New Mexico Geological Society Annual Spring Meeting — Abstracts

Morphological, textural, and chemical data for soils formed on andesitic alluvium form the basis for a preliminary soil chronosequence for the piedmont and shoreline deposits of San Agustin Plains, west-central New Mexico. Middle Holocene soils (Aridic Haplustolls) on inset valley fill terraces (Q3) possess thick mollie epipedons (0.6-2.8% organic carbon), cambic or argillic horizons (max. clay % increase = 11.6%; max. reddish color = 10YR 5/3) and stage I secondary carbonate morphology. Early Holocene soils (Typic Calciustolls) have developed on shoreline deposits of the late Pleistocene Lake San Agustin (11000-13000 yrs B.P.) and have thin, organic matter-rich ochric epipedons, moderately developed argillic horizons (max. clay %increase = 15.1 %; max. reddish color = 7.5YR 5/4) and stage II secondary carbonate morphology. Late(?) Pleistocene soils (Typic Paleargids) on older inset fill terraces (Q2) exhibit thin, organic matter-rich ochric epipedons, strongly developed argillic horizons (max. clay % increase; 20.3%; max. reddish color = 5YR 4/6) and massive, stage III secondary carbonate morphology. Mid-to early (?) Pleistocene soils (Ustollic Haplargids) on extensive fan deposits (Q1) exhibit only weakly developed ochric epipedons and weak Bt horizons, but overlie a stage IV, laminated calcic horizon.

The present ustic, mesic climate of the study area clearly favors rapid accumulation of organic matter in the gravelly soils and accumulation of silicate clay in the B horizons. Additionally, calcic horizons at (10-60 cm deep) have formed despite the relatively moist climate, significant vegetal cover, and noncalcic parent materials. This suggests a locally high influx of externally derived carbonate, clay and silt; possible sources include playalacustrine deposits of San Agustin basin or erosion of a formerly well developed soil on the oldest piedmont surface (Q1).