A preliminary investigation of hardpan horizons in north-west New South Wales

Abstract

Micromorphological, scanning electron microscope, electron microprobe, X-ray diffraction and chemical analyses of morphologically differing hardpan horizons show a wide range of constituent materials and interparticle cements. A number of different fabric elements occur within the hardpans. These include porphyroskelic zones with amorphous silica in the s-matrix, zones composed almost entirely of amorphous silica, chlamydic zones with clay coatings on skeleton grains, and zones of calcareous material filling fissures. A further porphyroskelic fabric type, in which the plasma consists of strongly oriented clay intimately mixed with isotropic material containing amorphous silica, was also recognized in one type of hardpan. Amorphous silica is the cementing agent within some of the fabric zones identified, but in the chlamydic zones, at least, clay minerals enriched in silica, iron and titanium, and depleted in aluminium, appear to be the cementing medium. Micromorphological evidence indicates a complex development of the hardpans with alternating phases of silica, clay and carbonate deposition.