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Nodular calcareous paleosols are common in the upper member of the Upper Triassic Dolores Formation in the San Juan Mountains of southwestern Colorado. These soils are developed in reddish brown, very fine-grained sandstone and siltstone of a sand-sheet facies that was deposited by eolian and aqueous processes on the margins of a large Triassic erg. Characteristics of these paleosols include nearly complete destruction of physical sedimentary structures, extensive mottling associated with burrows and root trace fossils, poorly sorted textures, and abundant carbonate nodules.

Vegetative stabilization of the sand sheet is recorded by trace fossils of long, monopodial root systems, and fine networks of rootlets. Distinctive purple pigmentation of the large root mottles appears to have been produced by more coarsely crystalline hematite, which precipitated in the presence of root-derived organic compounds.

Faunal bioturbation in these soils takes the form of meniscate and structureless burrows of the Scoyenia ichnofacies. The meniscate burrows are common in recent soils and pre-Holocene paleosols, and probably represent sediment reworking by arthropods.

Carbonate nodules in these soils are composed of micrite and microspar, and they contain sparry calcite crystallaria and septaria. These glaebules occur as individual “floating” entities and as stacked columns. Burrows cross-cut some nodules, indicating that at least some of the pedogenic carbonate accumulations were relatively unlithified at the time of deposition.

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