ABSTRACT

Field observation suggests five distinct crustal types among the playas of the Mojave Desert, southern California, as follows:

(1) Hard, dry, smooth, compact and impermeable crusts, generally with low water tables, ordinarily contain in excess of 50 percent clay-size particles, average about 7.0 percent carbonate, and contain about 5.0 percent or less soluble salines.

(2) Soft, porous and moderately permeable surfaces ordinarily contain less that 35 percent clay-size particles, average about 2.0 percent carbonate, and contain more than 5.0 percent soluble salines. When clay is covered, introduction of moisture to the surface produces a "self-rising" ground or "puffy" surface. High water tables are common to these surfaces and vehicular traffic is poorly supported.

(3) Salt pavements, (4) calcareous crusts, and (5) permanently wet crusts constitute three additional types recognized in the field. Combinations of two or more types may also occur in one playa. Salt pavements and wet crusts are often found in the same playa basin with puffy crustal types. These playas, although not distinct crustal types, have been referred to as compound surfaces. Contacts between surface types are gradational and appear to follow definite trends suggesting mineralogic control.

Some crusts contain 35-49 percent clay-size particles and moderate amounts of carbonate and soluble salts. Although these surfaces are megascopically indiscernible from clay crusts, they are more susceptible to physical change when exposed to precipitation. These surfaces are considered as a variation of the clay crustal type. Laboratory examination indicates that type depends in a large measure upon the nature, proportions, and particle-sizes of the mineral constituents combined with the presence or absence of moisture.

Strength, compaction, and permeability are shown to be interrelated with mineral content. The mineral content of crusts depends in a large measure on lacustrine conditions of sedimentation, largely Pleistocene.

The high-carbonate clay crusts as now found appear to demonstrate slow flocculation of clay particles in relatively low alkali-chloride basins. Carbonate or possibly bicarbonate phases may have acted as peptizers which inhibited clay flocculation and led ultimately to a wide, homogeneous, distribution of clay particles. Slow settling of clays resulted in a dense, compact clay mass with a high degree of particle alignment. Mutual clay-carbonate precipitation may have resulted in a surface interaction between the phases.

Basin water of high alkali-chlorinity electrostatically induced rapid coagulation of fine particles. The resulting sediment texture was heterogenous, with clay particles aligned in a haphazard manner. Salt, clay, and silts were intermixed. These factors facilitate brine percolation and result in a mineral complex producing puffy, saliniferous, wet, and compound crusts.

Local geologic factors may influence ground water and brine movement in crusts so that secondary mineralization may alter original assemblages. These altered areas are generally of minor aerial extent as compared to the playa as a whole, and are easily recognized.