A PRELIMINARY INVESTIGATION OF HIGHLY INDURATED PETROCALCIC SOIL HORIZONS ALONG THE LOWER COLORADO RIVER CORRIDOR IN NORTHWESTERN ARIZONA

Highly developed calcium carbonate soils are recorders of past climate change and surface processes over the past several million years. A 2-3 meter thick calcium carbonate deposit that caps alluvial landforms adjacent to the lower Colorado River north of Bullhead City, Arizona in the Lake Mead National Recreation Area (LMNRA) is preliminarily identified as one of these soils. The objectives of this study were to (1) determine if the material is pedogenic, (2) describe the macro-features present in the horizons, (3) use these features to determine the past erosional/depositional events that have affected its development, and (4) compare its development to similar soils in the region. Three profiles were sampled and color, size, shape and arrangement of macroscopic features were described and measured. Pedogenic features present include two types of pisoliths, horizontal and (rare) vertical lamina, cracks and linear voids that cut across the indurated materials, partially dissolved volcanic clasts (parent material), and pendant lamina commonly observed on the undersides of the parent material clasts. One type of pisolith is observed as older, angular petrocalcic fragments that represent erosion, brecciation and subsequent eolian deposition and re-cementation by calcium carbonate into the upper horizons. Pisoliths with diffuse edges and color differences from the surrounding cementing materials could represent mineralogical changes within the horizons. Evidence of dissolution of parent material clasts indicates soil water mobility within these highly indurated materials. This study is important because understanding the genesis and development of these unique soils benefits land use planners, flood and earthquake hazard assessment, and our overall understanding of geomorphic processes related to soil development in these arid and semi-arid regions.