminated sedimentary rocks and organic-rich horizons. These exposures are dominantly finer-grained than the rest of the sedimentary rocks in the Park, and represent a region inferred to have been distal to the bulk of an aggradational fluvial system. Water was abundant in the Plateau region, and during much of the time the landscapes were submerged. At least twice, large forests developed in soils forming on floodplain sediments. These paleosols are characterized by organic surficial horizons and gleyed subsoils. Coniferous stumps from two successive forests constitute the Petrified Forest preserved in TRNP today. The modern environments of the Petrified Forest Plateau contrast sharply with those of the ancient; they are characterized by intense seasonality, semi-aridity, steep topography, a degradational land surface, and low organic productivity. Despite this, the modern soils show an imprint deriving from ancient environmental conditions that is as strong as any imprint from Holocene pedogenic processes; the soils that are presentlymore » forming retain much of the waterlogged features of the Paleocene soils. While it is clear that soils do respond to environments, it is also clear that the response may vary, depending upon the nature of the soil material and morphologies inherited. Here, the development of a gleyed morphology has proven largely irreversible.« less