Triassic paleocaliche in red beds of Dolores Formation, southwestern Colorado

Early diagenesis of Upper Triassic lacustrine mudrocks and sandstones in southwestern Colorado includes pedogenic accumulation of calcium carbonate. Petrocalcic horizons, 0.4 to 1.5 m thick, are dominated by unlaminated micritic caliche concretions which range from pellets 1 mm in diameter to nodules 30 mm in diameter. Evidence for in-situ origin of these limestone concretions includes: irregular shape, floating texture, inverse grading, and truncation of back-filled burrows. Mature paleocaliche profiles are capped by calcium carbonate plugged horizons which contain rootlets 1 to 2 mm in diameter. These plugged horizons also contain paleofracture systems filled with reddish brown mudrock. Locally paleofracture morphology suggests plant-root control in its development. In very fine sandstones, micritic caliche nodules coalesce in upward coarsening, sinuous vertical stacks that suggest a rhizo-concretion origin. A maximum of nine petroclastic profiles were observed over a vertical interval of 56 m in one outcrop north of Durango, Colorado. The most mature profile observed exhibited the Stage III development of Gile and others. Caliche nodules and pellets occur throughout the Dolores Formation in southwestern Colorado either in pedogenic profiles or as transported clasts in fluvial, lacustrine deltaic distributary, or fan delta deposits. Literature descriptions suggest that caliche nodules are also present in the correlative Chinle Formation of New Mexico, Arizona, Utah, and Colorado. Based on the areal distribution of modern caliche, a semiarid climate is suggested for the Late Triassic in the Four Corners region of the western United States.