Problem: Age of Rooiberg “felsite.” If pre-BIC, it would be the roof of an intrusive Bushveld Complex. The controversial impact origin (suggested by E. H. L. Schwarz, 1909; R. S. Dietz, 1963, W. Hamilton. 1970; elaborated by R. C. Rhodes, 1975) would be precluded; it requires an open system.
Its 1890s discoverer, G. A. F. Molengraaff (Dutch; State Geologist, Transvaal Republic), called the BIC basin a laccolite; E. T. Mellor (1907) gave it a pre-BIC Waterberg roof, with Rooiberg felsite as base. Lopolith was introduced by A. L. Du Toit (1926), by analogy with Sudbury.
During the 1922 Shaler Memorial Expedition, R. A. Daly (Harvard), guided by Molengraaff, found a Rooiberg-Waterberg unconformity. They concluded that “felsites belong to the Bushveld Complex” which had “…no continuous sedimentary roof at all…a colossal lava flow…” (1924, J. Geol. 32, 1-35). When later workers rightly discredited the lava flow, the roofless idea was forgotten. In 1980, the authoritative S. A. Commission on Stratigraphy ruled that “…the Rooiberg is not part of the Bushveld Complex” and assigned it to the pre-BIC Transvaal Supergroup. This misconception persisted in spite of evidence by C. J. Hatton and J. K. Schweitzer (1995) that the Rooiberg roof accumulated while mafic and granite sills invaded its base. In the 2006 Geology of South Africa, Cawthorn et al. cautiously conceded that the Rooiberg “…may be…more closely related to the Bushveld event than to the Transvaal Supergroup.”
Vindication of the 1924 Daly-Molengraaff interpretation of a roofless open-system Bushveld removes one objection to the impact hypothesis. Many others remain. A century of research and >1600 Georef entries have brought no consensus on Bushveld origin. A unique phenomenon may require evaluation by unique criteria. One historical hint: after long debates, the Sudbury “lopolith” is now widely accepted as an impact structure.
Preferred Presentation Type: Oral