Mineralogy of 140-Foot Core from Willcox Playa, Cochise, Arizona: ABSTRACT

Abstract

The Willcox basin in southeastern Arizona is a closed drainage system of approximately 1,600 square miles. A core, consisting almost entirely of black mud with a silt-clay ratio of 3:7, was recovered from the playa at the south end of the basin. The diagenetic environment of the core sediments is characterized by high pH (9.0-9.5) and negative Eh (-100 to -300 mv) values. The dominant clay mineral is illite, with montmorillonite, mixed-layer illite-montmorillonite, and vermiculite in decreasing order of abundance. Two distinct monoionic divalent montmorillonite complexes occur with basal spacings of 14.2-14.7A and 15.2-15.4A, respectively. A moderately well-crystallized trioctahedral vermiculite occurs in about half of the core samples. Kaolinite and chlorite are present only in trace amounts, the kaolinite occurrences being restricted to the upper few feet of core. Comparison of the clay minerals of the core with those of the source area suggests that the former are of detrital origin. End_Page 478------------------------------ The clay mineral assemblage at the playa surface is dominated by illite with less than 5 per cent expansible phases present. Illite also is the predominant mineral in the source area; however, significant amounts of montmorillonite, chlorite, and kaolinite are being generated in the drainage basin. Discrepancies between the clay minerals of the lake bed and those of its source area are best explained by selective removal of finer colloidal material at the playa surface. The coarse sediment fraction contains detrital quartz, feldspar, heavy minerals, authigenic calcite, and ostracod valves. Saline minerals occur only as crusts at the playa surface. Authigenic analcime is an ubiquitous constituent of the -2-micron fraction in amounts of the order of 10 per cent by weight. The absence of tuffaceous sediments in the core precludes alteration of volcanic glass to form analcime. Evidence is presented which suggests that analcime is a reaction product of kaolinite in the diagenetic sedimentary environment. End_of_Article - Last_Page 479------------