THE 1992 DRILL CORE FROM THE KALKKOP IMPACT CRATER, EASTERN-CAPE PROVINCE, SOUTH-AFRICA - STRATIGRAPHY, PETROGRAPHY, GEOCHEMISTRY AND AGE

New drill core data are provided which support earlier interpretations that the Kalkkop structure, a 600-630 m wide, near-circular feature s outh-southwest of Graaff-Reinet in the Eastern Cape Province of South Africa, is a meteorite impact crater. Shock metamorphosed clasts in su evitic crater fill and Re-Os isotope data of this breccia indicate the presence of a minor (0.05%) meteoritic component in the suevite. The new data come from a 1992 borehole, which transected the complete crat er fill and extended from about 160 to 380 m depth into the sedimentar y basement belonging to the Koonap Formation of the Beaufort Group (Ka roo Supergroup). Dyke breccias were found in the otherwise coherent Be aufort Group sediments forming the floor to the Kalkkop Crater. Mostly narrow zones of different breccia types, including injections of lith ic impact breccia, a possible pseudotachylite veinlet and cataclasite occur predominantly in an approximately 65 m wide zone below the crate r floor, with a few other cataclasite occurrences found lower down in the basement. Stratigraphical crater constraints provide information f or the depth-diameter scaling and breccia volumes associated with such small, bowl-shaped impact craters formed in sedimentary targets. U-Th series dating of limestone samples from near the top and the bottom o f the crater sediment fill constrains the age of the Kalkkop impact ev ent to about 250 +/- 50 ka, similar to the age of the Pretoria Saltpan impact crater, also located in South Africa. The variety of different breccia types (polymict and monomict impact breccias; local formation s of pseudotachylitic and cataclastic breccias) observed in the crater fill of the Kalkkop Crater indicates the need to carefully distinguis h different breccia types in order to assess the respective importance of each formation. (C) 1998 Elsevier Science Limited.