TERRACE STRATIGRAPHY AND SOIL CHRONOSEQUENCE OF CAñADA ALAMOSA, SIERRA AND SOCORRO COUNTIES, NEW MEXICO

A total of six major terraces of probable climatic origin incised into the Palomas Formation of the Santa Fe Group make up the terrace stratigraphic framework for Cañada Alamosa, a large tributary of the Rio Grande, whose mouth lies some 25 km upstream of Truth or Consequences, New Mexico. The tread heights of these terraces extend from 9-16 m to 76-83 m above the modern floodplain and thicknesses range from 4.7 m to 18 m or more. Quaternary tectonism along the Cuchillo Negro fault zone has offset or warped the oldest four terraces and created two localized terraces, suggesting relative age of movement along faults from ~0.55 to ~0.32 Ma. Minor structural perturbations and/or complex response of transport/depositional systems are responsible for additional local terrace surfaces along the lower four terrace levels. A total of 11 terraces were mapped.

The effects of intercanyon and intraterrace spatial soil variations somewhat diminished the effectiveness of the soil chronosequence developed from profiles examined on five of the six terraces in this study. Soil profile CA3 on terrace Qt3 shows the maximum amount of development, more so than the profile CA1 on the oldest, highest terrace Qt1. Nevertheless, soils generally exhibit increases in pedogenic fines (silt + clay percentages increasing from 7.5 to 26.4 %) and carbonate development (stages I to III) with age. Soil Development Indices reflect this as well and correlate (r² = 0.776) with values derived from the Desert Project soils with known ages, indicating that Cañada Alamosa terraces fall within the established southern New Mexico alluvial morphostratigraphy.

A relative age chronology is suggested, with ages based upon the rate of incision, degree of pedogenesis, and the timing of glacial cycles recorded in the marine oxygen isotope record of the equatorial Pacific. Following cessation of Palomas Formation aggradation at ~0.8 Ma, the Rio Grande has downcut some 90-100 m into the abandoned Cuchillo geomorphic surface in a series of climatically-driven episodes of incision punctuated by periods of partial backfilling. Cañada Alamosa terraces Qt1-Qt6 likely formed following tread abandonment at ~0.67-0.63, ~0.58-0.56, ~0.45-0.43, ~0.38-0.32, ~0.15-0.12 Ma, and 17 to 15 ka.