Pseudomorphs of calcite are present in lacustrine strata and travertine of the Oligocene Creede Formation, the sedimentary moat fill of the Creede caldera in the Tertiary San Juan volcanic field. The pseudomorphs, previously thought to have formed after gaylussite (CaCO 3 Na 2 CO 3 .5H 2 O), are interpreted to have formed after ikaite (CaCO 3 .6H 2 O) on the basis of their modes of occurrence, pseudomorph morphology, and textural and chemical aspects of the calcite replacement. The extent of their exposure and degree of preservation in the Creede Formation allow elucidation of the depositional conditions in which the ikaite formed and was subsequently pseudomorphically replaced. The revised paleoenvironmental interpretation of the Creede Formation also has implications for understanding of the late Oligocene climate of the southwestern United States. Formation of ikaite requires water temperatures of 0-3 degrees C, supersaturation with respect to calcite, and the presence of a chemical inhibitor, such as dissolved phosphate, to prevent growth of calcite or aragonite (Shearman and Smith 1985). Geochemical calculations and hydrogeologic considerations suggest that waters with dissolved phosphate contents sufficient to inhibit calcite growth and stabilize ikaite could have developed in the Creede moat lake, although a chemical remnant of high phosphate content is not observed in the strata. The granular texture of the Creede pseudomorphs is common to calcite pseudomorphs after ikaite from many localities. A model is proposed in which slow pseudomorphic replacement (over a period of a day or more) results in sand-size calcite grains formed directly from the ikaite. This model has important implications for the interpretation of isotopic and chemical data obtained from pseudomorphs (and thinolitic tufa).