Abstract

Orange calcareous nodules are widespread in Lower Tertiary alluvial paleosols in the Bighorn Basin, Wyoming. The nodules are abundant at specific stratigraphic levels where they form distinctive and laterally extensive bands. Many are preserved as erect, crudely shaped cylinders a few centimeters to more than 1 m high and generally 10-30 cm in diameter. At two stratigraphic levels in the sequence, hands of cylindrical nodules occur in close association with upright and well-preserved fossil tree stumps. The stumps, which show a distinctive tree-ring growth pattern, are orange in color and permineralized with calcite. The physical proximity and morphologic similarities between the calcified tree stumps and calcareous nodules suggest that the nodules are simply additional, albeit more poorly preserved, tree stumps. Their widespread occurrence in specific horizons suggests that the nodules are the remains of Tertiary forests that grew on stable land surfaces. Although fossil forests can be readily recognized by the presence of upright permineralized trees, such occurrences are rare. The nodular horizons described here provide a new means of determining the location and stratigraphic position of ancient forests.

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