Abstract

Observations and measurements on hardpan calcrete (nari) from chalk indicate that three distinct horizons can be distinguished in the nari profile, (a) a very hard thin laminar crust (500-800 kg/cm 2 ), capping (b) a thick strongly indurated upper nari (100-500 kg/cm 2 ) and (c) a thick soft moderately indurated lower nari (20-120 kg/cm 2 ). Moisture content affects greatly the hardness value. If dry, the underlying chalk is harder than the lower nari. The Schmidt Test Hammer Type N was used for non-destructive measurements of relative hardness (strength) in the field. Calibration against unconfined compression strength on rock cubes in laboratory tests produced a highly significant relationship. Porosity has been shown to be the major controlling factor in determining hardness, while CaCO 3 content and its microtexture are contributing factors. This indicates that induration of the nari is due to infilling of CaCO 3 , partial recrystallization in fissures and cracks, and some displacement of non-carbonate material. It is suggested that the process of infilling and crystal growth of carbonate in the host sediment, with some domains of recrystallization, is characteristic of calcrete formation in consolidated, but porous sediments. In unconsolidated sediments the deposition of carbonates can be accompanied by volume expansion and an increased separation of the non-carbonate grains.

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