Abstract

Silcrete forms a remnant siliceous caprock over extensive areas of the Australian interior. Thin sections reveal considerable preservation of primary textural and mineralogical characteristics. Other features including abundant turbid, cryptocrystalline matrix, examples of dissolution embayments and floating textures are indicative of intense weathering and silicification. Cryptocrystalline silica is concentrated in random patches, continuous horizons, intergranular voids and immediately above larger quartz clasts. Observations on nodular to concretionary silcrete suggest that the cryptocrystalline silica was deposited during a main phase of silcrete development and as a result of continued, post-silicification near-surface weathering. Slides of specimens collected from well exposed silcrete developed in and directly overlying adamellite at one locality provide evidence of three distinct phases of silicification, which occurred both in regolith and in underlying parent bedrock. A close correlation between primary lithology and mode of occurrence of silica is noted wherein amorphous silica is associated with claystones while quartzose cement typifies overlying Tertiary sandstones. Petrographic observations serve to distinguish silcrete from other silicified sediments encountered during the study.

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