PEDOGENIC SILICA AND CARBONATE ALONG A CHRONOSEQUENCE IN NORTHERN PAHRANAGAT VALLEY, NEVADA

Arid and semi-arid soils occupy approximately 36% of the earth’s surface. Agriculture will heavily depend upon these increasingly important soils as world populations grow. Soils found in Pahranagat Valley of southeastern Nevada may have an important roll in the urban and agricultural future of Southwestern Nevada. Located approximately 120 miles north of Las Vegas, Nevada, the Pahranagat Valley is bounded by two north-south trending mountain ranges formed from Cenozoic extension of the western United States.

This study focused on five alluvial landforms and associated soils that are mid-Pleistocene to late Holocene in age. Goals of the project were to use macroscopic, microscopic and Scanning Electron Microscopy (SEM) analyses to determine structure, chemistry and relationships between secondary carbonate and silica found throughout all soils (excluding late Holocene) in the area. Also, two lithosequences and one chronosequence were chosen to provide a look at changes in silica and carbonate genesis and concentration over time and changes in lithology. Ten soil profiles were dug and described in the field. Soil texture, pH and EC analyses were completed in the UNLV Pedology Laboratory. Soils were classified according to USDA Taxonomy. Field and laboratory work has revealed that silica is present in all soils of the field area excluding late Holocene soils. Silica presence ranges from durinodes in early Holocene soils to opaline laminae found in pendants and hardpans in older soils. Secondary carbonate stages I-IV are also present. In arid environments, chronosequence studies are important in comparing the rate of accumulation and changes over time of silica, secondary carbonate and clays in soil. Silica and carbonate have a large influence on water movement in soils which is an important factor for agriculture production and land development. This study hopes to contribute to an overall understanding of silica and carbonate genesis in the soils of southern Nevada.