Genesis of typic paleorthids and petrocalcic paleargids on the same fan terrace in the Avra Valley near Tucson, Arizona

Abstract

Two soils, both underlain by petrocalcic horizons at shallow depths, occur together on the same geomorphic surface, an old fan terrace located in the Avra Valley near Tucson, Arizona. Both soils have formed from similar parent materials and are assumed to be of the same age. One of the soils, a Petrocalcic Paleargid, occupies the broad, flat interfluvial areas of the fan surface. It contains an argillic horizon above the hardpan and is carbonate free in the upper part. The lower portion of the argillic horizon has been engulfed by the petrocalcic horizon. The other soil, a Typic Paleorthid, occurs above and adjacent to the narrow drainage ways that incise the fan terrace. This soil lacks an argillic horizon and is calcareous to the soil surface. Although no direct evidence of a former argillic horizon remains, it is believed that an argillic horizon was once present and then was truncated. Truncation was followed by brecciation of the exposed pan surface and subsequent recementation. The observed differences between the two soils: calcium carbonate content, particle size distribution, color, morphology and size of calcite crystals, and clay mineralogy are a result of the pedogenic processes of argillic horizon engulfment and truncation followed by brecciation and recementation.