Microtopography and surface features of the mid-Pleistocene Kalkkop impact crater and Palaeolake, South Africa

• Kalkkop crater redefined as a 1.2 km eroded impact structure. • Carbonate benches trace palaeolake margins with microbialite-rich dips. • Isotopes show lake desiccation and radial hydrological gradients. • Fossil Cyperaceae extend palaeo-range in mid-Pleistocene deposits. • Terrain metrics reveal subtle crater structure beyond carbonate exposure. Meteorite impact structures are rare yet significant components of Earth’s geological record. Kalkkop, a mid-Pleistocene crater in South Africa’s semi-arid Eastern Cape, was confirmed as extraterrestrial in origin during the 1990s through deep coring that revealed a brecciated basement beneath lacustrine limestone. However, the surface morphology of both the impact structure and the palaeolake has remained insufficiently resolved. High-resolution terrain analysis—integrating sub-metre digital elevation models, orthophotography, and geomorphometric indices—reveals an ovate crater of ∼1200 m across, nearly double the size than previously recognised. The carbonate ring, once interpreted as the crater rim, instead marks the infilling palaeolake margin. The true rim extends over 1.2 km and is delineated by concentric elevation anomalies, slope breaks, and terrain roughness peaks. Shoreline-aligned carbonate benches dip inward, with steeply inclined microbialite-rich margins forming under shallow, evaporative conditions. δ 13 C and δ 18 O values reflect a radial hydrological gradient, transitioning to thicker, flat-lying central carbonates deposited in deeper water. Silicified remains of Pseudoschoenus inanis (Cyperaceae) from the central basin represent the first known fossil occurrence of the taxon and signal a shift from lacustrine to palustrine conditions. U–Th dating of various carbonate benches ranges from >650 ka to approximately 120 ka, which confirms progressive crater infilling and refutes earlier hypotheses suggesting a Miocene origin. Collectively, these findings reframe Kalkkop as a larger, eroded, relief-inverted impact structure and underscore the efficacy of high-resolution surface analysis in reconstructing impact morphology and palaeoenvironmental evolution in low-relief landscapes.