Morokweng impact structure, South Africa: Geologic, petrographic, and isotopic results, and implications for the size of the structure

Abstract

A large impact structure has been identified around Morokweng in the Northwest Province of South Africa. The central aeromagnetic anomaly of this structure is caused by a ~30-km-wide melt body. Textural, mineralogical, and chemical analyses of many drill-core samples of this melt rock are reported. Shock metamorphism in granitoids drilled below the melt body confirms the origin of this melt rock by impact melting. The age of the Morokweng impact melt rock is well-constrained at 145 Ma from single zircon U-Pb isotopic analysis. This age is indistinguishable from the biostratigraphic age of the Jurassic-Cretaceous boundary. Rb-Sr isotopic data of whole-rock and mineral specimens of impact melt rock show that the melt-clast system was not completely isotopically equilibrated. Secondary alteration caused, at least partial, open system behavior. Petrographic studies of the granitoids below the melt body suggest that this ~50-m-wide zone represents a megabreccia and not, as previously suggested, Archean basement. Analysis of a suite of dike breccia samples from this zone revealed the presence of injected impact melt rock, cataclastic breccia, and altered melt breccia, which could have resulted either from impact melting of highly shock-metamorphosed material or from frictional melting. The occurence of several distinct breccia types does not allow us to classify these dike breccias summarily as pseudotachylite, as earlier workers were prone to do. New surface and subsurface information, especially with regard to the stratigraphic thicknesses of the preimpact Dwyka Group and postimpact Kalahari Group in the region, suggest that the Morokweng impact structure could originally have been at least 200 km wide.

Pseudotachylitic breccia from the Dhala impact structure, north-central India: Texture, mineralogy and geochemical characterization

“…In addition, impact melt injected into the crater floor may macroscopically closely resemble some types of pseudotachylitic breccia (e.g., as diagnosed in the Morokweng impact structure - Reimold et al, 1999). Lambert (1981) provided a detailed report of very different types of melt-bearing or cataclastic breccias occurring within the crater floor rocks of the Rochechouart impact structure in France.…”

Impact-generated pseudotachylitic breccia in drill core BH-5 Hättberg, Siljan impact structure, Sweden

“…Gibson & Reimold 2008;Reimold & Koeberl 2014). Reimold et al (1999) demonstrated in a detailed petrographic and chemical investigation of dyke breccias from the crater floor of the large Morokweng impact structure in South Africa (Reimold & Koeberl 2014) that both melt-bearing impact breccias and impact-generated cataclasite (monomict lithic impact breccia) were present. This represents a typical example that a range of clast-based or melt-bearing breccias may occur in such settings and megascopically closely resemble friction melt.…”

Woodleigh impact structure, Australia: Shock petrography and geochemical studies

“…The diameter of this possible impact structure was estimated at 120 km from interpretation of regional geophysical data, which would make Woodleigh one of the 5 largest impact structures (after Vredefort, Sudbury, Chicxulub) known from the terrestrial impact cratering record (compare Grieve 1991; Grieve et al 1995). Mory et al (2000a) also placed the Morokweng impact structure, South Africa, in a similar size category, but initial indications of a >100-200 km diameter of this structure (e.g., Reimold et al 1999) have been refined to about 70 or 80 km based on more recent studies (e.g., Henkel et al 2002;Reimold et al 2002).…”