Cemented ultramafic till beneath a podzol in southeast alaska
1994
ALEXANDER, E. B. | GRAHAM, R. C. | PING, C. L.
A thick grayish to light olive brown (2.5Y 5/3, 7/3 dry) hardpan was found in till in a cold perhumid climate. It was sampled to determine the nature and conditions of cementation. The hardpan is very gravelly, has a mean particle density of 3.0, a bulk density of 2.6 Mg m-3, including pebbles, and a dry compressive strength about 20–50 MPa. Olivine, clinopyroxene, serpentine, and magnetite are the dominant minerals in the hardpan. Only serpentine was detected in the clay fraction. The nature of the cement is not discernible in thin-sections. Silicon, Mg, and Fe are the main elements in the cemented matrix. Dissolution studies indicated that basic (NaOH) and reducing (dithionite) solutions effected more disintegration of the hardpan than acid (Na-acetate, pH 5) solution. All of these solutions dissolved comparable amounts of Si, but NaOH solution dissolved little Fe. Therefore, the key element in the cement is probably Si. Na-acetate (pH 5) is more effective in dissolution of the ferromagnesian silicate minerals than in dissolution of the cement. Si-cemented hard-pans, or duripans, occur where weathering releases Si more rapidly than it is incorporated into secondary silicate minerals or is leached from the soil. These conditions are most extensive in seasonally dry soils with volcanic ash. Weathering may release Si from ultramafic materials rapidly too, because olivine and serpentine are more readily weathered than other common silicate minerals. The cement in the ultramafic till may be either silica or some combination of serpentine and silica.
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