Terrace stratigraphy and soil chronosequence of Canada Alamosa, Sierra and Socorro counties, New Mexico
David J. McCraw and Shannon F. Williams
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. 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 eleven terraces were mapped.The effects of intercanyon and intraterrace spatial soil variations somewhat diminished the effectiveness of the soil chronosequence developed in this study. Soil profile CA3 shows the maximum amount of development, more so than the oldest profile, CA1. 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.The data obtained in this study permit a relative age chronology to be 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. Rio Grande downcutting into the Cuchillo surface at the top of Palomas Formation aggradation began ~0.8 Ma and has progressed some 90-100 m. Cañada Alamosa terraces Qt1- Qt6 likely formed 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.
- McCraw, David J.; Williams, Shannon F., 2012, Terrace stratigraphy and soil chronosequence of Canada Alamosa, Sierra and Socorro counties, New Mexico, in: Geology of the Warm Springs region, Lucas, Spencer G.; McLemore, Virginia T.; Lueth, Virgil W.; Spielmann, Justin A.; Krainer, Karl, New Mexico Geological Society, Guidebook, 63rd Field Conference, pp. 475-490.