Abstract

The 650 m wide Kalkkop meteorite impact crater is situated in the Karoo, ~51 km southwest of Graaff-Reinet, South Africa. A core drilled through the centre of the crater passes through 89 m of variously laminated and massive, carbonates that overlie brecciated sandstone and shale of the Beaufort Group. Kalkkop has been previously dated at ~250 Ka using the U-Th method, similar in age to the Tswaing Crater, 40 km northwest of Pretoria. However, the inferred crater rim at Kalkkop has been almost entirely eroded and, given the low regional erosion rates (<4 m/million years), would appear to be much older than Tswaing. The carbonates are dominated by dolomite in the upper 35 m and by calcite below 35 m. Carbonate δ18O values vary between -3.85 and 6.64‰ (relative to PDB), and generally increase upwards. These data are consistent with precipitation from water ranging in δ18O between -3‰ (typical of meteoric water) at the base, and ~+5‰ (typical of highly evaporated water) in the upper part of the succession. The lower part of the succession shows a strong correlation between δ13C and δ18O values, suggesting non–equilibrium precipitation during periods of high evaporation rate. The upper part shows no correlation between δ13C and δ18O, which is consistent with equilibrium between the carbonates and the lake water, suggesting that recharge of water became more continuous. The Kalkkop Crater carbonates have great potential as a proxy for past climate, but first a detailed dating study is required.

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