Abstract

Several types of calcium carbonate cement occur on alluvial fans in southern Nevada. Secondary calcium carbonate deposits of pedogenic origin begin as coatings on pebble-size and larger clasts, and progress through calcic horizons to petrocalcic horizons and laminar layers. Nonpedogenic deposits include some laminar layers, gully-bed cementation, and case hardening.

The extent and development of cementation are greatest on fans composed of carbonate and basic igneous rock detritus, less on fans built of siliceous sedimentary detritus, and least on fans composed of acid igneous rock material. In a single fan, the best-developed cementation is in the poorly sorted layers of alluvium containing more than 25 percent material coarser than pebbles. The major source of calcium carbonate deposited on noncarbonate fans is apparently wind-blown silt and sand and, as a result, fans of noncarbonate detritus are best cemented downwind of playas high in carbonates.

Only fans composed of carbonate and basic igneous rock detritus show clearly defined, older, abandoned surfaces and extensive areas of the strongest forms of cementation; petrocalcic horizons and laminar layers are found only on these surfaces. It is suggested that formation and preservation of these surfaces are contingent upon development of strong carbonate cementation.

Petrocalcic horizons and laminar layers apparently formed over a wide climatic range, and variation of the other factors controlling the degree of cementation, therefore, appears to govern the formation and preservation of abandoned surfaces.

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