The chemistry and mineralogy of some South Australian calcretes and associated soft carbonates and their dolomitisation

Abstract

The chemistry and. mineralogy of calcretes and associated calcareous fine earth from the lower River Murray Basin and Yorke Peninsula in South Australia suggest that the carbonate material was deposited as a continuous unit and subsequently modified. At each site examined the Ca/Mg ratio progressively decreases with increasing depth, and where data are available, a similar decrease in the Ca/Sr ratio is observed. This sequence is independent of the hardness or physical condition of the carbonate. In some places the carbonate from the lowest horizons is nearly pure dolomite. The change in Ca/Mg ratio with depth is considered to be due to leaching, probably during periods of the past when the climate was less arid and the rainfall penetrated to about 2 m. In the lower Murray Basin the Ca/Mg ratio of the surface calcrete horizon progressively increases from Tailetn Bend N to Sedan, and the silicate clay fraction changes from sepiolite at Tailem Bend, to palygorskite at Murray Bridge and illite at Sedan. The bedrock also varies, from magnesium‐rich amphibolites at Tailem Bend to granite at Sedan and, further, there is an apparent relation between the chemistry of the non‐carbonate fraction of the calcretes and the underlying rock. The original carbonate deposit probably formed in lakes of different chemistry, and has been modified by solution and precipitation.