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Geomorphic Age and Genesis of Some San Luis Valley, Colorado, Soils

Natl. Soil Survey Lab., Lincoln, NE 68508-3866

J. M. Yenter

USDA-SCS, Alamosa, CO

T. W. Priest

USDA-SCS, Denver, CO.

* Corresponding author.

ABSTRACT

San Luis Valley is a semibolson in south central Colorado that has varying geomorphic and soil properties. Three sets of geomorphic surfaces were identified corresponding to young, intermediate, and old surfaces. A ¹⁴C date of 11 170 YBP on a peat deposit on the valley floor was used to separate Holocene geomorphic surfaces (Set 1) from late-Pleistocene ones (Set 2). The oldest-Pleistocene surfaces (Set 3) studied are believed to be Illinoian in age. They are above the late-Pleistocene age valley floor and are more dissected than the other Pleistocene surfaces. Entisols have formed on the Holocene surfaces (Set 1). These Entisols have some accumulation of organic C and movement of carbonate, but none have calcic horizons. Their sand grains lack clay cutans or other evidence of soil formation. Most of the soils on late-Pleistocene geomorphic surfaces (Set 2) have argillic and calcic horizons, and some have mollic epipedons. Grain argillans on sands are the most common form of illuvial clay and there are calcans in the calcic horizons. Some of the soils have natric horizons. The soils on mid-Pleistocene geomorphic surfaces (Set 3) have a greater clay accumulation than any of the other soils and have calcic horizons. Clay accumulation is largely masked by the carbonate accumulation. The distribution of salt for the most part is in balance with today's arid climate in the valley. The occurrence of carbonate in horizons with illuvial clay, especially in the soils on the oldest surfaces, suggests an arid climate following one or more Pleistocene pluvials. Some of the salt and carbonate may have been added as dust from playas on the valley floor.

Contribution from USDA-SCS, Natl. Soil Survey Lab. Lincoln, NE, and Nat. Cooperative Soil Survey Alamosa and Denver, CO

Journal Series no. 275.

Received for publication March 1, 1988.