New evidence supports the theory that the hematite pigment in many red beds, particularly those associated with evaporites and aeolian sandstones, formed after deposition in hot arid or semiarid climates. The evidence includes field, petro-graphic, and chemical data collected from studies of two stratigraphic sequences that contain red beds: (1) Recent, Pleistocene, and Pliocene deposits in the Sonoran desert of northeastern Baja California, Mexico, and (2) late Paleozoic deposits in Colorado.

The sequence in the Sonoran desert contains examples of red beds forming today in a hot dry climate. They are associated with bedded evaporites and occur where regional faunal, floral, and pedological evidence indicates rainfall has been low throughout the postdepositional history of the sediments. The facies associations reflect deposition in fluvial and fluvial-marine transitional environments. Red arkose fanglomerates occur on the flanks of the highland source areas, and red muds, probably of intertidal and shallow subtidal origin, occur in the transition sediments. Both of these red-stained facies show progressive stages of in situ alteration of nonred sediments to hematite-stained red beds, and in each, the iron in the stain is derived from intra-stratal alteration of iron-bearing detrital grains, particularly iron silicates such as hornblende and bio-tite.

The late Paleozoic red beds of Colorado contain rock types and facies associations similar to those of the Sonoran desert; they are interpreted as ancient counterparts of those red beds. Numerous lines of field and petrographic evidence indicate that the hematite pigment in the late Paleozoic red beds formed in place after the sediments were deposited in desert basins.

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